an introduction to groundwater in crystalline bedrock - ngu · page 1 an introduction to...

An introduction toGroundwater in CrystallineBedrock

David Banks & Nick Robins

Norges geologiske undersøkelse

© 2002 Norges geologiske undersøkelse

Published byNorges geologiske undersøkelse(Geological Survey of Norway)N-7491 TrondheimNorway

All rights reserved

ISBN 82 7386 100 1

Editor: Rolv Dahl

Design and print: Grytting AS

Recommended bibliographical reference for this volume:Banks, D. and Robins, N. (2002): An introduction to Groundwater in Crystalline bedrockNorges geologiske undersøkelse, 64ppISBN 82 7386 100 1

An introduction toGroundwater in CrystallineBedrock

David Banks1, Nick Robins2

1 Geological Survey of Norway, N7491 Trondheim, Norway

(Current address:Holymoor Consultancy, 86 Holymoor Road, Holymoorside, Chesterfield, Derbyshire, S42 7DX, United Kingdom).

2 British Geological Survey, Maclean Building, Crowmarsh Gifford,Wallingford, Oxfordshire, OX10 8BB, United Kingdom.

"Therefore a miner, since we think he ought to be a good and seriousman, should not make use of an enchanted twig, because if he is prudentand skilled in the natural signs, he understands that a forked twig is of nouse to him.....So if Nature or Chance should indicate a locality suitable formining, the miner should dig his trenches there; if no vein appears, hemust dig numerous trenches until he discovers an outcrop of a vein."

Agricola (1556). De Re Metallica.

PrologueDavid Banks

Having been employedwith the Thames WaterAuthority, the NationalRivers Authority and theUniversity of Sheffield inthe UK, David Banks workedfor six years with theSection for Geochemistryand Hydrogeology at NGU.

In 1998, he returned to Chesterfield in England, wherehe runs his own international "Holymoor Consultancy"business, through which he has gained hydro-geological experience from regions as diverse as theBolivian A ltiplano, Afghanistan and South Yorkshire.

Nick Robins

Dr Nick Robins works withthe British Geological Surveyat Wallingford in Oxfordshire.He has extensive experienceoverseas on hard rock hydro-geology and has also beenresponsible for promoting

interest in groundwater throughout Scotland andNorthern Ireland. Other interests include groundwater inChalk, in Quaternary deposits, mine water arisings andisland hydrogeology, as well as groundwater manage-ment.

ContentsContents...........................................................................................................................................3

Introduction .....................................................................................................................................5

1. What is Groundwater in Crystalline Bedrock?................................................................................7

2. How to Get Groundwater out of the Ground ..................................................................................8

3. Well Drilling in Bedrock. Bingo, Poker or Chess ? ..........................................................................12

4. Where Should I Drill My Borehole?..............................................................................................15

5. Fracture Zones ..........................................................................................................................17

6. Superficial Deposits...................................................................................................................20

7. Fracture Mapping......................................................................................................................21

8. Geophysics................................................................................................................................23

9. Stress........................................................................................................................................26

10. Borehole Orientation .................................................................................................................27

11. Drilling .....................................................................................................................................28

12. Borehole Yield Stimulation ........................................................................................................31

13. Test Pumping ............................................................................................................................33

14. Water Quality............................................................................................................................38

15. Water Treatment .......................................................................................................................43

16. Vulnerability and Source Protection ...........................................................................................44

17. Maintenance & Rehabilitation ...................................................................................................47

18. Ground Source Heat...................................................................................................................49

19. Conclusion ................................................................................................................................52

20. References ................................................................................................................................53

21. Glossary....................................................................................................................................61

Cover photos:

1. Shedding light on rock and water. Corbiere Lighthouse, Jersey, Channel Islands. Photo by Joe Bates.

2. Geologist Helge Skarphagen examines springs from gneisses exposed in a road cutting near Herefoss, southern Norway. Note that groundwater tends to emerge along the boundaries of pegmatites (light rocks).Photo by David Banks.See Figures 25a,b for locations of sites mentioned in the text.

3. Winter drilling, near Glasgow, Scotland.Photo by Nick Robins.

PAGE 4

This small book has a bigambition. It aims to presentpractical information and a little

philosophy to those involved in locatinggroundwater resources in areasunderlain by crystalline bedrock, that isto say:

• Private groundwater users, potential well owners and water bottlers

• Local authorities• Water companies and local water

supply undertakings• Drillers• and Consultants

Each of these users will inevitably havedifferent requirements and this volumemay be considered to be a "maximumversion", hoping to provide somethingfor everyone. We have consciouslymixed practical advice with somehydrogeological theory. Pick andchoose the parts that you find useful.We have also provided a comprehensivereference list for those of you who wishto delve further into the subject. Weaim to try to communicateScandinavian findings (often largelypublished in Nordic languages) to aninternational audience. A Norwegianversion of this book will be publishedlater.

Almost all of Norway (Figure 25b) isunderlain by some type of crystallinebedrock, and groundwater from suchrocks is an important drinking waterresource in rural areas, with over100,000 bedrock wells thought to existin a country with a population ofsomewhat over 4 million! In the UnitedKingdom, crystalline bedrockgroundwater is probably an underusedresource. Such rocks underlie much ofthe U.K.'s "Celtic Fringe" - Cornwall,Wales, Scotland and parts of NorthernIreland (see Figure 25a: Robins 1990,1996a,b, Robins and Misstear 2000), aswell as the Channel Islands (Robins &

Smedley 1994, Blackie et al. 1998). Anumber of small British communitiesare almost entirely dependent onbedrock groundwater, such as severalof the islands of Scilly (Banks et al.1998e), and such groundwater providesan attractive alternative resource forother communities with a currentlyunsatisfactory water supply (Ellingsen& Banks 1993).

Bedrock aquifers are also exploitedwidely in tropical climes; in much ofAfrica and India, for example. There,however, the hydrogeologicalconditions are very different. The rocksare deeply weathered and rainfallrecharge may be scarce. We will thuslargely, though not exclusively, restrictourselves to consideration of bedrockaquifers in the glaciated terrain ofNorway and the northern U.K., whererock outcrops are relatively fresh andwhere the quantity of precipitation isdepressingly abundant.

Groundwater in bedrock is a difficultresource to understand and pin down.It is very difficult to predict the yield orwater quality of a new borehole withany degree of certainty. It is, however,possible to quantify the chances ofbeing successful. We will attempt toguide you through the maze offractures and uncertainties comprisinga bedrock aquifer in such a way as toallow you to make an informed choiceabout its potential as a water resource.

Introduction

There are, of course, two parts to thisquestion:

1.1 What is Groundwater?Groundwater is simply water thatoccurs in the ground; in the pore spacesbetween mineral grains or in cracks andfractures in the rock mass. It is usuallyformed by rain water or snow melt-water that seeps down through the soiland into the underlying rocks.Unfortunately, we have a very poorunderstanding of exactly whatproportion of rainfall ends up enteringa crystalline rock aquifer, althoughRobins & Smedley (1994), Blackie et al.(1998) and Olofsson (1993) shed somelight on the problem. Sometimes, wherea pumping well is close to a river orlake, a well may also "suck" river- orlake-water into the river banks and bed,so that it enters the adjacent sedimentsand rocks and becomes groundwater.

In recent sediments, such as sands orgravels, groundwater flows through themany pore spaces between sand grains.The permeability of the sediment isgoverned by the distribution of grainsizes in the sediment and the yield of awell in such deposits is relatively easy topredict.

1.2 What is crystalline bedrock ?When we use the term crystallinebedrock (or hard rock or bedrock) in thisbook we refer to igneous or metamor-phic rocks, such as granites, basalts,metaquartzites or gneisses, where theintergranular pore spaces are negligibleand where almost all groundwater flowtakes place through cracks and fracturesin the rocks.

As fractures are not homogeneouslydistributed in the rock mass, andbecause the permeability of the fracture

system is very sensitive to the fractureaperture and degree of fractureconnectivity, it is very difficult topredict the yield of a well or borehole incrystalline bedrock. To be successful, weneed to understand, as Agricolarecommended in 1556 (see Prologue)both Nature (in the guise of geology)and the element of Chance.

Further reading on groundwater:Banks & Banks (1993a), Domenico &Schwartz (1990), Downing (1998),Ellingsen (1992a), Ellingsen & Banks(1993), Fetter (1994), Grundfos (1988),ISIS (1990), Knutsson (2000), Lloyd(1999), Olsson (1979), Price (1996),Robins (1990, 1996a,b), Todd (1990).

Hydrogeological Maps ofGroundwater in Crystalline RockIn Norway:Ellingsen (1978), Rohr-Torp (1987).In Sweden: Maps for each county, ofwhich Karlqvist (1985) is an example.In the UK: BGS (1990)

1. What is Groundwater inCrystalline Bedrock?

Figure 1. Schematic diagram of boreholes in acrystalline bedrock aquifer (from Eckholdt & Snilsberg1992). Borehole 1 intersects a thrust fault betweengranite and gneiss and may thus have a good yield.Nevertheless, a stream receiving agricultural run-offruns along the fault outcrop, rendering the wellvulnerable to pollution. Borehole 2 is less vulnerablebut does not intersect a fracture zone and may thushave a lower yield. Borehole 3 is located up-gradientof polluting activities and intersects a fracture zone(expressed in the topography as a linear valley).

2.1 SpringsUnder natural conditions, groundwaterflows from regions of high groundwaterhead to low groundwater head. Inpractice, this usually means, from areasof high topography to the coast or toriver valleys. Because rainfall is enteringthe bedrock aquifer, groundwater has tocome out somewhere. Very often itemerges as springs in low-lying areas,which springs typically drain intostreams or to the sea. Alternatively,groundwater may discharge directlyinto the bed of a stream. In either case,this groundwater baseflow maintainssome degree of flow in the streamsduring prolonged dry weather.

Groundwater flow generally followsthe gradient of the water table. This isessentially the surface separating water-saturated from unsaturated rocks. Inother words, it is the level of water inthe huge natural storage tank that anaquifer represents. In crystalline

bedrock of low permeability, the watertable reflects a subdued version of thenatural topography. A spring dischargearea can be thought of as a locationwhere the water table intersects groundlevel.

Springs have historically beenimportant water supplies. Very oftenthey have been excavated, lined withtimber, brick or stone and maybecovered by a roof or small house toform a well that is protected fromcontamination by surface run-off andanimals. Today, they can still be idealwater supplies, provided that the land-use in the surrounding area is such thatit does not contaminate the spring.

2.2 Wells and boreholesUnfortunately, springs only occur at thewhim of nature and topography. While,in historic times, "Mohammed hascome to the mountain" and people havesettled around springs, more recent

Figure 2. (a) A spring from Precambrian Hecla Hoekmarbles, Bockfjord, Svalbard (photo: David Banks).

2. How to Get Groundwaterout of the Ground

settlements have grown up in areasdevoid of springs and it has beennecessary to use technology to accessthe water table.

In many rocks (e.g. the Chalk ofsouthern England), wells may be dug toconsiderable depths to reach the watertable, but this is not possible in the hardcrystalline rocks we are considering. Inhard rock terrain, dug wells are at bestdug down through superficial soils andsediments to reach a bedrock spring, orare excavated to a few metres depth bythe judicious use of explosives.

In crystalline bedrock it is normal todrill a narrow (e.g. 150 mm diameter)borehole to several tens of metres depthbelow the water table. A pump maythen be installed in the borehole. As itpumps out water, the water level in theborehole is depressed, lowering thegroundwater head in the adjacent

Figure 2. (b) The Maharajah's Well, Stoke Row U.K. Adeep dug well in the Chalk, donated to the drought-stricken villagers of the Chilterns by the Maharajah ofBenares (photo: David Banks).

Figure 2. (c) an artesian (overflowing) borehole inCarboniferous rocks at Catcraig, near Dunbar, Scotland.The photo features the geologist C.T. Clough andderives from c. 1908 (after Robins 1990). Printed withpermission from British Geological Survey.

Figure 2. (d) A modern, angled borehole in granite, Hvaler Islands, Norway (photo: David Banks).

aquifer. This causes groundwater flowto be induced towards the borehole andalters the natural groundwater flow andwater balance in the aquifer. Providedwe do not try to take too much water,the aquifer will settle down to a newdynamic equilibrium situation. Thisequilibrium will govern the long-termyield of the borehole. Usually, the longterm yield is somewhat lower that theyield initially estimated by drillers onthe basis of short-term testing, becausein the latter case, the aquifer has nothad time to reach its new equilibrium.

Further reading on springprotection, groundwater abstractionand borehole drillingClark (1988), Commonwealth ScienceCouncil (1987), Lloyd (1999), Skjeseth(1955), Waterlines, UNESCO (1984).

Figure 3. Groundwater flow in the vicinity of apumped borehole, RWT = rest water table beforepumping, PWT = water table during pumping (afterBanks 1992d, printed with permission from BlackwellScience Ltd.).

It is tempting to regard well-drilling inbedrock as a game where the prize is ahigh quality, cheap water supply. But isit a game like chess, where a geologist'sskill and knowledge can find the rightborehole location and drilling strategy,or is it like bingo, where the outcome issolely determined by a randomselection of unpredictable numbers?Most hydrogeologists would probablymake a comparison with poker, whichis mostly determined by a blindselection of random cards, but where asensible playing strategy can increaseour chances of success. And likehardened card-sharps, many profes-sional hydrogeologists and water-witches have expended considerableeffort in building up a reputation andbluffing that their "infallible systems"

can overcome the random element.You, as customers and well-drillers,should treat such claims with greatcaution. Well drilling in bedrock alwaysbears a greater or lesser risk: what wehydrogeologists can do is estimate thatrisk and quantify your chances ofsuccess.

3.1 Yield distribution curvesIf we examine a particular rock type,such as the Iddefjord Granite ofsouthern Norway, we can take the yieldsof all the boreholes in the granite andplot them on a cumulative probabilitydiagram, such as that in Figure 4. Fromsuch a diagram, we can see that themedian yield is 600 l/hr (follow the redline horizontally from the 50% mark tothe curve for the Iddefjord Granite, and

3. Well Drilling in Bedrock.Bingo, Poker or Chess?

Figure 4. Cumulative frequency diagram showing yielddistribution curves for Norwegian wells in theIddefjord Granite ("Iddefjord"), Cambro-Silurianmetasediments of the Norwegian Caledonian terrain("Cambro-Silurian") and Precambrian gneisses("Precambrian").The added purple guide-line showsthe approximate 10% yield for most lithologies (i.e.90% of wells yield better than this figure).The red andorange guide-lines show the median yield (50%, red)and the 72% yield (orange) for the Iddefjord Granite.(Figure prepared by Geir Morland, using data from histhesis of 1997).

then vertically down to where it meetsthe x-axis at 600 l/hr). For a well drilledrandomly in the granite, there is thus a50% chance that a yield of 600 l/hr willbe achieved. Similarly, we can assess the25 % or 75 % yields. Or if, we wish toobtain 1200 l/hr we can see (byfollowing the orange line vertically upfrom the 1200 l/hr mark to theIddefjord Granite curve, and thenhorizontally across) that we have a 72 %chance of not achieving this amount(28 % chance of achieving it).

However, different rock types havedifferent yield distribution curves. Forexample, Caledonian slates and schistsof Norway have a lower yield distri-bution. This is because permeability isdetermined by fracture aperture, whichis, in turn, governed by the rock'sgeomechanical properties. In fact,theory can show that a single fracture of 1 mm aperture can transmit morewater than 900 planar, parallel fracturesof 0.1 mm aperture (the transmissivityof such fractures is proportional to thecube of the aperture). Brittle, hardrocks, such as granite, are better able tosustain fractures with wide aperturesthan soft, deformable rocks, such asshales and slates.

A word of caution, however. Theconstruction of such yield distributioncurves pre-supposes the existence of anadequately comprehensive well data-base (data for the UK, for example, arenot good enough to be used for thispurpose). The yields submitted bydrillers to such databases are often short-term yields. The sustainable, long-termyields may be considerably less.Additionally, dry boreholes may nothave been reported at all, so inducing apositive spin to the statistics. It is alsoimportant also to know whether suchdatabases include wells whose yield hasartificially been stimulated by explo-sives or hydraulic fracturing (seeChapter 12).

3.2 Dowsers and Water-WitchesIn hard-rock terrain, whether it be inNigeria, Norway or Cornwall, dowsersare very often used to locate under-

ground water and often seem to achievesimilar results to hydrogeo-logists (seeText Boxes 1 and 2). The authors wouldventure to argue that this is not becauseof special prowess on behalf of thedowser, but often because of lackinginsight on the part of the hydrogeo-logist. Dowsers will often try to locatewater using forked twigs, bent clotheshangers, German sausages or pendula(Figure 5). Some may even ply theirskills in the office over a map, withoutventuring into the terrain. The mosthonest dowsers will admit that it isdifficult to conceive of a physicalexplanation for dowsing and that theirskill is purely "spiritual". Agricola (1556)tells us that, "..wizards, who also makeuse of rings, mirrors and crystals, seekfor veins with a divining rod shaped like a fork; but its shape makes nodifference... for it is not the form of thetwig that matters, but the wizard'sincantations which it would notbecome me to repeat". It is likely thatcustomers use dowsers for threereasons:

(i) They are cheaper than hydrogeologists

(ii) The subject of groundwater is difficult to understand and has always been associated with magic and mysticism – see Kubla Khan by Samuel Taylor Coleridge

(iii) In a field where the uncertainty ofthe result is so great, people feel drawn to mystical, rather than scientific, methods.

Nevertheless, some dowsers often seemto enjoy considerable success (althoughothers, in the authors' experience, havecost their clients large sums of money).Why should this be so ? We offer twoexplanations:

(i) Most dowsers work for domestic clients where the water demand is only maybe 100 l/hr (most people use 300-400 l water every day).From Fig. 4, it will be seen that there is usually a c. 90 % chance ofachieving this yield wherever one drills (follow the purple line from the 10% mark).

Figure 5.Three subtle and esoteric implements of theHermetic art of dowsing: (a) the Knackwurst (orGerman Sausage); (b) the Liechtputze (or candletrimmer); (c) the Schneiderscheer (the tailor'sscissors) (from Zeidler 1700, reproduced in Prokop &Wimmer 1985).

(ii) Most dowsers operate in a geo--graphically limited area. They get to know their terrain and gain an instinctive (often subconscious) feel for how the hydrogeology ofthe area functions.

Dowsers can, however, create and per-petuate grave misconceptions. On theChannel Island of Jersey, for instance,rainfall on the Pyrenees is reputed toflow underground and beneath the Bayof Biscay to rise up onto Jersey to dis-charge as springs 200 m above sea level.This is an improbable situation giventhe abundance of local rainfall and localrecharge and the friction (or head loss)in driving the water underground allthe way from the Pyrenees!

Our advice: for small domestic supplies,the best person to site a borehole isoften a local well driller. He usually hasa reasonable hydrogeologicalunderstanding and is able to assess thelogistical and well-head protectionfactors that maybe far more importantthan the merely geological. For largersupplies, use the services of ahydrogeologist with experience of hardrock terrains.

Further Reading on Dowsing andWell Yield StatisticsAgricola (1556), Banks (1998),Henriksen (1995), Knutsson (2000),Morland (1997), Persson et al. (1985),Prokop & Wimmer(1985), USGS(1993), Wladis & Gustafson (1999).

The Unacceptable Face of Dowsing

While we would argue that many dowsers are honest and have a good intuitive understanding of groundwater, a few dowsers can cause great distress to theirclients. A Norwegian dowser is cited in the newspaper "Agder" (20/4/90) as saying (in English translation):

"The dowsing twig is on the way out, many say.The reason is supposedly that there is no scientific evidence for its many uses. In my opinion, that is utterrubbish....Hundreds of years before we began drilling boreholes, we used the forked twig to find water....No, the dowsing rod is by no means outdated."

Fair enough, one might say, but now things become macabre:

"the dowsing rod can be used .....to find all sorts of radiation. One thing's for sure. Many people have back problems due to "veins of water", which run under theirhouse.The radiation from these can be drawn away by cheap and effective means."

Luckily, the solution to such problems does not involve digging up the foundations of the house to find the offending fracture. A simple "radiation damper" canbe placed under the bed ! One may find such psychobabble amusing, but for some it is definitely not a joke. One of the authors was contacted in the UK by adistraught woman several years ago, whose husband was suffering a serious illness. In desparation, she turned to a dowser who told her that the illness wascaused by a "groundwater vortex" beneath the property. So, as well as her uncertainty and distress over her husband's health, she now was being asked toconsider moving house or some serious engineering geology. Our advice to you is the same at that we gave to her - "Stay away from such practicioners andtrust your doctor. Dowsing and medicine do NOT mix".

Water Divining in Kosova

Our colleague Habib Meholli tells us about the following methods for locating groundwater inKosova.

Method 1: First catch your chicken. Remove its head with an axe. Let the headless chicken run around for a few minutes and where it falls motionless, dig your well.

Method 2: Provide your horse with salty feed.The horse will soon become thirsty. It will start looking around for water or damp soil.The place where it starts pawing the ground with its hooves may be a good place to dig a well, as groundwater is likely to be close to the surface.

The location of a borehole should takeinto account at least three factors:

(i) Logistical factors, including access(ii) Vulnerability factors(iii) Geological factors

For small domestic supplies, where it ispossible to drill a well with satisfactoryyield almost anywhere, the first twofactors are likely to be paramount. Forlarger supplies, where the yield of thewell is critical, finding a sensiblegeological location becomesincreasingly important.

Logistics and access prevent a numberof sources from being developed. In theNorth-Western Highlands of Scotland,the Precambrian limestones of theDurness and Assynt areas offer karstconditions and the prospects of high-yielding groundwater sources. These arelittle used simply because few peoplelive in these areas.

4.1 Logistical FactorsIt is necessary to consider:

• proximity of the borehole to the point of use, or

• proximity of the borehole to an existing water distribution network

• availability of a power supply for the pump.

• ease of access for a drilling rig.

4.2 Vulnerability FactorsHere, one should consider potentialsources of pollution; the boreholeshould not be located in the immediatevicinity of:

• sewerage pipes, which may leak.• pit latrines, cesspits, septic tanks,

leaking tight tanks • unbunded oil or paraffin storage

tanks

• land subject to intensive use oforganic or inorganic fertilisers,pesticides or other chemicals

• surface waters, particularly those known to be bacterially (or otherwise) contaminated

• the sea.

If possible, boreholes should be locatedat least 50 m (and preferably more,depending on aquifer characteristicsand yield) up any topographicalgradient from the above, or any otherforms of contaminative human activity.In the case of surface waters, thelocation of a borehole will always be acompromise between the hydraulicadvantages that location near a river orlake can offer (a plentiful source ofgroundwater recharge) and thedisadvantages in terms of vulnerabilityto pollution.

Where an aquifer is covered by asignificant thickness of low-permeability materials (e.g. boulderclay), less stringent conditions mayapply to locating a borehole near topotential contaminant sources. Thereshould be a presumption against such alocation, however, unless it can beclearly demonstrated that the coveroffers adequate protection.

It should be noted that the concept ofsource vulnerability also applies toexisting surface water sources (eg. riveror stream intakes). It is often these that,due to their vulnerability to pollution,need to be replaced by newgroundwater sources. Care needs to betaken to ensure that the newgroundwater well does not draw on thesurface water, unless a sufficientresidence time and "filtration effect" arepresent to ensure that water quality issafeguarded. Some fluvial sand andgravel deposits act as efficient water

4. Where Should I Drill MyBorehole?

purifiers, however, and peat-stainedriver water may appear as crystal clear"bank infiltration" in a well only 5 mfrom the bank of the river (a fact nowbeing exploited in the replacement ofrural village supplies in Scotland,wherever peat stained surface watersupplies existed and which are nowoutlawed by current EC legislation).Note, however, that appearances may bedeceptive, and thorough chemical,bacteriological and hydraulic testingmay be necessary before a source can beapproved for supply. Fractured bedrockdoes not necessarily have these samepowers of attenuation and purificationand riverside boreholes into bedrockexposures should be avoided unlesswater treatment is available.

4.3 Geological FactorsAs previously explained, the hydrogeo-logical factors influencing well yield canbe difficult to predict. Ideally one wishesa borehole to intersect one or more

fractures of high groundwater trans-missivity, which also are interconnectedwith a wider system of fractures or withsuperficial deposits that provide adequategroundwater storage. Most hydrogeo-logists agree that it is sensible to target:

• zones of intense fracturing. Thesemay be vertical, horizontal or with anintermediate dip and are generallyreferred to simply as fracture zones.

• areas with a moderate (2-5 m thick)cover of superficial deposits (e.g.moraine). These deposits confer adegree of protection to theunderlying bedrock groundwater, andmay also act as a reservoir for water.The superficial deposits should not betoo thick though - they areconsiderably more costly to drillthrough than the bedrock itself.

Further Reading on Well LocationRobins & Ball (1998).

A fracture zone is a planar feature,where the density of fractures is veryhigh. It is usually an abortive or actualfault where some shear movement hasoccurred between the rocks on eitherside. If the fracture zone is vertical orsub-vertical it may be seen as anapproximately linear depression orvalley in the terrain, on maps or onaerial photos. If the fracture zone ishorizontal it may not be seen at thesurface at all!

Aerial photographs of Loch Fleet insouth-west Scotland revealed a distinct,but short lineament in granite whichcontains tight joints 0.5 to 3 m apart(Figure 7a). Seepages of soil water andgroundwater flowing into the loch werelocated early one frosty morning usinga thermal imaging or thermal infra-redlinescan camera which highlighted therelatively warm groundwater (c 8°C asopposed to the loch water's 2°C). Thelargest inflows were discovered aroundthe lineament intersection with the lochshore, but underwater discharge wasalso suspected as this corner of the lochhad an enhanced temperature of some3°C. Total discharge into the lochamounted to some 2 l/s (Cook et al.1991).

The fractures that make up a fracturezone may not have the same orientationas the zone itself (Fig. 6). The mostintense fracturing is usually at thecentre of the zone. It may be so intensehere that the rock has been broken intoa fault gouge consisting of rockfragments set in a fine grained, lowpermeability rock "flour".

Furthermore, some faults have beensubject to hydrothermal activity orsecondary mineralisation after theirformation. This seems to be particularlycommon in the region of the Oslo rift.Hot and/or mineralised waters passedthrough the fractures and alteredexisting minerals to clays or deposited

new minerals. In this way fractures mayhave become sealed and fault gougemay have been turned into very lowpermeability clays.

The phenomena of fault gouge andsecondary clay mineralisation are tworeasons why not all fracture zones arevery permeable. In a sub-sea tunnel inthe islands of Hvaler in SE Norway, thebiggest water leakages came not fromthe biggest fracture zones but fromsmaller fractures between them (Fig.7b). On the islands themselves, many

5. Fracture ZonesFigure 6.The development of a fracture zone (afterSelmer-Olsen 1976, printed with permission fromTapir forlag): (a) the development of en-echelontension gashes (fiederspalten), (b) the developmentof shear fractures across the fiederspalten, (c) and (d)breakdown of the central portion of the zone to yieldfault breccia and gouge.

wells drilled into apparently majorfracture zones have had verydisappointing yields.

There is some structural geologicalevidence to suggest that the margins ofa fracture zone, where fracturing is lessintense, may be more permeable thanthe core of the zone, where fracturingand secondary mineralisation may havebeen so intense that permeability hasbeen reduced.

5.1 DykesDykes are essentially extensionalfractures that have been filled withmagma. Because of the intensetemperature difference between the

magma and the host rock duringemplacement, a chill margin may havedeveloped, where expansion andcontraction have caused the host rockand the dyke rock to crack. These chillmargins may be very permeable and, atleast in the Oslo area, are good targetsfor boreholes for water supply.

Further reading on fracture zonesand dykesBanks et al. (1992a, 1994), Braathen etal. (1998), Braathen & Gabrielsen(2000), Cloos (1955), Cook et al.(1991), Hancock (1985), Leveinen et al.(1998), Mead (1920), Selmer-Olsen(1976).

Figure 7a. Map of Loch Fleet, Scotland, based in theresults of an infra-red linescan survey, showingboreholes, sample sites and inflows of groundwater(and soil water from peaty soils), which are greatestaround the fracture zone at the north of the Loch(after Cook et al. 1991, printed with permission fromElsevier Scientific Publishing Company).

Figure 7b.Water leakages in the Hvaler tunnel (after Banks et al. 1992a, printed with permission from Geological Society Publishing House). Note that zones of water leakage andinjection grouting do not necessarily coincide with fracture zones as detected by geophysics and in the tunnel itself.

The importance of deposits that overliebedrock, such as moraine material,marine or beach deposits or glacio-fluvial sand and gravel, is fourfold:

(1) Groundwater often flows moreslowly in granular aquifers than inbedrock fractures. Granular aquifersoffer greater potential to attenuateand retard pollutants. Overlyingdeposits may thus protect a bedrockaquifer from pollution.

(2) Superficial granular deposits tend tohave larger groundwater storagecoefficients than bedrock aquifers.They may thus represent asubstantial reservoir of groundwaterto feed the underlying bedrock.

(3) Some superficial deposits, especiallymarine clays, may affect underlyinggroundwater quality due to leachingof salts.

(4) In bedrock, the shallowest sub-horizontal fractures in the top 1 - 2metres are often very permeable. Inexposed bedrock, however, they areeither dry, or their water is vulner-able to surface pollution or they areexcluded from the borehole bysurficial casing. In bedrock belowa thickness of superficial deposits,however, these fractures are morelikely to be exploitable, and maymake a significant contribution toa well's yield (Fig. 8).

The various roles that the superficial(or "drift") deposits fulfil are currentlythe subject of intense investigation. Thenature and integrity of the deposit,particularly in the case of a glacial till, isoften not known, and the effect whichclay horizons may have on inhibitingrecharge or protecting the bedrock fromsurface pollutants is largely unquantifi-able. Considerable efforts in drift anddrift 'domain' mapping will help ourunderstanding of this complex area ofhydrogeology (McMillan et al. 2000).

Recent work in west Wales has revealeda complex system of glacio-lacustrinedeposits in a number of valleys includingAfon Teifi above the town of Cardigan.Granular deposits are, for the most part,saturated and capable of yielding smallamounts of water for domestic and farmuse. However, difficulties with actuallydrilling the deposits mean that mostboreholes case out the drift and drawwater from fractures in the underlyingLower Palaeozoic shales and grits. Tothe north, in the Rheidol valley, how-ever, a high yielding shallow boreholesupplies the town of Aberystwyth withfiltered river water from terrace gravels.

Further Reading on the Role ofSuperficial DepositsFoster (1998).

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6. Superficial Deposits

Figure 8. Exploitation of fractures near the bedrocksurface. In (a) without a superficial cover, these aredifficult to exploit (they may be dry or cased out) andthe hydraulic gradient achievable between fractureand borehole is low. Below superficial deposits (b)they are exploitable and a significant hydraulicgradient (Dh) may be applied.

Fracture zones and fractures may bemapped (Fig. 9):

• from satellite images or aerial photographs

• from maps• by direct measurement, in the field

On maps and aerial photos, sub-verticalfracture zones will usually appear aslineaments. Beware, however, not alllineaments are fracture zones; they mayalso represent lithological boundaries,beds of readily-weathered rock, glacialfeatures or anthropogenic features such

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7. Fracture MappingFigure 9. Fracture zones on (a) a topographic map(fracture zones marked as broken lines) printed withpermission from the Norwegian Mapping Authorityand (b) an aerial photo (photo: Fotonor AS). (c) Afracture zone in the field (photo: D. Banks) and (d) aweathered-out basalt/dolerite dyke at Skams Klove,Kjøkøy (photo: E. Rohr-Torp). All examples come fromthe Iddefjord Granite terrain of the Hvaler Islands,Norway. ) a and b with permission from theNorwegian mapping authority.

as trenches, roads or zones of clearedvegetation along power lines.

Fractures are measured, in the field,with a compass and clinometer. Theapparent frequency of fractures will,however, be determined by the anglebetween the fracture and the surfacebeing observed. For example, on ahorizontal land surface, verticalfractures will be very well observed buthorizontal fractures will not be seen.

7.1 Presentation of fracture dataFractures may be represented asdirections on a rose-diagram, where thelength of the "petal" is proportional tothe frequency of fractures in thatdirection. They may also be presentedas "poles to planes" on a lowerhemisphere stereographic projection.Here, horizontal fractures plot in thecentre of the hemisphere, while anorth-to-south vertical fracture plots atthe edge of the diagram (equator) inthe east or west position (Fig. 10).

7.2 Common fracture patternsA group of fractures or fracture zonesof the same direction is known as afracture set. Normally it is possible toidentify two or three main fracture"sets" or orientations in a terrain. Thesemay vary from scale to scale, such that

the fracture zone directions identifiedfrom an aerial photo are likely to bearsome relation to, but not be identical to,the fracture sets mapped at field scale.

One common pattern of fracture zonesin ancient crystalline terrain is a patternconsisting of four sub-verticaldirections at c. 45° to each other Tirén& Beckholmen (1989). There may bevery good structural geological reasonsfor this: such a pattern can "absorb" awide variety of tectonic stresses.

Two commonly occurring fracturepatterns at a more detailed scale are(Figure 11):

• the orthogonal pattern, consisting ofone horizontal and two verticalfracture sets at 90° to each other.

• the classic conjugate shear patternconsisting of a tension fracture andtwo shear fracture directions at anacute angle to this. A horizontal stressrelease fracture set may also occur.

Further Reading on Fracture Zoneand Fracture MappingBanks et al. (1992b), Braathen &Gabrielsen (2000), Broch (1979),Ellingsen (1978), Lloyd (1999),Ramberg et al. (1977), Rohr-Torp(1987), Tirén & Beckholmen (1989).

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Figure 10. Fracture orientations (n = 105) representedon (a) a fracture rose and (b) a lower hemispherestereographic projection. In the latter, poles (i.e. thedirection at right angles) to the fracture planes areplotted and contoured. From Jondal, HordalandCounty, Norway. After Midtgård et al. (1998).

Figure 11.Two common fracture patterns.(a) Orthogonal fracture pattern and (b) idealised conjungate shear / tension fracture pattern. In (b), the shear fractures are blue, the tension fracturesare green and the horizontal stress release fracture set red.The angle a is acute, typically around 30°.sH and sh are maximum and minimum horizontal stress directions, respectively.

8.1 Surface Geophysical MethodsIn terrain where no deep weatheringprofile occurs, the following methodsare likely to be of use in locating zonesof intense fracturing.

• Very Low Frequency (VLF)electromagnetic induction profiling.This measures the electromagneticresponse of the rocks toradio/microwave signals from anetwork of powerful transmitters

placed around the globe. A negativeanomaly indicates conductive (water-or clay-rich) rocks (Figure 12).

• Total magnetic field anomaly. Inrocks rich in magnetite, a negativeanomaly may be observed overfracture zones, where the magnetitehas been oxidised to non-magneticiron (III) oxides (Figure 12).

• Resistivity profiling. Here, electricalresistivity between two electrodes ofconstant separation is measured to

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8. GeophysicsFigure 12.VLF and magnetic field profiles acrossfracture zones in the Iddefjord Granite of the HvalerIslands, Norway.The negative anomalies in the VLH(real signal) and magnetic field define the zone (afterBanks et al. 1991).

give a lateral profile of resistivity. Anegative anomaly (low resistivity)indicates conductive rocks, as in VLF.

• Vertical electrical resistivity sounding(VES). Here the separation of anelectrode array is varied, such that avertical profile of resistivity is builtup. This is a common technique inweathered terrain in Africa, but inglaciated terrain, its main use is tolocate depressions (possible fracturezones) in the bedrock surface beneathsuperficial deposits.

• Georadar (ground radar). This workson the radar principle and detectsreflecting underground surfaces. Itmay be used to map the bedrocksurface beneath covering sedimentsor to identify low-angle fractures(Figure 13b).

It should be remembered that manymajor fracture zones form valleys whichmay be useful communication conduitsfor roads, buried services (pipes, cables)and for overhead telephone or powerlines. All of these may disturb electricalmethods.

Secondly, you should be aware thatnone of these methods can adequatelydistinguish between water-filled(permeable) and clay-filled(impermeable) fractures.

8.2 Downhole Geophysical LoggingAfter a well has been drilled, variousgeophysical probes or sondes may belowered into the borehole, to ascertainthe depths of water-bearing fractures(Figure 14). These sondes include:

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Figure 13. (a) The use of electromagnetic inductance(EM31) equipment in the field to measureconductivity (photo: Ingemar Aamo); (b) a Georadarprofile and its interpretation, showing the bedrocksurface below superficial deposits (hatched) and lowangle fractures in the underlying bedrock (fromHvaler, Norway - Banks et al. 1993b)

• the caliper sonde, where sprung armsmeasure the diameter of the borehole.Fractures can be seen as irregularitiesof larger diameter in the boreholewall.

• resistivity sondes, which measure theelectrical resistivity of the boreholewalls and which readily detect zonesof intense fracturing (but cannotdistinguish whether these are clay- orwater-filled).

• fluid sondes, which measure thetemperature and electricalconductivity of the water column inthe borehole. A jump in these signalsat particular depths can indicateinflowing groundwater from apermeable fracture.

• the gamma sonde, which detects rocktypes, clays and fracture infillminerals or pegmatites which are richin radioactive elements: uranium,thorium or potassium.

• The complex acoustic televiewer tooluses acoustic signals to profile thewall of the borehole. Fractures showup very clearly, and from the profile itis possible to calculate theirorientation.

Further Reading on GeophysicsBlikra et al. (1991), Davis & Annan(1989), Griffiths & King (1981),Houtkamp & Jacks (1972), Lloyd(1999), Mullern & Eriksson (1981),Olesen et al. (1992a), Sclumberger(1969), Tate et al. (1970).

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Figure 14. Drillers' logs , fluid resistivity log andresistivity logs of boreholes at Hvaler, Norway. Inborehole 1 there is a clear low-resistivity anomalyadjacent to the clay filled fracture zone at 54 - 62 m.There is abrupt change in fluid resistivity at 62 mindicating an inflow of water (after Banks et al.1992b).

The rocks below the earth's surface arein a constant state of stress due to:

• gravitational forces. The deeper onegoes, the greater the weight of theoverlying rocks. The weight fromthese tends to close fractures up andpermeability decreases with depth(Fig. 17).

• tectonic forces. Continents collideand break up. These immensedisturbances may induce very strongregional horizontal stresses in thelithosphere. In Southern Norway forexample, the lithosphere is in a stateof high stress (around 10 Mpa havebeen measured at only several tens ofm depth - Banks et al. 1996) due tothe opening of the Atlantic Ocean.

• topographic stresses. The weight ofmountains exerts stress on the rocksof the valley below. For example, nearthe foot of a mountain there will be alarge stress parallel to the slope of themountainside. Near the top of themountain slope there may be reducedstress (or even tension) parallel to theslope as the mountainside tries to slipaway.

Large compressional stressesperpendicular to a fracture will tend toclose it up and reduce its permeability.Compressional stresses parallel to afracture will, however, tend to open it.Thus, permeability will be enhanced in

a direction parallel to the mountainslope near the foot of a mountain, andparallel to the contours near the top. Ifone is able to drill angled boreholes,one should drill parallel to the contoursat the base of a hillside and parallel tothe slope near the top to encounter themost open fractures.Recent research using computer modelsshows that this directionalpermeability-enhancing effect is likelyto be real, but relatively small incomparison with other factors. Odling(1993) argues that the decisive factorfor permeability anisotropy (ordirectionality) is not stress, but the wayin which fractures are connected.

9.1 Regional StudiesRegional overviews of rock stressmeasurements do exist (Klein & Barr1986, Stephansson et al. 1986). One ofthe most interesting recent findingsrelates, however, to post-glacial stressrelease. Rohr-Torp (1995) and Morland(1997) found a very clear correlationbetween well yield and amount of post-glacial isostatic rebound in Norway.Such isostatic rebound is a geologicallyrecent and rather traumatic event,probably involving the creation of largenumbers of sub-horizontal stressrelease fractures and the reactivation ofolder fracture systems. In practice, thisfinding means that rocks in inlandNorway have, on average, higher yieldsthan those on the coast (Fig. 15).

Further Reading on Stress andNeotectonicsBanks et al. (1996), Bott & Kusznir(1984), Klein & Barr (1986), Knutsson(2000), Midtbø (1996), Morland(1997), Mörner (1979), Myrvang(1979), Odling (1993), Olesen et al.(1992b, 1997), Rohr-Torp (1994),Stephansson et al. (1986).

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9. Stress

Figure 15. Correlation between well yield and totalpostglacial isostatic uplift; the diagram shows theyield of wells in Norwegian Precambrian rocks permetre drilled depth as a function of annual land uplift.The figure was prepared by Geir Morland using datafrom his thesis (Morland 1997).

Most modern down-hole hammerdrilling rigs are able to drill at angles ofup to 45° from the vertical in anydirection (Fig 16a). Some rigs can evendrill horizontal holes. Boreholes do nothave to be vertical and a non-verticalborehole offers significant advantagesover a vertical one. Boreholes should beorientated with the following in mind,in order of priority:

1) A borehole should encounter afracture zone with certainty, at thedesired depth and so as to insert thegreatest possible drilled length intothe zone. It is recommended to drill,with a fall of some 45°-60°, at anacute angle across the zone. Theangle should not be too acute,however, for fear of "missing" thezone if it is not completely vertical(Fig. 16b).

2) A borehole should be oriented awayfrom possible sources of pollution,if these exist.

3) A borehole should encounter thegreatest possible number offractures. A vertical borehole will

not cross many vertical fractures.A borehole should ideally be drilledperpendicularly to the main fracturedirection or, if there is more thanone fracture set, should be drilledto approximately bisect the obtuseangle between the fracture sets.The optimal angle can, in fact becalculated relatively easily (Banks1992), although a fall of 45° is likelyto be a good compromise in the caseof poor fracture information.

4) Theory suggests that a boreholedrilled perpendicular to the greatesthorizontal compressive stress willencounter the most open fractures.Odling (1993) notes, however, thatthis effect is likely to be subordinateto other factors (see Ch. 9).

Further Reading on BoreholeOrientationBanks (1992c)

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10. Borehole Orientation

Figure 16. (a) A modern downhole hammer rig drills adeviated borehole in the gneisses of Flatangermunicipality, Norway. (b) Schematic diagram showingoptimum borehole orientation to meet a fracturezone.The angle a (borehole fall) should be such as tomeet the fracture zone at a depth of some 40 - 50 m.The lower the angle b, the longer the portion of theborehole within the zone, but the greater the angle b,the less the danger of missing the zone altogether.

In the past, boreholes have been drilledin hard rocks using simple cable toolrigs or conventional mud-lubricatedrotary rigs. These techniques are slowand labour intensive, and it is nowusual to drill with a down-hole airhammer.

The hammer is tipped with tungstencarbide buttons and powered bycompressed air. During drilling it isrotated slowly. The compressed air

which powers the hammer is blownthrough the bit and carries the drilledrock flour and cuttings to the top of thehole where they may be inspected andlogged by the driller or site geologist.Small quantities of water are oftenadded to the drill stem above the watertable to cool the bit and dampen thecuttings. Typically, a well is drilled at127-152 mm diameter.

If commenced directly on a rock

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11. Drilling

Figure 17. Diagram illustrating the trend to decreasingmean permeability with depth in bedrock (althoughwith very large scatter !), on the basis of data from theAltnabreac granite in Scotland (Holmes 1981).Thediffering colours refer to different boreholes, A, B andC. Printed with permission from British GeologicalSurvey.

outcrop, the upper 3-5 m of theborehole are usually cased with plaincasing (more if the rock is of poorquality). This is often installed bydrilling at c. 200 mm diameter using anODEX ex-centred bit which carries the(e.g. 152 mm diameter) casing downwith it. If superficial sediments arepresent, the casing is usually carrieddown through these to maybe 1 m intothe underlying bedrock (or at least 3 minto bedrock if the superficial depositsare thin, i.e. < 2 m). When the casing isinstalled it should be grouted intoplace, preferably by injection fromwithin the casing, and preferably usingan impermeable, durable and slightlyelastic grout. Drilling may then progresswith a bit of, say, 140 mm through theinstalled casing. Drilling progress maybe as fast as 1 m per 4-5 minutes. Mostbedrock boreholes may be drilled "openhole". In extremely fractured rock, aninner string of slotted casing may bedesirable to support the borehole wall.

The above paragraph describescommon drilling practice today. Thereare indications, however, that manyScandinavian bedrock boreholes areinadequately protected against surficialcontamination. Korkka-Niemi (2001)and Gaut et al. (2000) found many suchboreholes with bacterial contamination,while Frengstad et al. (in press) decribeboreholes subject to periodic run-in oflow-pH surficial water. One possiblereason for these observations isinadequate length and sealing of plaincasing in the uppermost section of theborehole.

During drilling, one may note thepresence of potentially water-bearingfractures by (see Fig. 14):

• "rough" drilling progress, with the bitappearing to catch on broken rock

• very fast drilling progress• red-brown drilling cuttings,

indicating oxidising, circulatinggroundwater

• the appearance of water or damp drillcuttings in the compressed air.

A driller will be able to estimate the

short-term yield of the borehole by theamount of water blown up with thecompressed air during drilling. He mayalso measure the rate of rise of water inthe hole on completion of drilling andmake an initial estimate of the yieldfrom this. Samples of water takenduring or shortly after drilling arealmost never representative of the finalwater quality taken from the hole -don't waste money on water samplingat this stage.

11.1 How Deep Should I Drill?This question is governed by geology,logistics and economics:

• Permeability decreases with depth(Fig. 17, and Carlsson & Olsson 1977)

• Rate of drilling decreases with depth• A rig carries only a finite amount of

drill string

but also

• There are extra costs associated withrelocating a rig and starting a newhole

Most drillers drill to depths of between50 and 90 m on the basis of experience.At depths greater than this it isprobably better to start a new boreholethan continuing to drill. Thus, twoboreholes of 50-60 m are probablypreferable to a single hole of 120 mdepth.

You should decide the borehole depthbefore commencing to drill. Never betempted to abandon an apparently dryhole at less than the agreed depth. Evenif no water strike is made before fulldepth, water may be obtained byartificial stimulation techniques(Chapter 12).

On Jersey (Channel Islands), theaverage depth of boreholes is muchshallower than in Norway, at just lessthan 30 m. Bearing in mind that thedepth to the water table varies by up to7 or 8 m, this suggests that the top 25 mor so of the saturated aquifer is themain productive zone in manyboreholes on Jersey (Robins & Smedley

PAGE 29

1994). Nevertheless, there exist twoconsiderably deeper boreholes onJersey, penetrating Precambrian shales,and yielding good quality water fromdeep-seated fractures. Other deepboreholes on the island include onewhich is 145 m deep but situatedadjacent to the shore and pumping saltwater and another 92 m deep,effectively dry borehole in solidunfractured granite. It is suspected that

these may reflect the needs of a drillerwho has been contracted to drill on a"drilled metres"-related payment systemrather than the more normal (andsafer) yield guarantee system.

Further Reading about BoreholeConstructionClark (1988), Driscoll (1986), Lloyd(1999), UNESCO (1984).

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The Luftwaffe School of Well Drilling

One of the most unusual well-drilling methods practised in the UK is documented in the BritishGeological Survey's well database, under index number SX-45/1. It concerns a well in Devonian TamarSlates in Plymouth Devonport Dockyard:

"Royal Naval Barracks - Devonport: At a point in the shillet [slate] near the flagstaff of the Barracks, ashaft made by an unexploded bomb has been explored and a considerable quantity of fresh water foundat a level just above the 'tide' apparently floating on top of the salt water.The Superintending CivilEngineer made a pumping test at the rate of 300,000 gallons per day [57,000 l/hr] and the level wasreduced by six inches [15 cm]. He did not consider it advisable to pump any heavier owing to the dangerof drawing in sea water.The Naval Medical Officer says that this water is potable, although not quiteequal to town water.

Dated 7th April 1943."

It's a great story, although we are somewhat suspicious of the enormous yield. Did the bomb in factfracture a water main ?

As the outcome of drilling is uncertain,it is worth budgeting at the outset forsome form of well-stimulation, for allexcept the very smallest domesticsupplies. Well-stimulation is forcingnature's hand in the hydrogeologicalpoker game. It gives us an ace up oursleeve. If transmissive fractures do notexist naturally, we can make them bybrute force. There are two main formsof well-stimulation:

• Explosives. These should be used by a skilled operator who can place theright charge at the right level in theborehole. A tamp of sand or similar is often placed above the charge todirect its force into fracturing therock rather than blowing water out of the borehole. The technique can be effective, but a more subtle andcontrolled method is

• Hydraulic fracturing (or hydro-fraccing). Here, water is injected intothe borehole at such a high pressurethat the strength of the rock and theambient stresses are exceeded, and anew fracture is propagated.(Alternatively, existing small fracturesmay be jacked open and made moretransmissive).

For hydraulic fracturing, the best rigshave a dual pump system. One pumpapplies a high pressure to initiate thefracture, while the secondary pump hasa high volume capacity, injecting largeflows of water to propagate the fractureas far as possible (Fig. 18). Dispersingagents (to remove clay minerals), orsmall amounts of sand or glass beads(to hold fractures open) may be addedto the hydrofraccing fluid.

Two systems of hydraulic fracturingmay be used. The most basic forminjects water below a single packer. It isbest used in dry or poorly yieldingholes. A fracture may be inducedanywhere in the section below thepacker. In highly-yielding boreholes orboreholes with clay-filled fractures, thismethod may simply open the existingwater-bearing fractures. After thehydrofraccing these may then close upagain with little net effect. In suchholes, hydraulic fracturing between apair of packers is recommended. Thisallows the targeting of hitherto poorlyfractured sections of borehole.

In no case should hydraulic fracturingtake place at less than 25-30 m depth.

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12. Borehole Yield Stimulation

Figure 18. (a) a hydraulic fracturing rig (photo: David Banks).

Shallow hydrofraccing runs the risk ofcreating fractures to the surface, whichwould be vulnerable to contamination.

Even for boreholes which yieldsatisfactorily after drilling, some formof yield stimulation may be beneficial.It will usually result in increased wellefficiency (yield/drawdown ratio),which reduces pumping costs, althoughstimulation techniques are themselvesexpensive..

Further Reading on Well StimulationLess & Andersen (1994), Smith (1989),Banks et al. (1996).

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Figure 18. (b) schematic diagram of hydraulicfracturing with a dual pump, dual packer system.

13. Test Pumping

Figure 19. A wind-pump used for pumping groundwater at Stonehaven, Scotland (photo: Nick Robins).

13.1 Drillers' EstimatesExperienced drillers are usually able togive a reasonable assessment of aborehole's short-term yield on the basisof the quantities of water blown up outof the borehole during drilling or bymeasuring the rate of rise of water levelin the borehole following drilling.

Remember, however, that suchestimates are likely to overestimate theborehole's long-term yield. Suchestimates will often only be sufficientfor making a decision about wellstimulation and for designing a longer-term test pumping.

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Figure 20. (a) Yield/drawdown curves for aconventional step test: in the left-hand method, arecovery period is allowed between each step anddrawdowns (s1-s4) are measured directly for eachpumping rate (Q1-Q4). In the right-hand method,steps follow directly on from each other, and the2-hour drawdown for a rate Qn is calculated by

sn = ∆s1 + ∆s2 +.....+ ∆sn. (after Banks 1992a).

Figure 20. (b) Idealised inflow / water level curves fora rising level test, where three water-yieldingfractures, with specific capacities C1, C2 and C3 occurat differing levels, and where Haq = aquifer head /rest water level (modified after Banks 1992b).

TIME

WA

TER

LEV

EL

13.2 Short-Term Test PumpingIf a more accurate measurement of yieldis necessary, a short-term test pumpingmay be necessary. For larger municipalsupplies, such a test is regarded as anecessary documentation of theperformance of a capital investment. Touse the analogy of a car, test pumping islike logging the fuel consumption, toensure the vehicle is properly tuned andperforming at optimum efficiency. Sucha test may take two forms:

1) For high-yielding boreholes, aconventional step test may becarried out (Fig. 20a). This involvespumping the well at four different(increasing) rates (Q) for two hoursteps, and measuring the water levelduring pumping. The specificcapacity Q/s may be determined foreach step, where s is the drawdown(total decrease in water level inborehole, relative to natural waterlevel, at the end of each step). Thistest is described by Kruseman & deRidder (1987).

2) For moderate or low-yielding

boreholes, a pump is set in theborehole at, say, 50 m depth. It ispumped until the water level isdrawn down to the intake (thepump draws air). The pump is thenoperated for another 1 hour and therate of water pumped (Q) ismeasured using a bucket / barreland stopwatch (Fig. 21a). Thedrawdown is known (50 m lessnatural water level) and the specificcapacity may be calculated. Thepump is then turned off andremoved from the borehole. The risein water level (Dh/Dt) is thenregularly measured and the inflowrate (Q) can be estimated by:

Q = π.r2.(∆h/∆t), where r is the radius of the bore.

This rise in water level may be used tocalculate the specific capacity for variousdrawdowns. Abrupt changes in the rateof rise indicate the presence of yieldingfractures and can be seen on a plot of Qvs. water level (Fig. 20b). The yields andapparent transmissivities of these

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Figure 21. (a) An elegant Steinar Skjeseth assists a driller in measuring water flow during the test-pumping of a well.

fractures may be estimated from therising-level test using the method ofBanks (1992b).

13.3 Long Term Test-PumpingA longer term test pumping of a durationof several weeks to one year is necessaryto ascertain the water quality and toensure that the long-term yield is sustain-able. An extended period of pumpingwill also ensure that the borehole isadequately cleared of drilling cuttings.

The length of test pumping for publicwater supplies may be determined bynational standards or legislation. Forpublic supplies in Norway, a period of6 months - 1 year is desirable in orderto encompass both a major rechargeevent (snow-melt or autumn rain,when boreholes may be susceptible tobacterial contamination) and a lowrecharge period (summer or, in inlandNorway, midwinter) when the yield willbe at its most critical).

PAGE 36

Figure 21. (b) test-pumping a borehole at Turriff,North-East Scotland (photo: Nick Robins).

During long term pumping, the pumpoperation is usually controlled by waterlevel-sensitive switches in the borehole,while the yield may be measuredmanually, via an on-line flowmeter or(for small supplies) by a tipping bucketgauge.

After the water has become clear ofcuttings, a programme of regular watersampling for microbiologicalcomponents may commence. A lessregular programme for chemicalconstituents should also be undertakenwhich should, in addition to standardmajor and minor parameters, alsoinclude fluoride, radon, and uranium asthese may present particular problemsin groundwater from bedrock. Waterquality may change throughout thetest-pumping period (Fig. 22).

Further Reading on Test PumpingBanks (1992b), Banks et al. (1993a,1994), Driscoll (1986), Jetel & Kràsny(1968), Kruseman & de Ridder (1989),Wladis & Gustafson (1999).

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Figure 22.Water level responses in three observationboreholes during the long term pumping of aborehole at Pulservik on the Hvaler Islands, Norway. Allthe boreholes are around 70 m deep and within c. 100m of each other.The dug well is shallow and insuperficial Quaternary deposits. Only borehole 2obviously affected by pumping at around 212 l/hr inborehole 1.The small fluctuations in the hydrographsaround the end of April are probably rainfall events.Note the yield along the base of the diagram. Duringthe test, the hydrochemistry of the water changedfrom sodium-bicarbonate to sodium-chloride due tothe wells drawing either on seawater or deep "fossil"saline water.

The chemical quality of groundwaterfrom bedrock is often very differentfrom that of water from superficial driftdeposits. Bedrock groundwater is oftenmore mature, more basic, morereducing, more sodium-rich andcontains more of most minor/traceelements than drift groundwater. InNorway, the median pH of groundwaterfrom bedrock is around 8.1, almostirrespective of lithology. Except incoastal areas, where sodium chloridemay dominate, the most importantanion is usually bicarbonate (HCO3-),while the main cation is calcium in lessmature waters or sodium in moremature waters.

14.1 Natural Health-RelatedParametersNorwegian bedrock groundwaters cancontain significant concentrations of

parameters which, from the point ofview of health or aesthetic acceptability,can be undesirable. For example, in arecent survey (from 1998) of bedrockboreholes in Norway:

• 115 of 1604 boreholes (7%) had a pHoutside the Norwegian acceptablerange (6.5 - 8.5). Most of these had apH that was too high.

• 222 of 1601 samples (14%) had aradon (Rn) concentration over theNorwegian recommended norm of500 Bq/l.

• 258 of 1604 (16%) had fluoride (F-)concentrations exceeding theNorwegian drinking water maximumof 1.5 mg/l.

• 46 of 1604 (3%) had sodium (Na)concentrations in excess of 150 mg/l.

• In 58 of 476 (12%) samples, uranium(U) concentrations were above the

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14. Water Quality

Figure 23. Cumulative probability plots forgroundwater chemistry (radon, fluoride, pH andcalcium) in Norwegian bedrock groundwater wells forall analysed bedrock wells (All; n=1604), wells inCaledonian metasediments (Rock group 74; n= 114),wells in Precambrian granites (Rock group 92; n= 76)and wells in Precambrian anorthosites (Rock group93; n= 34). Based on the dataset reported by Bankset al. (1998a,b,c,d). Note that fluoride and radonconcentrations below the analytical detection limitare plotted at a value of half the limit.

American drinking water norm of 30µg/l, while only 3% exceeded the lessconservative Canadian norm of 100µg/l.

• Only 1 of 476 samples exceeded theAmerican limit of 4 µg/l for beryllium(Be).

• Barium (Ba) concentrations exceededthe Norwegian guideline value (100µg/l) in 122 of 1604 sources (8%).

Iron (Fe) and manganese (Mn) werealso problematic in a substantialnumber of boreholes. All the aboveparameters should be analysed during

the long term test-pumping of a newborehole.

Although it is possible to say that theabove problem parameters exceeddrinking water norms more frequentlyin particular areas and rock types (e.g.particularly granites in the case of Na,Rn, F, U and Be, it is impossible topredict the water quality of anindividual well in just the same way as itis impossible to predict the yield. Wecan, however, estimate probabilities ofviolation of drinking water norms fromcumulative probability curves (Fig. 23).

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Figure 24. Statistical boxplots comparingconcentrations of various parameters (Rn, F, U, Na/Clratio, alkalinity and silicon) for groundwaters from fourdifferent granites, (i) the British Isles of Scilly granite,(ii) the granites of the Shira region, Khakassia,southern Siberia, (iii) the Norwegian Iddefjord graniteof Hvaler and (iv) the Permian granites of the OsloRift, dominated by the Drammen Granite. Data fromBanks et al. (1995), Morland et al. (1997), Banks et al.(1997), Parnachev et al. (1999). No radon data areavailable from Siberia.

Although it is possible to make somegeneral statements about groundwaterchemistry related to rock-type, suchgeneralisations can be very misleading.Fig. 24 compares groundwater chemistryfrom the British Isles of Scilly Granite,granites from the Shira region of sout-hern Siberia, the Norwegian IddefjordGranite of the Hvaler Islands and theNorwegian Drammen granite of the OsloRift. It will be seen that the Scilly Granitegroundwaters contain far less radon,fluoride, uranium and sodium (relative

to chloride) than the Norwegian waters.The Scilly waters are far less hydrochemi-cally mature. This may be due to:

• more permeable fractures and steepertopography at Scilly causing fastergroundwater flux and less maturewaters

• the Scilly granite not having beenintensively glaciated. Basic and trace-element-bearing minerals may thushave been removed by prolonged sub-aerial weathering to a greater extentat Scilly than on Hvaler (Banks et al.1997, 1998e).

In the UK, drinking water standardscomply with maximum admissibleconcentrations (MAC) or prescribedconcentrations of values (PCVs) laiddown by the European Union andinterpreted by the UK Government formany individual chemical constituents.For several trace consitituents (e.g.uranium, thallium) no European MACsor PCVs have yet been developed.

14.2 Pollution-Related ParametersPollution from anthropogenic, ratherthan natural, sources may be of manyforms and come from many sources. Inthe rural areas where bedrock boreholesare most common, the following typesof pollution should be considered:

• Pollution from sewage, cesspools,slurry lagoons or pit latrines,indicated by faecal bacteria, highconcentrations of nitrate and/orammonium and maybe potassium.One common source of suchpollution is farmyard run-offentering a borehole because of apoorly sealed and protected well top.

• Pollution from leaking fuel tanks orlines. Indicated by high hydrocarboncontents, oily smell or taste.

• Pollution from agricultural activity(fertilisers, manure, silage liquor,pesticides). May be indicated by thepresence of faecal bacteria, highconcentrations of nitrate and/orammonium, dissolved organic carbonand maybe potassium. If these arefound, pesticides should also beanalysed.

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Figure 25a. Map showing locations of some spas in theU.K. and other locations mentioned in the text.

• Road salt. Characterised by sodium(or maybe potassium) and chloride.

14.3 Spas and deep groundwatercirculation, bottled watersAlthough the pressure of overlying rocktends to reduce the numbers of openfractures with depth, there may be somedeep groundwater circulation, althoughgenerally of modest volume, alongselected flow paths. The emergences ofsuch deep flow paths often representmineral water springs or spas. Suchflowpaths may occur in an area oftectonic disturbance such as a majorfault, but also require sufficient head todrive water down into the earth. It cantake many years for the water to re-emerge from a deep circulatory system;radiometric dating estimates of thegroundwater rising at Buxton in theEnglish Peak District suggest an age of10,000 years. Clearly, such old watershould be free from modern daycontaminants, and is likely to be inmature hydrochemical equilibriumwith the rocks through which it haspassed. It could, therefore, be quitesaline, but happily the Buxton water isonly modestly mineralised and is widelyenjoyed as a bottled table water.

In Central Wales, the spa resorts ofBuilth Wells, Llandrindod Wells and thelesser known Llanwrtyd Wells andLlangammarch Wells (Figure 25) allrelied on old upwelling groundwaterfrom Silurian/Ordovian rocks, drivenby the head provided by thesurrounding hills. The sources are ofvariable hydrochemical type, fromsaline to iron-rich (chalybeate) andsulphur-rich. These small springdischarges relate geologically to theTywi Lineament with groundwatercirculating down to 300 m before risingto mix with shallower waters. Mixingwith the shallower groundwaters tendsto disguise the chemistry of the deepercirculating waters (Edmunds et al,1998). Known to the Romans for theircurative powers, the spa waters weredrunk warm and by the pint by theVictorians, and are now a novelty ondisplay for visitors. Treatments availableat the spas also included the needle

shower (high pressure needles of salinewater jetted at the naked patient) andother rather odd Victorian remedies.

Other similar deep-seated salinegroundwater systems occur in Britishbasement rocks in the Lake District, atWentnor near the Shropshire LongMynd and in Scotland.

Few British or Norwegian bottledwaters derive from old, deep ground-water circulation, other than that atBuxton. Most come from relativelyshallow sources. For example, watersbottled as Natural Mineral Waters(according to EC labelling require-ments) in Scotland issue from springsor are pumped from boreholes inDevonian and Carboniferous sandstoneand lavas, one (Caithness Spring,

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Figure 25b. Map of Norway showing some of the sitesmentioned in the text.

Berriedale) from the Precambrian andone (St Ronan's Spring, Innerleithen)from a borehole in Silurian shales.Another source in lavas is extremelyweakly mineralised and represents avery young water from a very shortflowpath: its attraction as a bottledwater eludes the authors who wouldrather add safe (but boring) tap waterto their whisky!

Further Reading on Water QualityAastrup et al. (1995), Asikainen &Kahlos (1979), Banks et al. (1993a,1995, 1997, 1998a-e, 2000), Bucher &Stober (2000), Frengstad & Banks(2000), Frengstad et al. (2000, 2001, inpress), Gaut et al. (2000), Lahermo et al.(1990), Morland et al. (1997), Reimannet al. (1996), Sæther et al. (1995).

Further Reading on Mineral Waters and SpasAlbu et al. (1997), Edmunds et al.(1969, 1998), Robins & Ferry (1992)

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Water Into Wine - Trondheim Police Pollute Aquifer with PotentiallyNarcotic Fluids.

The following story was reported in the Norwegian newspaper "Verdens Gang" (14/10/92), in Englishtranslation:

"Moonshine on tap

It's swimming with fusel and moonshine liquor in Osveien (Trondheim)...The brew available on tap fromthe Foss and Kristiansen families is derived from a police raid of over 3500 litres of sats*.The fermentingfluid which the police chucked out has ruined the well supplying the two families...."It's bubbling likeAlka-Seltzer", sighs Tor Kristiansen and a black thought wings its way to the boys in blue at TrondheimPolice Station.The police found an illegal still in the neighbouring house...The fire brigade were calledout to dispose of the offending mash and pumped it right out into the ground....

But it's not just from the 60 m deep borehole that the sats is pumped up. From a rocky slope behind thehouses a steady stream of water, with a familiar odour, trickles out.'"

*sats = mash, must...the mixture of sugar and yeast so beloved of the practitioners of the traditionalNorwegian art of home-distilling.

Most "problem parameters" may betreated. When considering treatment,please remember the following:

(i) Minimise your exposure. If youhave fluoride-rich water, you maynot need to use fluoride toothpasteor other supplements. Seek advicefrom your dentist or doctor.

(ii) You are the customer. Don'tbelieve the sales brochures. Insistthat a supplier proves that atreatment system is satisfactory byanalysing the water, "before" and"after" treatment.

(iii) Most water treatment systemsrequire maintenance. This mayentail cleaning, changing of filters,or regeneration of ion exchangeresins. Old filters can be breedinggrounds for bacteria.

(iv) You don't need to treat all yourwater - there is often acorrespondence betweentreatment capacity and price. Ifyou have fluoride-rich water, youneed only treat the water used fordrinking, not that used forshowering! Water hardness(calcium and magnesium), on thecontrary, is possibly quite good foryour health (according to somestudies), but may need to beremoved before feeding boilers,washing machines and otherheated appliances.

(v) Some treatment methods can havenegative health consequences.Some water softeners exchangecalcium ions for sodium. This isnot desirable for people requiringa low-sodium diet.

More specifically, for the most commonparameters requiring treatment:

• Bacteria may be treated by chemicaldisinfection (ozone, chlorinatingagents) or ultra-violet treatment. Thebest technique depends on waterquantity and water chemistry.

• Radon may best be treated by aerationfollowed by a short (c. 1 hour) storageto allow decay of daughter nuclides.On a domestic scale, several cycloneor filter-cascade units are available,providing effective treatment.

• Fluoride may be treated by reverseosmosis, anion exchange or activatedalumina adsorption techniques. On alarger scale, aluminium flocculationand precipitation can be effective.

• Nitrate may be treated by reverseosmosis or activated carbon filtration,as may uranium.

• Undesirable concentrations of sodiumor calcium/magnesium may be treatedby cation exchange.

Further Reading on WaterTreatmentBanks et al. (1998a, 2000), Ellingsen(1992b), Grundfos (1988), StatensStrålevern (1996).

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15. Water Treatment

We have already said that boreholesshould not be located too close to pitlatrines, agricultural land or oil tanks,but how many metres is "too close" ?The trouble is, we don't really know.

In relatively homogeneous granularaquifers (sand or gravels), we can derivea permeability and porosity fromlaboratory studies or pumping tests andcalculate how far groundwater travels in50 days (which UK practice suggests isabout the time needed for bacterialcontamination to "die out" in ground-water; Norway uses a figure of 60 days)or 400 days (the time estimated forsome other pollutants to degrade).Using analytical equations or numericalmodels, we can draw source protectionzones based on these distances.

In hard rock aquifers, however, we don'tknow very much for certain. We haveonly statistics on fracture patterns andstatistics on well yields. We can derivesomething called an "apparenttransmissivity" from pumping tests - akind of average of the permeability ofall the fractures feeding the borehole.But it's not the average that's importantin this context, it's the travel timethrough the most permeable fracturepathway. We often assume an effectiveporosity of 1% in crystalline bedrock,but there is very little basis for thisfigure.

The only way to approach a solution isvia a statistical or "stochastic" approach.Fracture statistics can be used to generate a range of possible ground-

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16. Vulnerability and SourceProtection

Figure 26. Comparison of groundwater catchmentareas (defined by flow paths) for a borehole drilled ina dolomite aquifer in the USA using (left) adeterministic porous medium model (contours inmetres OD) and (right) a stochastic model based ontwenty realisations of a fracture network. Note thatthe borehole's capture zone is larger (and the traveltimes faster) when modelled using the fracturenetwork model. After Bradbury & Muldoon (1994).Printed with permission from Springer Verlag.

water models for the aquifer, allowingmedian, best-case and worst-casesolutions to be examined. Guérin &Billaux (1994) used such an approachand found difficulties in calibratingmodels - adequate simulation of waterlevels and groundwater fluxes did notimply adequate simulation ofgroundwater transport times. Bradbury& Muldoon (1994) also used stochasticfracture network generation modelsand found that source protection zoneswere considerably bigger thanconventional porous medium modelswould predict (Figure 26).

What to do ?

• Use common sense• Use conventional techniques (porous

medium models) cautiously andmultiply the results by a significantsafety factor.

• Support research into development ofuser-friendly stochastic fracturenetwork models.

16.1 Groundwater ProtectionPractice in the UK and NorwayIn the UK, groundwater vulnerabilitymaps are available at a scale of1:100,000 for England and Wales andparts of Scotland. These provide a firsttier in the risk assessment proceduretowards deciding what activities arepermissible to avoid serious risk ofcontaminating groundwater resources.However, for much of the hard rockterrain of these areas, the maps showthat vulnerability is "negligible" on the(mistaken) belief that there is nogroundwater available within them tobe contaminated. These maps are,therefore, of limited value in hard rockareas, although they form a valuablepart of the decision support systemover the more permeable aquifers.

In Norway a similar situation exists: intheory, wells should be surrounded by"sanitary zones" within whichpotentially contaminating activities areprohibited or limited. These zones arebased on the well's catchment area andon groundwater travel times. Insedimentary aquifers these can be

reasonably estimated. In hard rockaquifers, however, it is recognised(Eckholt & Snilsberg 1992, Robins1999) that such calculations are almostimpossible to make. Only very general"common sense" guidance is given fordefining sanitary zones in hard rockaquifers and, as we have seen, commonsense is not always a particularlyreliable guide in such complexhydrogeological environments.

16.2 Soakaways, Septic Tanks andPit LatrinesOne of the commonly asked questionsis, "How far do I have to place my wastefacility, be it soak-away, septic tank orpit latrine, from my groundwatersource?" There is no single correctanswer. The optimum spacing betweenlatrines and water sources is often givenas 30 - 50 m as a hard and fast rule. Inreality, the distance is a function of theprevailing rock type and cover material,the depth to the water table, the localhydraulic gradient, and the design ofsanitation and groundwater source. Afractured rock aquifer, such as granite,behaves differently from a poroussandstone aquifer. A fracture may runbetween groundwater source and wasteareas to provide a hydraulic connectionand it will thus often be advisable tolocate waste facilities down-gradient ofand/or approximately perpendicularlyto any known fracture trends withrespect to a groundwater borehole.Greatest care must be taken where thewater table is shallow (ie less than about3 m below ground surface) as theopportunity for attenuation of wastematerial in the unsaturated zone issmall.

In the Republic of Ireland there are anestimated 300 000 septic tanks serving apopulation of 1 million people anddischarging some 80 million m_ peryear of effluent into the ground. Theeffluent contains faecal bacteria, andhigh levels of nitrogen and phosphorus.Recent investigations revealed that thesafe distance between source and septictank varied between 30 m and 60 mdepending on the prevailing geology,and that the worst conditions were thin

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soils over fractured bedrock, a situationcommon in hard rock areas. The studydefined a site unsuitable for a septictank as one:

– where the percolation rate ofwastewater to the ground is so fastthat it puts groundwater at risk,

– where the percolation rate is so slowthat it puts surface water at risk,

– where the water table is within 1.5 mof the surface,

– where the bedrock is within 1.5 m ofthe surface.

The basis to this importantconsideration is common sense. Ingeneral, the further the waste disposalfacility is from the water source thebetter. In practice, land boundaries maylimit available options. The sensibleplacement of waste down gradient ofthe water source, with source and septictank placed at right angles to theprevailing fracture orientation areadvisable precautions.

Further precautions include well-headsanitary seals and other means ofpreventing contaminated surface run-off, for example, getting down theoutside of the well casing tocontaminate the source. Do not siteyour borehole in the middle of afarmyard - you would be surprised howmany such boreholes exist and howmany of them receive nitrogenoussurface water as a result. Furthermore,ensure that your fuel tanks are as farfrom your groundwater source aspossible, that they are satisfactorilybunded and check the tanks regularlyfor spillage and leakage - a very smallconcentration of hydrocarbons indrinking water taints it and doesn’t doyou any good either.

Further Reading on SourceProtectionBanks et al. (in press), Bradbury &Muldoon (1994), Burgess & Fletcher(1998), Daly et al. (1993), Daly &Warren (1998), Eckholdt & Snilsberg(1992), Guérin & Billaux (1994), NRA(1992), Palmer & Lewis (1998), Robins(1998, 1999).

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A borehole is just like any other piece ofequipment. It requires regular checksand maintenance to function at opti-mum efficiency and to prolong its life.It is a good idea to regularly measureand record the yield of the borehole.For municipal supplies this may best bedone by means of a yearly, short termpumping test (see above) to measurethe borehole's specific capacity.

Many municipal waterworks do nothave the possibility to measure waterlevel or drawdown in their boreholes.This is a mistake - it is like driving a carwithout a "low oil" warning light. Water

levels can be measured most effectivelyvia either pressure transducers coupledto data loggers, or simply by anelectrical "dipper" through a speciallyinstalled "stilling pipe" to avoid tanglingthe dipper with rising main or cable.

It is also advisable to remove the pumponce every few years to inspect it forsigns of clogging or corrosion. Provideda flexible rising main hose has beenattached to the pump, this should notbe a problem for most users.

If a decline in borehole yield is noted, itis important to find out if this is due to:

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17. Maintenance &Rehabilitation

Figure 27(a) Iron bacterial biofilm growths in theHvaler subsea road tunnel, Granite, (b) growths ofGallionella on a microcope slide suspended in agranite borehole on Hvaler. Both photos: David Banks

• poor performance of the boreholeitself (low yield with high drawdown,i.e. declining specific capacity)

• or poor performance of the pump orrising main (low yield, butcorrespondingly low drawdown, i.e.no decline in specific capacity).

Poor pump or rising main performancemay be caused by leakages in the risingmain, clogging of the pump or risingmain or wear of the pump impellers.The solution is usually straightforward;replacement of worn/corroded parts orsome form of physical/chemicaltreatment to remove clogging.

If the problems lies in the borehole,however, it is important to ascertainwhether the low yield is due to:

• abnormally dry / low rechargeweather conditions

• increased abstraction from othernearby users

• a problem of fracture clogging ordegradation in the borehole.

Water-yielding fractures (and pumps orrising mains) may become clogged withparticulate matter, chemical precipitatesor, most often, by some sort of bacterialmat comprised of metal-immobilisingbacteria (often called iron bacteria) andiron oxyhydroxide and/or calciumcarbonate precipitates. These look likean orange slime at first, but can hardento a brown crust, and can often be seenin down-hole closed circuit televisionsurveys. Another tell-tale sign of thesebacterial biofilms are occasional, very

high total bacterial counts in wateranalyses as pieces of these mats sloughoff. The bacteria are not dangerous forhumans, but cause engineeringproblems.

If such bacteria and precipitates arefound, a course of treatment involving:

• shock chlorination of the borehole tokill the bacteria

• physical agitation, e.g. jetting or wirebrushing to remove physicalencrustation

• treatment with concentratedhydrochloric, sulphonic orhydroxyacetic acid to dissolve ironhydroxide or carbonate precipitates(this should only be undertaken byskilled personnel)

• clearance pumping to removedisplaced and dissolved biofilm

• repeat chlorination to kill anyremaining bacteria. (The pump andrising main should also be disinfectedbefore re-emplacement in theborehole).

Some hydrogeologists also recommendhydraulic fracturing in cases ofborehole clogging to re-open cloggedfractures or create new fractures. Insuch cases, ensure the water andequipment used is sterile, and disinfectthe borehole after treatment.

Further Reading on WellMaintenance and RehabilitationBanks (1992a), Banks & Banks (1993b),Driscoll (1986), Howsam (1988, 1990).

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Rocks, minerals and groundwater havea huge capacity to store heat. They havean approximately constant temperaturethroughout the year, although heatfrom the sun will tend to warm them inthe summer. The rocks cool very slowlyand they are generally warmer than theair in winter. Conversely, rocks aregenerally cooler than the air in summer.It is thus possible to extract some of thisstored solar heat (and a component ofgenuine geothermal heat - see 18.4) viaboreholes during the winter. This heatenergy may be tapped either by:

• pumping groundwater from aborehole and extracting heat from itvia a heat pump. This method is best

suited to permeable rocks and wellswith a high yield.

• circulating a fluid through a closedhose system down the borehole. Thefluid is warmed to the temperature ofthe rocks and, on its return to thesurface, may be sent through a heatpump.

A heat pump (Figure 28) needs a smallamount of electricity to run, andfunctions like a refrigerator. It takesheat from a low-temperature medium(e.g. Norwegian groundwater at 5 -6°C) and transfers it to a hightemperature space-heating medium at,say, 25°C. The electricity is used to"push" the heat "up" the temperature

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18. Ground Source Heat

Figure 28. Schematic diagram of a groundwater-basedheat pump system. A carrier fluid (e.g. glycol) with alow freezing point is circulated in a closed loop in theborehole (left), being warmed to the temperature ofthe groundwater.This passes through the heat pumpevaporator where it heats and vaporises the exchangerfluid (with a very low boiling point), which is in turnfuther heated by compression.This heat is thentransferred to a heating fluid (e.g. circulating air)which heats the room.

Evaporator(Exchanger fluid warmed and evaporatedby carrier fluid)

Condenser(Exchanger fluid cooledand condensed byheating fluid / air flow)

Expander(Carrier fluid coolsduring expansion)

Compressor(heats exchanger fluidby compression)

Heat Pump

Air flow

gradient. Heat pump systems may bebased on many different sources, suchas sea-water, unfrozen rivers, deep lakesor even sewage. However, in inland,rural areas of Norway or Scotland,geology may be the most accessibleresource. Heat pumps may be used towarm domestic properties, but areprobably most effectively utilised atlarger public buildings, vehicle depots,rail stations or residential blocks.Circulating groundwater may also beused for de-icing pavements in winter.

18.1 But is it Ecologically Friendly ?About as ecologically friendly as ispossible ! The heat extracted is essenti-ally solar energy (and a small proportioncontributed by the earth's geothermalgradient). Instead of using solar cells,we are using the earth's surface as ahuge solar energy collector. The energyis thus sustainable, provided we do notremove more than is replen-ished bynature. Other nations, notably Swedenand the USA, have actively promotedheat pumps as one of the most attrac-tive alternative technologies availablefor space heating. The only drawback toheat pumps is that they require a smallelectrical energy input to extract theground-source heat. However, the netenergy benefit is huge, and the electricityconsumed by heat pumps is outweighedby savings in electrical energyconsumption in conventional heating.

Not only are heat pumps ecologicallyfriendly, they can rapidly save theconsumer money. And, as opposed tomany alternative energy technologies,heat pumps are proven, and they maybe purchased today from a local waterengineer. It is also likely that heat pumpsystems may be implemented at sitessuch as landfills or abandoned mines,where heat extraction could efficientlybe combined with contaminant controlprogrammes.

18.2 How Much Energy can beExtracted ?The energy extracted from a "heat well"depends on:

• the thermal properties (heat capacityand conductivity) of the rock

• the temperature of the subsurface• the heat "catchment area" and surface

area available for exchange betweenrock and borehole fluid.

• the solar (major) and geothermal(minor) inputs to the system.

• the thermal efficiency of the heatpump and extraction system

The thermal properties of geologicalmaterial vary according to mineralcomposition (Table 1). Quartz contentis a decisive factor.

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Table 1.The thermal conductivity of selected rocks and minerals (after Sundberg 1991).

Rock Mineral Conductivity in W/(m.°C)

Limestone 1.5 - 3.0Shale 1.5 - 3.5Sandstone 2.0 - 6.5Granite 3.0 - 4.0Diorite 1.7 - 3.0Quartzite 5.5 - 7.5Gneiss 2.5 - 4.5

Quartz 7.7Plagioclase 1.5 - 2.3K-feldspar 2.5Mica 2.0 - 2.3Olivine 3.1 - 5.1

Of course, over the average year, theenergy extraction cannot exceed theenergy input to the system. During thewinter, however, the extraction canexceed the input, provided a sufficientperiod of recovery is allowed during thesummer. In fact, as hydrogeologists willbegin to recognise, the physicsgoverning heat flow is highly analogousto that governing groundwater flow,and the scheme just described isdirectly similar to seasonal exploitationschemes for groundwater management.

As an example, an installation inKristiansund, mid-Norway, comprising3 boreholes (two to 50 m, one to 37 m)is reported to have yielded 9 kW spaceheating effect, (2.5 times more energywas provided by the heat pump thanwas used to operate it - Oterholm1990). In the Oslo area, typical yieldsare reported as 45 W per metre ofborehole (Skarphagen 1996), which isnot dissimilar to the Kristiansundexperience.

In fact it is likely that the heat yield canbe significantly enhanced by

• pumping the borehole at a low rate,inducing groundwater flow to theheat borehole. The groundwatertransports additional heat to theborehole by advection, effectivelyincreasing the borehole's heatcatchment area.

• drilling in areas of significanttopography with a high naturalgroundwater flow and thus higherheat recharge.

• hydraulic fracturing to increasepermeability and thus groundwaterflow to the borehole. The hydraulicfracturing also increases the fracturecontact area between rock andgroundwater, permitting moreeffective exchange of heat between theprimary heat store (the rocks) and theheat transport medium(groundwater).

18.3 A Cooling ResourceCirculating groundwater may also beused to cool equipment (computers) oroffices. The circulating groundwater

will thus be warmed up. This "waste"heat may then be reused to heat otherparts of a building complex or may bere-injected to the ground via injectionboreholes to be used later (e.g. inwinter). This is thus a form of artificialheat recharge to the ground. For suchheat storage to work, there must not bea high natural groundwater through-flow which can disperse and advect theheat away.

18.4 Geothermal EnergyIn sections 18.1 - 18.3, we have discus-sed the abstraction of dominantly solarenergy which is stored in rocks. Ofcourse, there will also be a small com-ponent of genuine "geothermal" energy,derived from nuclear decay reactionswithin the earth. This componentbecomes more significant in deeperboreholes. Temperature increases withdepth according to the geothermalgradioent – typically at least 20°C forevery km in many basement rocks.

In some particular areas, the geother-mal temperature gradient may beespecially high and it may be possibleto extract geothermal energy fromboreholes (Lindblom 1978, Baria 1990).The possibility of doing this has beenexplored both in the Bohus Granite ofSweden (Landström et al. 1980) and inthe Carnmenellis Granite of Cornwall,U.K. (Downing & Gray 1986). In bothcases, although some success wasenjoyed during pilot projects, thecommercial exploitation of this energywas judged economically inefficient intoday's energy climate.

Further Reading on Ground SourceHeat & Geothermal EnergyAndersson (1996), Baria (1990),Downing & Gray (1996), Hilmo et al.(1998), IEA Heat Pump Newsletter,Kitching et al. (1992), Landström et al.(1980), Lindblom (1978), Morgan(1997), NGU/NVE (2000),NTH/SINTEF (1992), Oterholm(1990), Skarphagen (1995, 1996),Sundberg (1991), Wikström (1995).

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Groundwater in crystalline bedrock is amisunderstood resource. Althoughdifficult to predict its behaviour,groundwater in such aquifers remainsan excellent solution for potable /agricultural water supply for domesticproperties and farms. It is also anattractive, environmentally friendly,decentralised energy resource for spaceheating, via the use of heat pumps. Thelow temperature of groundwater,especially in Northern Britain andScandinavia, renders it suitable forcooling and air-conditioning purposes.

The major challenges in the field ofhard-rock hydrogeology today can besummed up as:

• Understanding mechanisms ofgroundwater recharge and quantitiesof water entering hard rock aquifers

• Building up statistically significantquantities of data on yield and qualityof water from wells in bedrock

• Developing low cost treatmentsystems for tackling the somewhat"unusual" quality problems (radon,fluoride, uranium) which may occurin hard rock groundwater.

• Promoting ground source energymanagement and developing off-the-shelf heat pump solutions based ongroundwater, in order to minimisethe capital investment required toinstall such systems.

• Developing methodologies forassessing aquifer vulnerability anddelineating source protection zones inhard rock terrain.

Groundwater in crystalline rocksremains an under-used resource,particularly in Britain. Hard rockaquifers contain enormous quantities offresh groundwater, a small fraction ofwhich is used today.

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19. Conclusion

Aastrup, M., Thunholm, B., Johnson, J.,Bertills, U. & Berntell, A. 1995. Groundwaterchemistry in Sweden. NaturvårdsverketFörlag, Report 4416, 52 pp.

Agricola 1556. De Re Metallica. Translated(1912) by H.C. & L.H. Hoover in TheMining Magazine, London.

Albu, M., Banks, D. & Nash, H. 1997.Mineral and thermal groundwater resources.Chapman & Hall, 447 pp.

Andersson, O. 1996. Kyl- och varmelagring iberg och grundvatten [Cold- and heat storagein rocks and groundwater - in Swedish].Borrsvängen, 3/96, 10-13.

Asikainen M & Kahlos H 1979. Anomalouslyhigh concentrations of uranium, radiumand radon in water from drilled wells in theHelsinki region. Geochimica etCosmochimica Acta, 43, 1681-1686.

Banks, D., Lauritsen, T., Skarphagen, H. &Rohr-Torp, E. 1991. Hvaler prosjekt -grunnvann i fast fjell. Boring ogkapasitetstesting av fire hull, ved Pulservik,Kirkeøy [Hvaler project - groundwater inbedrock. Drilling and capacity testing of fourboreholes, Pulservik, Kirkeøy - in Norwegian].Norges geol. unders. report, 91.215, 108 pp.

Banks, D. 1992a. Grunnvannsbrønner.Kontroll, vedlikehold, rehabilitering[Groundwater wells. Control, maintenanceand rehabilitation - in Norwegian]. Norgesgeol. unders. Skrifter, 108 (GIN Veileder no.13), 19 pp.

Banks, D., 1992b. Estimation of apparenttransmissivity from capacity testing ofboreholes in bedrock aquifers. AppliedHydrogeology, 1(4), 5-19.

Banks, D., 1992c. Optimal orientation ofwater-supply boreholes in fracturedaquifers. Ground Water 30(6), 895-900.

Banks, D. 1992d. Aquifer management - anintroduction to hydrogeology. GeologyToday, 8(2), Earth Reference No. 6, i-iv.

Banks, D., Solbjørg, M.L. & Rohr-Torp E.,1992a. Permeability of fracture zones in a

Precambrian granite. Quarterly Journal ofEngineering Geology, 25, 377-388.

Banks, D., Rohr-Torp, E., & Skarphagen, H.,1992b. An integrated study of a Precambriangranite aquifer, Hvaler, SoutheasternNorway. Norges Geologiske UndersøkelseBulletin, 422, 47-66.

Banks, S.B. & Banks, D. (eds.) 1993a. Mem.24th Congress of International Association ofHydrogeologists, 28th June- 2nd July 1993, Ås(Oslo), Norway ‘Hydrogeology of HardRocks’, Norges geologiske undersøkelse/IAH.

Banks, S.B. & Banks, D. 1993b. Groundwatermicrobiology in Norwegian hard-rockaquifers. In Banks & Banks (1993a), 407-418.

Banks, D., Rohr-Torp, E. & Skarphagen, H.1993a. Groundwater chemistry in aPrecambrian granite island aquifer, Hvaler,Southeastern Norway. In Banks & Banks(1993a), 395-406.

Banks, D., Lauritsen, T., Rohr-Torp, E.,Rønning, J.S. & Skarphagen, H. 1993b.Groundwater in bedrock - Hvaler project.Investigations at testsite Utengen. Norgesgeol. unders. report, 93.117, 29 pp.

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Aquifer. A sediment or rock unit that hassufficient groundwater storage and which issufficiently permeable that it can be used forthe viable exploitation of groundwater.

Baseflow. The portion of flow in a river orstream that is maintained during prolongeddry periods. Under natural conditions, thisbaseflow is normally supported bygroundwater entering the watercourse viasprings or leakage through the channel bed.

Borehole. See well.

Crystalline bedrock. In this book, the termis used interchangeably with hard rock andbedrock. Rocks comprised of interlockingcrystals with very little intergranularporosity. Most groundwater flow thus takesplace in fracture systems. These are typicallymetamorphic (e.g. gneisses) or igneous(basalts, granites) rocks.

Drawdown. When a well or borehole ispumped, the groundwater level is depressed.The drawdown is the difference between thenatural non-pumping level and the pumpedlevel.

Dyke. A sheet-like body of igneous rock(such as dolerite) that has been intrudeddiscordantly (i.e. cutting across bedding andother structural features) along a fracture.

Fault gouge. Fine-grained rock debrisoccurring in a fault zone, and produced bythe grinding action of fault motion on thewall rocks.

Fracture set. A collective term encom-passing all fractures in a rock unit that havea similar orientation and form and, usually,a common genesis.

Fracture zone. A planar zone of rock whichis characterized by an increased density offractures or joints. A fracture zone may be afault zone and its core may be comprised offine-grained fault gouge or rock flour.

Groundwater. Water that occurs in porespaces and fractures in sediments and rocksin the subsurface. The term groundwater isusually used to refer to water which occursin saturated strata below the water table, and

which can be abstracted from wells, springsor boreholes.

Head. A measure of the potential energy ofgroundwater at any point in an aquifersystem. Head is essentially composed of thesum of pressure head and elevation (orheight) head. Groundwater always flowsfrom regions of high head to low head.

Hydraulic conductivity. See permeability.

Lineament. A linear geographical featurethat can be observed on maps, aerial orsatellite photos. In hard rock terrain, thelineament may correspond with a fracturezone. Alternatively it may correspond with alinear rock outcrop, a lithological boundary,a pipeline trench or other man-madefeature.

Moraine. A sediment which was transportedby or deposited in contact with a glacier orice-sheet. A basal moraine or basal till is adeposit formed at the base of a glacier. Itoften comprises very poorly sorted materialwith a high content of pebbles or bouldersset in a fine-grained clayey matrix.

Permeability. The ease with which a fluidcan pass through a porous or fracturedmedium under a head gradient. Whenconsidering water as the fluid in question,the term permeability is effectivelyinterchangeable with hydraulic conductivity(expressed in m/d or m/s).

Post-glacial isostatic rebound. During theice age, the huge weight of ice coveringFennoscandia pushed the continental crustdownwards by a vertical distance of severalhundred metres. Following the melting ofthe ice sheet, the crust started to recover toits original level, a process that continuestoday. This rebound is accompanied byneotectonic activity, including faultreactivation.

Specific Capacity. The ratio of the yield(or pumping rate) of a well to thecorresponding drawdown in the water level.Specific capacity is an expression of theproductivity of the well and is related toaquifer transmissivity. Units are typicallym2/s or m2/d.

21. Glossary

Spring. A location where groundwateremerges naturally at the earth's surface. Thespring often coincides with the intersectionof the water table and the earth's surface.

Storage. The ability of an aquifer to storewater within its structure under conditionsof increasing groundwater head, and torelease it under conditions of decreasinghead. The storage coefficient will bear somerelation to the rock's porosity and to itselastic properties.

Transmissivity. The product of an aquifer'sthickness and its hydraulic conductivity. Theease with which water can flow through anaquifer unit. Expressed in m2/d.

Water table. The surface in an unconfinedaquifer at which the pore water pressureequals atmospheric. Below the water table,an aquifer is fully saturated withgroundwater. The water table is analogous tothe free water surface in a tank of water(representing an aquifer).

Well. A hole in the ground which providesaccess to an aquifer and through whichwater can be abstracted. Wells are typicallyeither dug by hand (dug wells) or drilledwith a drilling rig (borehole).

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The authors wish to acknowledge the assistance, discussion and inspiration of the followingin their studies of the hydrogeology and hydrochemistry of hard rock aquifers and during thepreparation of this volume:

Sheila Banks, John Barker, Anatoly A. Bedernikov, Peter Bennett, Aleksander Y. Berezovsky,Kjell Bjorvatn, Vidar Brekke, Donal Daly, Janice Doherty, Yekaterina M. Dutova, KnutEllingsen, Bjørn Frengstad, Dieter Garbe-Schönberg, James Marshall Hendrix, Wayne Holden,Olga V. Karnachuk, Lars Anders Kirkhusmo, Jan Reidar Krog, Nikolai A. Makarenko, BruceMisstear, Geir Morland, Noelle Odling, Valery P. Parnachev, Denis Peach, Dmitry S.Pokrovsky, Clemens "Regnskabets time" Reimann, Erik Rohr-Torp, The Sedatives, Stepan L.Shvartsev, Ulrich Siewers, Helge "Heat Pumps" Skarphagen, Aase Kjersti Skrede (néeMidtgård), Terje Strand and colleagues, Dave Watkins.

Finally Dave Banks wishes to thank the Department of Dynamic Geology at Tomsk StateUniversity, Russia, who provided the ideal working environment for the preparation of muchof this volume.

Acknowledgements

This small book has a big ambition. It aims to present practical informationand a little philosophy to those involved in locating groundwater re-sources in areas underlain by crystalline bedrock, that is to say:

• Private groundwater users, potential well owners and water bottlers• Local authorities• Water companies and local water supply undertakings• Drillers• and Consultants

We have consciously mixed practical advice with some hydrogeologicaltheory.We have also provided a comprehensive reference list for those ofyou who wish to delve further into the subject.We will largely, though not exclusively, restrict ourselves to considerationof bedrock aquifers in the glaciated terrain of Norway and the northernU.K.

Almost all of Norway is underlain by some type of crystalline bedrock, andgroundwater from such rocks is an important drinking water resource inrural areas. In the United Kingdom, crystalline bedrock groundwater isprobably an underused resource.

ISBN 82-7386-100-1

N-7491 Trondheim, Norway

Telephone +47 73 90 40 00Telefax +47 73 92 16 20

E-mail: [email protected]

Norges geologiske undersøkelse

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