The main characteristics of the groundwater in Bahi-Manyoni

Basin in Tanzania Knivsland, S., Bakari, S., Aagaard, P.

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Geology • Basement of granite

and gneisses • Sedimentary bedrock • Alluvial and playa

lake sediments • Inselsbergs • Faulting and

earthquakes • Heights ranging from 800 – 2400 m.a.s.l. • Chenene hills stands

at about 1500 m.a.s.l.

Bahi-Manyoni basin • Internal drainage • Two rainfall seasons • High potential evaporation • Actual evaporation balance annual rainfall • Lake Sulunga/Bahi swamp get recharge from: 1. Approx. 60 % from direct rainfall on lake area 2. Approx. 40 % from ephemeral rivers like Bubu and

Mponde • Area around the lake is used for rice cultivation • The ephemeral lakes also contain fish

Table 1: Bahi Meteorological Station database, years 1962-2005

Challenges in the area

Distribution of water samples

• Large pH variation within villages • pH not consistent with large amount of Fe or Al in water

Piper diagram

80 60 4 0 2 0 2 0 4 0 6 0 8 0

2 0

4 0

6 0

8 0

2 0

4 0

6 0

8 0

2 0

4 0

6 0

8 0

2 0

4 0

6 0

8 0

C a N a H C O 3 C

M g S O 4<=C

a + Mg

Cl +

SO

4=>

6 6 12 12 18 18 24 24 30 (meq/L 30

Na Cl

Ca HCO3

Mg SO4

6 6 12 12 18 18 24 24 30 (meq/L 30

Na Cl

Ca HCO3

Mg SO4

6 6 12 12 18 18 24 24 30 (meq/L 30

Na Cl

Ca HCO3

Mg SO4

3 3 6 6 9 9 12 12 15 (meq/ 15

Na Cl

Ca HCO3

Mg SO4

4 4 8 8 12 12 16 16 20 (meq/ 20

Na Cl

Ca HCO3

Mg SO4

4 4 8 8 12 12 16 16 20 (meq/ 20

Na Cl

Ca HCO3

Mg SO4

4 4 8 8 12 12 16 16 20 (meq/L 20

Na Cl

Ca HCO3

Mg SO4

4 4 8 8 12 12 16 16 20 (meq/L 20

Na Cl

Ca HCO3

Mg SO4

4 4 8 8 12 12 16 16 20 (meq/L 20

Na Cl

Ca HCO3

Mg SO4

Stiff diagrams: Deep wells

Mitoo wells are in granite Londoni wells are in shales Ivandaa well are in granite Chibumagwa and Mbwasa 1 and 2 are close to or in a fault Udimaa, Sanza 1, Chidilo and Mpalanga are probably in sediments or sedimentary rocks

6 6 12 12 18 18 24 24 30 (meq/L 30

Na Cl

Ca HCO3

Mg SO4

0

1

2

3

4

5

6

-4,5 -4 -3,5 -3 -2,5 -2

log[

Mg2

+/H

+]

log [aSiO2(aq)]

Aktivitetsdiagram for Magnesium-Silika systemet ved 25 °C

Kaolinitt

Gibbsitt

Mg-mont.

Amorf Silika metning

Kvarts metning

0

1

2

3

4

5

6

-4 -3,5 -3 -2,5 -2

log[

a K+/a

H+]

log [aSiO2(aq)]

Aktivitetsdiagram for Kalium-Silika systemet ved 25 °C

Kaolinitt

K-mont. Amorf Silika metning

Kvarts metning

Mikroklin

0

1

2

3

4

5

6

-4,5 -4 -3,5 -3 -2,5 -2 -1,5

log[

a Ca2

+/aH

+]

log [aSiO2(aq)]

Aktivitetsdiagram for Kalsium-Silika systemet ved 25 °C

Kaolinitt

Ca-mont.

Kvarts metning

Amorf Silika metning

0

1

2

3

4

5

6

7

8

-5 -4 -3 -2 -1

log[

Na+

/H+]

log [aSiO2(aq)]

Aktivitetsdiagram for Natrium-Silika systemet ved 25 °C

Na-mont. Kaolinitt

Gibbsitt

Kvarts metning

Amorf Silika metning

Lav-Albitt

SEM results for sediments in the water samples after filtering with 1 or 0.45 µm filter

Sample 9: Kisedet project Kaolinite Albite Iron

Sample 26: Kipondoda Kaolinite Illite Iron Titanium Barium

Sediment samples mineralogy from XRD results

• Used XRD on 30 crushed, bulk sediment samples • Uses MacDiff version 4.2 for prosessing the raw

data files • Results until now: 1. Main mineral: K-feldspar and Quartz

Aluminium Calcium

Iron

Potassium

Magnesium Phosphor

Sulphur

Silica

Titanium

The others

Sediment sample 1.1 Liwali river XRF

Cupper 21 ppm

Chrome 80 ppm Lead 8,7 ppm Litium 9 ppm

Vanadium 29 ppm

Manganese 184 ppm

9,5 2,2

Rubidium 91,5 ppm

Strontium 89 ppm

Yttrium 10,6 ppm

Barium 435 ppm

Lantanium 14,8 ppm Cesium

30,7 ppm

12

Thorium 6,3 ppm

Uranium 2,7 ppm

Sediment sample 1.1 Liwali River: The others

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

XRF

Uranium in deep, drilled wells and shallow wells/springs/rivers/lakes

Values given in µg/L on the map

WHO drinking water standard: < 15 µg/L

Lake Chibumagwa: 314 µg/L Lake used for fishing, bading, water for livestock, washing clothes and so on

Mitoo deep well area: ~200 µg/L 6 deep drilled wells that are to supply The whole western part of the basin. Wells in Granite with mafic dykes.

2 shallow wells stands out: Farkwa in Chenene hills: 174 µg/L Maweni A: 194 µg/L (Cond. >10000 µS/cm)

Planned work 1) Grain size distribution on 2011 samples and estimation

the hydraulic conductivity on both years samples 2) Analysis of 12 more sediment samples for trace elements 3) Analyse the mineralogy of more sediment samples (XRD) 4) Soil water extraction with 0.01 M CaCl have been done

on 2010 sediment samples. Have had the major ions analysed. Need to work on the results

5) PHREEQC modelling of the activity of the ions. Activity diagrams, saturation indices for minerals

6) Statistical analyses on the element results 7) Try to correlate sediment/rock samples and water

samples 8) Show results in readable Gis maps

Thank you for your attention!