my paper 1995 riethm iahs 232 0211

Application of Tracers in Arid Zone Hydrology (Proceedings of the Vienna Symposium, August 1994). IAHS Publ. no. 232, 1995. 211 RIETHM: Radioisotope Environmental Tracer Hydrology Model using exponential and dispersion methods FAHMY M. HUSSEIN Cairo University, Faculty of Agriculture, Soil and Water Department, Cairo, Egypt Abstract A method for quantitative estimation of aquifer and ground water hydrological parameters using radioactive environmental isotopes is presented in the form of a menu-driven interactive program , written to solve l umped parameter mathematical problems, using the EXCEL* soft ware, which can run on Macintosh and PC-compatible computers. The input function and mathematical formulae are stored in a work sheet for updating computed values and the macro selects the calculati on pathway. The program solves the direct problem by a selected model using input concentrations, the response funct ion and radioactive decay. The inverse problem could be sol ved through a trial and error technique. With higher number of tracer data points sampled at longer observation tim e, a closer estimation of the residence time and the dispersion parameter could be obtained. The program is intended to be used with the assumptions: the system is a homogeneous porous aquifer, water flow is in the steady state, the tracer is "operationally" ideal with respect to the calculated parameters, the tracer natural injection is proportional to water inflow, the tracer output is proportional to water outflow (except for the C FR dispersive model), the number of the "fitting" parameters is small, the precision of the input function is high enough (either affected or not by thermonuclear tests), the weighting function of the tracer and that of groundwater are similar, the tracer residence time is equivalent to that of groundwater (except for the C F R dispersive model) and sources and sinks of the tracer do not disturb the system. The model has been tested using the input function of an intrusive river in the southern hemisphere. Tritium data collection and interpretation is recommended in these regions in order to make use of the last favourable concentrations of this tracer. INTRODUCTION Before and after the large-scale thermonuclear injection of tritium into the atmospheric moisture, this environmental tracer has widely been used in studying many hydrological systems. Beginning from the 1960s, its presence in considerable amounts in groundwater samples is consi dered as a clear qualitative indication of recent recharge and, in some cases, as a sign of probable recent pollution events. On a quantitative basis, it can be employed in the calculation of the mean residence time of the stor ed groundwater as well

my paper 1995 riethm iahs 232 0211

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