Application of Tracers in Arid Zone Hydrology

Abstract A method for quantitative estimation of aquifer and groundwater hydrological parameters using radioactive environmental isotopes is presented in the form of a menu-driven interactive program, written to solve lumped parameter mathematical problems, using the EXCEL* software, 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 calculation pathway. The program solves the direct problem by a selected model using input concentrations, the response function and radioactive decay. The inverse problem could be solved through a trial and error technique. With higher number of tracer data points sampled at longer observation time, 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 FR 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