IMPACT OF GEOLOGIC HETEROGENEITY ON RECHARGE ESTIMATION USING ENVIRONMENTAL TRACERS - NUMERICAL MODELING INVESTIGATION

This paper presents a numerical modeling approach for assessing the im pacts of geologic heterogeneity on groundwater recharge estimates deri ved from environmental tracers. Common to many of the environmental tr acer methods used to infer recharge in arid environments is an assumpt ion of one-dimensional, vertical downward flow of water and solutes. H owever, in recent years there has been a growing recognition that flui d flux rates through geologic materials can be spatially variable owin g to heterogeneities in porous media properties. Previous studies have suggested that local flow directions are dependent upon the stratigra phic and sedimentologic characteristics of the medium and may not be v ertical even when application of water at the;surface is spatially uni form and hydraulic gradients are vertical. Consequently, environmental tracer movement will also be spatially variable, and the one-dimensio nal assumption invoked to interpret unsaturated environmental tracer c oncentration profiles may be unrealistic. Two different numerical simu lation experiments were performed to investigate the problems with usi ng environmental tracers to infer recharge. The first involves simulat ion of flow and tracer transport through heterogeneous Aat-lying media , and the second considers dipping layered anisotropic media. The resu lts of these simulations show that recharge inferred using environment al tracer methods is also highly spatially variable and that recharge estimates obtained by tracer profiles tend to overestimate recharge an d should be considered accurate only to within an order of magnitude, particularly in situations with significant media heterogeneity. Conse quently, recharge estimates obtained from tracer profiles should be cr itically evaluated with regard to impacts of spatially variable flow.