ENVIRONMENTAL TRACER IN ARID AND SEMI-ARID BASINS: HISTORICAL PERSPECTIVES AND FUTURE TRENDS (Invited Presentation)

Determining recharge to inter-montane basin aquifers is inherently difficult using water budgets because the storage volumes are large and the fluxes into and out of these systems are small. Furthermore, interbasin flow makes closing the water budget problematic. In 1967 J. C. Vogel proposed the use of “point” measurements of ¹⁴C on DIC in groundwater to estimate recharge to unconfined aquifers and this basic approach has since been utilized with other age-dating tracers. While the environmental tracer methods avoid the inherent difficulty in water budget approaches (i.e. computing recharge as the difference between large numbers) they introduce a variety of transport issues such as matrix diffusion, spatial variability, dispersion, complex geochemistry, and sampling bias that in part results from a gap in age-dating tracers between about 100 and 1,000 years. Nevertheless, an exponential increase in environmental tracer studies has occurred over the past 25 years examining recharge provenance, interbasin flow, recharge rates, and residence times. While environmental tracers have provided significant understanding of basin aquifers, advances in conceptual and numerical models are needed to more fully utilize tracer data. For example, steady state models may not be appropriate over the large space and time scales associated with inter-montane basin aquifers and recent pumping and irrigation return flows have dramatically altered natural flow in some systems. Future studies could benefit from recent advances in ³⁹Ar (t_½ =269 yr) and ⁸¹Kr (t_½ =229,200 yr) measurement technology, and by the installation of multi-level scientific monitoring wells. As the need to expand agriculture and develop municipal water supplies in arid and semi-arid regions increases, so will the need to better quantify both past and present rates of recharge to inter-montane basin aquifers.