THE EDWARDS AQUIFER – EVOLVING CONCEPTUAL MODELS

The Edwards Aquifer of Texas is a major water in a rapidly growing, urbanizing region; its management is complicated by environmental, political, and legal issues. Stakeholders include domestic and municipal users, industry, agriculture, and downstream and coastal water users. The historical evolution of aquifer conceptual models demonstrates that management systems should be sufficiently flexible to meet an evolving understanding of the aquifer and related water resources. The general location and geology of the aquifer have been known for over a century, but the “devil is in the details.” The conceptual view of the Edwards Aquifer as a conduit-dominated aquifer in the mid 1880’s and early 1990’s changed to a primarily porous media system in the 1980’s until groundwater tracing beginning in the late 1990’s demonstrated the importance of conduits in groundwater transport. The formerly common practice of disposing trash into sinkholes, diverting stormwater into sinkholes, and direct discharge of wastewater to the creeks is no longer accepted over the Aquifer. The role of soil and epikarst in attenuating contaminates is poorly understood, although soil infiltration is commonly assumed to attenuate urban storm water completely. The hydrostratigraphy has been redefined; the aquifer can include adjoining portions of at least 4 formations. Recharge is primarily from losing streams, but the effects of urbanization, leakage from adjacent units, revised estimates of direct recharge, and shifting groundwater divides must now be considered. Recharge from streams has been shown to be non-uniform spatially and temporally. Dye tracing studies have documented previously unknown discharge sites and unexpected flow directions. The factors influencing spatially-variable permeability and storativity are still being discovered. With the requirements to protect endangered aquatic species in springs and the rivers that are fed by them (all the way to the Gulf of Mexico), details are needed for management models that protect property rights, allow utilization of this vital groundwater system, preserve critical environments, and meet stakeholder concerns. Rigid decisions based upon past conceptual models can be counterproductive.