THE CENTRAL TEXAS EDWARD AQUIFER: INSIGHTS FROM REGIONAL GEOCHEMISTRY INTO AQUIFER PROCESSES AND VULNERABILITY

The Edwards aquifer in central Texas, a highly productive fractured karst aquifer, is a designated Sole Source Aquifer for 1.5 million people. Groundwater sampling of 136 wells and springs between 1996 and 2006 as part of the U.S. Geological Survey National Water-Quality Assessment Program yielded an extensive amount of data on regional groundwater geochemistry. Results for a broad range of geochemical constituents provide insights into geochemical evolution processes, occurrence of natural and anthropogenic constituents, and the relation between geochemistry and hydrologic conditions. Geochemical tracers indicate that longer residence times affect fluid compositions as groundwater evolves from shallow unconfined to deeper confined groundwater. Mixing with downdip saline water affects the aquifer on a local scale. Five organic contaminants (atrazine, deethylatrazine, simazine, chloroform, and tetrachloroethene) were routinely or frequently detected, particularly in the shallow/urban unconfined part of the aquifer, and indicate that the aquifer is vulnerable to anthropogenic effects. The Edwards aquifer, like many karst systems, responds rapidly to changes in hydrologic conditions with changes in water level, spring discharge, and geochemistry. Temporal variability in Mg/Ca ratios and atrazine and nitrate concentrations suggest that atrazine enters the aquifer with surface-water recharge, whereas background nitrate concentrations in the aquifer are diluted by surface-water recharge. Higher nitrate concentrations and Mg/Ca ratios occur in conjunction with drier hydrologic conditions and lower aquifer levels. These results indicate that timescales of hydrologic and geochemical variability should be considered in designing and implementing monitoring programs in karst aquifers to assess long-term trends.