TRENDS IN GROUNDWATER QUALITY OF THE EDWARDS AQUIFER AND POTENTIAL IMPACTS OF CHANGING LAND USAGE

The Edwards Aquifer (EA) in south-central Texas is one of the most productive karst aquifers in the US and covers approximately 8,800 square miles. Groundwater from the EA is the primary drinking water source for the fast-growing city of San Antonio and surrounding areas (~2.3 million people). At present, the groundwater quality of the EA is considered to be excellent, with low concentrations of regulated contaminants. Such good groundwater quality is attributed to a favorable combination of hydrogeology and land use where most of the urban and agricultural areas overlie the confining unit, except ranching and agricultural activities in the contributing zone. Recent studies have shown, however, that there is a presence of emerging contaminants (such as VOCs, pesticides, and herbicides) even in the confined zones of the aquifer. In this study, we investigate the impacts of various land uses and management practices on the EA groundwater quality. Analyses of historical groundwater data (2008-present) indicates that the average groundwater pH is near-neutral (7.2±0.3), total dissolved solids (TDS) of 316±93 mg/L, and average concentrations of major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃^-, Cl^-, and SO₄^2-) were 13, 1.6, 77, 18, 272, 23 and 35 mg/L, respectively. Whereas the average concentrations are below safe limits, the data suggests temporal increase in trace elements like As (maximum 9.7 µg/L and close to safe limit of 10 µg/L), Cu (maximum 0.5 µg/L below safe limit of 1.3 mg/L), and occasional elevated concentrations above safe limits of Pb, F^-, and NO₃^-, as high as 28 µg/L, 4 mg/L and 22 mg/L, respectively, in certain monitoring wells. Sampling from different land use areas, analyses of trace elements of human health concern and dissolved organic matter are underway to better understand impacts of land use changes on the EA groundwater quality.