GROUNDWATER CHEMISTRY CHANGES DURING A RECHARGE EVENT IN THE KARSTIC EDWARDS AQUIFER, SAN ANTONIO, TEXAS

The Edwards Aquifer, located in south central Texas, is the primary water source for more than 1.7 million people in the San Antonio region and provides nearly all the water for agricultural, industrial, and municipal users. Water quality degradation from rapid development in the recharge zone has become a significant concern. Water quality monitoring has traditionally occurred on a quarterly or yearly basis for most constituents. Rapid changes in water chemistry point to the need to reevaluate sampling intervals and the role of stormwater in water quality monitoring. Nationally, stormwater runoff is often monitored for water quality changes related to land use. Impacts to groundwater have been less documented, especially in vulnerable karst aquifers such as the Edwards which is characterized by rapid infiltration of surface water with little or no filtration.

This study was designed to determine the changes in groundwater chemistry related to surface water infiltration during a storm event. Water samples were collected from a monitoring well and creek located below a watershed of mixed land use (20% high-density urban and 80% undeveloped land) in the Edwards Aquifer recharge zone in the San Antonio area. The well was equipped with dedicated sampling equipment and a continuous conductivity/temperature data logger. Several days of heavy rainfall fell on San Antonio in July 2002 causing the creek to flow. Groundwater levels in the well rose over 110 feet in less than four days. Conductivity in the well indicated a significant influx of surface water for the seven days during which the creek flowed. The steep rise in water levels and rapid changes in conductivity are typical of conduit flow in karst aquifers and indicate significant surface water infiltration. Authority staff analyzed four groundwater samples and one surface water sample during the recharge event spanning the entire storm hydrograph. Samples were analyzed for major anions and cations, metals, volatile organic compounds, nutrients, and herbicides and pesticides. The results indicate the varying composition of groundwater and surface water influences.