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The Edwards Aquifer Authority (EAA) and other public agencies have collected and analyzed thousands of groundwater, surface water, and spring water samples to evaluate water quality in the San Antonio segment of the Balcones Fault Zone Edwards Aquifer of south central Texas. In July 2009, EAA staff prepared a report evaluating water quality data collected through August 2006. This update includes water quality data collected through June 2013. Like the original report, the purpose of this report is to examine this body of water quality data to characterize historical water quality, existing conditions, and water quality trends, as well as to identify specific constituents of concern (COCs) within the aquifer. Emphasis is placed on evaluating the presence of COCs that may have resulted from human activity in the study area. Water quality data were collected by the EAA, its predecessor agency, the Edwards Underground Water District (EUWD); the United States Geological Survey (USGS); and the Texas Water Development Board (TWDB) since 1913. More than 13,000 samples totaling more than 700,000 individual parameters were compiled for this report. Analytical data evaluated for the update of this report since 2006 has not changed the conclusions of the original report. The results are characterized by infrequent detections of potential contaminants in groundwater and surface water from multiple sources. Vulnerability of the aquifer to contamination has been demonstrated by the detection of parameters that do not occur naturally in the aquifer. The organic compounds were detected mainly in urban areas, whereas nitrate was detected throughout the aquifer. Despite the additional data, they are insufficient to determine whether concentrations are increasing or decreasing. Based on all of the historical analyses through June 2013, concentrations of 28 parameters exceeded protective concentration levels (PCLs) in one or more samples, which includes one additional parameter (nitrite) from the post-2006 analyses. The parameters were selected as COCs because their concentrations exceeded the assimilative capacity of the aquifer, which is the capacity of the aquifer to attenuate the concentrations of contaminants to acceptable levels before they reach a well or spring.
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