Technical Reports Document Library

The Edwards Underground Water District (EUWD) has been involved in an investigation to determine the source of volatile organic compounds (VOCS) in the Edwards aquifer in the Uvalde, Texas area (fig. 1). Existing Edwards aquifer wells were used to trace voc contamination up the hydraulic gradient in a west-northwest direction from the point of initial discovery to an area where highly contaminated wells exist. Wells further up the hydraulic gradient and in a distinct area around the contaminated area are free of VOC contamination. A monitor well drilling and sampling program was employed to better define the source area. Surface and subsurface geophysical techniques were used to reinforce monitor well data and further delineate a source area.

From September 1983 through February 1984 the u.s. Geological Survey (USGS) collected water samples from selected wells to provide baseline data on volatile organic compounds (VOC) in the Uvalde area. The samples were collected under a cooperative contract with the EUWD. Most samples were collected from public supply wells, however, some samples were collected from private wells. In February 1984 five wells were sampled for VOCs in the Uvalde area (see Appendix B for details of all sampling events).

The only VOC found was tetrachloroethylene (PCE) at a level of 6.4 micrograms per liter (ug/l) in a municipal well located at the Uvalde County airport (Garner Field). This VOC contaminated groundwater sample prompted further investigation.

To verify contamination in the vicinity of the municipal airport two sampling events were conducted in April and August, 1984. A total of eight wells were sampled including the airport well. Of the eight wells four showed PCE contamination.

Four wells drawing water from the Edwards aquifer were polluted with the same compound. This was sufficient evidence to rule out contamination by well construction materials or laboratory error. At this point it was reasonable to assume the volatile organic compound tetrachloroethylene (PCE) existed in the Edwards aquifer in the vicinity of the municipal airport.