In 1991, the U.S. Geological Survey (USGS) implemented the National Water-Quality Assessment (NAWQA) Program to describe the status and trends in water quality of a large, representative part of the Nation's surface- and ground-water resources. This program, when fully implemented, will be accomplished through investigation of 60 study units ranging in size from 1,200 to 60,000 mi2. Twenty study-unit investigations began in fiscal year 1991, 16 additional study units began in fiscal year 1994, 17 are scheduled to begin in fiscal year 1997, and the remaining study units are not scheduled yet. The South-Central Texas (SCTX) study unit is in the second group (starting in 1994) of study units in the NAWQA Program (fig. 1). The first phase of this study includes evaluation of existing water-quality data.
Analysis of available volatile organic compound (VOC) data for surface water and ground water was done as part of the evaluation of existing water-quality data for the SCTX study unit. Contamination of water by VOCs can pose a threat to the health of humans when concentrations greater than 1 part per million are ingested or inhaled (Bloemen and Burn, 1993). Possible effects include eye, nose, and throat irritation and central nervous system responses such as dizziness, headaches, and loss of short-term memory. Some VOCs are considered to be human carcinogens (benzene, vinyl chloride), and others are animal carcinogens (chloroform, methylene chloride, tetrachloroethene, and trichloroethene) that also might be human carcinogens (Bloemen and Burn, 1993). Therefore, the presence and extent of VOCs in surface and ground water is a public health issue.
The purposes of this report are to (1) describe the occurrence and distribution of VOCs in the San Antonio region of the SCTX study unit and (2) discuss implications of this information for planned monitoring of VOCs by SCTX NAWQA in the San Antonio region of the SCTX study unit based on occurrence and distribution of VOCs within the study unit. VOC data from the USGS WATer STOrage and REtrieval (WATSTORE) data base from 1983 to 1994 for both surface and ground water are reviewed.
The San Antonio region of the SCTX study unit (hereafter called the study area) is a 10,500-mi2 area that comprises the Edwards aquifer in the San Antonio region and its catchment area (fig. 2). The study area includes parts of two other major aquifers, the Edwards-Trinity and the Trinity. The entire study unit extends beyond the San Antonio region to the Gulf Coast of Texas to include the watersheds of three major rivers (Nueces, San Antonio, and Guadalupe Rivers).
The city of San Antonio and the surrounding area contain several large military installations, manufacturing industries, and a tourism industry. Away from the San Antonio area, the study unit consists mainly of rangeland with some agriculture and small urban areas.
The Edwards aquifer is the source of water for about 1.3 million people in and near San Antonio and for ranchers and farmers in the region. Water from the aquifer provides habitat for threatened and endangered species associated with major springs in the region. The Edwards aquifer is a sequence of extensively faulted, fractured, and dissolutioned limestone and dolostone that yields large quantities of water to wells and springs. The aquifer crops out and is unconfined in the recharge zone. The aquifer is confined (artesian zone) beneath much less permeable rocks downdip from the recharge zone. Further downdip, where the rocks are virtually impermeable, they contain moderately saline to very saline water (saline-water zone).
The study area comprises parts of three geographic subareas: the Edwards Plateau, the Hill Country, and the Gulf Coastal Plain (fig. 3). The Edwards Plateau is characterized by rolling hills capped with a thick mantle of limestone rocks and thin soils. The Hill Country consists of rugged terrain where upland areas have been extensively eroded, leaving deeply incised alluvial valleys with limestone caps. The Gulf Coastal Plain is characterized by rolling prairies with thick, fertile soils suitable for farming.
The data used for the analysis of VOCs in surface water were collected by the USGS. The River Authorities in the study unit (Nueces, San Antonio, and Guadalupe-Blanco) have not collected VOC data. The majority of VOC analyses in the SCTX study area are from water samples collected as part of the National Pollutant Discharge Elimination System (NPDES) program of the U.S. Environmental Protection Agency (USEPA). The NPDES work was done in cooperation with the San Antonio Water System from August 1992 through September 1994. Six NPDES urban-stormwater sampling sites were selected on the basis of three criteria: drainage-area characteristics; hydraulic factors; and accessibility and safety factors. All six NPDES sites are within the San Antonio city limits (fig. 4). Descriptions of the sites, including drainage area, land-use category, and sampling dates, are listed in table 1. Drainage areas of the NPDES sites range from 11 to 178 acres. Most of the drainage areas were characterized by a single land-use category, predominantly residential, commercial, or light industrial. Each site was fitted with a flow-control device, either a Palmer-Bowlus flume or sharp-crested rectangular weir for computation of streamflow. A straight, uniform channel the length of at least six outfall conduit diameters upstream from the flow-control device was required to insure complete mixing of stormwater. Good accessibility required that the sites be located at outfalls or at manholes 18 in. or greater in size.
Samples were collected according to the USEPA stormwater-sampling criteria: (1) the dry period preceding the storm is at least 72 hours; (2) the depth of precipitation over the basin is at least 0.10 in.; and (3) if possible, precipitation does not vary by more than 50 percent from the average precipitation amount and duration. Quality-control/quality-assurance procedures included analyses of equipment blanks and spike samples throughout the data-collection phase of the study. VOC samples were collected within the first 30 minutes of runoff, when possible, using discrete (grab) sampling techniques. The grab samples were analyzed at the USGS National Water Quality Laboratory (NWQL) in Arvada, Colo. The VOC compounds and minimum reporting levels (MRL) are listed in table 2.
VOC data collected between December 1991 and December 1994 are available from 15 additional USGS surface-water sites (fig. 4). Many of these sites were previously sampled in January 1985. These USGS sites, on larger stream channels than the NPDES sites, are not characterized by land use as are the NPDES sites, and therefore, detailed analysis of VOC detections for these sites comparable to that for the NPDES sites is not presented.
VOC data from ground-water samples were obtained from the USGS data base WATSTORE. All ground-water analyses are from samples of the Edwards aquifer. Available data comprise 307 sample analyses from 157 wells completed in the Edwards aquifer in the SCTX study area from 1983 to 1993. Most of the samples were collected during investigations done in cooperation with the Edwards Underground Water District (EUWD). Locations of wells sampled are shown in figure 5. These samples were analyzed for 26 to 35 VOCs. Table 2 lists possible VOCs sampled for in ground water. All samples were collected from untreated well water. Sample documentation in the data base includes location by latitude and longitude, date sampled, and compounds analyzed. Information regarding land use, open interval, or population served for wells sampled is not available. Some of the Edwards aquifer wells can be greater than 1,000 ft deep, and most are usually unscreened, open-hole wells.
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