L. J. Judd | Geology Reports

USGS1997

Water-quality assessment of south-central Texas: Occurrence and distribution of volatile organic compounds in surface water and ground water, 1983-94, and implications for future monitoring

The study area of the South-Central Texas study unit of the National Water-Quality Assessment Program comprises the Edwards aquifer in the San Antonio region and its catchment area. The first phase of the assessment includes evaluation of existing water-quality data for surface water and ground water, including volatile organic compounds, to determine the scope of planned monitoring. Most analyses of volatile organic compounds in surface water are from the National Pollutant Discharge Elimination System sites in San Antonio, Texas. Nine volatile organic compounds were detected at the six sites. The three compounds with the most detections at National Pollutant Discharge Elimination System sites are 1,2,4-trimethylbenzene, toluene, and xylene. Analysis of volatile organic compounds in ground water was limited to Edwards aquifer wells. Twenty-eight volatile organic compounds were detected in samples from 89 wells. The five most commonly detected compounds in samples from wells, in descending order, are tetrachloroethene, trichloroethene, bromoform, chloroform, and dibromochloromethane. Detections of volatile organic compounds in surface water and ground water within the South-Central Texas study area are limited to site-specific sources associated with development; therefore, planned monitoring for possible detections of volatile organic compounds as part of the National Water-Quality Assessment Program will emphasize areas of expanding population and development. Monitoring of volatile organic compounds is planned at National Pollutant Discharge Elimination System sites, at basic fixed surface-water sites, and in the ground-water study-unit surveys.

Texas↗

USGS1995

Streamflow to the Gulf of Mexico

Fifty-four major streams discharging directly to the Gulf of Mexico and having drainage areas exceeding 200 square miles were identified in the United States. Forty-four U.S. Geological Survey streamflow-gaging stations along the Gulf of Mexico with at least 40 years of daily streamflow data also were identified. These stations include most of the major streams and comprise 95 percent of the drainage area to the Gulf from the United States. Daily mean values of continuously monitored streamflow were aggregated, annually and monthly, for selected stations. The mean, minimum, and maximum values were determined for the aggregated data for each station. Statistical and graphical representations of temporal trends in streamflow are given for stations included in this report. Substantial percentage changes in annual and monthly streamflow between early (before 1963) and late (1963-90) time periods describe long-term temporal trends in streamflow for most of the 44 long-term stations. Graphical representations of long- and short-term temporal trends are presented for total gaged annual mean streamflow for each of 12 segments dividing the Gulf Coast. Temporal trends in streamflow were related to major factors that affect streamflow: precipitation, land use, withdrawals of surface water, reservoir operations, and other factors. Low- and highstreamflow periods are related to extremes in precipitation; substantial increases in streamflow are associated with urbanization; decreases in streamflow are coincident with increases in withdrawals of surface water; and increases in minimum streamflows and decreases in maximum streamflows are associated with increases in the number of reservoirs. Other factors (springflow, soil composition, and effluent discharges) affect monthly minimum streamflow for some stations. Seasonal or monthly distributions of streamflow were determined for each of the 44 long-term stations; months with high or low streamflow are presented for each station. Precipitation is the primary factor that affects the distributions of streamflow. Other factors that affect streamflow distributions include land use in urban areas, withdrawals of surface water for irrigation, and reservoir operation.

Water-Resources Investigations Report↗