1. OIL PRODUCTION, AGRICULTURE, AND GROUNDWATER QUALITY IN THE SOUTHEASTERN GULF COAST AQUIFER, TEXAS Paul F. Hudak University a/North Texas

2. ABSTRACT Associations between groundwater quality and land use were evaluated in the southeastern Gulf Coast Aquifer, Texas. Data from 19,234 oil/gas wells and 256 water wells were mapped with a geographic information system (GIS) and statistically analyzed. Water wells near oil/gas wells had significantly higher levels of chloride, bromide, and total dissolved solids (TDS). Bromide-chloride ratios were also higher at water wells near oil/gas wells. Shallower water wells had significantly higher chloride, bromide, TDS, and nitrate concentrations. Nitrate concentrations were higher beneath cropland compared to other land uses. Results of this study suggest that oil/gas production and agriculture have impacted water quality in the Gulf Coast Aquifer.

3. INTRODUCTION Oil/gas production and agriculture are common land uses and potential sources of groundwater contamination in southeastern Texas. The objective of this study was to map and evaluate distributions of groundwater solutes potentially related to oil/gas production and agriculture, and compare them with digital land use classifications.

4. STUDY AREA Interbedded clay, silt, sand, and gravel deposited in the Tertiary and Quaternary Periods make up the Gulf Coast Aquifer (TWC, 1989). Maximum total sand thickness ranges from 400 m in the northeast to 210m in the southwest (TWDB, 1995). Regionally, total dissolved solids (TDS) concentrations increase southwestward. The hydraulically connected aquifer sediments form a leaky artesian system. Groundwater in the aquifer flows generally southeastward. Heavy pumping along the coast has induced saltwater intrusion at some locations. However, the problem has stabilized and in some cases improved over the past 15 years due to declines in groundwater pumping.

6. Thousands of oil/gas wells have been drilled throughout the study area. In 1969, the Texas Railroad Commission prohibited using unauthorized pits to dispose of oilfield brine (RCT, 1993). However, pits are still being used for drilling fluid and emergency saltwater storage. Potential routes through which brine can enter the Gulf Coast Aquifer include leaking casings in production wells, leaking injection wells, upward migration through abandoned wells, and surface infiltration from past dumping (TDA, 1985). METHODS A geographic information system (GIS), ARC/INFO (ESRI, 1998), was used to map and analyze oil/gas-well, water-well, and land-use data. Locations of 19,234 oil/gas wells were mapped from a database provided by the Texas General Land Office (Natural Resource Inventory). Digital data from the National GAP (Geographical Approach to Planning) Analysis Program were used to map land-use classifications (Scott and Jennings, 1997). Water chemistry data were obtained from the Groundwater Database of the Texas Water Development Board (TWDB). Data used in this study were collected in 1997. Concentrations of six solutes (chloride, bromide, sulfate, barium, iodine, nitrate),

8. Methods

9. TDS, and bromide-chloride ratios were compiled for 256 water wells, ranging from 8.5 to 615.9 m deep (median 179.3 m). The GIS identified land-use classifications and concentration categories for water wells, constructed 750-m buffers around water wells, and overlaid the buffers on a point map of oil/gas wells. From this overlay, the number of oil/gas wells within 750 m of each water well was calculated. (The 750-m buffer is comparable to the half- mile radius used by the Texas Natural Resource Conservation Commission when delineating potential impact zones around sites with contaminated groundwater.) MINITAB (Ryan et al., 1992) computed summary statistics for each solute, generated a rank correlation matrix of solute concentrations, well depth, and the number of oil/gas wells within the 750-m buffer, and ran Kruskal-Wallis tests to compare solute concentrations between water supply wells (1) near and away from oil/gas wells and (2) in different land use classifications. (Solute concentrations were not normally distributed.)