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A groundand surface-water quality study was conducted in 1981 and 1982 to identify brine-contaminated streams and shallow aquifers in five major oiland gas-producing areas in southern Mississippi. The study encompasses a total area of about 5,200 square miles in 17 counties. Water samples were collected from wells at depths of less than 300 feet and from streams at low flow during two field reconnaissances of the areas. The background concentrations of selected chemical constituents of uncontaminated native water and the location of apparent brine contamination were determined from analysis of the water samples plus data from published and unpublished reports. A chloride concentration greater than 50 milligrams per liter is the primary criterion used to determine brine contamination. Bromide and strontium concentrations, specific conductance, pH, and calculated sodium to chloride ratios are supplementary data used to substantiate petroleum brines as the contamination source of freshwaters. Brine contamination was found in shallow aquifers (less than 300 feet) and in streams in parts of all five study areas. Contamination was prevalent in the Brookhaven, Baxterville, Pistol Ridge, Little Creek, Tinsley oil fields. Numerous areas of contamination were also found in Adams County. Data also indicate that deeper aquifers were contaminated in the Brookhaven and Tinsley oil fields. As a result of the study, five sites for additional detailed geohydrologic studies were identified to determine the extent and movement of contaminants in order to plan for present and future protection of water samples. INTRODUCTION Oil and gas occur at depths greater than 2,000 ft in most of the geologic units in Mississippi. Since the first successful oil well was drilled in Yazoo County in 1939, thousands of wells have been drilled and the producing areas have expanded into many parts of the State. Petroleum production is accompanied by the production of brine. Usually, newly completed wells have a lower saltwater to oil ratio than wells which have been pumping a number of years. This ratio may vary from almost no saltwater to more than 20 barrels of saltwater per barrel of oil (Mississippi State Oil and Gas Board, 1981). The most common suggested method of saltwater disposal in the past was by ''evaporation pits*', which in most cases, actually discharged water to underlying unconfined aquifers or nearby streams. Although this practice is now prohibited, thousands of pits were constructed and their impact on the hydrologic system is largely unknown. Brine is now reinjected into an aquifer with water having a dissolved solids concentration of 10,000 mg/L or greater. However, saltwater contamination of freshwater aquifers is still possible from improperly constructed or leaking injection wells. In order to determine the extent of brine contamination in oilproducing areas of Mississippi, a data-collection program was undertaken by the U.S. Geological Survey in cooperation with the Mississippi! Department of Natural Resources, Bureau of Pollution Control. Historical data collected by State agencies and the U.S. Geological Survey and data collected during the study were used to define and map areas of brine contamination. Finally, suggestions were made to further study the geohydrology in several areas where brine contamination poses a threat to the water supply. The writer acknowledges the cooperation of well owners who allowed sampling of their wells and who provided information on potential sources of contamination. A special thanks is extended to the people offering suggestions in the writing of this report and to those who reviewed it. Objectives and Scope The main objectives of the study are: (l) determine background chemical quality of shallow aquifers and streams in southern Mississippi; (2) determine and delineate specific areas of oil-field brine contamination; and (3) outline needs for more detailed site studies. Background or "natural" water quality that represents water in uncontaminated shallow aquifers and streams in southern Mississippi was determined by using a combination of previously published data from the U.S. Geological Survey WATSTORE file and data obtained during the study. The data were compiled and analyzed to determine background levels of selected major ion concentrations. Samples used to determine the levels of dissolved ion concentrations during the study were collected from several wells and surface-water low-flow sites in areas of no known oil production or brine contamination. Areas of possible contamination were determined by reviewing reports published by the Mississippi Department of Natural Resources, Bureau of Geologyj U.S. Geological Survey Open-File Reportsj and unpublished reports documenting the presence of oil-field brine contamination. Water samples were collected in five study areas encompassing a total of approximately 5,200 mi2 (table 1). Each study area was divided into two or more subareas of related data. Data were collected in two phases the reconnaissance phase and documentation phase. In 1980 and 1981 during the reconnaissance phase, 190 stream sites were visited during periods of low streamflow. Samples were collected for chloride analysis if the specific conductance of the water was greater than 100 yS/cm (microsiemens per centimeter). Specific conductance and chloride concentrations were determined at all 224 well sites visited during the reconnaissance phase (Kalkhoff, 1982). Table 1. Location, size, and major oil and gas fields in the study areas Oil and Gas Area Study Area 1 2 County Yazoo, Madison Adams, Jefferson Fields Included Pickens, Tinsley Numerous (miZ) 250 1,800 4 Franklin, Amite Lincoln, Pike, Amite Marion, Lamar Forrest Pearl River Clarke, Wayne Jones, Jasper McComb, Smithdale 470 Brookhaven, Little Creek, Mallalieu Baxterville 700 Pistol Ridge Numerous 2,000

Brine contamination of freshwater aquifers and streams in petroleum producing areas in Mississippi