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In 2000-2001, water-quality data were collected from 60 randomly selected domestic wells in the Acadian-Pontchartrain Study Unit, as part of the National Water-Quality Assessment Program. The data were collected from wells screened in shallow sands (less than 350 feet below land surface) in two major aquifer systems--the Chicot aquifer system in southwestern Louisiana and the Chicot equivalent aquifer system in southeastern Louisiana and southwestern Mississippi. The Chicot equivalent aquifer system is part of the Southern Hills regional aquifer system, and both the Chicot aquifer system and the Southern Hills regional aquifer systems are designated as sole-source aquifers by the U.S. ·Environmental Protection Agency (USEPA). The well depths ranged from 40 to 340 feet below land surface with a median depth of 120 feet. The ground-water-quality data included 5 physicochemical properties, dissolved solids, 9 major inorganic ions, 24 trace elements, 6 nutrients, dissolved organic carbon, 109 pesticides and degradation products, and 85 volatile organic compounds (VOC's); and a subset of the wells were sampled for radon, chlorofluorocarbons, and stable isotopes. Water from 35 of the 60 domestic wells sampled had pH values less than the USEPA Secondary Maximum Contaminant Level (SMCL) range of 6.5 to 8.5 standard units. Specific conductance ranged from 17 to 1,420 microsiemens per centimeter at 25 degrees Celsius. Dissolved-solids concentrations in water from two wells exceeded the SMCL of 500 mg/L (milligrams per liter); the maximum concentration was 858 mg/L. Sodium and calcium were the dominant cations, and bicarbonate and chloride were the dominant anions. One chloride concentration (264 mg!L) exceeded the SMCL of 250 mg/L. One arsenic concentration (55 .3 J..Lg/L [micrograms per liter]) exceeded the USEPA Maximum Contaminant Level (MCL) of 10 J..Lg/L. Iron concentrations in water from 22 wells exceeded the SMCL of 300 J..Lg/L; the maximum concentration was 8,670 J..Lg/L. Manganese concentrations in water from 26 wells exceeded the SMCL of 50 J..Lg/L; the maximum concentration was 481 J..Lg/L. Health Advisories have been established for six of the trace elements analyzed; no concentrations were greater than these nonenforceable standards. Radon concentrations in water from 9 of 50 wells sampled were greater than the proposed US EPA MCL of 300 picocuries per liter. Concentrations of ammonia, ammonia plus organic nitrogen, and nitrite plus nitrate in water from four wells were greater than 2 mg/L, a level that might indicate anthropogenic influences. The median dissolved organic carbon concentration was an estimated 0.30 mg/L, which indicated naturally occurring dissolved organic carbon conditions in the study area. Eight pesticides and two degradation products were detected in water from five wells. Twenty-four VOC's were detected in water from 44 wells. All concentrations of pesticides and VOC's were less than USEPA drinking-water standards. Quality-control samples, which included fieldblank samples, replicates, and field and laboratory spikes, indicated no bias in ground-water data from collection procedures or analyses. Variance between the environmental samples and the corresponding replicate samples was typically less than 5 percent, indicating an acceptable degree of laboratory precision and data collection reproducibility. The Mann-Whitney rank-sum test was used to compare depth to top of screen and selected physicochemical properties and chemical constituents between six groups of wells. Values for selected physicochemical and chemical constituents were typically greater in wells located in the Chicot aquifer system than in the Chicot equivalent aquifer system. Values for specific conductance, pH, calcium, sodium, bicarbonate, chloride, dissolved solids, and iron were typically greater in wells located south of the outcrop areas of the Chicot and Chicot equivalent aquifer systems than in wells located in the outcrop areas. The increase in concentrations are considered normal because dissolved solids and other constituents generally increase down gradient along ground-water flow paths. Most constituents were not significantly different between the two outcrop areas or the two areas south of the outcrop areas. The results of statistical analyses indicated that water quality in a well is influenced by the location of the well relative to the outcrop area.

Quality of water in domestic wells in the Chicot and Chicot equivalent aquifer systems, southern Louisiana and southwestern Mississippi, 2000-2001