Quality of Water from Shallow Wells in Urban Residential and Light Commercial Areas in Lafayette Parish, Louisiana, 2001 through 2002

In 2001-02, the U.S. Geological Survey installed and sampled 28 shallow wells in urban residential and light commercial areas in Lafayette Parish, Louisiana, for a land-use study in the Acadian-Pontchartrain Study Unit of the National Water-Quality Assessment (NAWQA) Program. The wells were installed in the Chicot aquifer system, the primary source of water for irrigation and public-water supplies in southwestern Louisiana. The purpose of this report is to describe the quality of water from the 28 shallow wells and to relate that water quality to natural factors and to human activities. Ground-water samples were analyzed for general ground-water properties and about 240 water-quality constituents, including dissolved solids, major inorganic ions, trace elements, nutrients, dissolved organic carbon (DOC), radon, chlorofluorocarbons, selected stable isotopes, pesticides, pesticide degradation products, and volatile organic compounds (VOC’s). Dissolved-solids concentrations for two wells exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Level of 500 mg/L (milligrams per liter). Concentrations for major inorganic ions, trace elements, pesticides, degradation products, and VOC’s were less than the Maximum Contaminant Levels for drinking water. Manganese concentrations for 18 wells exceeded the Secondary Maximum Contaminant Level of 50 micrograms per liter. Arsenic concentrations increased with depth and with increased pH, bicarbonate, calcium, and magnesium concentrations. Six pesticides and three degradation products were detected in the ground-water samples. Ten VOC’s also were detected in the ground-water samples. One nutrient concentration (that for nitrite plus nitrate) was greater than 2 mg/L, a level that might indicate contamination from human activities, and was greater than the Maximum Contaminant Level of 10 mg/L. The median DOC concentration was an estimated 0.3 mg/L, which indicated naturallyoccurring DOC conditions in the shallow ground water in Lafayette Parish. Quality-control samples indicated no bias in ground-water data from collection or analysis. Radon concentrations for 19 of 20 wells sampled were greater than the U.S. Environmental Protection Agency Maximum Contaminant Level of 300 picocuries per liter (piC/L). Radon concentrations ranged from 280 to 2,220 piC/L and had a median of 389 piC/L. Radon concentrations were correlated moderately and inversely to the depth to the top of the screened interval. Chlorofluorocarbons indicated the apparent age of the ground water varied with water level and ranged from about 12 to 50 years. The Mann-Whitney rank-sum test was used to compare water-quality data in the Chicot aquifer system between four groups of wells from three NAWQA studies. The means for most constituents were less for the urban wells than for wells in the rice-growing areas. The larger dissolved-solids concentrations, particularly sodium and chloride, for samples from wells in the rice-growing areas might be a result of heavy irrigation pumpage in southwestern Louisiana that causes movement of the constituents from deeper ground-water sources. The means for most constituents were greater for the urban wells than for wells in the outcrop area of the Chicot aquifer system and less for the urban wells than for wells south of the outcrop area. Because concentrations of dissolved solids and other chemical constituents generally increase along ground-water flow paths, concentrations of many of the selected chemical constituents were expected to be larger for samples from the urban wells than from wells in the outcrop area. The larger concentrations for samples from the wells south of the outcrop area compared to those for the urban wells might be explained similarly. The wells south of the outcrop area are deeper than the urban wells, thus increasing the time the water has to react with aquifer sediments. The lack of correlation between the four groups of wells suggests that spatial distribution of wells and the depth to the top of the screened interval affected the quality of water in shallow wells in southwestern Louisiana.