FACTORS AFFECTING WATER QUALITY IN SELECTED CARBONATE AQUIFERS IN THE UNITED STATES, 1993-2005

Based on data collected by the U.S. Geological Survey National Water-Quality Assessment Program, confinement, land use, and sinkhole density are major factors affecting water quality in ten carbonate aquifers in the United States. The carbonate aquifers are the Basin and Range, Biscayne, Edwards-Trinity, Floridan, Mississippian, Castle-Hayne, Ozarks, Piedmont, Silurian-Devonian, and Valley and Ridge. Data from 881 wells and 178 springs collected from 1993 to 2005 for field characteristics, major ions, nutrients, 82 pesticides, and 54 volatile organic compounds (VOCs) provide the basis for the study. Wells ranged in depth from 10 to 2,700 feet, with a median depth of 150 feet. Variations in water quality among these ten aquifers were related to physical characteristics such as confinement and sinkhole density. The median nitrate concentration was 1.4 mg/L as N in unconfined aquifers and < 0.06 mg/L as N in confined aquifers. Nitrate concentrations were also higher in wells in areas with a high sinkhole density than in areas with a low sinkhole density. Atrazine detection frequency was 46 percent for wells in unconfined aquifers and 11 percent for confined aquifers. At least one VOC from a schedule of the 54 compounds analyzed was detected at > 0.2 micrograms per liter in 25 percent of the wells in unconfined aquifers and 20 percent of the wells in confined aquifers. Land use within 500 meters of the well was also an important explanatory variable. Nitrate concentrations were correlated to percentage of agricultural land use and manure application rate near the well. Detection frequency of VOCs was related to urban land use. Atrazine concentrations were correlated to the percentage of agricultural land use near the well. Correlations between land use and water quality were stronger in unconfined aquifers than in confined aquifers. The combined effects of land use, sinkholes and confinement provided the best explanation of the patterns of contaminants. Wells in urban areas in unconfined aquifers had higher numbers of VOCs detected than in rural areas or in confined aquifers. Wells in areas with a high density of sinkholes in agricultural areas had higher concentrations of nitrate and more frequent detections of agricultural pesticides than wells in areas with either lower density of karst features or less agricultural land use.