GEOCHEMISTRY AND CONTAMINANT OCCURRENCE IN UNCONFINED AND CONFINED PARTS OF THE FLORIDAN AQUIFER SYSTEM

The Floridan aquifer system is a highly productive carbonate aquifer that provides drinking water to approximately 10 million people in Florida, Georgia, and South Carolina. Water samples were collected from 149 domestic wells in these three States from 1998 through 2005 and analyzed for major ions, trace elements, nutrients, volatile organic compounds and pesticides. Geochemical differences were noted between hydrogeologically distinct areas based on aquifer confinement. In most samples from unconfined areas, waters are undersaturated with respect to calcite, and dissolved oxygen concentrations are greater than 1.0 milligrams per liter (mg/L). In areas that are mostly confined, waters are near saturation with respect to calcite, and dissolved oxygen concentrations are mostly less than 0.5 mg/L. In confined parts of the aquifer in South Carolina, the presence of sodium-bicarbonate water types in seven samples indicates cation exchange with clay materials.

No primary drinking-water standards were exceeded although secondary drinking-water standards were exceeded for 13 and 3 percent of samples for iron and manganese, respectively. Nitrate concentrations greater than 1.0 mg/L were found mostly in unconfined parts of the aquifer where dissolved oxygen concentrations were greater than 1.0 mg/L and agricultural land use was common. One or more pesticides were detected in about 22 percent of samples, but pesticide detection rates were high (69 percent) in unconfined areas where nitrate concentrations were mostly greater than 1.0 mg/L. Atrazine, deethylatrazine, and metolachlor were the most frequently detected pesticides. Volatile organic compounds were detected in about 64 percent of all samples and detection rates in confined and unconfined areas varied from 60 to 66 percent. Chloroform, carbon disulfide, and 1,2-dichloropropane were the most frequently detected compounds. Statistical comparisons were made between samples with chloroform detections (28 percent of samples) and samples without chloroform detections. Results showed significant differences in dissolved oxygen and nitrate concentrations and well depth between the two groups, with higher dissolved oxygen and nitrate concentrations in the samples with chloroform detections.