National Water Quality Assessment Program; preliminary assessment of nitrate distribution in ground water in the Georgia-Florida Coastal Plain study unit, 1972-90

Abstract

The U.S. Geological Survey has implemented the National Water Quality Assessment program to describe the quality of the surface- and ground- water resources in 60 large areas or study units in the Nation. The Georgia-Florida Coastal Plain study unit was one of the first 20 selected for study when the full-scale program was implemented in 1991. The study unit has an area of about 54,000 square miles and is located on the south- eastern coast of the United States. The primary source of water supply in this study unit is ground-water from the Upper Floridan aquifer of the Floridan aquifer system. The Upper Floridan aquifer is unconfined or semiconfined in some parts of the study unit, but in other parts is confined by the overlying surficial aquifer system and other confining units. The surficial aquifer system is also used for water supply in some parts of the study unit. Three land-resource areas have been delineated in the study unit on the basis of generalized soil classifications: the Central Florida Ridge, Coastal Flatwoods, and Southern Coastal Plain. Predominant land use and land cover as classified in the 1970's, are forest, agriculture, wetlands, and urban. Nitrate data for water from the Upper Floridan aquifer and the surficial aquifer system were obtained from the National Water Information System data base of the U.S. Geological Survey for the years 1972- 90. In the Upper Floridan aquifer, the highest median nitrate (as nitrogen) concentrations (0.43 and 0.26 milligrams per liter) were in water samples from wells in agricultural and urban areas where the aquifer was unconfined or semiconfined. The maximum contaminant level for nitrate (as nitrogen) in drinking water of 10 milligrams per liter was exceeded in 25 of the 726 water samples from this aquifer. These 25 samples were from wells in urban areas. In water samples from the surficial aquifer system, the highest median nitrate concentration, 8.7 milligrams per liter, was for water samples from agricultural areas in the Central Florida Ridge area. Nitrate (as nitrogen) concentrations exceeded 10 milligrams per liter in 50 of the 421 water samples from wells completed in the surficial aquifer system. Most of these 50 water samples were from wells in agricultural and urban areas (sewage spraying areas) in the Central Florida Ridge.