FINDINGS FROM THE NATIONAL-WATER QUALITY ASSESSMENT PROGRAM: GEORGIA-FLORIDA COASTAL PLAIN, 1992-96
By Marian P. Berndt
The water quality in the Georgia-Florida Coastal Plain was evaluated from 1992-96 as part of the National Water-Quality Assessment (NAWQA) program of the U.S. Geological Survey. NAWQA began in 1991 to assess the status and trends in the Nation's water quality and to determine the effects of human actions and natural factors on water quality. The Georgia-Florida Coastal Plain study area, one of the first of the areas studied, encompasses nearly 62,000 square miles and contains several major rivers, including the Suwannee River in Georgia and Florida and the Altamaha River in Georgia. About 80 percent of the 9 million people residing in the study area obtain drinking water from groundwater.
Evaluation of the quality survey of water was done through a background water-quality survey and through studies of land-use impacts. Land-use study areas for groundwater included a row-crop agricultural area in the upper Suwannee River basin and two urban areas (Ocala and Tampa, Florida). Groundwater/surface-water interaction was studied in a karst area in the lower Suwannee River basin. Streams were sampled regularly at nine fixed sites distributed among major land uses and were sampled for three years, in time periods ranging from weekly to quarterly. Stream synoptic studies, short-term investigations focusing on specific hydrologic conditions and land uses, supplemented the fixed site information. Constituents analyzed in ground- and surface-water samples included nutrients, pesticides, and major ions; streambed samples were analyzed for pesticides, trace elements, and major ions. Stream ecological assessments, which integrated physical, chemical, and biological factors, were done at eight of the stream sampling sites.
Nitrate concentrations (as nitrogen) exceeded the U.S. Environmental Protection Agency (USEPA) drinking-water standard of 10 milligrams per liter (mg/L) in more than 20 percent of groundwater samples from surficial aquifers in agricultural areas. In the 23 groundwater samples from the row-crop agricultural area in the upper Suwannee River Basin, 33 percent exceeded the drinking-water standard. These samples were from aquifers that overlie the Upper Floridan aquifer, the major source of drinking water for the study area. Nitrate concentrations in streams did not exceed drinking-water standards or guidelines, but were higher in streams draining basins with agricultural and mixed land uses. Phosphorus concentrations in nearly 30 percent of stream samples were greater than 0.1 mg/L, the USEPA guideline for the prevention of nuisance algal growth.
Nitrate concentrations (as nitrogen) in the lower Suwannee River are affected by a cycle of water exchange between the river and the adjoining aquifer. During low flow in the river, groundwater containing nitrate enters the river, increasing river nitrate concentrations. During high flow river water enters the aquifer, resulting in a decrease in nitrate concentrations in the aquifer adjacent to the river. The dynamics and extent of the groundwater/surface-water interaction along the river is dependent on groundwater levels in the aquifer and surface-water levels in the river.
Of 85 pesticides and degradation products analyzed, 21 were detected in groundwater and 32 were detected in streams. Pesticide concentrations did not exceed any USEPA drinking-water standards, but criteria for protection of aquatic life were exceeded in some streams. Diazinon concentrations exceeded the aquatic-life criteria of 0.08 micrograms per liter in 20 percent of samples from an urban stream. The most frequently detected pesticides in groundwater and streams were three herbicides—atrazine, metolachlor, and prometon. No insecticides were detected in groundwater. The kinds and frequency of pesticides detected differed in agricultural and urban areas.
Concentrations of organochlorine pesticides in 22 percent of streambed-sediment samples exceeded aquatic-life criteria. Most exceedances were for chlordane and DDT and their degradation products, which were also the organochlorine pesticides most frequently detected in streambed sediments. Polychlorinated biphenyls (PCBs) were detected at only one site (in an urban basin) at a concentration below the aquatic-life criterion.
Findings from this 1992-96 sampling, plus data collected during the less intensive sampling period (1996-2000) and data collected by other agencies in the Georgia-Florida Coastal Plain, will be used to determine the sampling frequencies and locations when intensive studies resume in the year 2002.