SHALLOW GROUND-WATER QUALITY IN AGRICULTURAL AREAS OF SOUTH-CENTRAL GEORGIA, 1994

By Christy A. Crandall

ABSTRACT

The Georgia-Florida Coastal Plain National Water-Quality Assessment program began an agricultural land-use study in March 1994. The study area is located in the upper Suwannee River basin in Tift, Turner, Worth, Irwin, Wilcox, and Crisp Counties, Ga. Twenty-three shallow monitoring wells were installed in a 1,335-square-mile area characterized by intensive row-crop agriculture (peanuts, corn, cotton, and soybeans). The study focused on recently recharged shallow ground water in surficial aquifers to assess the relation between land-use activities and ground-water quality. All wells were sampled in March and April (spring) 1994, and 14 of these wells were resampled in August (summer) 1994. Constituents analyzed included: pH, specific conductance, dissolved oxygen, temperature, major ions, nitrogen, phosphorus, dissolved organic carbon, 47 pesticides, 60 volatile organic compounds, uranium, radon, and nitrogen isotopes.

Shallow ground water in the study area is characterized by oxic and acidic conditions, low bicarbonate, and low dissolved-solids concentrations. The median pH of shallow ground water was 4.7 and the median bicarbonate concentration was 1.7 mg/L (milligrams per liter). Dissolved oxygen concentrations ranged from 3.0 to 8.0 mg/L (14 samples). The median dissolved solids concentration in samples collected in the spring was 86 mg/L. Major inorganic ion composition was generally mixed with no dominant cation (13 of 23 samples collected in the spring). Nitrate was the dominant anion (greater than 60 percent of the anion composition) in 14 of 23 samples collected in the spring. Comparison of these data to data from background samples collected from 19 wells in areas of similar soils and hydrogeology showed little difference in major ion composition. Only concentrations of bicarbonate, dissolved organic carbon, and nitrate had significant differences in concentrations between samples collected in the spring and the background samples. However, median concentrations of some of the major ingredients in fertilizer (including magnesium, chloride, nitrate, iron, and manganese) were higher in water samples from agricultural wells than in background samples. The median concentration of dissolved solids in ground-water samples collected in the spring (86 mg/L) was more than double the median concentration (41 mg/L) of the background samples.

The median nitrate as nitrogen concentration of 6.7 mg/L in the spring samples reflects the effects of agricultural activities on ground-water quality. Samples from 30 percent of the wells exceeded the Maximum Contaminant Level (MCL) for nitrate in drinking water (10 mg/L as N). Nitrogen isotope ratios, analyzed in 14 samples collected in the summer, ranged from 2.4 to 9.0 parts per thousand and indicate that most nitrogen in shallow ground water is probably from inorganic fertilizer. In addition, nitrate concentrations were positively correlated (p-values all less than 0.01) with concentrations of some of the major ingredients in fertilizer, such as potassium, calcium, magnesium, manganese, and chloride, and with values of specific conductance.

Concentrations of pesticides and volatile organic compounds, detected in samples from 11 wells, were all below the MCL's. Of these constituents, only alachlor, metolachlor, metribuzin, toluene, benzene, and methyl chloride were detected in ground water at concentrations that ranged from 0.01 to 1.0 g/L (micrograms per liter). Maximum concentrations of 1.0 g/L of metolachlor and toluene were detected in two wells. Radon concentrations ranged from 530 to 1,400 pCi/L (picocuries per liter), exceeding the proposed MCL of 300 pCi/L in all samples; the median concentration was 1,000 pCi/L.


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