ABSTRACT: The surficial aquifer system beneath citrus groves in Indian River, Martin, and St. Lucie Counties, Florida, was studied to determine the effects of citrus agriculture on ground-water quality. The surficial aquifer is the primary drinking-water source for Martin and St. Lucie Counties and furnishes about 33 percent of the drinking-water for Indian River County. Water-quality samples and water-level data were collected from December 1996 through October 1998.
Nitrate concentrations in ground water exceeded 10 milligrams per liter (mg/L), the U.S. Environmental Protection Agency’s maximum contaminant level for nitrate reported as nitrogen, in 5 percent of the samples from citrus groves. These exceedances occurred in samples from wells with depths of 10 feet or less at citrus groves, and mostly in samples collected during or immediately following fertilizer applications. Samples from wells with depths of 20-25 feet contained little or no nitrate. The decreased nitrate concentrations in ground water with depth was not consistent with chloride and dissolved-solids concentrations, two other common indicators of agricultural activity.
Chloride and dissolved-solids concentrations remained elevated in ground-water samples from all depths at citrus groves; median chloride and dissolved-solids concentrations in samples from citrus sites were 125 and 779 mg/L, respectively. In comparison, samples from the reference site had maximum chloride and dissolved-solids concentrations of 61 and 366 mg/L, respectively. Based on the age of ground water at 20-25 foot depths (3-50 years, measured with tritium and helium-3 concentration ratios), nitrate concentrations also should have remained elevated with depth because fertilizers have been used for at least 20-30 years at these citrus groves. Nitrate concentrations decreased with depth as a result of denitrification. This could have occurred because favorable conditions for denitrification existed in the aquifer, including high concentrations of dissolved organic carbon and iron (median concentrations of 25.5 and 1.75 mg/L, respectively at citrus sites) and low concentrations of dissolved oxygen (median concentration of 0.9 mg/L at citrus sites), which indicates that reducing conditions were present.
Evidence that denitrification occurred included the enrichment of ground water with depth in the heavier isotope of nitrogen, nitrogen-15 (15N). Ground water from wells screened 10-15 feet below land surface had a median d 15N value of 24.6 per mil, whereas ground water from wells screened 5-10 feet below land surface had a median d 15N value of 9.4 per mil. Fertilizer samples had a median d 15N value of 3.0 per mil. Increased d 15N values coincident with decreased nitrate concentrations with depth indicates that fractionation occurred during denitrification reactions. Finally, excess nitrogen gas, a byproduct of denitrification reactions, was detected at concentrations ranging from 0-8 mg/L in samples from wells screened 10-25 feet below land surface.