The environmental fate of nitrogen in ground water resulting from leaching of chemical fertilizer and manure applied to farm fields was investigated as part of the U.S. Geological Survey National Water-Quality Assessment program. The study site is located on a dairy farm in Orangeburg County, South Carolina, and consists of a cornfield, hay field, managed pine forests, and a forested riparian zone adjacent to Cow Castle Creek. A series of shallow wells were initially installed to monitor water-table elevations and to obtain preliminary data on nutrient concentrations. Additional wells were installed that contained from three to five sampling ports located at depths ranging from 2 to 30 feet below land surface. Sites for these wells were selected to sample along a groundwater flowpath that begins in the upland area located beneath the cornfield, continues laterally through an area of planted pines, a hayfield, a forested riparian zone, and ends where ground water discharges to Cow Castle Creek. Samples from these multiport wells were collected in 1997-98 and analyzed for field parameters, major ions, organic carbon, nutrients, pesticides, pesticide metabolites, dissolved gasses, and chlorofluorocarbons.

Nitrate-nitrogen was the dominant form of nitrogen in ground water at the study site. Ammonia plus organic nitrogen concentrations were generally low and ranged from below detection to 2.21 milligrams per liter (mg/L). The median ammonia plus organic nitrogen concentration was below the detection limit of 0.1 mg/L. Concentrations of nitrate-nitrogen ranged from below detection to 24.3 mg/L. The median concentrations of nitrate samples collected in the spring and fall were 4.46 and 4.00 mg/L, respectively. Nitrate-nitrogen concentrations exceeded the U.S. Environmental Protection Agency’s drinking water standard of 10 mg/L in 8 of the 25 fall samples and 7 of the 29 spring samples.

Concentrations of nitrate were highest beneath the cornfield where the most fertilizer was applied. Concentrations of nitrate beneath the cornfield decreased with depth from over 20 mg/L at the water table to less than 5 mg/L at 25 feet below the water table. Beneath the hayfield, nitrate concentrations ranged from 12 to 14 mg/L and decreased with depth to about 4 mg/L. The shallowest floodplain sediments were organic-rich, anaerobic, and contained ground water with nitrate concentrations of about 1 mg/L. Beneath this zone, the sediment becomes gradually coarser-grained, has less organic matter, and contains ground water with dissolved oxygen concentrations that increase with depth from below detection to about 5 mg/L. Nitrate concentrations in this lower zone ranged from 2-3 mg/L. Ground water discharging in the center of the channel to Cow Castle Creek was well oxygenated and had a nitrate concentration of about 3 mg/L. Ground water discharging near the streambank was anoxic and contained no detectable nitrate. Cow Castle Creek is a gaining stream where the samples were collected.

Recharge dates for ground water were determined by measuring concentrations of chlorofluorocarbons (CFCs). These concentrations were compared to equilibrium air concentrations from historical atmospheric sampling to obtain estimates of recharge dates. Beneath the cornfield, hayfield, and upland pine areas, the shallowest water was the most recently recharged and recharge age increased with depth. At the water table in the upland areas recharge dates were all about 1990, indicating that water took approximately 7 years to move through the unsaturated zone. Ages increased with depth and were oldest in the most upland position, beneath the cornfield, with ages of 1965-67. Beneath the hayfield, the deepest samples were recharged in 1980. Beneath the floodplain, adsorption of CFCs onto organic material, biodegradation, or gas stripping resulted in overestimates of recharge dates. The recharge age of ground water in the shallowest floodplain sediments was 1976 and dates with increasing depth were 1978, 1981, and 1977. The 1977 date may represent water that is largely unaffected by the organic adsorption. Ground water discharging to Cow Castle Creek was recharged in the mid-1970’s.

Much of the nitrate that enters the groundwater system in the cornfield and hayfield is removed through denitrification before it discharges to Cow Castle Creek. Decreases in nitrate concentration were significantly correlated with decreases in dissolved oxygen concentrations, suggesting that denitrification, which occurs under anaerobic conditions, accounts for the decrease in nitrate concentration. This process occurs with increasing depth beneath the cornfield and hayfield as oxygen is depleted and conditions begin to favor denitrification. Denitrification also occurs horizontally along the flowpath and is especially significant in the shallow riparian zone. Water that passes through the shallow, organic-rich, fine-grained sediments contributes little or no nitrate to Cow Castle Creek as it discharges. Water flowing through the coarser-grained sediments below this zone contains 2-3 mg/L nitrate as it discharges to Cow Castle Creek. Dilution of nitrate enriched water is an unlikely explanation for the observed decreases in nitrate because conservative species such as chloride show little variability in concentration with depth or landscape position.

This information is significant to environmental planning and management in coastal areas because of the emphasis by many federal and state programs on developing or maintaining forested buffers. Buffers have been shown to be extremely useful in controlling inputs of sediment, nutrients, and other anthropogenic compounds to streams from runoff. They can also aid in the removal of nitrate where conditions support denitrification or where root penetration can provide for direct uptake of nitrogen. This study indicates, however, that forested buffers may not provide complete protection against discharge of nitrate enriched ground water to surface water.

--- March 1999

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Celeste A. Journey
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cjourney@usgs.gov
(803) 750-6141