Sources and distribution of nitrate in ground water at a farmed field irrigated with sewage treatment-plant effluent, Tallahassee, Florida

Abstract

The City ofTallahassee began applying effluent from a secondary sewage-treatment plant to a spray field site southeast of the city in 1980. Fertilizers containing inorganic nitrogen were also applied in conjunction with the operation of a commercial farm at this site. Analyses of ground water from both the surficial aquifer and the Upper Floridan aquifer have shown nitrate enrichment in some wells that exceeds the prescribed drinking water maximum contaminant level of 10 milligrams per liter (nitrate as nitrogen). Nitrate concentrations above the maximum contaminant level were not detected in samples from monitoring wells located outside the sprayfield boundary. The distribution of nitrate concentrations in ground water has probably been affected by the ground-water flow systems of the surficial aquifer and the Upper Floridan aquifer at the site. The flow system is controlled by the combined recharge from precipitation and effluent application in the surficial aquifer and by the high volume of ground water flowing roughly north to south in the Upper Floridan aquifer. Samples of water from the unsaturated zone, collected in lysimeters, showed that conversion of organic nitrogen and ammonia to nitrate was complete before the nitrogen-enriched water reached the water table. Water samples from wells completed in the surficial aquifer or at depths less than 100 feet in the Upper Floridan aquifer and located inside sprayed areas had mean nitrate concentrations of 7.9 and 4.4 milligrams per liter, respectively. This was higher than concentrations in samples from wells of similar depth located outside sprayed areas at the southeast sprayfield (0.5 and 0.6 milligrams per liter, respectively). Wells located inside the sprayed areas were the only wells in which the maximum contaminant level for nitrate in drinking water was exceeded. Water samples from wells greater than 100 feet deep in the Upper Floridan aquifer had a lower mean nitrate concentration (1.0 milligrams per liter) than did the shallower wells. Samples from this depth, located in the southern, downgradient part of the sprayfield, had a higher mean nitrate concentration than samples from wells in the northern, upgradient part of the sprayfield (3.8 compared to 0.78 milligrams per liter). Analyses of the nitrogen isotope ratios, 815Af, in nitrate in ground water were used to determine if the major source of nitrogen was treated sewage, inorganic fertilizers, or both. The 8 15Af values in nitrate in nitrate-enriched ground water at the southeast site were compared to 8 15Af values in ground water at another sprayfield southwest of the city where effluent was the sole source of nitrogen. Statistical analyses of the isotope ratios at the two sites showed a significant difference in the 815Af values, indicating that the contribution of nitrogen from fertilizers was significant at the southeast sprayfield.