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Elevated nitrate concentrations are present in shallow ground water in selected lake basins in the Central Lakes District of Polk and Highlands Counties, Florida. Because there are no significant natural mineral sources of nitrate in Florida, the presence of nitrate in ground water is indicative of land use activity. Other factors that affect the concentration of nitrate in ground water are the rate of recharge, the concentrations of nitrogen species in atmospheric deposition, and the amount of dissolved oxygen in the ground water. A technique based on nitrogen isotopes was used to identify the likely sources of nitrate in ground water in the study area. Nitrogen isotopes in nitrate are distinctly different for organic (septic and animal waste) and inorganic (fertilizer) sources of nitrate. Water samples collected from wells that tap the surficial aquifer system in the lake basins studied indicated varying concentrations of nitrate in different areas of the basins. Samples collected from wells in undeveloped land parcels had very low nitrate concentrations, less than 0.002 milligram per liter. The nitrogen-isotope ratios of nitrate in these samples ranged from +9.0 to +16.6 per mil and indicate that fractionation, probably of soil-derived nitrogen, has occurred. Water samples collected from wells in citrus land-use areas had the highest nitrate concentrations, ranging from 4.9 to 57 milligrams per liter. The nitrogenisotope ratios for these samples ranged from -0.7 to +10.9 per mil, indicating that the nitrate is probably of inorganic (fertilizer) origin. In residential areas, nitrate concentrations ranged from less than 0.02 to 4.6 milligrams per liter and nitrogen-isotope ratios ranged from -6.8 to +13.4 per mil. These data indicate that the source of nitrate was either inorganic or organic, or a combination of both. In areas of mixed land use (undeveloped, citrus, and residential), nitrate concentrations ranged from less than 0.02 to 37 milligrams per liter and nitrogen-isotope ratios ranged from -7.0 to 15.3 per mil. The nitrogen isotopes were used in combination with ground-water head distributions, land-use information, and other geochemical data to better understand ground-water recharge, flow patterns, and chemical reactions that occur along ground-water flow paths.

water-resources investigations report

Evaluation of nitrate sources using nitrogen-isotope techniques in shallow ground water within selected lake basins in the Central Lakes District, Polk and Highlands Counties, Florida