Marine and estuarine ecosystems in the region surrounding the Florida Keys may be sensitive to minor changes in the abundance of nitrogen in surface water and discharging ground water. Because there are many potential natural and anthropogenic sources of nitrogen, the origins and isotopic characteristics of nitrogen species were investigated in various water sources in the region including the Florida Keys, Florida Bay, and the offshore reef tract. The ¹⁵N values of ammonium in saline ground waters in karst aquifers underlying most of the region ranged from about +2 to +7 ‰ and were correlated spatially with the ¹⁵N values of organic matter in overlying sub-aqueous carbonate sediments. Those data are consistent with the hypothesis that much of the ground-water ammonium was derived naturally from diagenesis of organic matter in the overlying sediments and transported downward in hypersaline bay water and offshore seawater. In contrast, wastewater from small-scale sewage treatment plants and wastewater injection sites in the Keys generally had large concentrations of waste-derived nitrogen (mainly nitrate, generally less ammonium) with ¹⁵N 7 ‰. Tap water imported to the Florida Keys from the south Florida mainland also contained substantial amounts of nitrate and ammonium, representing as much as 5 to 10 % of the anthropogenic load of fixed nitrogen to the Keys. Chemical and isotopic analyses indicate that the tap-water nitrate (¹⁵N = 16 to 22 ‰) was derived from nitrate-bearing ground water that was pumped and processed for distribution, whereas the tap-water ammonium (¹⁵N = 0 to 5 ‰) was largely the result of the water treatment. Fresh ground water at the water-supply well field on the mainland, and brackish ground water beneath the Keys and near-shore areas, commonly had excess non-atmospheric nitrogen gas (¹⁵N = -0.7 to +5.0 ‰) that is attributed to denitrification, which also increased the ¹⁵N values of nitrate after it was recharged on land or injected with wastewater.

These results indicate that some of the anthropogenic sources of nitrogen in the Keys that are abundant locally in shallow fresh and brackish ground waters (largely nitrate and derivative nitrogen gas) may be distinguished chemically and isotopically from major natural sources of fixed nitrogen that are widely distributed in saline and hypersaline ground waters underlying the region (mainly ammonium). Overall, the data do not indicate direct anthropogenic contributions to nitrogen in ground water beneath the reef tract or most offshore areas including Florida Bay, but they do indicate localized anthropogenic contributions to nitrogen in shallow ground waters near onshore development. Denitrification reduced substantially the concentration of nitrate in the Keys water supply before it was pumped from the source, and it reduced the concentration of nitrate in treated wastewater after it was injected beneath the Keys. Nevertheless, nitrogen was moving through both ends of the anthropogenic water cycle, some being introduced with the drinking water supply and some ultimately discharging to near-shore surface waters after use and disposal.

Contact: John Karl Böhlke, U.S. Geological Survey, 431 National Center, Reston, VA 20192, Phone: 703-648-6325, Fax: 703-648-5274, jkbohlke@usgs.gov, Question 2

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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