NITROGEN SOURCES AND FATE IN A LARGE KARSTIC SPRINGS BASIN

A nitrogen (N) mass-balance budget was developed to assess the sources and fate of nitrogen affecting elevated groundwater nitrate concentrations in the 960-km² karstic Ichetucknee Springs basin in northern Florida. Budget components included direct measurements of N species in rainfall, groundwater and spring waters, along with estimates of N loading from fertilizers, septic tanks, animal wastes, and land application of treated municipal wastewater and residual biosolids. Based on estimates of N leaching, N loads to groundwater ranged from 262,000 to 1.3 million kg/yr; and were similar to N export from the basin in spring waters (266,000 kg/yr) when 80-90% N losses were assumed. Fertilizers (cropland, lawns, and pine stands) contributed about 51% of the estimated total annual N load to groundwater in the basin-- other sources contributed the following percentages: animal wastes (27), septic tanks (12), atmospheric deposition (8), and land application of treated wastewater and biosolids (2). Generally low N-isotope values for six spring waters (δ¹⁵N-NO₃=3.3 to 6.3‰) and elevated potassium concentrations in groundwater and spring waters were consistent with the large N contribution from fertilizers. Given groundwater residence times on the order of decades for spring waters, possible sinks for excess N inputs to the basin include uptake by vegetation, denitrification, and N storage (in the unsaturated zone and parts of the aquifer with relatively sluggish groundwater movement). Groundwater dentrification estimates derived from direct measurement of N₂ ranged from 27 to 64% of the NO₃-N load for the six major springs, and totalled 182,000 kg N/yr basinwide. In the upper 5 km of the springfed Ichetucknee River, nitrate-N removal averaged 43,000 kg N/yr (about 25% of the N load to the river from springs discharge) over a 20-year period with more than 90% of the nitrate-N removal attributed to denitrification. Areas in the springs basin most vulnerable to nitrate contamination of groundwater were located in closed depressions containing sinkholes and other dissolution features in the southern half of the basin, based on a spatial loading model for fertilizer N.