Estimating Nitrogen Loading to Ground Water and Assessing Vulnerability to Nitrate Contamination in a Large Karstic Springs Basin, Florida¹

Abstract

A nitrogen (N) mass‐balance budget was developed to assess the sources of N affecting increasing ground‐water nitrate concentrations in the 960‐km2 karstic Ichetucknee Springs basin. This budget included direct measurements of N species in rainfall, ground water, and spring waters, along with estimates of N loading from fertilizers, septic tanks, animal wastes, and the land application of treated municipal wastewater and residual solids. Based on a range of N leaching estimates, N loads to ground water ranged from 262,000 to 1.3 million kg/year; and were similar to N export from the basin in spring waters (266,000 kg/year) when 80‐90% N losses were assumed. Fertilizers applied to cropland, lawns, and pine stands contributed about 51% of the estimated total annual N load to ground water in the basin. Other sources contributed the following percentages of total N load to ground water: animal wastes, 27%; septic tanks, 12%; atmospheric deposition, 8%; and the land application of treated wastewater and biosolids, 2%. Due to below normal rainfall (97.3 cm) during the 12‐month rainfall collection period, N inputs from rainfall likely were about 30% lower than estimates for normal annual rainfall (136 cm). Low N‐isotope values for six spring waters (δ15N‐NO3 = 3.3 to 6.3‰) and elevated potassium concentrations in ground water and spring waters were consistent with the large N contribution from fertilizers. Given ground‐water residence times on the order of decades for spring waters, possible sinks for excess N inputs to the basin include N storage in the unsaturated zone and parts of the aquifer with relatively sluggish ground‐water movement and denitrification. A geographical‐based model of spatial loading from fertilizers indicated that areas most vulnerable to nitrate contamination were located in closed depressions containing sinkholes and other dissolution features in the southern half of the basin.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

“…Upchurch et al, 2007), and springs discharge accounts for a large proportion of the N load to estuarine and coastal waters (Pittman et al, 1997). Despite the perceived vulnerability of the UFA to nutrient enrichment, river export accounts for only a moderate proportion of N inputs to North Florida landscapes, and considerable proportions of inputs remain unaccounted for (Katz et al, 2009). Landscape-scale mass balance generally suggests inorganic fertilizer as the primary source of N enrichment (Katz et al, 2009), but isotopic studies (that assumed negligible denitrification) have indicated a greater role of organic N from animal or human waste water (Katz et al, 2001).…”

Contaminant Transport in Two Central Missouri Karst Recharge Areas

“…There have been only a few studies that have reported nutrient fluxes or loads for karst aquifers Currens, 2002;Panno and Kelly, 2004;Katz et al, 2009). Of these studies, only two, Boyer and Pasquarell (1996) and Panno and Kelly (2004), were conducted in recharge areas with agricultural land use intensity comparable to the Devils Icebox and Hunters Cave recharge areas.…”

Cumulative Spring Discharge and Survey Effort Influence Occupancy and Detection of a Threatened Freshwater Mussel, the Suwannee Moccasinshell

“…Highly elevated nitrogen levels have existed in the mid to lower main-stem Suwannee River Basin for some time (Pittman et al 1997;Katz and Bohlke 2000;Katz et al 2009). Springs have been shown to be primary vectors of this nitrogen.…”

“…The spring with the largest discharge (but not the highest nitrogen concentration) in that study contributed 550 kg of nitrates daily. Ammonia levels in springs are typically lower than U.S. Environmental Protection Agency Aquatic Life Ambient Water Quality Criteria, but ammonia is usually present in most groundwater wells and springs tested Katz et al 2009;EPA 2013). Ammonia is extremely toxic to larval and juvenile freshwater mussels (Wang et al 2007), and may persist longer in springs than in other systems because of low dissolved oxygen and eutrophic conditions limiting bacterial nitrification (Haag 2012).…”

“…Hypoxic conditions or elevated levels of ammonia in spring outflows may explain why we observed minimal occurrences of Suwannee Moccasinshell within 100 m and 500 m of springs. Agricultural fertilizers, septic tanks, and animal waste are believed to be the primary sources of nitrogen in springs (Andrews 1994;Pittman et al 1997;Katz and Bohlke 2000;Katz et al 2009). The sites monitored by Pittman et al (1997) were~32 km upstream of any tidal influence with a maximum SpringQ of 32 cms.…”