Identification of nitrogen sources and transformations within karst springs using isotope tracers of nitrogen

Abstract

Isotope analyses of nitrate and algae were used to gain better understanding of sources of nitrate to Florida's karst springs and processes affecting nitrate in the Floridan aquifer at multiple scales. In wet years, d 15 N and d 18 O of nitrate ranged from ?3 to ?9% in headwater springs in north Florida, indicating nitrification of soil ammonium as the dominant source. With below normal rainfall, the d 15 N and d 18 O of nitrate were higher in almost all springs (reaching ?20.2 and ?15.3%, respectively) and were negatively correlated with dissolved oxygen. In springs with values of d 15 N-NO 3 and d 18 O-NO 3 greater than ?10%, nitrate concentrations declined 40-50% in dry years and variations in the d 15 N and d 18 O of nitrate were consistent with the effects of denitrification.Modeling of the aquifer as a closed system yielded in situ fractionation caused by denitrification of 9 and 18% for D 18 O and D 15 N, respectively. We observed no strong evidence for local sources of nitrate along spring runs; concentrations declined downstream (0.42-3.3 lmol-NO 3 L -1 per km) and the isotopic dynamics of algae and nitrate indicated a closed system. Correlation between the d 15 N composition of nitrate and algae was observed at regional and spring-run scales, but the relationship was complicated by varying isotopic fractionation factors associated with nitrate uptake (D ranged from 2 to 13%). Our study demonstrates that nitrate inputs to Florida's springs are derived predominantly from non-point sources and that denitrification is detectable in aquifer waters with relatively long residence time (i.e., matrix flow).

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

“…However, several recent studies have found isotopic evidence for denitrification in karstic aquifers, including those in southwestern Illinois, USA (Panno et al, 2001), southern Germany (Einsiedl and Mayer, 2006), and the Floridan Aquifer (Albertin et al, 2011). One feature of karst aquifers that may facilitate denitrification is large difference in hydraulic conductivity, and thus water age and chemistry, between karstic aquifer matrices and conduits.…”

“…Although insufficient to directly estimate rates of denitrification, coupled enrichment of δ 15 N NO3 and δ 18 O NO3 is now widely used to infer the occurrence of fractionating processes (Burns et al, 2009). Among these are recent studies that suggest denitrification in the Upper Floridan Aquifer and other karst groundwater systems (Panno et al, 2001;Hackley et al, 2007;Albertin et al, 2011). One potential value of dual isotopic measurements is the ability to partition removal into its component processes (e.g.…”

Hydro-Geochemical Characteristics and Health Risk Evaluation of Nitrate in Groundwater

“…In the early 1990s, research indicated that 0.8 million people in France were threatened by nitrate-polluted drinking water, 0.85 million in the UK, and 2.5 million in Germany [5]. Thus, issues of groundwater quality are gradually attracting more attention from researchers worldwide [2,[6][7]. Subsequently, many scholars carried out large-scale studies on the detection, prevention, and control of nitrogen in groundwater, and the three types of nitrogen-composed elements (nitrate, nitrite, and ammonium nitrogen) have become topics…”

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“…In many spring runs in Florida, proliferation of epiphytic algae, as well as benthic macroalgae, have been observed concomitantly with declines of SAV communities (Stevenson et al 2004;Frazer et al 2006;Pinowska et al 2007;Stevenson et al 2007;Quinlan et al 2008;Brown et al 2008). Water quality in groundwater discharged from many springs in Florida has shown significant increases in NO x -N concentration, attributed predominantly to fertilizer application and/or wastewater or manure sources in individual spring sheds (Jones et al 1996;Katz 2004;Albertin et al 2012). Odum (1957) reported mean NO x -N concentration of 0.45 mg L -1 for Silver Springs in the 1950s which had risen to over 1 mg L -1 by 2005 (Munch et al 2006;Quinlan et al 2008).…”