Monitoring groundwater nitrate attenuation in a regional system coupling hydrogeology with multi-isotopic methods: The case of Plana de Vic (Osona, Spain)

Abstract

This paper describes an integrated application of classical hydrogeological methods and multi-isotopic methods (δ15N, δ 18ON O3, δ34S, δ 18OS O4, δ13C) to assess the fate of groundwater nitrate in the Osona area, declared vulnerable to nitrate pollution by the Catalan Government in 1998, where nitrate is derived from intensive pig farming activities. Previous studies, involving a small area, indicated the occurrence of denitrification processes and their relationship with pyrite oxidation [Vitòria, L., Soler, A., Canals, A., Otero, N., 2008. Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: example of Osona (NE Spain). Appl. Geochem. 23, 3597-3611]. For the present study, groundwater samples were collected at 60 production wells at three different periods between April 2005 and May 2006 to confirm that denitrification takes place in a larger area than that studied by Vitòria et al. [Vitòria, L., Soler, A., Canals, A., Otero, N., 2008. Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: example of Osona (NE Spain). Appl. Geochem. 23, 3597-3611]. The aim of the study was to characterize the denitrification processes that control natural attenuation and to study their spatial and temporal variations. Nitrate concentration ranged from 10 to 529 mg/l, with 82% of the wells above the drinking water threshold of 50 mg NO3/l. Nitrate isotopic composition ranged from +5.3‰ to +35.3‰ for δ15N and from +0.4‰ to +17.6‰ for δ 18ON O3, and the samples showed a positive correlation between δ15N and δ 18ON O3, with a εN/εO ratio of 1.8, consistent with denitrification processes. The link between denitrification and pyrite oxidation is demonstrated by coupling chemical data with nitrate and sulfate isotopes. Furthermore, a spatial distribution of samples with significant denitrification was observed, allowing us to determine two main hydrogeological zones where natural attenuation was most effective. In several of the studied points, denitrification processes related to pyrite oxidation predominated and an estimation of the isotopic enrichment factors was performed using the temporal variations of nitrate concentration and the isotopic composition of dissolved nitrate (δ 15NN O3 and δ 18ON O3). Finally, using estimated isotopic enrichment factors, an approximation of the degree of natural attenuation of nitrate was performed on those samples showing clear denitrification, and a median value of 30% of contaminant diminution was obtainedThis work has been financed by CICYT projects CGL2005-08019-C04-01 and 02, CGL-2008-06373-CO3-01 and 02/BTE from the Spanish Government, the local authorities (Consell Comarcal d’Osona), the Catalan Water Agency (Age` ncia Catalana de l’Aigua), the local private association of livestock raising (ASSAPORC), the project SGR2005-00933 from the Catalan Government, the I3P Programme funded by the EU, and the ‘‘Jose´ de Castillejo’’ program of the Spanish Governmen

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

“…2a). These enrichment factors are consistent with values from denitrification studies that report ε15N ranging from -4.0 to -30.0‰ and ε18O ranging from -1.9 to -8.9‰ (Vogel et al, 1981;Mariotti et al, 1988;Böttcher et al, 1990;Spalding and Parrott, 1994;Mengis et al, 1999;Pauwels et al, 2000;Otero et al, 2009). The maximum NO3-N concentration in groundwater was measured in core sample DC15-23 (clay at 2 m bgl, 7 m hydraulically downgradient of DMW3).…”

“…Numerous studies have made qualitative assessments of denitrification using the stable isotope approach (Böttcher et al, 1990;Wassenaar, 1995;Singleton et al, 2007;Baily et al, 2011;Clague et al, 2015;Xu et al, 2015). However, very few published field studies report 35 quantitative assessments of denitrification based on isotopic enrichment of δ 15 NNO3 and δ 18 ONO3 and, to our knowledge, none account for uncertainties in source values or enrichment factors (Böttcher et al, 1990;Otero et al, 2009;Xue et al, 2009).…”

Modeling biogeochemical processes and isotope fractionation of enhanced in situ biodenitrification in a fractured aquifer

“…Because of dilution, a 86 decrease in nitrate concentration cannot always be attributed to degradation. Monitoring the 87 changes in the nitrogen and oxygen isotope ratios of nitrate (δ 15 N-NO 3 -and δ 18 O-NO 3 -) allows 88 the degradation to be identified (Otero et al, 2009;Puig et al, 2013;Carrey et al, 2014) remaining, f; R s,0 is the initial isotopic composition of the molecule; and α is the kinetic isotopic 97 fractionation factor of the transformation process, which is often represented as the kinetic 98 isotopic enrichment factor ε (in permil, ‰), where ε = (α-1). 99…”

Evaluation of the Fate of Nitrate and Analysis of Shallow Soil Water using Geo-electrical Resistivity Survey

“…The reduction of fertilizer practices showed a high potential of decreasing N 2 O emission and nitrate leaching [6]. Otero, et al [7] conducted a study on the monitoring of nitrate using a multi-isotope method, which provides an approximation of the degree of natural attenuation of nitrate. The result showed clear denitrification and contaminant diminution in the groundwater.…”