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
This study is based on in situ measurements of the soil and the vadose zone (<60 m) in a Mediterranean karst experimental site near Nerja Cave (a show cave in dolomite marbles in southern Spain). Air temperature, relative humidity, and CO2 concentrations are the main variables measured, especially their variations with depth in a number of boreholes. The CO2 content generally increases with depth. Our measurements indicate average vadose air CO2 concentrations of nearly 40,000 μL L−1, with a maximum of nearly 60,000 μL L−1 In this context, the cave itself appears to be a vadose subsystem above the groundwater level, with significantly lower CO2 concentrations (a few thousand microliters per liter maximum) due to its ventilation. The vadose air in the lower part of the boreholes also exhibits near‐saturation humidity and a quite stable temperature around 21°C, similar to the conditions inside the cave. The measured vadose conditions were simulated by a reaction‐path hydrogeochemical model that starts with the local rainwater composition and reproduces the chemistry of the cave drip water, particularly its high Mg2+ content. The soil cover, although very scarce, has a relatively high organic matter content. The δ13C‐CO2 data of the vadose air point to an origin of the gas mainly related to biological soil processes. This gas can diffuse or flow laterally, upward, or downward through karst conduits. Interactions between air masses of surface origin (relatively dry, with variable temperature and low CO2 content) and typical vadose attributes (relatively high CO2 content, near‐saturated humidity and 21°C temperature) produce clear ascendant or descendant air fluxes inside the boreholes, especially those that cross significant karst voids.
Using chemical, isotopic and microbiologic techniques we tested in laboratory experiments the extent to which the addition of pyrite to groundwater and sediments from a nitrate-contaminated aquifer could stimulate denitrification by indigenous bacteria. In addition to this biostimulated approach, a combined biostimulated and bioaugmented treatment was also evaluated by inoculating the well-known autotrophic denitrifying bacterium Thiobacillus denitrificans. Results showed that the addition of pyrite enhanced nitrate removal and that denitrifying bacteria existing in the aquifer material were able to reduce nitrate using pyrite as the electron donor, obviating the need for the inoculation of T. denitrificans. The results of the 16S rRNA and nosZ gene-based DGGE and the quantitative PCR (qPCR) showed that the addition of pyrite led to an increase in the proportion of denitrifying bacteria and that bacterial populations closely related to the Xanthomonadaceae might probably be the autotrophic denitrifiers that used pyrite as the electron donor. Not only autotrophic but also heterotrophic denitrifying bacteria were stimulated through pyrite addition and both populations probably contributed to nitrate removal. Isotopic analyses (δ 15 N and δ 18 ONO3) were used to monitor enhanced denitrification and the N and O isotopic enrichment factors (-26.3±1.8‰ and-20.4±1.3‰, respectively) allowed to calculate the degree of natural nitrate attenuation in the aquifer. Furthermore, flow-through experiments amended with pyrite confirmed the long-term efficiency of the process under the study conditions. Further research under field conditions is needed to determine whether stimulation of denitrification by pyrite addition constitutes a feasible bioremediation strategy for nitratecontaminated aquifers.
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