The role of Lower Cretaceous sediments in groundwater nitrate attenuation in central Spain: Column experiments

Endorheic basins located in semiarid or arid regions constitute one of the most vulnerable and exposed environments to NO 3 - pollution. The Pétrola basin (Central Spain) is an outstanding example of one of these endorheic system. Several constraints have been observed that impede correct identification of the denitrification pathway and its degree at the field scale. To better understand the key factors controlling NO 3 - attenuation, a five-stage column experiment was performed using organic and pyrite rich sediments from the Utrillas Facies (Lower Cretaceous) as a possible electron donor source to promote denitrification. A chemical and multi-isotopic characterization (δ15NNO3, δ18ONO3, δ13CDIC, δ34SSO4, δ18OSO4) of the outflow of the column experiment showed that NO 3 - attenuation was driven by organic matter. The amount of organic C consumed during denitrification was 2% of the total organic C present in the sediment. Both the degree and rate of denitrification were related to flow rate variations. Lower flow rates favored bacterial growth at the beginning of the experiment producing an increase of complete denitrification rate (CDR). When NO 3 - reduction was greater than 15%, CDR remained constant (around 30 μmol L−1 d−1) under different flow rates. In the last stage, flow rate had no effect on output NO 3 - concentration because the organic C supply was limited and had become the main kinetic factor in denitrification. Isotopic fractionations (ε of 15N– NO 3 - and 18O– NO 3 - between reaction products and remaining reactants were calculated using representative column samples. The results for the two attenuation stages observed were −11.6‰ and −15.7‰ for εN, and −12.1‰ and −13.8‰ for εO. Most of the field samples showed a percentage of attenuation ranging from 0% to 60% of NO 3 - removal.