Resumen de Induced attenuation of nitrate pollution: multi-isotopic study at laboratory and field scale
The contamination with different compounds of nitrogen, such as nitrate and ammonium, constitutes one of the most common problems of diffused groundwater contamination, especially from the point of view of the quality of the water bodies of Catalonia. The most important natural nitrate attenuation process is denitrification, i.e. the reduction of nitrate to dinitrogen gas by anaerobic facultative bacteria (that utilize nitrate as the electron acceptor). Nevertheless, this process is limited by the availability of electron donors in the aquifers. To overcome this natural limitation, different external electron donors can be added to promote denitrification. Permeable Reactive Barriers (PRBs) filled with organic or inorganic substrates can be used as induced remedial treatments to decrease nitrate contents in aquifers. In general, a PRB involves the placement of a reactive media perpendicular to the potential trajectory of the contaminated groundwater.
The general aim of this doctoral thesis is to improve the knowledge on induced nitrate attenuation processes using isotopic tools in laboratory and field scale studies. At laboratory scale, the feasibility to enhance denitrification of two low-cost organic substrates and one inorganic substrate is studied. At field scale, multi-isotopic tools are used to assess the effect on nitrate fate of two different PRBs, which were initially installed to treat other contaminants. The first one uses an organic substrate (vegetal compost) as reactive material and consists in a permeable reactive layer (PRL) located in the bottom of an infiltration pond installed in the managed aquifer recharge (MAR) system of Sant Vicenç dels Horts (Barcelona, Catalonia). It was implemented to increase groundwater resources and to improve water quality. The second PRB uses ZVI as reactive material and is located in the flow path of a contaminated plume in Granollers (Barcelona, Catalonia). It was initially installed to remove chlorinated solvents from groundwater.
In the first part of this thesis, the feasibility to enhance denitrification of two low-cost, easily available and easily handled organic substrates (commercial compost and crushed palm tree leaves) is examined. Chemical and multi-isotopic results demonstrate that both substrates are able to induce denitrification. The estimated isotope fractionation values can be used to assess the efficacy of nitrate remediation in future field MAR-PRL applications with these low-cost substrates.
The second part of this thesis is focused on assessing the long-term effectiveness promoting denitrification of the PRL of vegetal compost of the Sant Vicenç dels Horts site. Laboratory experiments and field campaigns confirm the long-term capacity of the PRL to enhance denitrification. The extent of denitrification enhanced by the reactive layer is estimated using the isotope fractionation values obtained in laboratory experiments with the material extracted from the PRL. It is demonstrated that the time of operation of the MAR system is the key to promote high percentage of denitrification, and thus continuous periods of recharge are recommended.
The third part of this thesis is focused on investigating the potential of isotope analyses to assess the effect on nitrate fate of the ZVI-PRB of the Granollers site. Combining chemical and isotope data, as well as isotope monitoring of laboratory experiments with ZVI, it is demonstrated that the ZVI-PRB installed in the study area locally induces nitrate attenuation, though its effect is limited (less than 15-20% of nitrate removal).
Overall, this thesis demonstrates the potential of isotope tools to assess the efficacy of PRBs filled with low-cost organic substrates or ZVI to remove nitrate from groundwater. This information might be crucial for mitigating existing pollution in water resources and improving remediation actions.
