Abstract:

Nitrate contamination is a major threat to groundwater quality worldwide, particularly in Catalonia, where nearly 40% of the territory is designated as a nitrate vulnerable one. This study investigates the potential for stimulating denitrification within a semiconfined alluvial coastal aquifer in the Argentona ephemeral stream catchment. A secondary objective was to evaluate methodological biases in characterizing aquifer microbial communities. We performed a series of push-pull tests to evaluate nitrate reduction rates during controlled aquifer recharge events.

Each cycle involved: (1) the injection of 50 m³ of reactive mixture, consisting of nitrate rich groundwater, acetate as electron donor and conservative tracers (Amino G, Rhodamine WT, Uranine): (2) a short incubation phase (0.5-2 days); and (3) extraction phase. To assess the validity of methods to characterize microbial communities, we compared six sampling techniques targeting both suspended and attached biomass, including standard groundwater sampling, downhole swabbing and pistoning, soil-core recovery, and passive biofilm sediment traps.

Field monitoring included continuous logging of hydraulic heads, redox potential (Eh), and physicochemical parameters. Groundwater samples were analyzed for major ions, nitrogen species, redox-sensitive elements (Fe, Mn, sulfide, As), and carbon fractions (DOC, TIC) to track geochemical evolution. By linking solute transport with biogeochemical transformations, this study quantifies how microbial community dynamics contribute to nitrate reduction and provides insights into the coupled mechanisms governing denitrification in alluvial aquifers.