Intrinsic potential and activity of nitrate turnover examined for different hydrogeological aquifer settings

Nitrate (NO₃^-) is one of the most serious contaminants in groundwater, frequently deteriorating water quality and groundwater use as drinking water. Nitrate can, at specific environmental conditions, be removed within the subsurface environment via natural biogeochemical processes, such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Traditionally, research has concentrated on the NO₃^- attenuation potential and microbial activity in groundwater, largely overlooking an aquifers’ sediment matrix as a reaction contributor. We hypothesize that the sedimentary deposits host the major potential for NO₃^- reduction, carrying the majority of microorganisms as well as different sources of electron donors. Moreover, diverse hydrogeological settings harbor different sources and varying amounts of electron donors such as reduced iron minerals and organic matter. Therefore, subsurface physicochemical heterogeneity, determined by sedimentology, controls the potential of an aquifer to reduce nitrate. We hypothesize that locations where physicochemical conditions favor NO₃^- reduction, we may expect microbes involved in these processes to be more abundant and highly active, leading to a fast NO₃^- removal from groundwater.

Here, we present results from temperature-controlled (12°C) batch incubations and flow-through experiments conducted over several months using fresh sediments from an alluvial, tufaceous unconfined aquifer, with embedded peat layers, in an agricultural landscape in southwest Germany. Batch experiments received an addition of NO₃^- of 50 mg L^–1. Sediment columns were supplied with nitrate-spiked groundwater collected from the site at a maximum inlet concentration of 150 mg L^– and run in continuous injection mode. Batch experiments showed a gradual decrease in the initially high hydrogen sulfide (H₂S) concentration, becoming undetectable after Day 7, compared to an untreated control, which revealed a slow conversion of HS^-. These preliminary findings indicate a strong autotrophic denitrification with NO₃^- reduction coupled to aqueous HS^- oxidation. In contrast to nitrate, NH₄⁺ concentrations remained stable in all experiments. In the column experiments, the sediment exhibited a substantial NO₃^- reduction capacity in the early phase but rapidly declined with time. We hypothesize that a dual contribution to denitrification via easily bioavailable electron donors in the groundwater followed by slower denitrification coupled to organic matter in the sediment matrix is responsible for the observed dynamics. Our ongoing experiments, including groundwater and sediments from other, hydrogeologically different aquifers, will dissect the individual redox processes and key microorganisms involved in turnover of prominent nitrate and carbon species in the context of different hydrogeological settings.