Sedimentary organic carbon contents control the nitrate reduction potential in a Holocene aquifer: Evidence from batch incubations

Abstract Groundwater quality is increasingly threatened by nitrate (NO 3 − ) contamination from agricultural activities. In many aquifers, NO 3 − is attenuated through microbially mediated reduction coupled to the oxidation of reduced constituents in the aquifer matrix. However, predicting NO 3 − turnover at the aquifer scale remains difficult because electron donors such as organic carbon and reduced sulfur species are heterogeneously distributed and reliable proxies for NO 3 − reduction potential are lacking. To address this challenge, batch experiments were conducted using eight facies types from a previously characterized tufaceous Holocene aquifer in southwestern Germany. The facies exhibited a wide range of solid-phase organic carbon contents (SOC 0.5–40.1 wt.%). NO 3 − reduction in batches was assessed by monitoring temporal changes in NO 3 − , sulfate (SO 4 2− ), and dissolved organic carbon (DOC) concentrations. NO 3 − reduction followed a two-phase pattern: An initial rapid, nonlinear decrease likely driven by readily bioavailable DOC, followed by a linear decline consistent with zero-order kinetics. Long-term NO 3 − reduction rates showed a strong positive relationship with solid phase organic carbon (SOC) ( R 2 = 0.69) across all microcosms ( n = 40) and facies ( n = 8). Experimental reduction rates were combined with facies-specific horizontal groundwater flow velocities to estimate NO 3 − turnover distances and times for an initial concentration of 50 mg L −1 NO 3 − . The results demonstrate that NO 3 − attenuation is controlled by both biogeochemical and hydraulic factors. The study highlights SOC as a key proxy long-term NO 3 − reduction potential and demonstrates the value of facies-based approaches combined with reaction-kinetic modeling for assessing groundwater quality at the aquifer scale.