Unravelling the Spatial Variability and Complexity of Denitrification in Heterogeneous Aquifer Sediments

Nitrate pollution of groundwater represents a critical environmental concern, with many aquifers exceeding ecological and health-related concentration limits. Microbial driven denitrification represents the primary mechanism for nitrate attenuation in aquifers. Despite a robust understanding of individual transformation steps, predicting nitrate turnover in the complex and heterogeneous subsurface environment of aquifers remains challenging. This is primarily due to the uneven distribution of potential electron donors (e.g., organic carbon) and the variability in local biogeochemical conditions on the aquifer scale. To assess the spatial variability of denitrification at field-relevant scales, we conducted laboratory microcosm experiments were conducted with distinct sedimentary facies from a Holocene aquifer which we previously characterized by paleo environmental reconstruction. The denitrification capacity of anaerobically incubated sediments (n=40) from eight distinct sedimentary facies with varying TOC contents was evaluated by monitoring the concentrations of NO₃⁻, NH₄⁺, N₂O, SO₄²⁻, and DOC over time. Microcosm replicates of the same sedimentary facies at a specific sampling location showed consistent nitrate removal. However, microcosms of the same sedimentary facies at different locations showed significant variability of nitrate removal. The observed variability of a particular sedimentary facies was found to be within the same range as the mean variability observed across different sedimentary facies. Our study indicates that the denitrification potential of heterogenous aquifers is far more complex and variable than commonly assumed and cannot be gauged by evaluating the electron donor distribution.