Historic and future classification of nitrate degradation for porous and fractured aquifers in terms of their denitrification potential using isotopic, hydrochemical and borehole data

Diffuse nitrate pollution in groundwater from intensive agriculture continues to contribute to a persistent risk to groundwater quality. While natural nitrate degradation processes can mitigate anthropogenic nitrate inputs, the available electron donors in the pore structure of aquifers, primarily pyrite (FeS₂) and organic carbon (Corg), are finite resources. Quantifying these resources is crucial for the long-term management of groundwater resources; however, the quality of this quantification is often limited by a lack of real-world data.This study presents a comprehensive approach to characterizing denitrification potentials in various aquifers (porous and bedrock) based on solid-phase analysis of drill core samples combined with hydrochemical and isotopic data in the state of Sachsen-Anhalt, Germany. The methodology integrates the geochemical quantification of reducing agents with hydraulic parameters to calculate a specific lifetime of nitrate degradation (denitrification potential) for the historical reference period (1961 – 2020) and for future projections (2020 – 2100) using modeled groundwater recharge rates for the RCP 2.6 and RCP 8.5 climate scenarios.The results show a clear division between aquifer types. Aquifers in bedrock (e.g., Keuper, bunter sandstone) exhibit high resilience with lifetimes > 10,000 years, primarily due to a high autotrophic denitrification potential. In contrast, porous aquifers, particularly the covered Pleistocene units, were identified as highly vulnerable groundwater systems. Historical analysis shows that these aquifers have significantly lower reducing agent reserves, resulting in lifetimes of less than 1,000 years. Future climate projections indicate a critical depletion of these resources, mainly driven by climate-related changes in groundwater recharge and continuous nitrate inputs. The remaining lifetime of porous aquifers is projected to decrease to less than 40 years in some cases this century under both climate scenarios, potentially leading to a large-scale nitrate breakthrough. The combination of rapidly depleting denitrification buffer and the hydraulic lag of the systems underscores the urgency of implementing strategies in vulnerable catchment areas, as reliance on natural attenuation is no longer a sustainable safeguard.