Technical design and hydrogeochemical evaluation of a stormwater-based managed aquifer recharge pilot site in an intensively cultivated region (Hallertau, Germany)

Abstract Managed aquifer recharge (MAR) with stormwater remains underexplored in rural settings due to uncertainties related to source-water variability, treatment performance, and groundwater-quality impacts. This study presents the design and hydrogeochemical evaluation of a stormwater-based MAR pilot site implemented in 2024 in Hüll (Bavaria, Germany), a flood-prone agricultural region underlain by a heterogeneous Tertiary aquifer. The system integrates a stormwater retention basin, treatment units, an infiltration well, and a comprehensive monitoring framework combining multi-parameter online sensors, detailed laboratory analyses, and contextual information on catchment activities. Hydraulic testing confirmed strong aquifer heterogeneity but also demonstrated high injectivity of the target formation. Hydrochemical monitoring evaluated raw water characteristics, treatment efficiency, and groundwater responses. The treatment unit effectively removed pesticides and substantially reduced suspended solids, while stormwater nitrate concentrations were consistently lower than those in groundwater, indicating dilution potential during recharge. The pilot demonstrates that rural stormwater can serve as a reliable MAR source under variable hydrological conditions. The integrated design and monitoring concept provides a transferable framework for similar agricultural regions and supports future calibration of site-scale hydrogeological and reactive transport models.