Export of groundwater-borne geogenic phosphorus from a drained wetland into surface water

Groundwater-derived geogenic phosphorus (P) plays a significant but often overlooked role in surface water eutrophication. Geogenic P differs from anthropogenic P in its release mechanisms, seasonal variability and magnitude of release. While many studies have addressed the spatial distribution of geogenic P,  its temporal dynamics, transport and export mechanisms remain insufficiently understood. This study was conducted in a small study site (~1ha) located in a drained riparian wetland in southeastern Germany, where anthropogenic P input is minimal. The combination of a P-rich geological background, dynamic redox conditions and a drainage network provided an ideal setting to study the P mobilization and export processes. Hydrogeochemical monitoring of groundwater and drainage water over two years, complemented by vertical profile sampling of dissolved and solid phases, revealed significant P enrichment in the subsurface. About 70% of groundwater and drainage water samples exceeded the German Environment Agency’s threshold of 0.1 mg/L. Soluble reactive phosphorus (SRP) concentrations in groundwater reached up to 16 µmol/L (0.5 mg/L) in two of four wells, showing minimal seasonal variation. Drainage water SRP ranged from 6 to 15 µmol/L, with some interannual variability due to dilution during wet periods, and closely matched the chemistry of high-P groundwater wells. Both high spatial and low temporal variability were attributed to the site-specific geochemical settings. A strong correlation between P and iron (Fe) in groundwater and drainage water highlighted the critical role of Fe-P interactions in controlling P dynamics. Electrical resistivity tomography confirmed a subsurface preferential flow channel aligned with the high-P wells. These findings proposed a conceptual model: geogenic P, probably originating from the weathering of P-bearing minerals, reductive dissolution of Fe oxides, and organic matter mineralization, is stored in the subsurface. Preferential flow paths transport Fe-P-rich, anoxic groundwater to drainage systems, which further accelerate P export by creating direct groundwater-surface water connections, reducing residence time, and acting as hotspots for P accumulation and event-driven transport. This study provides novel insights into the processing of geogenic P in groundwater and its continuous contribution to surface water eutrophication. While concentrations may be lower than those from surface runoff or agriculture, geogenic P remains a long-term and persistent source of P loading. These results underscore the need for eutrophication mitigation strategies to address both geogenic and anthropogenic P sources.