Geogenic enrichment of dissolved phosphorus in a calcareous aquifer: Surface adsorption versus organic-matter mineralization

Abstract

Calcareous aquifers represent vital water resources and may critically influence the retention, release, and transport of phosphorus (P) within catchments. To address the limited understanding of P cycling in pristine calcareous aquifers, where both P retention and release processes remain poorly constrained, we conducted a hydrogeochemical field study in the Ammer floodplain catchment in Southwest Germany. Here, we combined groundwater hydrochemical analyses, sediment geochemistry data (e.g., P speciation by sequential extraction) and geochemical modeling to examine controls on total dissolved P (TDP) in a shallow calcareous aquifer. Inorganic orthophosphate (PO₄³⁻) dominated within the aqueous phase and correlated with reducing redox conditions (Eh), DOC, and NH₄⁺ concentrations (p < 0.05, respectively), suggesting in situ microbial mineralization of sedimentary organic matter (OM) as a key mobilization mechanism. Sediment extractions revealed a strong positive correlation of surface-adsorbed PO₄³⁻ and organic P (P_org) with organic carbon (C) concentrations (p < 0.05), indicating that OM constituted an important sink for both PO₄³⁻ and P_org. Distribution coefficients (Kd values) further suggested that P_org was preferentially adsorbed over PO₄³⁻. Supersaturation regarding calcite (saturation indices > + 0.25) and a negative correlation between inorganic C and total P concentrations in the sediments (p < 0.5) suggest a limited role for Ca-mineral precipitation in active P immobilization. In summary, our study advances mechanistic insights into P cycling in calcareous groundwater systems, thereby emphasizing the dual function of OM as both a sink and a source for P, a dynamic previously overlooked in floodplain aquifers.