Phosphorus Dynamics in Aquifers: Processes, Pathways, and Links to Trace Elements

Understanding the biogeochemical processes that lead to the enrichment of dissolved phosphorus in groundwater is crucial, especially as concentrations exceeding 1 mg L^-1 occur in many floodplain and delta aquifers. These elevated concentrations of primarily inorganic orthophosphate can cause eutrophication in surface waters and mobilize toxic elements like arsenic.

Here, we provide a comprehensive overview of the biogeochemistry of phosphorus within groundwater systems, aiming to elucidate how phosphorus interactions with other elements influence its behaviour. Our methods include chemical analyses of groundwater and aquifer materials, laboratory and field experiments, isotope analysis, and geochemical modeling.

Our findings indicate three primary processes responsible for the release of phosphorus into groundwater: apatite weathering; microbial mineralization of organic matter, which generates dissolved phosphorus as a by-product; and the reductive dissolution of iron-(hydr)oxides that serve as hosts for phosphorus. Additionally, phosphorus undergoes microbial processing during its transport, as evidenced by phosphate-bound stable oxygen isotope analysis. Phosphorus may also be immobilized by (co-)precipitation of secondary minerals (e.g., calcium-phosphates or iron minerals) or through surface adsorption. Notably, phosphorus immobilization often takes precedence over arsenic immobilization, enhancing the mobility of the latter.

The occurrence of these mobilization and/or immobilization processes largely depends on local groundwater characteristics and the aquifer's mineralogical composition. Our results highlight the intricate links between the fate of phosphorus and the biogeochemical cycles of calcium, carbon, iron, arsenic, and possibly sulphur, suggesting potential strategies for in-situ groundwater remediation approaches.