Carbon and iron isotope approach elucidates the enrichment of geogenic phosphorus in alluvial-lacustrine sedimentary aquifers

Abstract

Iron (Fe) (oxyhydr)oxides and organic matter (OM) are important hosts of geogenic phosphorus (P) in groundwater systems. However, the coupled influence of Fe (oxyhydr)oxides and OM on the occurrence and mobility of P in groundwater remains unclear. In this study, we shed light on the underlying mechanisms of the control of Fe (oxyhydr)oxides and OM on P mobilization in the alluvial-lacustrine aquifers at the Dongting Plain (DTP), central China, using tracing techniques of isotopes (δ13C-DIC and δ56Fe), characterization of dissolved organic matter (DOM), and groundwater geochemistry. The results suggest that high concentrations of geogenic P up to 3.58 mg/L as total dissolved phosphorus (TDP) tend to occur in relatively reducing and sluggish hydrogeological environments experiencing longer time of water–rock interactions, and are closely related to the reductive dissolution of P-rich Fe(III) (oxyhydr)oxides as well as the mineralization of organic P. Our observations suggest that two different processes between Fe(III) (oxyhydr)oxides and OM contribute to the high concentrations of TDP in groundwater. Firstly, under moderately reducing conditions, labile OM is oxidized, with amorphous Fe(III) (oxyhydr)oxides acting as electron acceptors. Subsequently, the adsorbed and/or occluded P on/into OM or amorphous Fe(III) (oxhydr)oxides is concomitantly released into groundwater. Secondly, under strongly reducing conditions, the degradation of recalcitrant OM depends on the concurrent roles of crystalline Fe(III) (oxyhydr)oxides as electron acceptors and conduits, which further influence the mobility of P. The latter P-mobilizing redox processes were identified to prevail over the former in the groundwater of DTP. This study provides new insights into the anomalous concentration of P in groundwater of alluvial-lacustrine sandy aquifer systems.