Phosphorus in shallow and deep groundwater: Importance of P/Fe ratio in Fe(III) oxides in aquifer sediments

The causes of the differential enrichment of dissolved phosphorus (P) in the shallow and deep aquifers remain elusive. In this study, groundwater and aquifer sediments were sampled from the Songnen Basin to explore the behaviour and mobilization mechanism of P in a shallow sandy aquifer and a deep sand-gravel aquifer by means of hydrogeochemical analyses and characterizations of dissolved organic matter (DOM) and sedimentary Fe minerals. The results showed that both aquifers were enriched in dissolved P, being mainly related to the organic matter degradation and Fe(III) oxides reduction. Comparatively, total dissolved phosphorus concentrations in shallow groundwater were significantly higher than those in the deep one (p < 0.01), owing to the stronger extent of reductive dissolution of Fe(III) oxides with higher P/Fe molar ratio in the shallow aquifer. Moreover, groundwater DOM with more humic substances in the shallow aquifer promoted the P mobilization via electron shuttling and/or competitive adsorption. In the deep aquifer, reductive dissolution of Fe(III) oxides with high P content resulted in the enrichment of groundwater P. In the two aquifers, immobilization of P via Fe(II) mineral precipitation played a minor role in the fate of groundwater P. This study underlines that geogenic enrichment of P in groundwater can occur in the sand-gravel aquifers with high contents of P and Fe(III) oxides under reducing conditions, and highlights that P/Fe molar ratio in Fe(III) oxides is the most important factor controlling dissolved P in groundwater systems.