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

[Display omitted] •Groundwater P enrichment occurred via DOM degradation and Fe(III) oxide reduction.•DOM with more humic substances in shallow aquifer promoted P mobilization.•Higher P/Fe molar ratio in Fe(III) oxides caused higher P in shallow groundwater.•Immobilization of P via Fe(II) precipitation weakly constrained P enrichments. 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