158-1 The Mechanisms of Differential Enrichment of Dissolved Phosphorus in Groundwater of Floodplain Aquifers

Session: Redox-Driven Nutrient and Contaminant Dynamics in Terrestrial Systems

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Abstract:

In recent years, increasing number of studies have shown the importance of geogenic enrichments of dissolved phosphorus (P) in floodplain aquifers worldwide, among which an obviously differential enrichment was observed between the shallow Late Pleistocene-Holocene sandy aquifers and the deep confined sand-gravel aquifers. However, to date, the causes of the differential enrichment of dissolved P in floodplain aquifers remain elusive. To uncover this, we systematically analyzed the differences in hydrogeochemical compositions, organic matter (OM) molecular characteristics, and sediment mineral compositions between the shallow sandy and deep sand-gravel aquifers in the Songnen Basin, PR China. Our results showed that dissolved P was more enriched in shallow aquifers (552 μg L^-1 in average) than in deep aquifers (141 μg L^-1 in average). High P groundwater was characterized by high HCO₃^-, NH₄⁺, and Fe(II) concentrations, indicating that the degradation of organic matter and the reducing dissolution of Fe(III)-oxides in the aquifers were the main controlling processes for P enrichments in groundwater. Along the depth profile, both the proportions of biodegradable and P-containing molecular compositions of dissolved OM and of sedimentary OM continuously decreased. The characteristics of groundwater-surface water common OM molecules and of groundwater-sediment common OM molecules indicated that surface water DOM and sedimentary OM jointly determined the groundwater DOM compositions. Specifically, both surface water DOM and sedimentary OM released more biodegradable and P-containing DOM into shallow groundwater, which promoted stronger degradation of P-containing DOM and reductive dissolution of P-containing Fe(III)-oxides in shallow aquifers, resulting in higher P concentrations in shallow groundwater than in the deep one. Furthermore, compared with deep aquifers, Fe(III)-oxides in shallow aquifers were characteristics by weak crystallinity and high P/Fe molar ratio. The significant positive correlations between the proportions of non-crystalline Fe(III)-oxides in Fe(III)-oxides, P/Fe molar ratios, and corresponding groundwater P concentration within the same depth indicated that the degree of crystallization and P/Fe molar ratio of sediment Fe(III)-oxides were conducive to the reduction of Fe(III)-oxides and the release of P. Our study highlights that within the shallow Late Pleistocene-Holocene sandy aquifers, the occurred stronger degradation of biodegradable P-containing DOM and induced reductive dissolution of Fe(III)-oxides with low crystallization but high P/Fe molar ratio were the main mechanisms that result in the differential enrichment of dissolved P between shallow and deep floodplain aquifers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5796

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