Coupled Processes Involving Organic Matter and Fe Oxyhydroxides Control Geogenic Phosphorus Enrichment in Groundwater Systems: New Evidence from FT-ICR-MS and XANES

Revealing degradation pathways of soluble and dissolved organic matter in alluvial-lacustrine aquifer systems impacted by high levels of geogenic ammonium

The excessive presence of geogenic ammonium (NH4+) in groundwater poses a global environmental concern, commonly linked to the degradation of nitrogen-containing dissolved organic matter (DOM). However, there is a gap in systematic studies on the combination of soluble organic matter (SOM) in sediments and DOM in groundwater, with few indoor incubation experiments to validate their degradation pathways. This study utilized ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to analyze the molecular characteristics of DOM and SOM in aquifer systems affected by geogenic NH4+. Subsequently, indoor incubation experiments spanning up to 140 d were conducted to verify the degradation pathways. The experimental results revealed a two-phase degradation process for both the DOM and SOM. The initial stage was characterized by the degradation of aliphatic compounds (ALC) with the production of polyphenols (PPE) and highly unsaturated compounds (HUC). The second stage was dominated by the degradation of PPE and HUC, accompanied by the re-consumption of some ALC, while more recalcitrant HUC persisted. Notably, the first stage of SOM degradation exceeded that of DOM degradation, indicating that SOM exhibited greater resistance to aging. This phenomenon may be attributed to a wider range of active enzymes in sediments, the rapid replenishment of SOM by organic matter in sediments, or the accelerated degradation of DOM. The experimental results aligned with the molecular characterization of DOM and SOM in actual aquifer systems. It is hypothesized that NH4+ produced through the direct mineralization of SOM may contribute more to the enrichment of NH4+ in groundwater than that produced through the mineralization of DOM. This study is the first to analyze DOM and SOM together in aquifer systems and validate their degradation pathways through incubation experiments, thereby providing novel insights into the enrichment of geogenic NH4+ in groundwater.

Flood irrigation increases the release of phosphorus from aquifer sediments into groundwater

Nonpoint source pollution caused by agricultural activities has long attracted widespread attention from people in society and academia. Many studies have found that human activities not only convey exogenous pollutants into aquifers but also affect the mobilization and transport of geogenic pollutants in aquifers. Geogenic groundwater with high phosphorus concentrations has been found, but it is unclear whether the changes in hydrogeochemical conditions caused by flood irrigation in paddy fields affect the fate of phosphorus. We investigated the temporal and spatial distribution characteristics of phosphorus in groundwater under the influence of flood irrigation through laboratory experiments, proved its impact on phosphorus in groundwater, and explored the mechanisms influencing P concentrations. The results show that flood irrigation can increase the release of phosphorus in the aquifer media and greatly increase the phosphorus concentration in the groundwater of the study area, which has a negative impact on groundwater quality. The main mechanism of increase in phosphorus concentration in groundwater involves an increase in the reducibility of the aquifer via flood irrigation; as a result, iron oxides are reductively dissolved and iron-bound phosphorus is released into the groundwater. Changes in pH also result in the dissolution of calcium phosphate minerals and the release calcium-bound phosphorus. This study not only advances the theory of multielement-coupled hydrogeochemistry but also provides a reference for agricultural planning and groundwater pollution prevention and control in rice-growing areas.