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

Unraveling the fate of phosphorus in alluvial aquifers of the middle-lower Yellow River: Coupled natural and anthropogenic impacts

In recent years, groundwater phosphorus (P) contamination has received increasing attention, yet most studies focus solely on either anthropogenic or geogenic influences. This research addressed the combined effects of human activities and natural processes on P enrichment in the middle-lower Yellow River basin, where dissolved inorganic phosphorus (DIP) concentrations reached 0.59 mg/L. Hydrogeochemical analysis, along with multiple statistical methods and the Redfield ratio, revealed that geogenic processes were the dominant drivers of groundwater P enrichment, accounting for 77.5 % of the samples, while anthropogenic activities, particularly intensive agriculture, densely residential area and industrial development, contributed to P inputs in 22.5 % of the samples. Further analysis using dual isotopes (δ13C-DIC and δ56Fe) demonstrated that OP mineralization was the dominant geogenic P enrichment process, with the reductive dissolution of P-rich iron minerals serving as a secondary contributor. A comparative analysis between the middle-lower Yellow River basin and the central Yangtze River basin highlighted that the abundance of natural P-containing carriers and the closed or open nature of the groundwater environment jointly determined the extent of geogenic and anthropogenic P enrichment. This study provides valuable insights into the coupled impacts of natural and anthropogenic factors, enhancing our understanding of groundwater P dynamics.

Sewage Vertical Infiltration Introduced Polygenic Multipollutants into Groundwater

With the increasing environmental impacts of human activities, the problem of polygenic multipollutants in groundwater has attracted the attention of researchers. Identifying the hydrobiogeochemical characteristics of the surface sewage that replenishes groundwater is crucial to addressing this problem. The input of polygenic multipollutants into groundwater leads to not only the mechanical superposition of pollutants but also the formation of secondary pollutant types. The evolution of polygenic multipollutants is influenced by aquifer characteristics, carbon sources, microbial abundance, etc. Therefore, this study took a sewage leakage point in Northwest China as the research object, carried out a controlled laboratory experiment on the impact of sewage discharge on groundwater, and, combined with long-term field monitoring results, determined the main hydrobiogeochemical processes of polygenic multipollutants and their secondary pollutants. The results showed that the redox environment and the gradient change in pH were identified as the most critical controlling factors. In oxidative groundwater during the early stage of vertical infiltration, sewage carries a substantial amount of NH4+, which is oxidized to form the secondary pollutant NO3−. As O2 is consumed, the reduction intensifies, and secondary pollutants NO3−, Mn (IV), and Fe(III) minerals are successively reduced. Compared with the natural conditions of rainwater vertical infiltration, the reaction rates and intensities of various reactions significantly increase during sewage vertical infiltration. However, there is a notable difference in the groundwater pH between sewage and rainwater vertical infiltration. In O2 and secondary pollutant NO3− reduction, a large amount of CO2 is rapidly generated. Excessive CO2 dissolves to produce a substantial amount of H+, promoting the acidic dissolution of Mn (II) minerals and generation of Mn2+. Sewage provides a higher carbon load, enhancing Mn (II) acidic dissolution and stimulating the activity of dissimilatory nitrate reduction to ammonium, which exhibits a higher contribution to NO3− reduction. This results in a portion of NO3− converted from NH4+ being reduced back to NH4+ and retained in the groundwater, reducing the denitrification’s capacity to remove secondary NO3−. This has important implications for pollution management and groundwater remediation, particularly monitored natural attenuation.

Activation of iron oxide minerals in an aquifer by humic acid to promote adsorption of organic molecules

In aquifers, the sequestration and transformation of organic carbon are closely associated with soil iron oxides and can facilitate the release of iron ions from iron oxide minerals. There is a strong interaction between dissolved organic matter (DOM) and iron oxide minerals in aquifers, but the extent to which iron is activated by DOM exposure to active iron minerals in natural aquifers, the microscopic distribution of minerals on the surface, and the mechanisms involved in DOM molecular transformation are currently unclear. This study investigated the nonbiological reduction transformation and coupled adsorption of iron oxide minerals in aquifers containing DOM from both macro- and micro perspectives. The results of macroscopic dynamics experiments indicate that DOM can mediate soluble iron release during the reduction of iron oxide minerals, that pH strongly affects DOM removal, and that DOM is more efficiently degraded at low rather than high pH values, suggesting that a low pH is conducive to DOM adsorption and oxidation. Spherical aberration-corrected scanning transmission electron microscopy (SACTS) indicates that the reacted mineral surfaces are covered with large amounts of carbon and that dynamic agglomeration of iron, carbon, and oxygen occurs. At the nanoscale, three forms of DOM are found in the mineral surface agglomerates (on the surfaces, inside the surface agglomerates, and in the polymer pores). The microscopic organic carbon and iron mineral reaction patterns can form through oxidation reactions and selective adsorption effects. Fourier transform ion cyclotron resonance mass spectra indicate that both synergistic and antagonistic reactions occur between DOM and the minerals, that the release of iron is accompanied by DOM decomposition and humification, that large oxygen- and carbon-containing molecules are broken down into smaller oxygen- and carbon-containing compounds and that more molecules are produced through oxidation under acidic rather than alkaline conditions. These molecules provide adsorption sites for sediment, meaning that more iron can be released. Microscopic evidence for the release of iron was acquired. These results improve the understanding of the geochemical processes affecting iron in groundwater, the nonbiological transformation mechanisms that occur at the interfaces between natural iron minerals and organic matter, groundwater pollution control, and the environmental behavior of pollutants.