Inhomogeneous porewater geochemistry in unconsolidated argillaceous sediments: Implications for porewater extraction protocols and characterization of solute mobilization

Abstract

Unconsolidated argillaceous sediments play important roles in water-rock interactions and can significantly impact the geochemistry of groundwater and connected surface water bodies. Characterizing porewater geochemistry in these sediments and the geochemical processes regulating solute mobilization is challenging, however. While various porewater extraction methods have been used, the representativeness of the analytical results remains largely untested. Here, we carried out continuous and segmented porewater extraction experiments on clay-rich sediments from the Jianghan Plain in central China. Our data clearly show that the porewaters are mixtures, rather than homogeneous solutions. A first type of porewater inhomogeneity was associated with the abrupt increase in porewater binding strength separating free from bound porewater. The change in binding strength occurred at a loading stress of around 0.20–0.30 MPa, with the threshold value dependent on the particle size distribution of the sediments. The second type of inhomogeneity featured smooth changes in solute concentrations and isotopic compositions, likely reflecting variable contributions of water-sediment reactions. Solutes whose mobilization under increasing loading stress was primarily affected by the first type of inhomogeneity included ammonium and arsenic, those primarily affected by the second type of homogeneity included dissolved organic carbon, alkalinity, and strontium. Overall, the macroscopically observed geochemical variability of porewater discharged during the extractions was related to changes in moisture content and pore stress. Our findings shed light on the multiplicity of processes regulating solute mobilization from unconsolidated argillaceous sediments and emphasize the importance of selecting appropriate porewater extraction procedures for these sediments.