Linking water quality changes to geochemical processes occurring in a reactive soil column during treated wastewater infiltration using a large-scale pilot experiment: Insights into Mn behavior

Abstract

The spatial and temporal evolution of the physical properties and chemical composition of water and soil during the infiltration of treated wastewater through a reactive soil column at the pilot scale (25m3) were investigated for 18months. Major and trace elements, and organic carbon were measured monthly in treated wastewater, output water and pore water. O2, CO2, CH4, N2O, N2, H2S and H2 were measured occasionally in soil air. Geochemical processes occurring in the soil changed rapidly over the study period. Nitrification was effective in the top 1.5m of soil during the first 8months. Thereafter, the organic carbon load from treated wastewater and the treated wastewater left above the infiltration surface due to clogging (organic matter accumulation and precipitation of carbonates) created anaerobic conditions that led to denitrification and reductive dissolution of Mn oxides in the soil. The latter led to the release of both Mn (exceeding the WHO drinking water limit by a factor of up to 2) and Ni that was strongly associated with the Mn oxides. Forty-five percent of the Ni that had accumulated in the soil during the first year was released. Dissolved element concentrations were non-uniform because of the spatial and temporal distribution of redox reactions in the soil due to the heterogeneous distribution of organic matter and/or non-uniform flow (water saturation, residence time). The soil was very effective in removing phosphate, Fe, and Li, and removing/degrading organic carbon and NH4. In contrast, Ba, NO3 and, to a lesser extent, Ca were exported. Removal rates were moderate for As and total nitrogen, and low for K, Na, Ni and Mn. However, removal rates of nitrogen species (NH4, NO3), total nitrogen, Mn, Fe, Ni and As were strongly dependent on redox conditions which varied during the experiment. The evolution of geochemical processes occurring in the soil column therefore has significant implications on the quality of water moving from the soil to groundwater and provides key knowledge for the management of groundwater under artificial recharge.