Seasonal Oxygenation of Contaminated Floodplain Soil Releases Zn to Porewater

Soil zinc contamination is a major threat to water quality and sensitive ecosystems. While Zn itself is not redox-active in soils, transitions in soil redox conditions may promote mobilization of Zn from common Zn hosts, including Mn(IV)/ Fe(III)-(hydr)oxides and sulfide precipitates, leading to elevated concentrations of dissolved Zn in surface and groundwater and thus a potential increase in Zn transport and uptake. Here, we examined the impacts of hydrologic fluctuations and coupled redox transitions on Zn partitioning in contaminated riparian soil in a mountain watershed. We found that oxygenation of the soil profile during low water conditions caused a spike in porewater Zn concentrations, driven by oxidative dissolution of amorphous ZnS and weak partitioning of Zn to Fe(III)-(hydr)oxides, hydroxy-interlayer vermiculite, and vermiculite. In contrast to Pb, released Zn did not immediately adsorb to Fe(III)-(hydr)oxides or particulate organic matter due to less-favorable sorption of Zn than that of Pb and, further, decreased Zn sorption at slightly acidic pH. As aridification intensifies and groundwater levels decline throughout the western United States, contaminated floodplain soils in mountain watersheds may be frequently oxygenated, leading to increased mobilization of dissolved Zn, which will amplify the threat Zn poses to water quality and ecosystem health.

Automated summary

Soil zinc contamination poses a significant threat to water quality and sensitive ecosystems. Changes in soil redox conditions can lead to mobilization of zinc from common hosts, resulting in increased dissolved zinc concentrations in surface and groundwater. The oxygenation of contaminated riparian soil during low water conditions caused a spike in porewater zinc concentrations. As aridification intensifies, oxygenation of floodplain soils in mountain watersheds may lead to increased mobilization of dissolved zinc, amplifying the threat to water quality and ecosystem health.