MERCURY CYCLING IN COAGULANT-TREATED WETLAND SOILS UPON REWETTING: A CONTROLLED LABORATORY INCUBATION

Methylmercury (MeHg) bioaccumulation in aquatic biota is a major concern in the Sacramento-San Joaquin Delta, California, USA. To reduce mercury (Hg) export from historical Hg mines, metal-based coagulants can be added to improve Hg trapping in settling basins. However, there are some concerns about soils treated with coagulants enhancing MeHg production after rewetting. This study used soil-water microcosms from Cache Creek Settling Basin, Sacramento, California, USA, to examine whether soils treated with the coagulants ChitoVan™ (organic/shell amino-based), Ferralyte® (ferric-sulfate-based), and Ultrion® (polyaluminum chloride) could release MeHg upon rewetting. The study also assessed MeHg release under elevated organic matter loading (addition of spirulina powder). The findings showed that MeHg levels in water (~0.1 ng/L) and soil (~1.2 µg/kg dw) were not significantly higher in coagulant-treated soils compared to the control. Oxidation-reduction potential in sediment-water incubations was associated with manganese reduction, above levels related to Hg biomethylation in aquatic ecosystems. The addition of organic matter resulted in a depressed oxidation-reduction potential and a short-lived burst of MeHg production (~28-40 ng/L in water ~6-8 µg/kg in soil) in both coagulant-treated and control soils. Dissolved organic matter (DOM) fluorescence spectroscopy showed an increase in quinoid-like moieties and microbial-source DOM, which likely provided labile carbon and promoted inorganic Hg bioavailability, thereby promoting Hg methylation. Results highlight that algal organic matter loading to Hg-contaminated flooded soils appeared to stimulate microbial activity and generate an ephemeral window of MeHg production during mildly reducing conditions that may represent a period of Hg bioaccumulation. Findings also suggest that the optical properties of DOM could be useful indicators of MeHg production in aquatic ecosystems, providing valuable insights into Hg cycling.