Soil amendments and potential risk of mercury methylation in a constructed wetland designed for metals treatment

Constructed wetlands designed for metals removal attempt to capitalize upon the biotic and abiotic mechanisms within natural wetland systems that enhance immobilization of metal contaminants from the water column. When treatment systems are amended with sulfate, increased sulfate reduction may lead to an increase in microbially-mediated methylmercury production if inorganic mercury is bioavailable. This study used laboratory microcosms and an experimental field model of a constructed wetland to evaluate the risk of mercury methylation when the soil of one particular wetland system is amended with sulfate. In the laboratory, various sulfate treatments were combined with wetland-based soil in anoxic microbial slurry reactors with bioavailable mercury. When evaluated in conjunction with bacterial population dynamics and sulfate/sulfide chemistry, it was evident that methylation depended upon the growth of indigenous sulfate-reducing bacterial populations and subsequent sulfate reduction. The field model was planted with Schoenoplectus californicus, and the soil was varied during construction to provide a control and two sulfate treatments. This allowed characterization of sulfate's effect on the removal efficiency of mercury from the wastestream, as well as the risks from methylmercury formation. Mercury concentrations flowing into the constructed wetland model were reduced by an average of 37%. This was a significant reduction, and there were no differences between the control or the two sulfate treatments. This indicated that sulfate soil amendments made no difference in overall treatment efficiency. Although all porewater methylmercury concentrations were relatively low (<8 ng/L) throughout the study, concentrations in the sulfate treatments were consistently elevated over those in the control, leading to an overall statistically significant difference. There was no significant difference in porewater methylmercury between the low and the high sulfate treatments. The elevated porewater methylmercury concentrations noted in sulfate treatments were not detected in the surface water. The risk of sulfate addition to the soil of the constructed wetland model was most notable through the elevated mercury accumulation by periphyton and two small fish species, Gambusia holbrooki and Erimyzon sucetta.