Use of constructed wetlands with four different experimental designs to assess the potential for methyl and total Hg outputs

Abstract

This study used 10 parallel, smallscale constructed wetlands to investigate the potential for methylmercury (MeHg) production and water quality improvement using water and sediment from a creek that is a significant source of non-point nutrient, sediment and Hg pollution to a pristine river. The 4 replicated experimental designs utilized: (1) creek or Hg-contaminated water (25–320 ng Hg L −1) and creek or Hg-contaminated sediment (0.86 ± 0.52 μg Hg g −1) (MW-MS), (2) Hg contaminated water and clean sediment (0.09 ± 0.03 μg Hg g −1) (MW-CS), (3) clean water (effluent from a wastewater treatment plant; 4–16 ng Hg L −1) and Hg contaminated sediment (CW-MS), and (4) clean water and clean sediment (CW-CS). All designs functioned as sinks for N, P, sediment, and total Hg (THg). However, designs receiving clean water as the influent exhibited the least removal. Seasonal variations in net MeHg output were observed for designs with MW-MS and CW-MS, with concentrations peaking during warmer months. Designs with CS did not exhibit clear seasonal trends. Wetlands with CW and MS were the greatest MeHg sources. This was probably due to the fact that the treated wastewater had greater SO 4 2 - and total organic C (TOC) concentrations, lower pH, and, in general, higher temperatures than the creek water, and to the greater pool of Hg available in the Hg contaminated sediment, all of which could lead to enhanced Hg methylation. Temperature and SO 4 2 - correlated best with MeHg output in all designs. Although data from these small systems cannot be scaled up to predict the response in larger wetlands, results indicated that the benefits of a wetland, such as nutrient, suspended solids and THg removal, should be considered together with the risk of MeHg production.