Using Permanent Wetlands as Polishing Ponds to Remove Monomethylmercury: Results of a Large Scale Replicated Field Experiment

Abstract:

Seasonally inundated wetlands in the Yolo Bypass Wildlife Area (YBWA; Davis, CA) are producers of monomethylmercury (MeHg) during fall flood up. The export of MeHg from the YBWA is of serious concern to downstream habitats as it is a neurotoxin and poses a serious health concern to humans and wildlife. In this experiment, we routed water from a 40 hectare seasonal wetland through nine replicated 100 x 25 m constructed ponds to determine their efficiency at lowering MeHg. All of the sampling was conducted over a three year period (2011-2013) during fall months. We measured total and filtered (<0.45 µm) methyl and total mercury as well as nutrients in incoming tail water and at the outflow of each pond. Residence times in the ponds were modeled using rhodamine dye and ranged from 0.7 to 13.0 days in depending on flow (Q = 0.08 – 1.39 cfs). Reductions of dissolved (filtered) MeHg concentrations ranged from 0.024 – 0.455 ng L^-1 d^-1 and particulate (total – filtered) MeHg reductions ranged from 0.028 – 1.02 ng L^-1 d^-1 in the outflow of the ponds. We observed little to no change in dissolved MeHg concentrations at high flow rates, however, there was still a significant decrease in the particulate fraction of MeHg. In contrast, ponds with lower flow rates (e.g. longer residence times) were efficient at lowering both dissolved and particulate MeHg concentrations. During polishing periods when overlying water MeHg concentrations were highest dissolved MeHg flux was into sediments where we hypothesize bacterial demethylation was responsible for transformation back to inorganic mercury species. Loss of particulate MeHg from the water column was attributed to particle settling. Results from this experiment indicate small ponds with short residence times can be efficient at lowering MeHg concentrations. The design of the pond will vary depending on the volume of the seasonal wetland being drained and needs of the wetland manager.