Assessment of methyl mercury (MeHg) release from sediment formed in wetland treatment cells designed to remove mercury from surface waters using in situ coagulation

The use of in situ coagulation techniques to remove mercury (Hg) from surface waters has gained attention as a potentially viable control measure for reducing methylmercury (MeHg) loads from key source areas (wetlands, subsided Delta Islands, mine drainage) but concerns remain about the reactivity of the resulting sediment material that forms and whether it can be safely retained in a wetland environment. The material that that settles out is largely composed of "flocculant" which is a complex of the added coagulant, organic material and other compounds like Hg. Indirect evidence from an ongoing study on Twitchell Island suggests there may be differences in the stability of the flocculant formed under different coagulation treatments (iron sulfate versus polyaluminum chloride), despite their similar total Hg and MeHg removal rates from surface waters.

Our objective is to determine the relative propensity of sediment material formed under two different in situ coagulation treatments (iron sulfate, polyaluminum chloride) to release MeHg into surface waters under environmentally relevant conditions that may affect flocculant stability and MeHg production rates.

The Twitchell Island Low Intensity Chemical Dosing (LICD) project has been operating an experimental coagulation-wetland system since 2012 that demonstrates the feasibility of removing Hg from surface waters and sequestering it in wetland sediments. The field experiment includes three treatments which received water treated with either iron sulfate (FeS), polyalumnin chloride (PAC), or nothing (untreated control). All sediment samples will be collected from these wetland cells, homogenized and stored in glass containers with no headspace to maintain conditions at the time of sampling. The surface sediments, which in the treated wetlands are predominantly made up of flocculant, will be characterized prior to the start of the experiment in both the bulk solids (total, methyl, and reactive Hg) as well as an elutriated solution (filtered total, fTHg, and methyl Hg, fMeHg, extracted from the bulk sample when shaken with native water) to determine the mobile fraction of each Hg species. We will also characterize the flocculant using sequential selective extractions (SSE) to evaluate the potential stability and reactivity of the different coagulant-Hg complexes relative to previous characterizations of Hg source materials (Bloom et al., 2003). We will also have carbon, nitrogen, iron, and aluminum concentrations of all of these sediments analyzed as part of the larger study.

Based upon the results of the characterization, a known mass of each type of sediment (FeS, PAC, control) will be added to a set of jars (66) containing receiving sediment and allowed to incubate under controlled temperature through two wet-dry cycles of varying length and redox conditions. The jars will be monitored for redox potential (Eh), pH and temperature over the course of the incubation. At each pre-determined time step (and associated redox condition), ten samples (3 treatments in triplicate plus a blank) will be collected, extracted and preserved for analysis of THg and MeHg in the aqueous and solid-phases. The results will be statistically analyzed to determine differences in phase (solid vs aqueous) and speciation of the Hg (MeHg vs THg) due to redox, time, and treatment.

The research proposed by the USGS addresses two of the primary areas highlighted in the USGS Science Strategy 2007-2017 (USGS Circular 1309).: 1) Understanding Ecosystems and Predicting Ecosystem Change and 2) The Role of Environment and Wildlife in Human Health; also other federal and State programs (e.g., CWA §303d, TMDLs, Bay Delta Action Plan, and CERCLA).