Distribution and biogeochemical controls on net methylmercury production in Penobscot River marshes and sediment

Cynthia Gilmour; James Tyler Bell; Ally Bullock Soren; Georgia Riedel; Gerhardt Riedel; A Dianne Kopec; R A Bodaly

doi:10.1016/j.scitotenv.2018.05.276

Distribution and biogeochemical controls on net methylmercury production in Penobscot River marshes and sediment

Sci Total Environ. 2018 Nov 1:640-641:555-569. doi: 10.1016/j.scitotenv.2018.05.276. Epub 2018 Jun 1.

Authors

Cynthia Gilmour¹, James Tyler Bell², Ally Bullock Soren², Georgia Riedel², Gerhardt Riedel², A Dianne Kopec³, R A Bodaly⁴

Affiliations

¹ Smithsonian Environmental Research Center, 647 Contees Wharf Rd., Edgewater, MD 20657, United States. Electronic address: gilmourc@si.edu.
² Smithsonian Environmental Research Center, 647 Contees Wharf Rd., Edgewater, MD 20657, United States.
³ Penobscot River Mercury Study, Bangor, ME, US.
⁴ Penobscot River Mercury Study, Bangor, ME, US. Electronic address: drewbodaly@gmail.com.

PMID: 29864668
DOI: 10.1016/j.scitotenv.2018.05.276

Abstract

The distribution of mercury and methylmercury (MeHg) in sediment, mudflats, and marsh soils of the Hg-contaminated tidal Penobscot River was investigated, along with biogeochemical controls on production. Average total Hg in surface samples (0-3 cm) ranged from 100 to 1200 ng/g; average MeHg ranged from 5 to 50 ng/g. MeHg was usually highest at or near the surface except in highly mobile mudflats. Although total Hg concentrations in the Penobscot are elevated, it is the accumulation of MeHg that stands out in comparison to other ecosystems. Surface soils in the large Mendall Marsh, about 17 km downstream from the contamination source, contained particularly high %MeHg (averaging 8%). In Mendall marsh soil porewaters, MeHg often accounted for more than half of total Hg. Salt marshes are areas of particular concern in the Penobscot River, for they are depositional environments for a Hg-contaminated mobile pool of river sediment, hot spots for net MeHg production, and sources of risk to marsh animals. We hypothesized that exceptionally low mercury partitioning between the solid and aqueous phases (with log K_d averaging ~4.5) drives high MeHg in Penobscot marshes. The co-occurrence of iron and sulfide in filtered soil porewaters, sometimes both above 100 μM, suggests the presence of nanoparticulate and/or colloidal metal sulfides. These colloids may be stabilized by high concentrations of aromatic and potentially sulfurized dissolved organic matter (DOM) in marsh soils. Thus, Hg in Penobscot marsh soils appears to be in a highly available for microbial methylation through the formation of DOM-associated HgS complexes. Additionally, low partitioning of MeHg to marsh soils suggests high MeHg bioavailability to animals. Overall, drivers of high MeHg in Penobscot marshes include elevated Hg in soils, low partitioning of Hg to solids, high Hg bioavailability for methylation, rapidly shifting redox conditions in surface marsh soils, and high rates of microbial activity.

Keywords: Bioavailability; Estuary; Metal sulfides; Methylation; Nanoparticulate; Saltmarsh.