The environmental and geochemical controls on mercury and sulphur cycling in Prairie Pothole Region wetland complexes

Abstract

Wetlands are known to play important roles in the cycling of mercury (Hg), which may ultimately lead to the generation of methylmercury (MeHg), a neurotoxin that bioaccumulates in food webs. The Prairie Pothole Region (PPR) of North America hosts sulphur-rich wetlands that favour the production of MeHg. Within these wetland systems, dissolved organic carbon (DOC) and sulphate fuel bacterial sulphate reduction (BSR), resulting in increased Hg methylation in select PPR wetlands. Due to local variability in wetland geochemistry, a reflection of hydrologic processes and differences in surficial Quaternary lithology, understanding the biogeochemical cycling of Hg in these wetlands is inherently complex. In Saskatchewan, PPR wetlands vary with respect to surface water and sediment chemistry, groundwater interactions, topographic position, and permanence, among other factors. As a result, MeHg can vary significantly on a wetland-bywetland basis in even the smallest complexes. Here, the geochemical and hydrogeological controls of sulphur cycling on Hg methylation in the St. Denis National Wildlife Area (SDNWA), an analogue for the northern Prairie Pothole Region, were explored. This was done through evaluating groundwater–surface water interactions, predicted Hg speciation, and how the sulphur cycle related to Hg methylation in the SDNWA. Previously collected surface and groundwater chemistry datasets obtained from the Global Institute of Water Security (University of Saskatchewan) and previous studies conducted at the SDNWA were coupled with new water and bulk sediment geochemistry analyses and sulphur stable isotope analyses. Results indicate that varying topographic position, wetland type (recharge or discharge), and prevailing bottom water and sediment redox conditions fundamentally influence sulphur cycling and Hg methylation in the SDNWA. In addition to varying bottom water redox conditions, Hg-S speciation, and trace metal cycles in the wetlands, indicate that localized variability may strongly influence overall Hg methylation. In calcium sulphate wetlands, sulphur disproportionation and sulphide mineral formation may keep porewaters from becoming too concentrated in sulphide and stifling BSR and, by extension, MeHg production. In contrast, the Hg-S speciation, temporary wetland nature, and the lower concentrations of sulphate in the surface waters of calcium bicarbonate wetlands may be more favorable for BSR and result in transient periods of Hg methylation. Although porewater and surface water redox characteristics played a large role in Hg methylation, other processes could significantly influence net methylation rates and overall ecosystem health. As such, constraining the biogeochemical cycling of Hg in PPR wetlands is critical for understanding how high levels of MeHg may impact aquatic biota.