Mercury dynamics in groundwater across three distinct riparian zone types of the US Midwest

Abstract

Although the intense biogeochemical gradients present in riparian zones have the potential to affect mercury (Hg) cycling, Hg dynamics in riparian zones has received relatively little attention in the literature. Our study investigated groundwater filtered total mercury (THg) and methylmercury (MeHg) dynamics in three riparian zones with contrasting hydrogeomorphic (HGM) characteristics (till, alluvium, outwash) in the US Midwest. Despite high Hg deposition rates (>16 μg m⁻²) in the region, median THg (<1.05 ng L⁻¹) and MeHg (<0.05 ng L⁻¹) concentrations were low at the study sites. Methylmercury concentrations were significantly (p < 0.05) correlated to THg (R = 0.82), temperature (R = 0.55), and dissolved organic carbon (DOC) (R = 0.62). THg also correlated with groundwater DOC (R = 0.59). The proportion of MeHg in THg (%MeHg) was significantly correlated to temperature (R = 0.58) and MeHg (R = 0.50). Results suggest that HGM characteristics, the presence of tile drains, and the propensity for overbank flooding at a riparian site determined the extent to which stream water Hg concentrations influenced riparian groundwater Hg levels or vice versa. Differences in hydrogeomorphic characteristics between sites did not translate however in significant differences in groundwater MeHg or %MeHg. Overall, widespread Hg contamination in the most common riparian hydrogeomorphic types of the US Midwest is unlikely to be a major concern. However, for frequently flooded riparian zones located downstream from a potentially large source of Hg (e.g., concentrated urban development), Hg concentrations are likely to be higher than at other sites.