Iowa receives relatively high wet deposition of mercury (Hg), but the fate of this Hg in Iowa’s aquatic and terrestrial ecosystems is largely unknown. Here we describe geochemical controls on Hg speciation in ground waters of the Swamp White Oak Nature Preserve, a restored savanna oak floodplain of the Cedar River, a major river draining ~21,000 km2 of eastern Iowa. To evaluate the influence of microtopography on groundwater geochemistry, we sampled a ~100 m2 area including four topographically low wells in silty swales and two high wells in sandy ridges on an approximately monthly basis from May 2016 through August 2017. Low wells had higher total dissolved solids (TDS) and dissolved organic carbon (DOC) concentrations and were more reducing (lower dissolved oxygen, higher concentrations of dissolved Fe(II) and Mn) than topographically high wells. We attribute these differences to variation in sediment lithology (organic carbon content and quality) and longer travel times for groundwater extracted from low wells compared to high wells. Total filterable Hg (THg) concentrations in groundwater averaged 1.1±1.1 ng/L across all groundwater sites and sampling dates, whereas filterable methylmercury (MeHg) concentrations averaged 0.12±0.13 ng/L. No clear seasonal or spatial trends were evident for THg. However, MeHg concentrations showed a clear peak in late summer/early autumn and varied up to 40-fold (from less than 0.02 to nearly 0.80 ng/L) across monitoring wells sampled on the same date. Across all wells, total dissolved Mn concentration was the best predictor of observed filterable MeHg concentrations (r = 0.46, p &lt; 0.01). This finding suggests a possible role for Mn-reducing bacteria in Hg methylation in these groundwaters. Overall, our results suggest that riparian groundwaters can be an important source of MeHg to fluvial systems with high surface-groundwater interaction. Furthermore, we demonstrate that both dominant terminal electron acceptor processes and Hg methylation can demonstrate exceptional fine-scale spatial heterogeneity owing to complex microtopography and groundwater flowpaths. Microtopographic effects likely warrant consideration when designing robust monitoring programs for contaminant fate and transport in river floodplains.

Microtopographic control of groundwater geochemistry and Hg methylation in a river floodplain

255th National Meeting (2018)

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