The agriculturally-dominated watersheds of the Upper Midwest United States receive elevated Hg wet deposition (~10- 15 µg m-2 yr-1), and Hg biogeochemistry on multiple time and length scales within these systems deserves further attention. Here, we focus on spatiotemporal variability in methylmercury (MeHg) production within the Swamp White Oak (SWO) preserve, an oak savanna floodplain along the Cedar River (draining 20,200 km2) in east-central Iowa. Shallow alluvial groundwaters (4 “high” wells in sandy ridges and 2 “low” wells in silty swales) and wetland surface waters (2 ponds plus a stream emptying into the Cedar River), all within a small 0.025 km2 area, were sampled monthly from Jun-Oct 2016 for major and minor water constituents (including Hg and MeHg) to evaluate the importance of seasonal, hydrologic (surface-groundwater interactions), and microtopographic control on redox and Hg biogeochemical cycling. MeHg concentrations in groundwaters showed considerable variability on small spatial scales, ranging from &lt; 0.02 to 0.74 ng/L (mean = 0.14±0.16 ng/L). Concentrations of MeHg in shallow wetland surface waters enriched in DOC were typically higher, ranging from 0.14 to 0.68 ng/L (mean = 0.34±0.17 ng/L). Restored and conserved riparian wetlands likely play an important role in downstream MeHg export within the Cedar River watershed. Marked spatiotemporal variations in dissolved oxygen, nitrate, Fe(II), Mn(II), and sulfate suggest that shifts in dominant terminal electron acceptor processes (denitrification, iron, manganese, and sulfate reduction) caused by complex surface-groundwater interactions and microtopography-induced subsurface flowpaths exert significant control on MeHg production within the Cedar River floodplain.

Spatiotemporal variability in MeHg production in a Restored Oak Savanna Floodplain

253rd National Meeting (2017)

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