In this study we examined the biogeochemistry and historical behavior of Pb in an ombrotrophic peatland watershed (S2 peatland, Marcell Experimental Forest, MN, USA) and contrasted it with Hg. Both elements are only deposited at Marcell via long-range transport and atmospheric deposition, but Pb experienced dramatic decreases in deposition several decades ago while Hg deposition has decreased only slighly in recent years. We measured trace metal inputs to the peatland in surface and subsurface runoff, export from the peatland via surface water outflow, and Pb concentrations at depth in shallow peat cores. We found a strong relationship between DOC and Pb concentrations at the outflow of the S2 peatland, but no relationship between Hg and DOC. These results show key differences in the biogeochemical processing of Pb and Hg in the peatland watershed. Measurements of trace metals in upland runoff demonstrated that upland sources of Hg to the peatland are significant, while Pb transport from the upland to the peatland is minimal. A previous study in the early 1980s reported significant Pb inputs via upland runoff. Geochemical differences in binding to soil minerals and organic matter may explain the differences observed between Hg and Pb. In addition, we could be observing different ecosystem response times to dramatic decreases in Pb atmospheric deposition. The uplands have been depleted of Pb available for transport via runoff, while Pb in the bog continues to be mobilized by DOM, Pb inventory in recently collected peat cores was found to be similar or less than reported Pb inventory in cores collected in the early 1980s suggesting that Pb continues to be mobilized by DOM produced in the peatland decades after decreases in Pb deposition.

Pb and Hg mobilization by dissolved organic matter in an ombrotrophic peatland

253rd National Meeting (2017)

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