Chronology of atmospheric deposition of arsenic inferred from reconstructed sedimentary records

Vertical distributions of As, Fe, and organic C were determined in dated (Pb-210 and Cs-137) sediment cores obtained from two adjacent basins (one perennially oxic and the other seasonally anoxic) of an oligotrophic headwater lake where atmospheric deposition is the only input of anthropogenic As. Despite similar sources in the two basins, the As profiles in the sediments differed markedly. Differences include the following: (i) As concentrations increased sharply upward close to the sediment surface in the perennially oxic basin due to scavenging of upward diffusing As by Fe oxyhydroxides, whereas they decreased in the seasonally anoxic basin where As scavenging by Fe oxyhydroxides did not occur, and (ii) the magnitude and position of major subsurface As maxima differed between the two basins. We applied a one-dimensional transport-reaction equation to porewater As concentration profiles obtained at the two sites to estimate sedimentary As concentrations at the time of deposition as well as subsequent addition or removal of As at various sediment depths. By multiplying As concentrations at the time of deposition by sediment mass accumulation rates, we were able to estimate variations in As fluxes at the sediment surface over the last two centuries. These fluxes were then transformed into atmospheric As deposition fluxes by applying a correction for basin-specific processes using the ratio of expected from atmospheric deposition to measured unsupported Pb-210 inventories at the sampling sites. The resulting chronological profiles of atmospheric fluxes of As deposition are similar in both basins, and are consistent with both the history of specific markers for coal combustion and direct historical measurements of As in dry and wet atmospheric deposition in rural areas of North America. We conclude that the history of As inputs can be reconstructed from As sedimentary records using appropriate corrections for diagenesis and basin-specific processes.