Abstract

Many ecosystems throughout the world are experiencing significant biogeochemical perturbation from anthropogenic activities. As one example, manganese (Mn) is enriched in surface soils at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a small watershed in Huntingdon County, PA. Soils at the SSHCZO record the legacy of industrial Mn inputs from the numerous iron furnances operating in Huntingdon County during the mid-19th century and the widespread industrialization of PA into the 20th century. Atmospheric inputs of manganese to soils from disperse industrial sources have led to widespread but patchy contamination of soils with Mn in previously or currently industrialized regions throughout Pennsylvania and the United States. The enrichment of Mn in a soil profile relative to the parent lithology can be quantified to derive estimates of past atmospheric inputs. We establish that although atmospheric deposition rates of Mn have fallen with the decline of the iron and steel industries, soils, vegetation, and water systems in Pennsylvania exhibit high concentrations of Mn, and impacted ecosystems slowly release accumulated Mn from soils and vegetation into rivers.

The rate of transfer of Mn contamination from soils into rivers is strongly impacted by vegetation in SSHCZO. Specifically, trees take up dissolved Mn from the soil and store it in biomass for years to decades. Additionally, the soluble Mn that is stored in biomass is oxidized and immobilized as Mn-oxides in the soil during biomass decomposition. The continuing process of uptake, storage, and oxidation of Mn in vegetation leads to long-term retention of Mn within impacted ecosystems, and Mn is only slowly leaked from soils and vegetation into rivers. Conversely, high concentrations of soil organic matter in parts of the catchment lead to high concentrations of dissolved organic carbon that facilitate Mn mobilization from soils. Thus, the Shale Hills catchment is characterized by spatial and temporal heterogeneities in the rates that Mn is leached from soils into pore fluids and streams. Specifically, Mn is input to the SSHCZO stream dominantly from organic-rich swales during periods of low stream discharge, and Mn concentrations in the stream are diluted by inputs from organic-poor hillslopes during periods of high stream discharge. Overall, biological cycling increases Mn retention in ecosystems impacted by industrial inputs and concentrates the Mn into soils with high soil organic matter before the Mn is lost from the catchment. Vegetation therefore acts as a capacitor that concentrates Mn inputs in this temperate catchment.

Citation

Herndon, E. (2012): Biogeochemistry of manganese contamination in a temperate forested watershed. Doctor of Philosophy, Geosciences and Biogeochemistry, The Pennsylvania State University, p. 290..

This Paper/Book acknowledges NSF CZO grant support.