Many soils are enriched in trace elements due to atmospheric inputs from industrial sources but little is known about how long these contaminants persist in soils or the rates at which they are transferred into rivers. Modeling the movement of contaminants through the environment is complicated by the heterogeneity of soils and the variability of contaminant mobility across spatial scales. In this study, we use soil, water, and vegetation chemistry to compare rates of Mn contaminant mobilization and removal from soils at ridge, hillslope, and catchment-scales in the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). The SSHCZO is a first-order, forested watershed located within the Susquehanna River Basin (SRB) in Pennsylvania, U.S.A. Studies from the SSHCZO are compared to trends in long-term water quality measurements for the Susquehanna River to evaluate terrestrial inputs to the river system.
At SSHCZO, we find that Mn is being removed ~7x more quickly from soils in swales than soils on convex-upward hillslopes; thus, swales are a large source of dissolved Mn to the stream. Release rates of Mn from all soils are dwarfed by rates of uptake into vegetation, consistent with the hypothesis that trees temporarily slow the removal of atmospherically-deposited Mn from the soil by accumulating Mn in plant biomass. However, elevated levels of dissolved organic carbon in soil pore waters may enhance Mn release in the swales; therefore, vegetation may first decrease then increase rates of Mn removal from soils over the long-term. Unlike the major rock-derived elements which exhibit chemostatic behavior, Mn concentrations in the stream vary widely over a large range of stream discharge rates. High Mn fluxes in the stream occur in short pulses that only weakly respond to precipitation events, suggesting that dissolved Mn loads in rivers are not solely driven by the hydrology but are rather strongly impacted by processes in the soil and stream sediments. Current area-normalized release rates of Mn from soils in the Shale Hills watershed are consistent with rates estimated for the SRB; however, the Susquehanna River experienced a decline in dissolved Mn concentrations from the 1950s to the present. We propose that higher inputs of Mn to the Susquehanna River in the past reflect rapid leaching of Mn contaminants that declined as atmospheric inputs of Mn decreased.
Herndon, E., Brantley, S.L. (2012): Factors impacting manganese transport from soils into rivers using data from Shale Hills CZO . AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.