Hydropedologic Influence On Soil Manganese Distribution, Form and Reactivity Near Seeps in a Forested Catchment.

Groundwater seeps can supply persistent and enriched flow to upland catchments dominated by shallow flowpaths.  In watershed 3 of the Hubbard Brook Experimental Forest, we have found highly elevated concentrations of Mn in soils near perennial seeps.  This redox active element appears to be accumulating along surface and subsurface flow paths.  In addition to fine scale mapping of Mn distribution in a 0.5 ha around these seeps, we have studied the Mn oxides by both traditional and synchrotron-based x-ray diffraction in an attempt to determine mineral phases, x-ray absorption near-edge structure to determine oxidation state, and the Cr-oxidation test to determine reactivity.  Preliminary results show two distinct areas of Mn accumulation.  One appears to be primarily from subsurface flow with the Mn having low reactivity (i.e. low Cr-oxidation test) and likely in mixed oxidation-state minerals The other is found in and around surface discharge and the Mn appears highly oxidized and reactive.  In fact, the highest Cr-oxidation test was found in a sample taken from under flowing water.  These differences highlight the effect of different hydrologic flowpaths on soil chemical development and resulting reactivity.