Longitudinal variations in trace metal concentrations in a Northern forested ecosystem

Longitudinal variations in the concentrations of four trace metals (Fe, Pb, Mn, and Zn) in soil solutions and streamwater were investigated at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. The soils at Hubbard Brook are well-drained Spodosols. Concentrations of Pb and dissolved organic carbon (DOC) were elevated in soil solutions draining organic (Oa) horizons both in coniferous (red spruce, Picea rubens; balsam fir, Abies balsamea) and northern hardwood (yellow birch, Betula alleghaniensis; American beech, Fagus grandifolia; sugar maple, Acer saccharum) stands. In the coniferous zone, concentrations of Pb were high in streamwater, while DOC and Fe concentrations were elevated in the lower mineral soil (Bs2) solutions and streamwater. In hardwood stands, reduced DOC concentrations in Bs2 soil solutions and streamwater relative to Oa horizon concentrations coincided with lower concentrations of Fe and Pb, and apparent retention of these solutes in the mineral soil. Concentrations of Pb and Fe were highly correlated with DOC in soil solutions and streamwater suggesting that mobilization/immobilization of Pb and Fe were predominately controlled by transfer of DOC through complexation reactions. In contrast, concentrations of Mn and Zn were highest in streams and soil solutions in high-elevation hardwood stands. Elevated concentrations in deciduous vegetation and patterns of solution concentration suggest that vegetative cycling may be important in regulating transfer of Mn. Results of this study indicate that hydrologic pathways, vegetation, and soil characteristics related to hillslope position all contribute to regulate trace metal concentration in drainage waters. As a result, considerable variability in trace metal transport is evident within a relatively small catchment.