Spatial relationships of aluminum chemistry in the streams of the Hubbard Brook Experimental Forest, New Hampshire

Aluminum chemistry was evaluated in two headwater streams in the White Mountains of New Hampshire. Observed elevational trends in stream aluminum chemistry may be related to spatial variations of vegetation type and mineral soil depth within the watersheds. At the highest elevations maximum densities of spruce and fir vegetation occur and aluminum appears to be mobilized predominantly by transformations involving dissolved organic matter. At the mid-elevations hardwood vegetation predominates and the mechanism of aluminum mobilization shifts to dissolution by strong acids within the mineral soil. At the lowest elevations, relatively thick mineral soil seems to limit aluminum mobility, resulting in low concentrations in streamwater. Comparison of these results with an earlier study of an adjacent watershed, indicates that subtle differences in watershed characteristics such as tree species distribution and topography may cause significant variations in stream aluminum chemistry. Control of aluminum mobility by imogolite minerals was not indicated by the stream chemistry of these watersheds. To determine the relationship between acidic deposition and aluminum mobility, natural variations which occur in the aluminum cycle must be addressed.