QUANTITATIVE XRD ANALYSIS OF SPODOSOLS AS A MEANS OF ASSESSING RESPONSE TO ACID DEPOSITION

Given that the mineral content of soils is an important factor in determining soil acidification potential, accurate soil mineral quantification is essential to model the potential effects of acid precipitation. A model was developed by Ecosystems Research Group (ERG) of Norwich, Vermont, to predict mineralogical content of soils based upon the redistribution of bedrock by glacial erosion and till deposition. The model characterizes this deposition in fan-shaped plumes directed downgradient from bedrock sources. It is based on inferred glacial migration and has been used to develop predictions for soil acidification potential using source rock and assumed weathering rates to infer the buffering capacity of Green Mountain soils. In order to assess the accuracy of this model, X-ray diffraction (XRD) using reference intensity ratios was applied to quantifying mineral proportions downgradient from a point source of a distinctive mineral. Test sites were located in a 60^o fan originating from the kaolinite-rich Brandon Residual Formation.

The presence of kaolinite in trace amounts (<1%) in bulk powders downgradient from the source area (but not present outside the plume) indicates that the model accurately predicts bedrock redistribution. Kaolinite is easily identifiable in the <2-micron size fraction of the soils and it appears to be a good tracer mineral because (1) it is relatively stable in this weathering environment, and (2) does not appear to be forming in these soils via weathering. The lack of carbonate minerals (to a depth of 1 m) in spodosols proximal to the carbonate-rich Champlain Valley indicates carbonates play little or no role in buffering current acid deposition. The main weathering product in the spodosols of the study area is a mixed-layer dioctahedral mica/smectite. The presence of peaks (ethylene glycol-solvated [EG]) at 27 Å and 13.6 Å, that collapse to 10 Å when heated (300C, 1h) and an 060 at 1.50 Å, suggest R1-ordered illite/smectite (I/S). Peaks at 24 Å and 12 Å (EG) indicate coexisting hydrobiotite (R1 vermiculite/biotite), and a 17 Å peak in some specimens indicates discrete smectite. These phases appear to form via weathering of chlorite and biotite. Lacking carbonate, reactions such as chlorite to I/S and biotite to hydrobiotite appear to determine the buffering capacity of these soils.