* Rice, K C kcrice@usgs.gov
,USGS, P.O. Box B, Charlottesville, VA 22903
United States
Hornberger, G M, University of Virginia, Department of Environmental Sciences, Charlottesville, VA 22903
United States
Webb, J R, University of Virginia, Department of Environmental Sciences, Charlottesville, VA 22903
United States
Paine Run flows through a 12.4 km2 forested catchment located on the western flank of the Blue Ridge Mountains at the southern end of Shenandoah National Park. The catchment receives highly acidic precipitation that had an annual volume-weighted average pH of 4.4 during 1998. The catchment is underlain by Cambrian-age siliciclastic rocks that provide little neutralization of the acidic input; thus, streamwater has poor acid-neutralizing capacity (ANC). Discharge and ANC data have been collected at Paine Run at least weekly, and hourly during some storms, from 1992 to the present by the Shenandoah Watershed Acidification Study of the University of Virginia. The data indicate that the stream is subject to episodic acidification--a temporary decrease in stream ANC and pH during stormflow, with a return to normal ANC and pH as the stream returns to baseflow--which has been documented to have deleterious effects on aquatic biota.
ANC in weekly streamwater samples is inversely related to stream discharge, and the data show a typical dilution response throughout all seasons. Some scatter is observed in the points on a concentration-discharge (c-Q) curve, however, indicating that simple dilution does not account for the full range of variability. The variation of ANC with discharge over storm events suggests a dynamic mixing response: the c-Q curves for individual storms exhibit hysteresis (a "looped" pattern). For ANC at Paine Run, loops are clockwise for moderately large storms, falling into the Evans and Davies C1 or C3 classes. When we assume a 3-component mixing model (surface water, soil water, and ground water) with constant-concentration end members, we conclude soil water is the most acidic component and that flushing of soil water during large storms is responsible for the episodic ANC depression. This is consistent with analyses for streams in New York and in Pennsylvania. For at least one small summer event at Paine Run, however, the c-Q curve is anticlockwise, suggesting that the "surface-water" component is the acidic end member. We hypothesize that the hydrograph peak for such smaller events is dominated by direct throughfall on the stream surface.
Rice, K.C., Hornberger, G.M., and Webb, R.J., 2000, Episodic variation of acid-neutralizing capacity in streamwater at Paine Run, Shenandoah National Park, Virginia [abs.], American Geophysical Union 2000 Spring Meeting, Washington, D.C.: published as a supplement to EOS Transactions, May 9, 2000, p. S259.
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