Sensitivity of stream basins in Shenandoah National Park to acid deposition

Six synoptic surveys of 56 streams that drain the Shenandoah National Park, Virginia, were conducted in cooperation with the University of Virginia to evaluate sensitivity of dilute headwater streams to acid deposition and to determine the degree of acidification of drainage basins.Stream samples were collected from August 1981 through June 1982; each sample was analyzed for alkalinity, major anions and cations, silica, and pH.Flow-weighted alkalinity concentration of most streams is below 200 microequivalents per liter, which is commonly considered the threshold of sensitivity.Stream-water sensitivity is strongly affected by drainage basin bedrock type.Streams draining the resistant siliceous bedrocks show extreme sensitivity (alkalinity below 20 microequivalents per liter); streams draining granite and granodiorite show a high degree of sensitivity (20 to 100 microequivalents per liter); and streams draining the metamorphosed volcanics show moderate to marginal sensitivity (101 to 200 microequivalents per liter).The strong relation between bedrock type and stream-water chemistry in the Park is evaluated statistically by multiple-regression analysis.This technique indicates that concentrations of alkalinity, silica, and base cations are strongly related to bedrock type, and that sulfate concentration is strongly related to geographic location.The regression equation for alkalinity is shown to be a useful tool for predicting sensitivity of unsampled streams within the Park and for streams in areas with similar geology outside the Park.Predicted values are generally within 30 microequivalents per liter of the measured value.A comparison of current stream-water chemistry to that predicted by a model based on carbonic-acid weathering reactions suggests that all basins in the Park show signs of acidification by atmospheric deposition.Acidification is defined as a neutralization of stream-water alkalinity and/or an increase in the base cation weathering rate.These processes cannot ~>e distinguished with the available data, but both are detrimental to stream basins in the Park.Acidification averages 50 microequivalents per liter, which is fairly evenly distributed in the Park.However, the effects of acidification are most strongly felt in extremely sensitive basins, such as those underlain by the Antietam Formation, which have stream-water pH values averaging 4.99 and a mineral acidity of 7 microequivalents per liter.Acidification of basins in the other geologic formations also may be significant, but higher "preacidification" concentrations of stream-water alkalinity and base cations make it less apparent. Purpose and ScopeThe Appalachian Mountains are identified on national maps as potentially sensitive to acid deposition (Galloway and Cowling, 1978;Omernik and Powers, 1982).The purpose of this report is to describe the stream-water quality of a region of the Appalachian Mountains, Shenandoah National Park, Virginia, to determine more precisely the sensitivity of this area and to estimate how this area is responding to the current input of acid deposition.Specifically, the objectives are to: 830 39° 76° -y\ 7Y.......