VULNERABILITY OF STREAMS TO ACID RAIN IN SHENANDOAH NATIONAL PARK, VIRGINIA

Acidic deposition is a serious environmental problem affecting Shenandoah National Park, Virginia, the third most contaminated park in the National Park System because of the deposition of acid rain. Acid rain affects streams by temporarily reducing the acid-neutralizing capacity (ANC). The increased acidity of streamwater can have deleterious effects on aquatic biota. Although acidic deposition to the park is relatively uniform across its land area, the response of streamwater during rain events varies substantially, a function of the underlying geology and topography of the watersheds.

Geologic and topographic data for the park's 231 watersheds are readily available; however, long-term measurements of streamwater ANC and accompanying discharge are not and would be prohibitively expensive to collect. Modeled predictions of the vulnerability of streams to episodic acidification are an alternative to long-term water-quality monitoring.

We modeled the magnitude, frequency, and duration of ANC decreases by developing transfer function time series models to predict hourly ANC from discharge for 5 watersheds that have long-term records of ANC and discharge. Hourly ANC predictions over short time periods were averaged and distributions of the recurrence intervals of annual minimum ANC were modeled for periods of 6, 24, 72, and 168 hrs. The distributions were extrapolated to the 231 watersheds based on geology and topography. Quantitative assessments of watershed vulnerability can be made, which provides better information to decision makers than qualitative assessments that can be made from geology alone.

The results indicate: 1) large numbers of streams have 6- to 168-hr. periods of low ANC, which may stress aquatic species; 2) smaller watersheds are more vulnerable to ANC decreases than larger watersheds on the same bedrock; 3) watersheds with similar topography and area are more vulnerable if underlain by less basaltic bedrock; 4) 14% of the watersheds are predicted to have 72-hr. periods of average ANC < 0 microequivalents/liter at least once every 2 yrs. At this frequency, these watersheds may, with a probability > 90%, have 4 continuous yrs. of these conditions at least once in the next 40-100 yrs.