GEOLOGIC CONTROLS ON THE CHEMICAL STREAM WATER RESPONSE TO ATMOSPHERIC POLLUTION (ACID AND HG DEPOSITION) IN SHENANDOAH NATIONAL PARK

Terrestrial watersheds have the potential to mediate the response of aquatic ecosystems to atmospheric pollutants. Forested mountain watersheds in Shenandoah National Park (SHEN) have been subject to elevated levels of acid (sulfur dioxide and nitrogen oxides) as well as Hg deposition from coal burning power plants since the start of the industrial revolution. The Shenandoah Watershed Study (SWAS), first established in 1979 to study the impact of acid rain on native trout brook streams in SHEN, now monitors the response of these streams to reduced acid deposition. Additionally, a short-term study on streamwater mercury (Hg) dynamics was completed within three SWAS streams to evaluate the influence of watershed characteristics on Hg export. Initial findings from the acid monitoring program demonstrated streamwater response to acid inputs is strongly mediated by watershed bedrock. The three main bedrock types in SHEN, siliciclastic, felsic, and mafic, span a gradient in weathering rates and base cation content, rendering soils with different texture and chemical composition. Streamwater in watersheds underlain by siliciclastic bedrock (low base cation content) have a reduced acid neutralizing capacity (ANC) and lower pH whereas streams in watersheds underlain by mafic bedrock (relatively high base cation content) have relatively high ANC and circumneutral pH. A recent analysis of long-term (1987-2011) streamwater chemistry trends within SHEN and surrounding watersheds revealed the response to reduced levels of acid deposition is similarly tied to bedrock composition with declines in ANC associated with base-poor bedrock and increases in stream ANC associated with base-rich bedrock watersheds. Results from the Hg study found that bedrock also influenced the amount of Hg exported from a watershed, with sites underlain by siliciclastic bedrock exporting more Hg as compared to mafic or felsic; findings were attributed to differences in weathering rates and the resultant soil textures. These long and short-term studies within SHEN illustrate that bedrock composition, as it defines the physical and chemical soil characteristics, plays a role in the response of stream ecosystems to both acid and Hg atmospheric deposition.