Characterizing Episodic Stream Acidity During Stormflow in the Great Smoky Mountains National Park

Episodic stream acidification occurs as storm events temporarily reduce acid neutralizing capacity (ANC) and pH. Stream acidification is suspected to have damaging effects on the health of aquatic ecosystems and biota. The objectives of this research are to 1) characterize stream baseflow and stormflow chemistries in three watersheds in the Great Smoky Mountains National Park (GRSM), 2) understand potential mechanisms responsible for episodic acidification, and 3) understand the relationship between storm event magnitude, antecedent soil moisture condition, and the stream’s pH response. Three remote, forested, high-elevation streams (Middle Prong, Ramsey Prong, and Eagle Rocks Prong) were selected in the Middle Prong of the Little Pigeon River Watershed. Multi-parameter data sondes were installed at each site to record continuous stream data. Autosamplers were set up in connection with the sondes to collect samples during storm events. Stormflow, baseflow, and precipitation samples were analyzed for pH, ANC, and a broad spectrum of cations and anions that contribute to the ion balance.
During stormflow, ANC and pH depressions were observed for all storms at each study site. Sulfate, nitrate, and organic acid concentrations increased during each storm. Base cation concentrations generally increased during stormflow at Middle and Ramsey Prongs, but diluted occasionally on Eagle Rocks Prong. The relative changes in ion concentrations were used to determine which ions (acids) were most responsible for ANC depression. ANC contribution analysis indicates acid deposition may be the primary cause of episodic acidification, but it appears organic acids and cation dilution may also contribute. Pyritic geology is also suspected to contribute to baseflow and stormflow acidity in the Eagle Rocks Prong. Data exploration indicates large storms preceded by long, dry periods cause the largest pH depressions. It appears stream acidification may be driven by acid deposition, but additional inputs from varying vegetation and geology create unique and complex response to the observed stream acidification.