Chemical Flux During Event Stormwater Flows in the Great Smoky Mountains National Park: Comparison of Two Streams Varying by Drainage Area and Elevation

Streams in the Great Smoky Mountains National Park in North Carolina and Tennessee have been impacted by acid deposition from anthropogenic sources for decades. Recent declines in acid deposition throughout the eastern U.S. appear to be initiating improvements in stream water quality. However, watershed recovery could take decades due to regional differences in the hydrologic and biogeochemical processes that influence chemical fate and transport. These processes have been extensively studied at longer time scales (i.e. seasonally, annually), by long-term annual ion budgets, and study designs based on grab samples. Less known are the acidification effects on streams through rapid ion transport during storm events. Of particular importance is advancing our understanding of ion transport at the storm-event scale with respect to soil sulfate desorption, nitrate saturation, and base cation depletion. In this study automated samplers allowed for continuous and time dependent stream sampling during stormflow events, which collected samples used to characterize event-based flux of stormwater chemistry. Two streams, one in a small, high elevation watershed and one in a larger, low elevation watershed were selected for study because of their distinct differences in hydrology. Eight events were sampled at each site (average of 23 samples per event) and analyzed for anions, cations, dissolved metals, pH, and acid neutralizing capacity. Samples were organized into three hydrograph categories (rising limb, peak, and falling limb). Throughfall samples were also utilized for an ion event-based input and output mass comparison. On average, sulfate desorption was not observed at either site because concentrations were not significantly different between hydrograph rising and falling limbs. Nitrate saturation and base cation depletion appears to govern the storm-based acidification response because of increased concentrations of nitrate and decreasing concentrations of base cations during events at the low elevation site. Although acid deposition has declined, stream acidification was still observed with episodic drops in pH and ANC governed by the lack of base cations to buffer acid anions from inorganic nitrogen and possibly organic acids. Further study is needed on the influence of organic acids in the response to acidification in headwater streams.