Long-term effects of acidic deposition on water quality in a high-elevation great smoky mountains national park watershed: Use of an ion input-output budget

Impacts from acidic deposition on stream water quality in the Great Smoky Mountains National Park (GRSM) have long been reported; however, a better understanding of the biogeochemical processes that regulate stream acidification is needed for resource management. Water quality monitoring of Noland Divide Watershed (NDW), a high-elevation watershed in the GRSM, was used to generate an ion input-output budget in order to evaluate what processes have influenced stream pH and acid neutralizing capacity (ANC) over the long term. NDW was equipped with wet deposition, throughfall, soil lysimeters, and stream collection stations, and monitoring began in 1991 and continues to the present. Using data from 1991 to 2006, this study found annual deposition fluxes of SO₄ ^2- and NO₃ ^- averaged 1,735 and 863 eq ha^-1 year^-1, respectively. Data indicated that 61% of the net SO₄ ^2- entering the watershed was retained, suggesting soil adsorption dominates as a biogeochemical process. Although net SO₄ ^2- retention was observed, SO₄ ^2- appeared to move rapidly through NDW during large precipitation events causing stream acidification, as evidenced by significant inverse correlations between biweekly throughfall SO₄ ^2- flux and stream event pH and ANC. Nitrogen uptake by forest vegetation and nitrification play key roles in regulating NO₃ ^- export to the stream as observed by 32% retention of net inorganic nitrogen, and 96% of NH₄ ⁺ input was converted to NO₃ ^- in the uppermost soil horizon. Net export of base cations (Ca²⁺, Mg²⁺, Na⁺) is observed and apparently moderates stream acidification. In contrast, 71% of net K⁺ input was retained, which is likely due to forest vegetation uptake. Net export of Ca²⁺ was 867 eq ha ^-1 year^-1 compared to net throughfall of 790 eq ha ^-1 year^-1. Long-term cation depletion from the NDW soils could limit recovery potential in stream water quality. Findings from this NDW study suggest that future stream acidification conditions in high-elevation GRSM watersheds are dependent on interrelated biogeochemical processes and precipitation patterns, illustrating the need to better understanding potential impacts of climate variability on stream water quality.