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

Abstract

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 SO 4 2− and NO 3 − averaged 1,735 and 863 eq ha −1 year −1 , respectively. Data indicated that 61% of the net SO 4 2− entering the watershed was retained, suggesting soil adsorption dominates as a biogeochemical process. Although net SO 4 2− retention was observed, SO 4 2− appeared to move rapidly through NDW during large precipitation events causing stream acidification, as evidenced by significant inverse correlations between biweekly throughfall SO 4 2− flux and stream event pH and ANC. Nitrogen uptake by forest vegetation and nitrification play key roles in regulating NO 3 − export to the stream as observed by 32% retention of net inorganic nitrogen, and 96% of NH 4 + input was converted to NO 3 − in the uppermost soil horizon. Net export of base cations (Ca 2+ , Mg 2+ , 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 Ca 2+ 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.

Long-Term Annual and Seasonal Patterns of Acidic Deposition and Stream Water Quality in a Great Smoky Mountains High-Elevation Watershed

“…Soil lysimeters were also monitored in the NDW, but soil water chemistry data were not used in this study analysis. Cai et al (2010c) provides additional details on soil horizon and water chemistry characteristics.…”

“…Ions Na + and K + were measured by ion chromatography from 1993 to 1998, but Ca 2+ and Mg 2+ were not measured. Laboratory analyses met quality criteria set forth in a detailed quality assurance/quality control program (Cai et al 2010c). …”

“…Net ion fluxes as watershed retention/depletion were estimated by the difference between total deposition TF flux and stream export flux, summarized also by month. Only the SW stream data were used in this study because flow patterns and ion concentrations between NE stream and the SW stream were significantly correlated (Cai et al 2010c). …”

“…This analysis provides a temporal perspective to the ion input-output budget findings reported in Cai et al (2010c) and addresses the key regional question whether acid deposition has declined over the long-term and what changes in stream water chemistry has occurred. In addition, this study summarizes essential data for the development of a watershed model incorporating biogeochemical processes to predict future recovery from acid deposition.…”

“…Noland Divide watershed (NDW) site has been monitored for deposition and stream water chemistry since 1991. Cai et al (2010c) completed an ion input-output budget on NDW data from 1991 to 2006 finding an average net annual sulfate retention of 61%, nitrate retention of 32%, and net export of most base cations except K + . This NDW study did not report on temporal trends or seasonal patterns of net ion flux.…”