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Long-term response of surface water acid neutralizing capacity in a central Appalachian (USA) river basin to declining acid deposition
Abstract Long-term changes in acid-base chemistry resulting from declining regional acid deposition were examined using data from repeating synoptic surveys conducted within the 275 km2 Upper Savage River Watershed (USRW) in western Maryland (USA); a randomly-selected set of 40 stream reaches was sampled 36 times between 1999 and 2014 to: (1) repeatedly characterize the acid-base status of the entire river basin; (2) determine whether an extensive network of streams of varying order has shown signs of recovery in acid neutralizing capacity (ANC); and (3) understand the key factors controlling the rate of ANC recovery across the river network. Several non-parametric analyses of trends (i.e., Mann Kendall Trend: MKT tests; and Regional Kendall Trend: RKT) in streamwater acid-base chemistry suggest that USRW has significantly responded to declining acid deposition during the study period; the two most robust, statistically significant trends were decreasing surface water SO4 2− (∼1.5 μeq L−1 yr−1) and NO3 − (∼1 μeq L−1 yr−1) concentrations—consistent with observed downward trends in regional wet S and N deposition. Basin-wide decreasing trends in K+, Mg2+, and Ca2+ were also observed, while Na+ concentrations increased. Significant ANC recovery was observed in 10–20% of USRW stream reaches (depending on the p level used), but the magnitude of the trend relative to natural variability was apparently insufficient to allow detection of a basin-wide ANC trend using the RKT test. Watershed factors, such as forest disturbances and increased application of road deicing salts, appeared to contribute to substantial variability in concentrations of NO3 − and Na+ in streams across the basin, but these factors did not affect our overall interpretation of the results as a systematic recovery of USRW from regional acidification. Methodologically, RKT appears to be a robust method for identifying basin-wide trends using synoptic data, but MKT results for individual systems should be examined closely (e.g., to identify trends for specific subpopulations).
Highlights Data from multiple synoptic spring baseflow surveys were used to characterize the acid-base status of USRW streams. Decreasing trends in surface water SO4 2− and NO3 − were explained by declining S and N emissions and deposition onto USRW. We observed significant ANC recovery in 10–20% of the USRW stream network, despite not detecting a regional ANC trend. Surface water ANC recovery was significantly moderated by decreasing trends in K+, Mg2+, and Ca2+ concentrations. RKT appears to be a robust method for identifying basin-wide trends for some key chemical constituents.
Long-term response of surface water acid neutralizing capacity in a central Appalachian (USA) river basin to declining acid deposition
Abstract Long-term changes in acid-base chemistry resulting from declining regional acid deposition were examined using data from repeating synoptic surveys conducted within the 275 km2 Upper Savage River Watershed (USRW) in western Maryland (USA); a randomly-selected set of 40 stream reaches was sampled 36 times between 1999 and 2014 to: (1) repeatedly characterize the acid-base status of the entire river basin; (2) determine whether an extensive network of streams of varying order has shown signs of recovery in acid neutralizing capacity (ANC); and (3) understand the key factors controlling the rate of ANC recovery across the river network. Several non-parametric analyses of trends (i.e., Mann Kendall Trend: MKT tests; and Regional Kendall Trend: RKT) in streamwater acid-base chemistry suggest that USRW has significantly responded to declining acid deposition during the study period; the two most robust, statistically significant trends were decreasing surface water SO4 2− (∼1.5 μeq L−1 yr−1) and NO3 − (∼1 μeq L−1 yr−1) concentrations—consistent with observed downward trends in regional wet S and N deposition. Basin-wide decreasing trends in K+, Mg2+, and Ca2+ were also observed, while Na+ concentrations increased. Significant ANC recovery was observed in 10–20% of USRW stream reaches (depending on the p level used), but the magnitude of the trend relative to natural variability was apparently insufficient to allow detection of a basin-wide ANC trend using the RKT test. Watershed factors, such as forest disturbances and increased application of road deicing salts, appeared to contribute to substantial variability in concentrations of NO3 − and Na+ in streams across the basin, but these factors did not affect our overall interpretation of the results as a systematic recovery of USRW from regional acidification. Methodologically, RKT appears to be a robust method for identifying basin-wide trends using synoptic data, but MKT results for individual systems should be examined closely (e.g., to identify trends for specific subpopulations).
Highlights Data from multiple synoptic spring baseflow surveys were used to characterize the acid-base status of USRW streams. Decreasing trends in surface water SO4 2− and NO3 − were explained by declining S and N emissions and deposition onto USRW. We observed significant ANC recovery in 10–20% of the USRW stream network, despite not detecting a regional ANC trend. Surface water ANC recovery was significantly moderated by decreasing trends in K+, Mg2+, and Ca2+ concentrations. RKT appears to be a robust method for identifying basin-wide trends for some key chemical constituents.
Long-term response of surface water acid neutralizing capacity in a central Appalachian (USA) river basin to declining acid deposition
Kline, Kathleen M. (author) / Eshleman, Keith N. (author) / Garlitz, James E. (author) / U'Ren, Sarah H. (author)
Atmospheric Environment ; 146 ; 195-205
2016-07-12
11 pages
Article (Journal)
Electronic Resource
English
Acid deposition , Surface water recovery , Sulfate , Nitrate , CAAA , U.S. Clean Air Act Amendments of 1990 , SO<inf>2</inf> , sulfur dioxide , NO<inf>x</inf> , nitrogen oxide , μeq , microequivalents , ANC , acid neutralizing capacity, μeq L<sup>−1</sup> , DOA , dissolved organic acids, μeq L<sup>−1</sup> , DOC , dissolved organic carbon, mg L<sup>−1</sup> , SBC , sum of base cations, (Na<sup>+</sup> + K<sup>+</sup> + Mg<sup>2+</sup> + Ca<sup>2+</sup>), μeq L<sup>−1</sup> , MKT , Mann-Kendall Trend Test , RKT , Regional Kendall Trend Test , USRW , Upper Savage River Watershed , NADP , National Atmospheric Deposition Program , DI , Disturbance Index
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