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Long-term response of surface water acid neutralizing capacity in a central Appalachian (USA) river basin to declining acid deposition

K. M. Kline,K. N. Eshleman,James E. Garlitz,Sarah H. U'Ren

Published 2016 in Atmospheric Environment

ABSTRACT

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 km 2 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 SO 4 2− (∼1.5 μeq L −1  yr −1 ) and NO 3 − (∼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 + , Mg 2+ , and Ca 2+ 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 NO 3 − 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).

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