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Kline, K.M., K.N.Eshleman, R.P. Morgan II, and N. M. Castro. 2007. Analysis of Trends in Episodic Acidification of Streams in Western Maryland. Environmental Science and Technology 41(16):5601-5607. In a Nutshell: Problem - Reductions in emissions of acid rain precursors from power plants in the 1990's have produced significant declines in surface water sulfate concentrations. Few studies have documented significant increases in surface water alkalinity (acid neutralizing capacity, or ANC) which suggests that recovery of aquatic ecosystems from a long history of acid deposition may be appreciably delayed. Additionally, little research has focused on the impact of decreasing deposition on episodic acidification, which is defined as the transient loss of ANC during a hydrologic event. What - Using an extensive record of long-term, high frequency water quality monitoring from a small, forested watershed in western Maryland, Upper Big Run, we assessed whether there have been any changes in episodic acidification at this site from the early 1990s to the late 1990s. We also compared the mechanisms of ANC loss between the two study periods. Findings - Results suggest that streams in western Maryland are episodically acidified by different mechanisms and that the dominant mechanism and magnitude of episodic acidification have shifted over time as deposition declined and as recovery progressed. At Upper Big Run, we observed a shift from episodes dominated by sulfuric acid flushing, an anthropogenic source of acidification, to episodes dominated by base cation dilution, a natural source of acidification. Although significant recovery in annual average ANC and sulfate concentrations have been observed at Upper Big Run, our results suggest that the acute effects of elevated inorganic aluminum and depressed pH on biota that can occur during episodes must still be addressed. Many watersheds could still be at risk even after the reductions in emissions mandated by the Clean Air Act Amendments of 1990. |
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