Environ. Sci. Technol. 2007, 41, 5601-5607
Analysis of Trends in Episodic Acidification of Streams in Western Maryland KATHLEEN M. KLINE,* KEITH N. ESHLEMAN, RAYMOND P. MORGAN II, AND NANCY M. CASTRO University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, Maryland 21532
In this study we report on changes in the magnitude and mechanisms of episodic acidification of a small acidsensitive stream in western Maryland (U.S.) during the 1990s, a period in which wet sulfate deposition declined by 10-25% due to implementation of the Clean Air Act Amendments (CAAA) of 1990. We observed a relatively minor trend in the magnitude of episodic acidification over this period, as measured by transient changes in acid neutralizing capacity (∆ANC) and minimum values of ANC (ANCmin) during 22 events sampled prior to and following CAAA implementation. Any relationship to changes in atmospheric deposition appears to be confounded by large hydroclimatological variability between the two sampling periods. Nonetheless, results obtained prior to implementation of the CAAA indicated that the mechanism of episodic acidification was mostly attributable to flushing of accumulated sulfate from the watershed, whereas results obtained postCAAA indicated domination by base cation dilution. This shift in the mechanism of episodic acidification is qualitatively consistent with hydrochemical theory, as well as with empirical results from surface waters in other regions where dramatic declines in sulfate deposition have taken place.
Introduction The Clean Air Act Amendments (CAAA) of 1990 have contributed to reduced emissions of SO2 and NOx from stationary electricity-generation sources in the United States, resulting in reductions in wet deposition of acidifying pollutants throughout much of the country. By the late 1990s, some of the largest (10-25%) reductions in wet sulfate (SO42-) deposition were observed to have occurred in the midAppalachian and northeastern regions of the U.S (1).; more recent analyses suggest reductions of 24-32% in this region as of 2004 (2). Reductions in SO42- deposition have produced significant declines in surface water SO42- concentrations in most regions of the U.S., with reported rates of change in the range of 1-3 µeq L-1 yr-1 (3-9). Fewer experimental studies in the U.S. or elsewhere (6-9) have documented significant increases in surface water alkalinity (or acid neutralizing capacity, ANC)sa primary indicator of acid-base statuss thus suggesting that recovery of aquatic ecosystems from a long history of acid deposition may be appreciably delayed. The majority of experimental studies of aquatic recovery due to reductions in deposition have focused on long-term * Corresponding author phone: 301-689-7122; fax: 301-6897200; e-mail:
[email protected]. 10.1021/es070424u CCC: $37.00 Published on Web 07/07/2007
2007 American Chemical Society
chronic acidification (3-9). With the exception of research in Sweden documenting significant water quality improvements during spring flood (10), little experimental research has focused on the impact of decreasing deposition on episodic acidificationsa process that has been shown to have deleterious effects on aquatic biota (11). Episodic acidification, defined as the transient loss of ANC during a hydrologic event, occurs both naturally and through additions of anthropogenically derived mineral acids during the runoff process (12). Natural mechanisms that cause ANC loss include (1) production/release of organic acids; (2) nitrification/ release of nitric acid; (3) sea salt addition/production of acidity by cation exchange; and (4) hydrologic dilution of base cations (12-13). Transient increases in concentrations of strong acids (e.g., sulfuric acid) in surface waters are commonly attributed to anthropogenic sources of acidity and can also contribute to ANC loss (14-16). Evaluating the recovery of a surface water system from episodic acidification requires a considerable record of longterm, high-frequency monitoring of transient high flow events that is difficult and expensive to obtain. We have been able to develop such a record for a small, acid-sensitive, headwater stream (Upper Big Run) located in the Appalachian Plateau region of western Maryland (U.S.). Data from two separate projects were collected: sampling for the first project occurred during 1991 and 1992, whereas the duration of the second project was from 1996 through 1998. The resultant data from these projects provide a unique opportunity to investigate the potential impact of decreasing deposition on episodic acidification. We assessed whether there have been any changes in episodic acidification at this site from the early 1990s to the late 1990s and compared the mechanisms of ANC loss between the two study periods; we also used data from two other low-order streams in the region with varying sensitivity to acidification to interpret patterns observed in Upper Big Run.
Materials and Methods Site Description. Upper Big Run (BIGR) is located on the Appalachian Plateau in western Maryland within the boundaries of Savage River State Forest (Figure 1). BIGR drains a 162 ha, forested watershed and exhibits chemical characteristics typical of an acid sensitive stream; the mean discharge-weighted ANC during the first project (1991-1992) was estimated as 22 µeq L-1, but this value increased to 33 µeq L-1 during the second project (1996-98). ANC has continued to rise to a value of 42 µeq L-1 in 2005 (9). Two other streams were sampled (during the second episodic project only): (1) Black Lick Run (BLAC) and (2) an unnamed tributary to Herrington Run (HRTB). BLAC drains a 558 ha predominantly forested watershed located mostly within Savage River State Forest. HRTB drains a 255 ha predominantly forested watershed primarily within Garrett State Forest (Figure 1). BLAC exhibits the characteristics of a moderately well buffered stream (ANC values are typically in the range of 75-300 µeq L-1), whereas HRTB is extremely acid sensitive (baseflow ANC values typically
