Acid-Base Status of Western Maryland Streams

The Clean Air Act Amendments (CAAA) of 1990 contributed to reduced emissions of sulfur dioxide and nitrogen oxides 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 1990's, some of the largest reductions in sulfate deposition were observed to have occurred in the mid-Appalachian and northeastern regions of the U.S. Reductions in sulfate deposition have produced significant declines in surface water concentrations of sulfate in many areas of the U.S. Fewer experimental studies, though, have documented significant increases in surface water alkalinity (or acid neutralizing capacity, ANC), a primary indicator of acid-base status, in response to these reductions in acid deposition.

Since the early 1990's, AL researchers have been studying the impact of both chronic acidification and episodic acidification on western Maryland streams, an area where many streams have been impacted by acid deposition. Research has focused on answering the following questions:

  • What are the trends in acid-base conditions at acid-sensitive streams in western Maryland?
  • Have there been any changes in episodic acidification from the early 1990's to the late 1990's?
  • Have the mechanisms for episodic acidification change from the early 1990's to the late 1990's?

Locations of long-term sampling sites in western Maryland.AL researchers began monitoring acid-base chemistry at Upper Big Run (BIGR) in late 1989. In 1995, two additional monitoring sites were added: Black Lick (BLAC) and an unnamed tributary to Herrington Run (HRTB). BIGR and BLAC watersheds are both located primarily in Savage River State Forest, while HRTB watershed is located mostly in the Garrett State Forest. Discrete samples for analysis of acid-base chemistry have been collected at these sites at varying frequencies (anywhere from semi-hourly to monthly) through 2006.

Streamwater samples were analyzed for an extensive suite of analytes including: pH, specific conductance, ANC, acid anions, base cations, and dissolved organic carbon (DOC).

Computed annual discharge-weighted concentrations of sulfate, nitrate, ANC, and base cations using LOADEST and WATFLOW.To assess long-term trends in acid-base chemistry at BIGR and BLAC, two load estimation methods (WATFLOW and LOADEST) were used to calculate discharge-weighted ion concentrations. Then, trends in discharge-weighted concentrations and fluxes were analyzed for statistical significance using a t-test and the non-parametric seasonal Kendall tau (SKT) test. Results identified significant decreasing trends at BIGR for sulfate, nitrate, and base cations that are comparable to the literature for other streams in this region (see figure at right). An increase in ANC at BIGR was also observed at the rate of about 1.5 µeq/L yr indicating some recovery from acid deposition. Results suggest, though, that this recovery may be tapering off.

Given that the trends in ANC and other constituent concentrations at BIGR appear to be highly representative of median trends identified for acid-sensitive streams in the Northern Appalachians, our data may provide new evidence that recovery of streams from acidification in this region due to implementation of the CAAA has largely ceased. Continued monitoring at gaged watersheds will indicate whether these trends continue.

AL researchers have also been studying stream response to episodic acidification, or temporary decreases in ANC and pH occur during high stream flow events. Studying episodic acidification requires a considerable record of long-term, high-frequency monitoring of transient high flow events. Using such data from BIGR during the early 1990's and the mid- to late-1990's, scientists here have attempted to assess whether there have been any changes in episodic acidification at this site during this time period. They also compared the mechanisms of ANC loss between the two study periods.

Episodic acidification occurs both naturally and through additions of anthropogenically derived mineral acids ruing the runoff process. Natural mechanisms that cause ANC loss include: production/release of organic acids, nitrification, sea salt addition, and hydrologic dilution of base cations. Acidification by strong acids (e.g., sulfuric acid) is commonly attributed to anthropogenic sources of acidity from acid deposition.

Median relative proportional ion contribution ratios to ANC depression at BIGR, BLAC and HRTB. Median relative ranks of ion changes to ANC depression are shown in parentheses. Results from an analysis of transient depressions in ANC at BIGR suggest that streams in this region 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 (see figure at left).

In the early part of the 1990's, the dominant source of episodic acidification to BIGR was sulfuric acid flushing, a mechanism commonly considered to be a result of acid deposition. In the latter part of the 1990's, sulfuric acid flushing was the least common mechanism while more natural mechanisms like base cation dilution were the dominant source of episodic acidification.

The results from this project are important from the standpoint of biological recovery. Although significant recovery in annual average ANC and sulfate concentrations at BIGR have been observed through 2005, it is apparent that the acute effects of elevated inorganic aluminum and depressed pH on biota that can occur during episodes must still be addressed. Although the mechanism for episodic acidification may have shifted from anthropogenic sources to natural sources, many watersheds could still be at risk even after the reductions in emissions mandated by the CAAA have been achieved.

To Learn More:

  • Kline, K.M.; Eshleman, K.N.; Morgan, R.P., II; Castro, N.M.; Negley, T.L. Long-term changes in the acid-base status of western Maryland streams. EOS Trans. A.G.U. 2006, 87 (52), (Fall Meet. Suppl., abstract), H23B-1488. (download pdf)
  • 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. (nutshell page)
  • Eshleman, K.N.; K.M. Kline; R.P. Morgan II, N.M. Castro, and T.L. Negley. In prep. Contemporary trends in the acid-base status of two acid-sensitive streams in western Maryland, U.S.

Project PI's:

Dr. Keith Eshleman
Dr. Raymond P. Morgan II