Environ. Sci. Technol. 2008, 42, 56–61
Contemporary Trends in the Acid–Base Status of Two Acid-Sensitive Streams in Western Maryland KEITH N. ESHLEMAN,* KATHLEEN M. KLINE, RAYMOND P. MORGAN II, NANCY M. CASTRO, AND TIMOTHY L. NEGLEY† University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, Maryland 21532
Received May 21, 2007. Revised manuscript received September 21, 2007. Accepted October 22, 2007.
Recovery of streamwater acid neutralizing capacity (ANC) resulting from declines in regional acid deposition was examined using contemporary (1990–2005) data from two long-term monitoring stations located on the Appalachian Plateau in western Maryland, U.S. Two computational methods were used to estimate daily, monthly, and annual fluxes and dischargeweighted concentrations of ANC, sulfate, nitrate, and base cations over the period of record, and two statistical methods were used to evaluate long-term trends in fluxes and concentrations. The methods used to estimate concentrations, as well as the statistical techniques, produced very similar results, underlining the robustness of the identified trends. We found clear evidence that streamwater sulfate concentrations have declined at an average rate of about 3 µeq L-1 yr-1 at the two sites due to a 34% reduction in wet atmospheric sulfur deposition. Trends in nitrate concentrations appear to be related to other watershed factors, especially forest disturbance. The best evidence of recovery is based on a doubling of ANC (from 21 to 42 µeq L-1) at the more acid-sensitive site over the 16year period. A slowing, or possible reversal, in the sulfate, nitrate, and SBC trends is evident in our data and may portend a decline in the rate of—or end to—further recovery.
Introduction Atmospheric deposition of acidifying pollutants (i.e., acid deposition) originating primarily from fossil fuel combustion contributed to acidification of surface waters in many regions of Europe and North America during the 20th century. Using data from systematic surveys of the acid neutralizing capacity (ANC) of surface waters, the USEPA estimated that hundreds of lakes and thousands of kilometers of streams in the eastern United States have been chemically impaired by acid deposition onto their watersheds (1–3). Since implementation of phase I of the Clean Air Act Amendments (CAAA), signed into law in 1990, sulfate deposition (and sulfate concentrations in precipitation) has declined measurably throughout much of this region (4, 5). * Corresponding author phone: (301) 689-7170; Fax: (301) 6897200; e-mail:
[email protected]. † Present address: ARCADIS of New York, Inc., 6723 Towpath Road, Syracuse, New York 13214. 56
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Many published statistical analyses of surface water data collected within this region and elsewhere in North America and Europe have attempted to determine whether declining deposition has contributed to significant rates of chemical recovery. Owing to differences in available data that could be subjected to analysis, the analytical techniques that were employed vary widely and the results of these trend analyses are mixed. While most of the studies show that local and regional surface water sulfate concentrations have decreased due to declining deposition (6–13), fewer analyses indicate that surface water ANC (or alkalinity) has increased significantly since 1990 (9, 10, 14). Several explanations for this outcome have been suggested, including mobilization of stored sulfate from soils (9, 10), mobilization of nitrate from soil storage (7, 15), decreasing surface water base cation concentrations (6, 11), decreasing atmospheric deposition of base cations (16, 17), and hydroclimatological variability (18, 19). Trend analyses of acid–base recovery have been based almost exclusively on raw concentrations of key constituents measured in individual surface water samples at fixed intervals. While this technique is clearly appropriate for ungaged watersheds, we propose that the use of alternative techniques appropriate for gaged watersheds may be able to better constrain natural variability—the bane of temporal trend analysis—and improve the interpretability of identified trends. Analysis of trends in water quality constituent concentrations at different flow levels offers one possible way of addressing this problem (13). We suggest that an alternative approach that is less reliant on high-frequency sampling during stormflow periods is an analysis of trends in discharge-weighted concentrations based on flux estimates. This approach has the added advantage of overcoming requirements for fixed sampling intervals that are needed when working with raw concentration data. The mountainous mid-Appalachian highlands have historically received some of the highest acid deposition loadings in the eastern United States (20). In the late 1980s, the USEPA estimated that 1–5% of streams in this region were chronically acidic (ANC < 0) owing to acid deposition (2, 3); much higher percentages of streams in this area were considered episodically acidified during major stormflow events (21). As in other regions of the eastern United States, annual wet deposition (and concentrations) of hydrogen ion and sulfate in this region declined significantly during the 1990s, while atmospheric nitrate deposition (and concentrations) has reportedly remained relatively unchanged (4). In this paper we present results from a statistical analysis of contemporary trends in acid–base conditions in two acid-sensitive streams in western Maryland. The results of the analysis are compared and discussed in the context of declining regional acid deposition, and influences of other factors such as forest disturbance.
Methods Two small, acid-sensitive streams located on the Appalachian Plateau in western Maryland are the subjects of our study (Figure 1). Big Run (BIGR) drains a small, predominantly forested watershed (area ) 162 ha) underlain by sedimentary formations of layers of sandstone, shale, and siltstone. Discrete samples for analysis of acid–base chemistry have been collected from the same station on BIGR at varying frequencies (semihourly to monthly) since late 1989. A continuously recording stream gage (i.e., a stilling well equipped with an analog water level recorder) was established 10.1021/es071195e CCC: $40.75
2008 American Chemical Society
Published on Web 11/27/2007
FIGURE 1. Locations of stream sampling stations in western Maryland and NADP wet deposition monitoring stations in neighboring Pennsylvania and West Virginia. at this station in 1995; the gage was later upgraded by installing a digital meter. A stable stage-discharge rating developed from field discharge measurements has allowed us to derive continuous records of hourly and mean daily discharge for water years (October-September) 1996 through 2005 using conventional gaging methods (22). Discharge records for BIGR were extended back to 1989 using data from a gaging station on Savage River (USGS No. 01596500, located about 7 km from the BIGR gage), assuming that daily runoff at the two stations were the same. This assumption is justified by the fact that mean annual runoff from the two stations during water years 1996–2005 was virtually identical (Savage River ) 0.582 m; BIGR ) 0.585 m). Black Lick (BLAC) drains a somewhat larger (558 ha), mostly forested watershed, is less acid-sensitive, and is also underlain by sedimentary formations. Comparable water sampling and stream gaging at BLAC began in May 1996 and has continued through the end of the 2005 water year. The data used in this paper include analyses of 640 discrete (or time-composited) streamwater samples. Streamwater samples were analyzed for a complete suite of constituents using analytical methods approved by USEPA for acid deposition studies (23): pH (meter); specific conductance (meter); ANC (acidimetric Gran titration beginning in 1990); acid anions (i.e., SO42-; NO3-; Cl-; ion chromatography), base cations (i.e., Na+, K+, Mg2+, Ca2+; no data prior to 1995; ion chromatography from 1995 through 1999; atomic absorption spectrometry after 1999); and dissolved organic carbon (DOC; UV-assisted persulfate digestion, beginning in 1996). Blanks and check samples were used in each batch to ensure that our analyses met rigorous analytical quality control standards. Conductivity and ion balance checks were performed to validate the sample analyses. Both ion balances and DOC concentrations indicated that organic anion concentrations were very low in these streams (
