Assessing the regional effects of sulfur deposition on surface water


effects of sulfur deposition on surface water chemistry: the Southern Blue Ridge ... Acid-base chemistry of high-elevation streams in the great sm...
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Environ. Sci. Technol. 1988, 22, 685-690

Curl, R. L.; Keoleian, G. A.; Environ. Sci. Technol. 1984, 18, 916. Simoneit, B. R.T.; Philp, R. P.; Jenden, P. D.; Galimov, E. M. Org. Geochem. 1984, 7,173. Podoll, R. T.; Irwin, K. C.; Brendlinger, S. Environ. Sci. Technol. 1987,21, 562.

(65) Lee, M. L.; Novotny, M. V.; Bartle, K. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic: New York,

1981. Received for review July 1,1987. Accepted December 11,1987.

Assessing the Regional Effects of Sulfur Deposition on Surface Water Chemistry: The Southern Blue Ridge Keith N. Eshleman" Environmental Research Laboratory, Northrop Services, Inc., 200 S.W. 35th Street, Corvallis, Oregon 97333

Philip R. Kaufmannt Utah Water Research Laboratory, Utah State University, Logan, Utah 84322

A method was developed for quantifying the regional chronic acidification of surface waters which uses synoptic survey data and a conceptual titration model of acidification. The principal assumptions of the model are that streamwaters have been titrated by an amount of sulfuric acid equivalent to their current SO-: concentration and historical pH and acid-neutralizing capacity (ANC) can be calculated from current chemical data. The model allows for increases in S042-concentration to be compensated by increased production of base cations through use of a regional coefficient. Making "worst case" assumptions, the median historical decline in ANC and pH in streams in the Southern Blue Ridge was estimated to be 23 pequiv/L and 0.09 unit, respectively. An inverse correlation between the Sod2to base cation ratio and ANC is shown to be consistent with the use of the titration model.

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Introduction Quantifying the regional acidification of surface waters in the United States by acidic deposition remailns an important objective of environmental assessment. Analyses of data from historical surveys of lake water and streamwater chemistry have been undertaken as a means of assessing the regional extent of acidic or low acid-neutralizing capacity (ANC) surface waters (1-12). While the results of all of these studies are consistent with the hypothesis that surface waters in areas of North America have experienced declines in pH and ANC over the past half century, quantitative regional estimates of historical acidification by acidic deposition have been hampered by questionable analytical and sampling bias in these data sets (13). Estimates of historical acidification and predictions of future changes in aquatic chemistry are thus being made through application of state of the art mechanistic models on a site-specific basis (14-16). Data limitations have prohibited the explicit application of these models to the scores of systems that would comprise a reasonable sample from a regional population, although a regionalization method has been developed that uses Monte Carlo techniques to select combinations of input parameters for a mechanistic model of catchment geochemistry (17, 18). Another possible assessment technique is a combination ~~

Present address: Environmental Research Laboratory, U S . Environmental Protection Agency, 200 S.W. 35th St., Corvallis, OR 97333. 0013-936X/88/0922-0685$01.50/0

Table I. Chemical Characteristics of the National Stream Survey Population in the Southern Blue Ridge

parameter 20th percentile 40th percentile median 60th percentile 80th percentile

variable" SO:-

ANC

pH

86.6 102.6 119.6

6.86 6.97 7.03 7.06 7.23

18.6 22.4 22.9 27.5

134.3 58.9 197.7 "All variables expressed in bequiv/L except pH.

NOS3.0 4.4 7.6 10.9 23.4

of an empirical or conceptual acidification model with current regional water quality data sets. The ultimate goal of such an analysis would be to provide answers to the following types of questions with known confidence: (1)What is the likely proportion of surface waters in a region that has been acidified to a given extent by acidic deposition? (2) What is the median change in pH and ANC that has likely occurred in a region? (3) With 95% confidence, what is the maximum proportion of surface waters that has experienced chronic depressions in pH of at least 0.2 pH unit? (4) What proportion of surface waters are presently acidic as a result of atmospheric sulfur deposition? The purposes of this paper are to (i) derive an aquatic chemical model that is applicable to answering questions such as those stated above and (ii) apply the model so as to make quantitative,-statistically unbiased estimates of the historical acidification of streams in the Southern Blue Ridge of the United States (Figure 1)potentially attributable to deposition of atmospheric sulfur. Prior to the completion of Phase I of EPA's Eastern Lake Survey in 1984 (19) and a pilot of the National Stream Survey (NSS) in 1985 (20),this physiographic region had been shown to contain low ANC (-]:[CBC] were calculated for values of ANC representing the midpoints of the grouped data (Table 111). The results in Table I11 indicate reasonable agreement between the predicted and observed [S042-]:[CBC]values for five ANC classes that comprise nearly 80% of the target streams in the Southern Blue Ridge. These results implicate an acidic source of S042such as atmospheric deposition or sulfide weathering. Because widespread occurrence of sulfide-bearingminerals is not demonstrated, these results further support the hypothesis that S042-concentrations in streams in the Southern Blue Ridge are primarily attributable to sulfur deposition. The observed pattern of relatively uniform [S042-]from a nonterrestrial source also suggests relatively uniform SO-: retention in watersheds of the Southern Blue Ridge. Worst case model predictions of AANC and ApH (assuming F = 0 regionally) correspond to a H2S04titration scenario in which terrestrial alkalinity generation through base cation supply is presumed to have remained constant. Our worst case scenario could be underestimating historical acidification by atmospheric sulfur only if base cation supply to surface waters in the Southern Blue Ridge has actually declined (Le., F < 0). Data from Coweeta watershed W-2 within the Southern Blue Ridge do not appear to support the hypothesis that base cation concentrations in streamwater have declined dramatically (if at all) over the past 15 years (48). Results based on the application of a hydrochemical simulation model to a forested catchment in the Blue Ridge Mountains of Virginia suggest that the integrated value of F from 1844 to 1984 was about 0.6 (49). While our model predictions for the Southern Blue Ridge are not verifiable, they do not appear to be invalidated by limited field data or by our current understanding of the acidification process as represented in more complex mathematical models. Conclusions Application of a simple conceptual titration model to National Stream Survey (NSS) data in the Southern Blue Ridge was used to predict the maximum chronic acidification of target stream reaches potentially attributable to atmospheric sulfur deposition. Although no target reaches have been acidified to the point where their acid-neutralizing capacity (ANC) is zero, regional median worst case

historical declines in ANC and pH of 23 pequiv/L and 0.09 unit, respectively, were predicted by the model. While not providing a strict verification of the model, the predicted changes are of the same magnitude as postulated changes on the basis of an independent evaluation of historical data. An inverse relationship between [S042-]:[CBC] and ANC was shown to be consistent with the hypothesized sulfuric acid titration scenario, as opposed to a uniform mineral S042- weathering scenario. Oxidation of sulfide minerals present in the Anakeesta formation cannot be ruled out as a local source of sulfuric acid, but a regional phenomenon is not consistent with the fact that outcrops of the formation are mapped on the watersheds of only 2 of the 54 streams sampled during the NSS; in neither case was the measured [SO:-] found to be extraordinarily high. The observed pattern of relatively constant [SO,2-] from a nonterrestrial source suggests relatively uniform SO?retention in this region. The primary weakness in the overall approach is our inability to determine either regionally or locally whether sulfur deposition has increased the flux of base cations from soils and surficial materials to surface waters. Site-specific field manipulation experiments may provide the only direct method for measuring this process, and of ultimately quantifying-albeit at only a few sites-the impact of acid deposition on surface water chemistry.

Acknowledgments We acknowledge the scores of people whose coordinated efforts resulted in the National Surface Water Survey data bases. We thank M. Mitch, J. Coe, M. Sale, and H. Jager for performing data analyses. We thank M. Church, J. Eilers, A. Herlihy, P. Shaffer, and two anonymous reviewers for helpful comments on an earlier draft.

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Received for review August 18, 1987. Accepted December 15, 1987. The research described in this paper has been funded by the US.Environmental Protection Agency (EPA)and conducted at EPA’s Environmental Research Laboratory in Coruallis, OR, through Contract 68-03-3246 to Northrop Services, Inc. It has been subjected to EPA’s peer and administrative review and approved for publication.