E235
LETTERS
TABLE 1
The adjacent table was inadvertently omitted from the article, "Subsurface h s p r t of Contaminants" by John E McCarthy andJohn M. Zachara, which appeared in the May ES&T on page 496. The editors apologize for any confusion this may have caused.
Surface water chemistry Dear Sir: We found misinformation in ESdrT's feature article by Driscoll et al. (Vol. 23, No. 2, 1989). We would like to discuss the errors lest they be quoted as facts in the literature. The article cited Chen et al. (I) for the discussion of a correlation between acid anions and base cations in precipitation. Driscoll et al. claimed that the Hubbard Brook data did not support that contention. To provide legitimacy to their claim, they cited Hedin et al. (2). We found that their statement was contradicted by their own data shown in Figure la. We also found that their cited reference did not contradict Chen et al. We took the data from their Figure l a and ploned CB vs. sulfate (see Figure 1). The plot clearly indicates that the Hubbard Brook data confirm the observations of Chen et al. ( I ) . Since the data shows the existence of a relationship, we became curious as to what analysis was performed to suggest no relationship. We found that the data in Table 1 of their cited reference indicated a parallel decreasing trend of sulfate and base cations in the bulk precip itation data of Hubbard Brook. In the said reference, a hypothetical calculation was performed to determine the precipitation acidity if sulfate did not decrease in concert with cations. That is not what the data really show. In the confusing world of acid rain literature, it is very troubling when an unfounded assumption in one paper can be transformed into a factual statement in another paper. We must point out the differences between the Hubbard Brook and the Adirondack data. The Hubbard Brook 752 Envimn. Sci. Technol.. Vol. 23,No. 7 , 1989
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data are bulk precipitation, annual averages, excluding ammonium in CB and excluding nitrate and chloride in CA (acid anions). The Adirondack data are wet precipitation event samples, including ammonium in CBand including nitrate and chloride in CA. Another important difference is that Chen et al. ( I ) were only trying to show the apparent statistical relationship between CA and CBin precipitation data collected in the Adirondack from 1978 to 1981, during which there was no time trend in precipitation acidity. Chen et al. suggested mechanisms to explain the relationship. But they were careful not to claim that the relationship had a predictive power. On the other hand, the Hubbard Brook data covered 1964 to 1987, during which there was a decrease of precipitation sulfate and CBin conjunction with a decrease in sulfate emission. By putting the two together, the relationship perhaps may have some predictive power. In the article, Driscoll et al. played down the significance of nitrate by stating that there was no long-term trend in precipitation and streamwater nitrate. The statement contradicted the data presented by Likens et al. (3). Figure 17 on page 53 of this reference showed an increasing trend of nitrate in both precipitation and streamwater. Driscoll et al. postulated a number of hypotheses to explain the apparent decreasing trend of streamwater CB. They r ~ l e dout flowpath, weathering rate, and biomass accumulation as the possible cause. They could not evaluate the effect of cation exchange due to the lack of data and the difficulty in accounting for all the offsetting processes. At the end, they tried to explain the trend by changes in atmospheric input, even though precipitation CB accounted for only 15% of streamwater efflux in the early 1960s and 8% in the 1980s (their Figure IC). Driscoll et al. calculated the decreasing trend of precipitation CB concentrated by evapotranspiration (ET). They also calculated the decreasing trend of CB in streamwater. They then took the ratio of the two slopes. To us, the ratio indicates the relative magnitude of two trends. To the authors, the ratio (0.77) meant that 77% of the observed decrease of streamwater CB could be explained by the observed decrease of precipitation CB. It seems that the authors need to run a regression between the two trends in order to make such a statement. Driscoll et al. correctly pointed out the deficiency of the Hendricksen and Wright model which attributed surface water acidification exclusively to atmospheric sulfur deposition. We are surprised, however, by their failure to rec-
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ognize the existence of the Integrated Lake-Watershed Acidification Study (ILWAS) model (4, 5). To provide a linkage between acid deposition and surface water chemistry, the ILWAS model simulates the watershed processes of wet and dry deposition, ET, tlowpath, biomass accumulation, litter decomposition, organic acid production and decay, mineral weathering, cation exchange, sulfate sorption and desorp tion, nitrification, aluminum speciation, and others. The ILWAS model will adjust the watershed processes to calculate the new efflux of cations, anions, alkalinity, and pH in response to changes in the atmospheric loading of not only sulfate but also of all base cations and acid anions. An application of the ILWAS model to the Hubbard Brook watershed, if carried out, would predict a response consistent with their data. The model would predict that the change in the atmospheric input of cations and anions was too small to change the soil solu-
me authors reply: We would like to respond to the four issues that Chen and Gomez raised concerning our paper "Changes in the Chemistry of Surface Waters" (ES&I: February 1989, p. 137). 1. Concentrationsof both SO4and CB (sum of Ca, Mg, Na, and K) have shown significant decreasing trends during 1964 to 1987 in bulk precipitation at the Hubbard Brook Experimental Forest (HBEF).Because of these patterns, it follows that there exists a positive correlation between yearly concentrations of SO, and Ce, as noted by Chen and Gomez. These concurrent decreases in both CB and SO4 were indicated clearly in our paper (one of our major points), and several possible reasons for the declines were discussed. However, what is at issue is not the existence of a correlation between CB and SO,, but whether these long-term declines in Ce and SO4 are independent or, as argued by Chen and Gomez and in Chen et al. (I), causally dependent, with the decline in CBcontrolled by the decrease in SO,. We did not suggest a lack of correlation, as indicated by Chen and Gomez. Instead, we stated that we lack evidence to conclude that CB and SO4 are causally coupled at Hubbard Brook. In fact, our evidence suggests that CBand SO4 are not causally coupled. Our reasons are as follows: First, if there were a strong chemical coupling between CB and SO, in the atmosphere, as suggested by Chen and 754 Environ. Sci. Technal., Vol. 23, NO. 7 , 1989
tion pH. The weathering rate, which is pH-dependent, would therefore be maintained constant, and consequently so would the silica concentration in the streamwater. A reduced sulfate deposition would be accompanied by a decrease in H+ input, even though it would not be an equivalent decrease, according to Chen et al. (I). A lower input of hydrogen ions and cations (CB) would result in a higher retention of cations by cation exchange sites. Meanwhile, lower sulfate inputs would lead to a lower soil solution sulfate concentration causing some sulfate desorption to occur. The ILWAS model would predict lower CBand sulfate concentrations of streamwater, similar to the observed tight coupling of CBto the decline in streamwater sulfate, discussed in the article. This tight coupling effect was anticipated by the ILWAS model, but was unexpected by Driscoll et al. Since the alkalinity is the sum of cations less the sum of anions, parallel reduction of CB and sulfate
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would not change the streamwater alkalinity and pH very much. Thus, the IL-
WAS model would predict, as it was observed, the insensitivity of streamwater pH to the change of atmospheric deposition to the Hubbard Brooksystem.
References (l)Chen, C. W. et al. J. Environ. Engr Din ASCE 1987, 113, 919-93. (2)Hedin, L. 0.;Likens, 0. E.; Borman, E H. " W e 1987,325,244-46. (3)Likens, 0. E. et al. Biogeochemistryofn Forested Worershed; Springer-Verlag: New York, 1911. (4)Chen et al. nte lntegrored Lake Uhtershed Acidification Study, Vol. 1. Model Principles nnd Application Procedures: Electric Power Research Institute: Palo Alto, CA EA-3221, 1983. (5)Gherini. S. A. et al. Water,Air, SoilPollur. 1985,26, (4), 425-59.
Carl W.Chen Luis E. Goma Systech Engineering, Inc. Lafayette, CA 94549
Sulfate
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c Gomez and Chen et al. (I), we would expect that seasonal variations in concentrations of Ce would follow closely seasonal variations in SO,. However, this coupling is not evident. As shown in Figure 1, seasonal variations in CBat HBEF are markedly different from seasonal variations in SO,. Second, the long-term decreases in CB and SO4 at HBEF are not strictly parallel, as stated by Chen and Gomez, but show significant differences during
the 25-year record. As noted in Table 1 of Hedin et al. (Z), the SO, decline is best described by a linear regression function, whereas the decline in CB(including Nh) is best described by a curvilinear regression. Based on regression trends for the 25-year record, we calculate that 76% of the decline in CB between 1964 and 1987 took place during the early part of the record (19641972) (r2=0.72; p
