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. Concentrations of 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 CBand 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
0-0
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. Norure 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
I/
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
