Environ. Sci. Technol. 2002, 36, 4714-4720
Retrospective Analysis of the Response of Soil and Stream Chemistry of a Northern Forest Ecosystem to Atmospheric Emission Controls from the 1970 and 1990 Amendments of the Clean Air Act SOLOMON S. GBONDO-TUGBAWA* AND CHARLES. T. DRISCOLL Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244
The 1970 and 1990 Amendments of the Clean Air Act (CAAA) have resulted in a decline in acidic deposition in the northeastern United States. Results from the application of a biogeochemical model (PnET-BGC) at the Hubbard Brook Experimental Forest in New Hampshire suggest that, without the implementation of the CAAAs, soil base saturation and soil solution molar Ca/Al ratio would decrease to values below 6% and 1.0, respectively, while S would continue to accumulate in organic matter and adsorbed pools at rates of 2 and 3 kg of S ha-1 yr-1, respectively. This scenario of conditions without the CAAAs was projected to result in higher stream concentrations of SO42-, NO3-, and Ca2+; monomeric Al; pH below 4.8; and acid-neutralizing capacity (ANC) less than -15 µequiv L-1. The implementation of the CAAAs has led to a slight improvement in the soil base saturation, while recovery of soil solution Ca/Al cannot be fully assessed because of variability in observed values. Our evaluation of the relative benefits of the 1970 and 1990 CAAAs indicate that although the magnitude of the cumulative decrease in strong acid deposition was greater following the 1970 CAAA as compared to the 1990 CAAA, the extent of ecosystem recovery relative to the changes in acidic deposition suggests that the 1990 CAAA was also beneficial. The slow recovery rates might be the result of a legacy of chemical effects of acidic deposition for the last 150 years and suggests that additional controls in emissions might be required to show significant changes.
Introduction The deposition of strong acids (e.g., H2SO4, HNO3) and weak acids [e.g., (NH4)2SO4, NH4NO3] from the atmosphere has been implicated as a critical environmental stress that affects forested landscapes and aquatic ecosystems in North America, Europe, and Asia (1). Acidic deposition affects trees directly (including changes in nutrient uptake or leaching in the canopy, altered metabolism, and damage to tissues) and indirectly through changes in soil chemistry, altered mycor* Present address: TAMS/Earth Tech, 300 Broadacres Dr., Bloomfield, NJ 07003; telephone: (973)338-6680; fax: (973)338-1052; e-mail:
[email protected]. 4714
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 22, 2002
rhizal development, impaired root function, and increased nutrient losses from soil (2). Acidification has had marked effects on the trophic structure of surface waters. Decreased pH and increased Al concentrations contribute to declines in species richness and in the abundance of zooplankton, macroinvertebrates, and fish in surface waters (3). Critical chemical thresholds that appear to coincide with the onset of deleterious effects to biotic resources include (i) the molar Ca/Al ratio of soil water
