Environ. Sci. Technol. 1990, 24, 531-537
Acidification and Recovery of a Spodosol 6s Horizon from Acidic Deposition Randy A. Dahlgren,t Drew C. McAvo~,*and Charles T. Driscoll' Department of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, New York A laboratory study was conducted to examine acidification and recovery of a Spodosol Bs horizon from acidic deposition in the Bear Brook Watershed (BBW) in central Maine. A mechanical vacuum extractor was used to draw solutions through a soil column at three treatments containing 40,100, or 160 pmol/L SO:-. Following 44 days of leaching, all treatments were decreased to the 40 pmol/L SO:- level to examine recovery from acidification. Acid additions were initially neutralized by release of basic cations and sulfate adsorption. Following attainment of steady-state conditions for basic cations and S042-with respect to the soil adsorption complex, A1 dissolution was the primary neutralization mechanism. Aqueous A1 activities appeared to be regulated by equilibrium with an A1(OH)3 mineral phase. Following decreases in acid loadings, recovery was rapid resulting in retention of basic cations, reversible release of SO4*, and a marked reduction in the concentrations of soluble Al. Introduction
One of the most important issues currently facing environmental scientists is the need to predict the effects of acidic deposition on terrestrial and aquatic ecosystems. Considerable effort has gone toward quantitatively understanding soil chemical processes that regulate the acid/base chemistry of soil and drainage waters. This understanding has been motivated by the need to predict the chemical response of ecosystems to increases in acidic deposition as well as recovery following reductions in atmospheric emissions. Galloway et al. (1)proposed a conceptual model to describe the acidification and recovery of watershed ecosystems subjected to increases and decreases in atmospheric loadings of HzS04 They suggested that increases in SOz emissions and ,302- deposition would result in increases of S042-concentrations in drainage water and retention of Sod2on reactive soil surfaces through adsorption. Moreover, an increase in inputs of HzSO4 would coincide with displacement of basic cations (CB; sum of Ca2+,Mg2+,Na+, and K+ concentrations) from exchange sites and/or consumption of acid neutralizing capacity (ANC). This in turn would result in a decrease in pH and an increase in concentrations of inorganic A1 in drainage waters. The watershed will eventually attain a new steady state with respect to the higher loading of HZSO4 Under elevated HzS04inputs, drainage water S042concentrations are increased, and basic cation concentrations and ANC values are lower than during preacidification conditions. This results in drainage waters with lower pH values and higher concentrations of inorganic Al. Galloway et al. (I) also contended that when the H2S04 loading is reduced, the soil processes reverse and equilibrate with the lower inputs. They proposed that under lower H2S04 inputs, Sod2desorbs from soil surfaces, basic cations are retained on the exchange complex, and ANC is increased. 'Present address: University of California, Land, Air and Water Resources, Hoagland Hall, Davis, CA 95616. :Present address: The Proctor and Gamble Co., Ivorydale Technical Center, Cincinnati, OH. 0013-936X/90/0924-053 1$02.50/0
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As a result, the drainage water pH is increased and inorganic A1 concentrations are reduced. Although this conceptual model serves as the foundation for many of the computer models that quantitatively assess the chemical effects of watersheds subjected to acidic deposition and recovery (2-5), a number of important questions still remain. For example: (1) What is the contribution of individual processes (e.g., cation exchange, SO:- adsorption/desorption, A1 precipitation/dissolution) to the pH buffering of the soil following changes in acid loading? (2) Is the SO-: that adsorbs to soil surfaces entirely desorbed following decreases in HzSO4 loading? (3) What is the time frame of these acidification/recovery processes? Unfortunately, it is difficult to answer these questions for a given watershed, let alone for large acid-sensitive regions of eastern North American and Europe. Thus, the objective of this paper is to investigate the appropriateness of the conceptual model proposed by Galloway et al. (1) and to provide process-level information for the development and calibration of computer models used to predict the response of watersheds to changes in acidic deposition. A series of column leaching experiments were performed using soil from the Bs horizon of a Spodosol that is representative of acid-sensitive soils found in the northern hardwood forest ecosystems of northeastern United States. The Bs horizon was chosen to demonstrate the response of soil chemical processes to acidic deposition because it is the soil horizon thought to most influence surface water chemistry in Spodosols (6). Experimental Section
Study Site. Soil samples were collected from a Spodosol in the Bear Brook Watershed (BBW), ME. Bear Brook Watershed is located on the southeast-facing slope of Lead Mountain (44'52' N, 68'6' W), approximately 40 km from the Atlantic coast in central Maine. The elevation at the summit of Lead Mountain is 450 m, and the relief is approximately 210 m. This watershed receives a mean annual precipitation of 115 cm. The bedrock is composed of metasedimentary pelitic graded beds, metamorphosed and folded quartz sandstones, and intrusive granitic dikes. A shallow overburden of glacial drift (50-100 cm) overlays the bedrock. The soils are classified as well-drained Typic Haplorthods. Soil samples used in this study were collected immediately adjacent to the BBW at an elevation of 290 m. The vegetation surrounding the collection site is dominated by American beech (Fagus grandifolia) with a scattering of red maple (Acer rubrum),gray birch (Betula populifolia), and yellow birch (Betula alleghaniensis). Soil Column Leaching. Soil samples from the Bs horizons were passed through a 2-mm sieve to remove coarse fragments. The soils were maintained at field moist conditions and refrigerated at 3 "C to reduce biotic processes prior to experimentation. Selected chemical properties of the solid phase for the Bs horizon are summarized in Table I. Soil columns were prepared by packing 60-mL polypropylene syringes with 45 g (dry weight equivalent) of soil. The leachate solution was added to bring the soil column to saturation. At saturation, the soil columns contained
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 4, 1990 531
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Table I. Selected Characteristics of Data for the
