Soil: acidification chemistry

Giesler, R. et al., Reversing acidification in a forested catchment in southwestern Sweden Effects on soil solution chemistry, J. Environ. Qual., 25, 110, 1996. 

For a variety of practical reasons, great effort has been expended in recent years to understand controls on and to predict rates of chemical weathering in soils. A principle reason has been a need to assess the effects of acid precipitation on the chemistry of soils and thus on the health of affected plants and trees. Soil acidification from acid precipitation has been a serious problem in industrialized areas where soils are thin or absent and bedrock and resultant soils lack carbonate or reactive silicate minerals (cf. Likens et al. 1977 Berner and Berner 1996). Soil acidification in such areas has caused the acidification of adjacent streams and even underlying groundwaters (cf. Bottcher et al. 1985 Hansen and Postma 1995). 

In addition to what might be considered apparent effects acid precipitation contributes to unseen acidification of soil. In certain areas where the natural buffering capacity is low - see 8 below/ soil acidification leads to loss of certain tree and other plant nutrients and change in soil chemistry. In the long term (more than 50 years), this process could produce other apparent environmental change, and might be irreversible. 

The first publicised hypothesis by Ulrich is that tree damage is caused by soil acidification which induces aluminium attack on the roots of trees followed by pathogens (Section 6.3.1). Others have argued that the spatial occurrence of observed tree damage and evidence of soil chemistry do not support this hypothesis. 

Various process models have been used to estimate critical loads. These Include empirical models using process-oriented mechanisms in their structure, such as the MAGIC model (Cosby, Homberger and Galloway, 1984), the RAINS Lake Model (RLM) of Kamari and Posch (1987) and the Model to Assess a Critical Acid Load (MACAL) of de Vries (1988). More complex models such as the SteaSoil Chemistry Model (PROFILE) developed by Sverdrup and Warfvinge (1989) and RESAM, the Regional Soil Acidification Model (de Vries and Kiros, 1989) have been developed. 

Hultberg, H. and Grennfelt, P. (1986). Gardsjon project lake acidification, chemistry in catchment runoff, lake liming 2md microcatchment manipulations. Water, Air and Soil Pollution, 30, 31-46. 

The assumption employed in models is that equilibrium chemistry is applicable in all relevant situations. This implies that the reaction of soil pH and other parameters to a change in input is virtually instantaneous and that processes such as diffusion can be neglected. Long-term, large-scale acidification models are difficult to calibrate and validate because of the paucity of sufficient long-term (>50 yr) observations. 

Acidity is a major limitation to soil productivity in much of the world. Although acidification is a natural process in many soil environments, agricultural practices and pollution from industrial, mining, and other human activities have accelerated the process. It is important that acidity be understood in terms of its fundamental chemistry so that soil management and remediation schemes are based on sound principles rather than empirical knowledge that may only be locally relevant. This chapter attempts to provide this understanding, and uses some example field data to illustrate important principles. 

The hydrology and chemistry of lakes and streams are highly individualistic. Lakes surrounded by poorly buffered soil and underlain by granitic bedrock appear to be more susceptible to acidification when exposed to acid rain (Havas et al. 1984). Other lake characteristics such as dominance of atmospheric input or surface/subsurface runoff as the major source of water, type and depth of soil, bedrock characteristics, lake size and depth, area of the drainage basin, and residence time of water in the lake are all features that influence the response of a lake to acid rain. 

Figure 3.10. Time series of predictions with the acidification model PnET-BGC of changes in stream chemistry at Hubbard Brook to changes in past and potential future emissions of sulfur dioxide and nitrogen oxides, including the 1990 Amendments of the Clean Air Act and moderate and aggressive emission control scenarios. Shown are model-predicted stream concentrations of sulfate, nitrate, acid neutralizing capacity, pH and dissolved inorganic aluminum, and soil percent base saturation. Measured values are indicated for comparison...

Gunnar Jacks (hydrology hydrogeology hydrochemistry groundwater chemistry groundwater arsenic and fluoride acidification of soil and groundwater artificial... 

The Chemistry of Weathering—Long-term Control of Acidification SHORT-TERM AND EPISODIC ACIDIFICATION. 4.1 High Discharge from Snowmelt and Rain Pulsed Release of SO4 and NOsfrom Soils Marine Aerosols Organic Acidity... 

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