Acid rain and its geological impacts

Acid Rain and Geology

Acid rain is a term that refers to a mixture of dry and wet deposited materials that falls in precipitation from the atmosphere, containing "higher then normal amounts of nitric and sulfuric acids" (Environmental Protection Agency). Some of the "precursors, or chemical forerunners" of acid rain are from natural sources like volcanoes and rotting vegetation; other precursors are from man-made sources like nitrogen oxides (NOX) and sulfur dioxide (SO2). The man-made emissions result for the most part from the burning of fossil fuels, like coal and oil for electrical production, according to the EPA.

Wet Deposition: In the United States, about two-thirds of all the SO2 and one-quarter of all the NOX results from the burning of coal and oil. Acid rain is created when these gases -- released through the smokestacks of the coal and oil-fired plants -- mix with water and oxygen and other chemical typically found in the air, and rain down on the land, water and forests.

Dry Deposition: Acid rain can be formed in very dry climates when acidic chemicals are mixed in with smoke or dust and fall to the ground, sticking to the ground, trees, buildings and cars, the EPA explains. When rain does fall, the water that runs off becomes even more acidic.

Acid rain creates unnatural amounts of acidification in lakes and rivers and damages trees at higher elevations, the EPA explains. The results tends to accelerate the "decay of building materials and paints" -- and acid rain is unhealthy for trees and humans.

The Literature on Acid Rain and Geology

Acid rain has been impacting the land, forests and waters of some regions of the planet since as early as 1852, according to author S.V.S. Rana in his book, Environmental Pollution: Health and Toxicology. The impact of acid rain isn't entirely determined by the level of acid in the rain, but also on the "nature of the environment itself," Rana writes on page 25. In terms of geology, areas that are underlain by granite or quartatic bedrock are "particularly susceptible to damage," Rana explains. The reason this is true is that soils and water are already partly acidic, and hence they do not have the ability to "buffer or neutralize additional acidity" from the falling rain (Rana, p. 25). So when acid levels rise in the soil -- soil that is already acidic to a degree -- the environmental balance is "disturbed" and some "serious ecological damage is the inevitable result," Rana makes clear (p. 25).

Meanwhile if the area where acid rain is falling is underlain by chalk or limestone (what Rana calls "geologically basic" areas), that area of oil may well "benefit" from the falling acid rain, Rana continues (p. 25). In areas that feature "highly alkaline soils and water," when the acid rain is added to that environment the acid is "effectively neutralized," according to Rana's book. In areas where there has been "glacial drift" or another "unconsolidated deposit," the susceptibility of the environment to being damaged by acid rain "…will be determined by the nature of the superficial material rather than by the composition of the bedrock" (Rana, p. 25).

Forest Soil and Acid Rain

An article in the journal Communications in Soil Science and Plant Analysis (Kang, et al., 2005, p. 2129) discusses soils and nitrogen in a scholarly context. The authors explain that forest ecosystems are understood to be limited on nitrogen (N) as a general rule, hence productivity can be enhanced with additional nitrogen. However, when excessive amounts of nitrogen are deposited, it can adversely affect the soil, hence the geology. In Europe and North America, as well as the Far Eastern Asia and Korea there is known to be a problem with high levels of nitrogen (due to acid rain) (Kang, 2130).

Once the soil has been saturated with an excess amount of nitrogen, there is the chance that a "nutrient imbalance to vegetation" could occur. Also, with an overabundance of nitrogen in the forest soil (from acid rain) can make trees "vulnerable to frost" -- and places the forest ecosystems at risk (Kang, p. 2130). The authors explain that some soil on forest floors have developed "efficient retention" of nitrogen, and that retention could be a result of "mycorrhizal assimilation" -- a process through which the soil seamlessly absorbs and incorporates nitrogen from acid rain into its organic composition.

The science involved in Kang's research is interesting: organic nitrogen is known to be produced in forest soils through "microbial assimilation" or "litter production"; it can be mineralized in the soil by microbes that produce "extracellular enzymes," Kang writes (2130). This enzymatic process is part of nutrient cycling and so if there are impediments to enzyme activity too much nitrogen could possibly be retained. What would an impediment do? Acid rain could become an impediment to normal cycling of enzymes, hence, resulting in the retention of too much nitrogen (Kang, 2130).

In Kang's conclusion, the author asserts that there are debates related to nutrient cycles in forest soils, some claiming that "elevated CO2… may not directly induce greater primary production due to nitrogen limitation…" (p. 2133).

Another article (Geochemical Comparison of Stream Water, Rain Water, and Watershed Geology in Central Korea) discusses the poor acid-neutralizing capacity of granite and how that affects the aquatic systems in the granite watershed in central Korea.

The particular portion of central Korea called Chonju -- a "hilly to mountainous watershed" that is the focus of this research -- receives very little if any acid rain, authors Nakano and Jeon explain on page 739 of their scholarly article in Web of Science. The average pH in that district is pH: 6.2 (p. 740). However the water flowing down the stream -- in a granite watershed -- does have some acidity (6.4-6.7). In fact the watershed with granite as a bed has a "low concentration of calcium when compared with the stream water found running through sedimentary and volcanic rock watersheds," Nakano explains. That concentration of calcium is 6.8-7.6.

Given that the concentrations of calcium and strontium in the stream water do change due to the watershed geology at any particular time, the stream Sr.-87/Sr.-86 ratios are "…closer to the Sr.-87/Sr.-86 ratios of rain than to those of the substrate rocks," Nakano points out (p. 740). What does this suggest? The authors explain that the "selective but sluggish weathering" of the calcium-rich minerals in the streambed tend to neutralize the acid.