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Acid rain steals our heritage
In addition to affecting people, plants, and wildlife, air pollution also affects rocks and soils. One of the problems it causes is the degradation of buildings and monuments, especially those built out of limestone or marble. These rock types, both almost pure calcite (calcium carbonate), are commonly used throughout the world as a building stone.
These balusters, on the Pan American Union Building, Washington, D.C., were made from Georgia marble, and were installed in 1910. They demonstrate the effects of dry deposition of sulfur dioxide, which causes the formation of gypsum. Gypsum traps particulate matter to form heavy, black incrustation. In some areas, the gypsum crust has flaked off the balusters exposing a fresh but very rough surface.
Studies to determine damage caused by air pollution have pointed to changes in the acidity of the air and rain. In fact, the term acid rain is now commonly used in the media as well as scientific studies. Acid rain affects carbonate stone buildings and monuments in two ways. The first is by dry deposition of sulfur dioxide gas, increasingly contributed to the atmosphere by the combustion of fossil fuels. The gas reacts with calcium-carbonate building stone to form calcium sulfate (gypsum). As gypsum forms on the surfaces of the stone, it traps particulate matter, forming a blackened crust.
The second effect of acid rain is wet deposition. Natural rain water is a weak carbonic acid solution and all carbonate-stone surfaces that are washed by rainwater are subject to gradual erosion. This erosion is accelerated, however, by the increased acidity of rain in the eastern United States, which is often 10 times greater than in areas where acidic pollutants are absent.
Current research on acid rain is directed at defining the degree of stone damage due to both dry and wet deposition. Scientists are measuring the effects of acid rain on historic stone buildings and monuments across the country. They are exposing samples of marble and limestone to weathering at specific field sites and simulating depositional processes under highly controlled laboratory conditions.
The effects of both dry and wet deposition are evaluated by the chemical analyses of the stone surfaces before and after exposure and of rain run-off solutions collected from test slabs.
Recent research by the USGS and other agencies conducted under the National Acid Precipitation Assessment Program has shown that test samples of marble erode 15 to 30 micrometers per year, while limestone (which is less compact than marble) erodes from 25 to 45 micrometers per year. (These measurements are slightly less than those of the diameter of a human hair). Approximately 20 percent of this erosion is caused by acid rain. The remaining 80 percent is the result of the natural solubility of the stone in rain water. Because the effects of acid rain only develop over an extended period of time, high-precision analytical chemistry plays a central role in measuring these effects. Return to this point in index.
The chemistry of mine drainage
Mine drainage is water that drains from mines. The water can be of the same quality as drinking water, or it can be very acidic and laden with high concentrations of toxic, heavy metals. In general, the more acidic the water is, the poorer the water quality.
Because the chemistry of water samples can rapidly change if they are removed from the natural site, many measurements are made in the field. One of the first of these field measurements is for acidity, which is read by a meter and reported as the pH of the sample. Water with a pH of 2 has a high concentration of hydrogen ions and is acidic, whereas water with a pH of 7 is neutral. A study of mine drainage in Colorado, for example, shows that the pH of mine waters ranges from a low of 1.8 to a high of 8.
A companion field measurement made on mine water is for specific conductance. This property of water measures the electrical conductivity associated with a water sample and is useful as a quick estimate of total dissolved solids. A low number from 10 to about 200 microsiemens/centimeter (the unit of specific conductance measurements) could be considered to be drinking-water quality. Specific conductance measurement of mine waters in the Colorado study range from 100 to 38,000 microsiemens/centimeter.
The full characterization of mine water requires a number of other instrumental and analytical measurements that are carried out using both mobile and laboratory facilities. Three main, instrumental, analytical techniques are used to complete the characterization of mine-water samples. These techniques are: ion chromatography (IC), which is used to determine the concentration of fluoride, chloride, nitrate, and sulfate in aqueous samples; ICP-AES, which determines the concentration of major and trace elements(for additional discussion on ICP-AES and an illustration of the instrument, scroll down to \"10,000 element determinations a day\"); and liquid ICP-QMS , which is used to determine elements below the ppm level (for additional discussion and an illustration of a laser ablation ICP-QMS instrument, scroll back to the \"Disaster from space\" section).
Why is it so important to characterize mine drainage? Because mine- drainage water almost always flows into a stream where it can dramatically affect the aquatic organisms and the quality of the water received by downstream communities. To successfully reduce the effect of the toxic elements, their abundances must be known.
Mineral-laden water from the Argo drainage tunnel in Colorado, entering into Clear Creek, illustrates the possible environmental impact of untreated mine drainage.
From the analytical chemistry of mine drainage, scientists have concluded that the major cause of high acidity of the water is the bacterially catalyzed oxidation of the mineral pyrite. This acidity stimulates the dissolution of many other sulfide minerals, resulting in the high concentration of metals such as copper and zinc.
While it is difficult or impossible to stop mine drainage, it might be possible to cut back the rate of the introduction of toxic elements into the environment. This can be done by hindering the bacteria that speed up the oxidation of the pyrite or by neutralizing the drainage and extracting toxic elements. Recent studies have shown that wetlands can concentrate heavy metals from mine drainage. Constructed wetlands could, therefore, be used to accumulate the pollution from mine drainage. By analytical monitoring of the toxic, metal build-up in these wetlands we can avoid any impact on the wildlife that might try to live there. Return to this point in index.
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