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 (µS/cm, 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 (click here for previews of Chapter IIIb with discussion on ICP-AES and an illustration of the instrument); and liquid ICP-QMS, which is used to determine elements below the ppm level (click here to return to Section IIa for discussion and an illustration of a laser ablation ICP-QMS instrument).

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.

Photo of Argo Tunnel Adit with drainage. Mineral-laden water from the Argo drainage tunnel in Colorado, entering into Clear Creek, illustrates the possible environmental impact of untreated mine drainage. [77k]

Photo of drainage flowing across and through Argo Mine waste piles. [96k]

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.

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