Acid mine drainage, described

Because gravity is too weak to be used for removal of cakes in a gravity side filter (2), continuously operated gravity side filters are not practicable but an intermittent flow system is feasible in this arrangement the cake is first formed in a conventional way and the feed is then stopped to allow gravity removal of the cake. A system of pressure filtration of particles from 2.5 to 5 p.m in size, in neutralized acid mine drainage water, has been described (21). The filtration was in vertical permeable hoses, and a pressure shock associated with relaxing the hose pressure was used to aid the cake removal. 

Scorodite (FeAs04-2H20) is an important mineral in controlling the solubility of As(V) in acid mine drainage (Chapter 3). At pH conditions below about 2.4-3 (Krause and Ettel, 1988 Langmuir, Mahoney and Rowson, 2006), the dissolution of scorodite may be described with the following reaction (Harvey et al., 2006), 6709 ... 

From this assumption, inverse mass balance modeling is more likely to be applicable to large regional aquifer systems, but less applicable to aquifers with point source contamination. For example, in the acid mine drainage impacted aquifer described in Chapter 6, the chemistry at all points is changing with time. Applicability of inverse mass balance modeling to this type of system depends on the spatial locations of the flow path and the time-frame of interest. 

The second site is in Tennessee Park, a sub-alpine wetland near Leadville, Colorado. Thirteen wells were drilled here as part of a project to determine the effects of the wetland on the input from an alpine stream heavily contaminated with acid mine drainage (M). The site and the wells have been described in detail ( ). The water table is within one meter of the surface in all wells, which have screen depths less than four meters below the land surface. The composition of the well waters changes seasonally and in response to precipitation events. 

The most common source of pollutant acid in water is acid mine drainage. The sulfuric acid in such drainage arises from the microbial oxidation of pyrite or other sulfide minerals as described in Section 3.5. The values of pH encountered in acid-polluted water may fall below 3, a condition deadly to most forms of aquatic life except the culprit bacteria mediating the pyrite and iron(II) oxidation, which thrive under very low pH conditions. Industrial wastes frequently have the potential to contribute strong acid to water. Sulfuric acid produced by the air oxidation of pollutant sulfur dioxide (see Chapter 7) enters natural waters as acidic rainfall. In cases where the water does not have contact with a basic mineral, such as limestone, the water pH may become dangerously low. This condition occurs in some Canadian and Scandinavian lakes, for example. 

In addition to dilution of acid rock/mine drainage under oxidizing conditions, neutralization can occur under mildly or highly anaerobic conditions. This will create distinctive environments in which microorganisms thrive and nanoparticles form as a result of their activity. We describe two examples of such subsurface systems below. However, before turning to these topics, we note that Fe-based microbial ecosystems are not only found in association with metal sulfide deposits, but may be broadly relevant in the subsurface where Fe-rich minerals (biotite, olivine, pyroxenes, etc.) are present in reasonable abundance and dissolve, releasing aqueous ferrous iron. 

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