FeS2s14Feq8H20 a 15Feq 2S04aq 16Haq

At pH values much above 3 the iron(III) precipitates as the common iron(III) oxide, goethite (FeOOH):

The precipitated goethite coats stream beds and brickwork as a distinctive yellow-orange crust (Plate 5.2, facing p. 138), a very visible manifestation of the problem.

Bacteria use iron compounds to obtain energy for their metabolic needs (e.g. oxidation of ferrous to ferric iron). Since these bacteria derive energy from the oxidation of inorganic matter, they thrive where organic matter is absent, using carbon dioxide (CO2) as a carbon (C) source. Iron oxidation, however, is not an efficient means of obtaining energy; approximately 220 g of Fe2+ must be oxidized to produce 1 g of cell carbon. As a result, large deposits of iron(III) oxide form in areas where iron-oxidizing bacteria survive.

We should note that the common sulphide of iron (pyrite—FeS2) often contains significant amounts of the toxic semimetal arsenic, as impurities. As a result, when iron sulphides are oxidized (eqn. 5.15) arsenic is released along with the dissolved iron and sulphate. In very rare circumstances this arsenic release can result in groundwater contamination (see also Section 5.7.2).

The acidity caused by mining operations can be treated (neutralized) by adding crushed CaCO3 to the system and by removing dissolved trace metals. At active mines this is the responsibility of the mine operators. Abandoned mines, however, create a bigger problem because the source of leakage from the mine area is unpredictable, flowing out of various fissures and fractures in the rock as the mine fills with water. Furthermore, in abandoned mines there is often no operator to take responsibility for treatment. Moreover, as developed countries move away from coal as an energy source, more coal mines are being abandoned increasing the risk of acid drainage. There are various strategies being developed to create passive, low-cost treatments for acid waste including phytoremediation (see Section 4.10.4), where reed-beds are used to encourage oxidation and trap the solid iron oxides that precipitate.

5.4.3 Recognizing acidification from sulphate data-ternary diagrams

We have already seen how the factors regulating river chemical composition can be summarized using simple cross-plots of weathering and atmospheric (sea-salt) inputs (Fig. 5.3). Recognizing the significance of acidification either from acid rain or from acid mine drainage is aided using a ternary (triangular) diagram to allow for three inputs—weathering, sea-salt and sulphuric acid (Fig. 5.8). The diagram plots alkalinity, chloride and sulphate data to trace weathering, sea-salt and sulphuric acid inputs respectively for river systems discussed elsewhere in this chapter. Sulphate is a good tracer of acid mine drainage (eqns. 5.15 & 5.17), although not totally unique to this system (see below).

® Global average river

+ Groundwater o Acid mine drainage

♦ Adirondak Lakes (acid deposition)

® Global average river

+ Groundwater o Acid mine drainage

♦ Adirondak Lakes (acid deposition)

(Sulphuric acid)

40 50 60 % Alkalinity

(Sulphuric acid)

40 50 60 % Alkalinity

groundwater Schist groundwater

ALK (Weathering)

Fig. 5.8 Ternary plot of alkalinity, chloride and sulphate data to trace weathering, sea-salt and sulphuric acid (acid deposition/mine drainage) inputs to river- and groundwaters. River and groundwater data are from Tables 5.1-5.3 and Box 5.1. River Yare (eastern England) is representative of a river system developed on limestone (chalk) bedrock. The Adirondak Lakes data (from Galloway et al. 1983) represent response of nearby lakes to acid deposition. Woods Lake has pH of 4.7 and zero alkalinity; it cannot buffer effects of acid input. As alkalinity increases, due to increased carbonate weathering in the bedrock, the effects of acid deposition are progressively buffered (Sagamore (pH 5.6) to Panther Lake (pH 6.2)). Acid mine drainage (AMD) data (from Herlihy et al. 1990) are all averaged from streams in the eastern USA where (1) are strongly impacted acidic streams, (2) are strongly impacted non-acidic streams and (3) are weakly impacted non-acidic streams.

The Onyx River in Antarctica plots in the top corner of Fig. 5.8 as a pure chloride (sea-salt-dominated) system. As this river evolves and starts to weather rocks in the catchment, it picks up alkalinity moving on a trajectory toward the bottom right corner (weathering dominated). Upland systems heavily impacted by acid mine drainage or acid rain plot toward the bottom left corner of Fig. 5.8, their anion chemistry is dominated by sulphate with little or no alkalinity present. Again as these systems mature and acquire weathering products including alkalinity, they too move in trajectories toward the bottom right corner (weathering dominated). As noted earlier, HCO3- alkalinity is the dominant anionic component of most mature rivers and groundwaters, explaining their position at the bottom right of Fig. 5.8. The only exception is the Rio Grande that plots well away from most mature rivers. Although this river has the highest HCO3-alkalinity of those plotted on Fig. 5.8 (3.0mmoll-1 HCO-; Table 5.2), the weathering of evaporite minerals (NaCl and CaSO4) in the catchment, and the concentration of dissolved salts by evaporation, leads to much increased dissolved chloride and sulphate compared to all the other mature rivers, explaining its near central position on the diagram.

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