5 Coal: Non-acid forming
5.1 Introduction
Impacts on the aquatic environment from coal mining in non-acid-forming regions can include, but are not limited to, elevated concentrations of Fe, elevated concentrations of trace elements such as Mn and Zn, and elevated total suspended solids (TSS) (Shapely and Bishop 1965). High TSS can result from erosion of disturbed waste-rock material and from erosion of exposed coal seams and coal stockpiles. These sources produce TSS consisting predominantly of sand and clay particulates and coal fines (Osterkamp and Joseph 2000). High TSS concentrations can result in elevated turbidity in aquatic environments.
5.2 Predicted water quality
Few data exist with which to predict the chemistry of neutral mine drainage from coal mines (Pope et al. in press). In general, mine drainage from NAF coal measures rocks is well buffered by carbonates and therefore has circum-neutral pH. However, there can be elevated concentrations of Fe, as Fe2+, because many groundwaters are in equilibrium with Fe carbonate. When mine drainage leaves the mine environment or waste-rock dump, bright Fe oxyhydroxide precipitates form as the Fe is oxidised. In addition elevated concentrations of some trace elements such as Mn and Zn are possible. In general, the concentrations of these components are likely to be low (<0.2 mg/L). The highest trace element concentrations measured in samples collected to date (from a sample of pH 6.8) are 10 mg/L Fe, 0.15 mg/L Mn and 0.2 mg/L Zn.
Total suspended solids are generally expected to be elevated at mine sites, and management of TSS is routinely undertaken at all mine sites. The amount of TSS is site specific and varies with climate-controlled factors such as rainfall, and the presence of areas that can generate dust and the geotechnical properties of the rock; also, the efficacy of TSS management options in place may be variable. TSS can be predicted for Southland Coal Measures based on geological information (Craw et al. 2008), but these relationships have not been thoroughly researched for all rock types that are likely to be disturbed by mining.
As there is limited predictive capability of the likely extent of TSS, it is managed proactively (refer to Appendix B). When mining operations commence, suspended solid loads, in particular that present in the discharge from the treatment system, should be monitored to assess the effectiveness of treatment systems.
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5.2.1 Downstream water quality
Prediction of water quality downstream from a NAF mine site follows the process outlined in Figure 10 (section 3.3) and requires integration of data on the chemistry of the neutral mine drainage, site hydrogeology and background water quality. As discussed above, there are limited data with which to enable prediction of likely neutral mine drainage chemistry from coal mines. Collection of relevant site hydrological and background water quality is outlined in section 2.3.
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