4.5 Contaminated land

Minimising land contamination during operations and rehabilitating areas affected by mining remain the goal of leading practice mining. The post-mining land use will determine to a large extent the nature and type of contaminated land management and associated monitoring that is required.

The surface of an operating mining lease inevitably becomes impacted to a greater or lesser extent by mineralised material derived from the stockpiling of concentrates, ore, waste rock and mineral processing residues (tailings) on the site. Contamination of the surface and at depth may also occur as a result of spills or leakages of petroleum products (petrol, diesel, kerosene) or process chemicals (xanthates, cyanide). To address the latter two issues at source, there needs to be regular mass balance monitoring of inventory held in storage so that leakage can be detected and repairs effected as soon as possible.

Monitoring of shallow groundwater bores in these areas will indicate whether there has been contamination of groundwater. Best practice for storage of these materials requires that tanks be located on impermeable pads bounded by bunds high enough to contain the volume of the tanks.

A site assessment will be needed to determine the extent of contamination and to quantify the human health and ecological risks associated with it. The outcomes from the assessment inform rehabilitation or management plans needed to maintain or restore the current or future condition of the land. In this context, the metal content of material used to produce the final surface layers of rehabilitated landforms also needs to be consistent with the final land use. When considering the area of land that needs to be assessed, the propensity of mineralised dust to be carried considerable distances downwind should be taken into account.

A fundamental requirement for contaminated-land assessment is to do detailed site mapping that shows the distributions and levels of contaminants. Conventional soil sampling with post-collection analysis constrains the number of sampling points (because of cost) and potentially reduces the resolution of the assessment.

The development and ready availability over the past decade of field-portable metal and organics analysers with good detection limits have revolutionised the conduct of contaminated site assessments at mine sites. Handheld X-ray fluorescence analysers (Hall et al. 2012), in particular, can be used to screen for concentrations of relevant metals at dozens of grid points in a day. Thus, areas with metal concentrations above environmental screening thresholds can be rapidly identified and the initial sampling grid refined as the survey is being conducted, resulting in substantial increases in efficiency, a reduction in overall cost and a more rigorous assessment. Any relationships between concentration and depth can also be rapidly assessed by digging shallow holes, which is especially useful for estimating the areal extent of surface contamination from dust fallout.

This approach contrasts with past practices in which samples were collected and then sent for analysis in a laboratory, with weeks often elapsing before the results were obtained. Repeat infill sampling was then often needed to fill in gaps in the coverage introduced by imposing a relatively coarse random grid over the area to be sampled.

A leading practice mine site determines the baseline concentrations of metals in the surface soil before starting operations in order to provide the basis for developing robust closure criteria. It is especially important to do this in mineralised areas, where shallow outcrops of mineralised material could later be wrongly attributed to contamination from mining-related activity. Some of this baseline information may be available from the exploration database, but a purpose-commissioned survey may also be needed to complete the coverage required. Longitudinal surveys through time enable any trends in contamination to be determined. Such surveys are especially important in areas likely to be affected by metal-containing dust.

A leading practice mine site also minimises the extent of surface contamination by avoiding or minimising the use of mineralised material to construct roads and other infrastructure, such as water diversion bunds or the embankments of tailings dams.

The framework that has been accepted in all Australian jurisdictions for the assessment of site contamination is the National Environment Protection (Assessment of Site Contamination) Measure (NEPM). While not specifically developed for assessments of contamination by mining, the framework contained in the NEPM is directly applicable to the design, conduct and interpretation (in both human and ecological health risk contexts) of a mine site assessment. The most recent version of the NEPM and associated schedules (2013) is available for download through the Standing Council on Environment and Water. The most significant changes in the latest version of the NEPM that are relevant to mine site assessments are summarised below.

The 1999 NEPM ecological investigation levels (EILs) have been expanded and cover a range of soil types and constituents that apply for fresh and aged contamination in soil. The current NEPM requires that both the potential effects to human health and the environment (ecology) of metals and metalloids be fully evaluated.

The NEPM (SCEW 2013) now defines the EIL as the concentration of a contaminant above which further appropriate investigation and evaluation of the impact on ecological values is required. The EILs are calculated using EC30 or lowest observed effect concentrations toxicity data. EILs are the sum of the added contaminant limit and the ambient background concentration, and the limit is expressed in terms of total concentration. EILs depend on specific soil physicochemical properties and land-use scenarios and generally apply to the top 2 metres of soil.

The methodology used for deriving the EILs used in the ecological risk assessment framework is described in schedules B5b and B5c of the NEPM. Because the toxicity of some contaminants is affected by soil physicochemical properties, empirical relationships are used to model the effect of soil properties on toxicity so that soil-specific EILs can be developed. A supplied EIL calculation spreadsheet provides step-by-step guidance to enable the derivation of EILs specific to the site.

The tiered ecological risk assessment approach used in the latest framework facilitates the:

  • identification of the ecological receptors of concern
  • estimation of the concentration of a contaminant of concern to which the ecological receptors are exposed
  • consideration of the toxicity-modifying or toxicity-enhancing capacity of the receiving environment (whether that be soil, sediment or water)
  • determination of whether the ecological receptors and ecological values may be at risk
  • application of a multiple-lines-of-evidence approach to assess risks.

The tiered approach screens out those sites where the environmental risk is minimal and enables resources to be focused on those locations that pose the greatest potential risk.

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