Warrender, R, Prestia, A, Bowell, RJ, Byrns, C, Rakvica, B & Spidell, J 2013, 'Defining an approach for contaminated land management in the context of mine reclamation in the historic Comstock mining district, Nevada, USA', in M Tibbett, AB Fourie & C Digby (eds), Mine Closure 2013: Proceedings of the Eighth International Seminar on Mine Closure
, Australian Centre for Geomechanics, Cornwall, pp. 55-68, https://doi.org/10.36487/ACG_rep/1352_06_Warrender
Many abandoned mine sites are characterised by elevated concentrations of environmentally sensitive elements such as arsenic, lead and mercury that can cause contamination of surrounding soil and water resources and may have associated ecological and health impacts. Often these sites have important historic and cultural values that require access to be maintained, despite potential toxicity of the waste. Detailed site characterisation is therefore essential to ensure implementation of appropriate management measures and prevent potential impact to receptors.
This paper presents an approach to evaluating potential environmental impacts in a historically important mining district, the Comstock gold and silver mining district in Nevada, USA. For this site, geochemical soil surveys demonstrate elevated levels of arsenic, lead and mercury that are influenced by anomalous geological background concentrations connected to bedrock mineralisation, placement of mineralised waste rock and element redistribution around sites of mineral processing. The goal of the data evaluation is to develop screening levels against which geochemical data could be compared and classified for purposes of management of the materials on-site. A common approach for contaminated land is the application of normal probability plots. However, where materials have been influenced by different processes (e.g., hydrothermal mineralisation, supergene weathering, mining and processing), the inherent differences in these materials may mask or overshadow geochemical anomalies and such an approach is flawed. In order to compensate for these differences, a site-specific mineralogical and uni-variant statistical approach has been applied. This approach can be used as an initial (Tier 1) screening tool to isolate undisturbed mineralised outcrop from disturbed areas or mine waste.
Further evaluation of the geochemical data can then be undertaken as part of a Tier 2 assessment, including an assessment of contaminant bioavailability and leachability using selective extraction and physiologically based extraction tests. A Tier 3 assess would then involve the employment of geochemical predictive calculations to determine the potential of contaminants to disseminate in the environment. In this way, the areas demarked as having potential to impact the environment and interact with a receptor can be identified and appropriate management strategies implemented. This approach has the benefit of controlling costs and protecting the cultural value of historic mining areas while still allowing protection of the environment and mitigation of potential future environmental impacts.
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