EHS Support are currently engaged by the Department of Energy, Environment and Climate Action (DEECA)/Earth Resource Regulator (ERR) to undertake environmental monitoring at several of its legacy mine sites across Victoria. The Kangaroo Flat Former Gold Mine and Woodvale Evaporation Pond Complex (WEPC) formed an important part of Bendigo’s gold mining history. However, since the sites operators when into liquidation, ERR inherited the responsibility to manage and rehabilitate the sites under Section 83 of the Mineral Resources (Sustainable Development) Act 1990.
The Kangaroo Flat Former Gold Mine comprises of a network of mine shafts, a processing plant which historically used carbon-in-leach gold extraction methods (including use of cyanide), several tailing storage facilities and process water storage dams, and auxiliary buildings such as workshops, drillers yards and offices. The WEPC which is located approximately 15 km north of Bendigo was historically used to store groundwater extracted as part of historical gold mining operations across the Bendigo region. The WEPC comprises of a network of 7 main evaporation ponds, 2 of which have already been rehabilitated. Historically, seepage of the pond water has known to directly impact the underlying shallow and intermediate aquifers (Shepparton Formation).
To assist in the management of the sites and to ensure DEECA/ERR are meeting its obligations under the Environment Protection Act, 2017, including its duty to manage contaminated land, EHS Support were engaged to undertake routine environmental monitoring at the sites. The overall objective of the environmental monitoring was to monitor and manage potential environmental risk associated with the sites prior to, during and after rehabilitation.
The scope included the development of a comprehensive environmental monitoring program which was approved by an EPA appointed environmental auditor, quarterly surface water and groundwater monitoring, monthly depositional dust monitoring and ongoing continuous real-time dust monitoring.
The quarterly surface water and groundwater monitoring focused on assessing potential risk associated with seepage from surface water bodies including evaporation ponds and tailing storage facilities using a comprehensive network of groundwater monitoring wells which screen various aquifers including the Shepparton Formation, Deep Lead and the Castlemaine Group. Ongoing monitoring includes measuring depth to water, collection of field water quality parameters (such as temperature, pH, redox potential, electrical conductivity, total dissolved solids, turbidity, dissolved oxygen) and groundwater sampling using a variety of groundwater sampling methods in accordance with EPA Publication 669.2 including low flow sampling and hydrasleeves. Groundwater and surface water samples are analysed for key contaminants of potential concern (CoPCs) including arsenic (and other metals), cyanide, sulfate and salinity, along with major ions and stable isotopes.
Based on the results of the surface water and groundwater monitoring programs, EHS Support has been able to use general hydrogeological principles including groundwater elevations, conductivity, horizontal and vertical hydraulic gradient and seepage velocity to identify whether seepage from pond water has occurred, and which aquifers may potentially be impacted. Furthermore, EHS Support has used analytical results for stable isotope, major ions and Na/Cl ratios and CoPCs to identify which areas and aquifers have potentially been impacted by seepage, and the extent in which seepage water may be migrating from the site.
Moreover, the data from the surface water and groundwater monitoring programs, combined with reviews of surrounding groundwater users, groundwater dependent ecosystems and evaluation of localised natural groundwater quality of the Shepparton Formation and Castlemaine Group have been used to assess whether there is a potential risk to environmental values of water in accordance with the Environment Reference Standard, 2021. Based on the inferred extend of seepage impacts, the distance to the nearest groundwater dependent ecosystem, the absence of extractive groundwater users down gradient of the site, and the natural variability in groundwater quality, EHS Support concluded that the risk to environmental values of groundwater was low and acceptable.
The dust monitoring program comprises of monthly depositional dust monitoring including analysis of soluble an insoluble key contaminants of concern such as arsenic and the establishment of real-time continuous dust monitors in strategic locations. Depositional dust is collected and analysed at the site in accordance with AS/NZS 3580.9.15-2014 and EPA Publication 1961. Depositional dust gauges are located on each boundary of the site and within a background location, and are analysed on a monthly basis for total insoluble solids, soluble solids, total solids, ash content and combustible matter. In addition, the soluble and insoluble dust fractions are also analysed for CoPCs including arsenic and cyanide. The depositional dust monitoring program provides information which can be used to assess the origin of dust and whether its likely to pose a risk to receptors via various migration pathways.
The real-time dust monitoring includes the establishment of real-time dust monitors which continuously reports PM2.5, PM10 and TSP dust fractions in real-time via a cloud base platform. Monitors are placed in areas of the site most conducive of dust generation and adjacent to potential receptors. The real-time nature of the monitoring networks has allowed for timely and informed decisions to be made with respect to adaptive dust mitigation management measures.
The results of the depositional dust and real-time dust monitoring programs are used to assess potential risk to environmental values via various transport mechanisms. These includes inhalation by adjacent residents or on-site workers, deposition of dust onto soils and direct contact by ecological and human health receptors, deposition of dust onto roofs and accumulation in tank water potentially used for drinking water, and deposition into surrounding water bodies or swimming pools used for recreation. EHS Support has been able to maximize the use of the available data to estimate concentrations in air of key CoPCs and compare the concentrations to air pollution assessment criteria (APACs) defined in EPA Publication 1961. In addition, estimated air concentrations have also been compared to concentrations previously used to model deposition rates into rainwater tanks and dam water. This has allowed an assessment of potential risk to be undertaken, and has set up the foundations for risk based dust monitoring when rehabilitation activities are undertaken in the future.
In addition to the monitoring program, EHS Support developed a data dashboard/web-base mapping tool with an integrated automated reporting function. This tool was developed as it allowed key stakeholders to view and interact with site analytical results in real-time (rather than wait for reports to be issued). This essentially reduced the need for quarterly reporting and as such resulted in both direct and indirect cost savings.