7.19 Screening level risk assessment - a tool for optimising closure and
mitigation strategies at the Big Springs Mill Site
Throughout the world, regulators are reluctant to grant certificates of closure for mining operations. The main concern centres on the uncertainty about the development of any potential future impacts from the site and who then carries the responsibility, and financial liability, for addressing these. By undertaking a risk assessment methodology to identify and predict the likelihood and consequences of any future impact, and thereby provide an improved level of risk assurance, AngloGold Ashanti USA aims to address these concerns in respect of the Big Springs Mill Site.
Part of the USA's operations, the Big Springs project was situated near Elko in Nevada, where operations began in 1988 and ended in 1999. Two gold recovery processes were used during the life of the Big Springs project: one was a conventional CIL (carbon-in-leach) circuit (with associated tailings storage facility) and the other a heap leach process.
Mill site closure and reclamation activities began soon after the cessation of operations, and included the construction and operation of two evapo-transpiration basins (and associated drainfields) to manage the process water associated with the leach facility. Both the basins and the drainfields were constructed as part of final reclamation activities to manage the near-term drain-down (0-3 years) and long-term flow from the reclaimed heap leach pad and tailing storage facility
(TSF) (See diagram 1). To address concerns about the long-term risks to the plant communities, livestock, avian and terrestrial wildlife, a screening-level ecological risk assessment (SLERA) was carried out. The aim of this was to provide assurance about the long-term effectiveness of the closure activities and to achieve closure of the project in 2004.
Jonathan Gorman, environmental manager at AngloGold Ashanti (Nevada) Corporation explains the main objectives of the risk assessment:
- to identify the constituents (metals and salts such as antimony, selenium, cobalt, molybdenum, manganese, sulphate, and nitrate) and their concentration in the heap pad and TSF water, as well as the heap pad solids, that could affect vegetation, livestock, and terrestrial or avian wildlife;
- to identify and establish a plant community that would grow (and possibly thrive) if exposed to these metals or salts in the water or soil; and
- to quantitatively and/or qualitatively assess the risks to the vegetation, livestock, or terrestrial or avian wildlife exposed to metals or salts in the water.
The SLERA incorporated site-specific data from five sources, namely process water quality analyses; chemistry of the soils, heap, and tail material; leaching and attenuation testing of the soils, heap, and tail materials; vegetation chemistry and metals bioavailability assessment; and baseline soils, vegetation, and wildlife inventories.
To inform the study, data from risk and closure studies carried out at similar mining sites and agricultural studies with elevated metals and/or salts were used.
The potential impacts to ground water, from the near-term drain-down and long-term flow, from heap pads and TSF had previously been analysed by the company and a consulting firm. This had found that there would be no impairment to potable water supply uses. Attenuation (the process by which metals and salts are adsorbed by the unsaturated soils) and dilution processes (such as seasonal rainfall) in the vadose zone (the unsaturated soils above the ground water table) below the basins (and associated drainfields) would reduce all chemical constituents in the drain-down water to drinking water standards, or better.
Three potential exposure pathways (conservative 100% exposure) to the vegetation community were considered
(see diagram 2), namely: direct contact between heap leach solids and vegetation (via roots); indirect contact between heap leach solids and vegetation (via pore water interaction with roots); and direct contact between heap leach and TSF water and vegetation (through irrigation).
Three potential exposure pathways to wildlife/livestock were also considered (see diagram
2), namely: direct ingestion of water by livestock, and avian and terrestrial wildlife; consumption of vegetation (forage/diet) over the basins and drainfields by the livestock or wildlife, where the vegetation has been exposed to the drain-down water; and ingestion of heap material by the selected livestock and wildlife.
The SLERA concluded that concentrations of arsenic, antimony, selenium, cobalt, molybdenum, and manganese could exceed one or more relevant ecological screening-level benchmark criteria for vegetation for either the water or the solids; and concentrations of arsenic and selenium were the only trace element constituents that could exceed one or more ecological screening-level benchmark criteria for wildlife or livestock for either the water or the solids. Based on the realistic exposure assessment (water and vegetation from the evapotranspiration basins would constitute significantly less then 100% of the exposure) and site-specific data, the risks posed by these constituents under the implemented closure water management strategy were judged to be negligible.
Thus, the SLERA methodologies applied at the Big Springs mill site provide a powerful tool to identify and evaluate ecological risks at mining operations. Based on this realistic risk identification and prioritisation process, AngloGold Ashanti can optimise closure and mitigation strategies, maintain biodiversity, and protect local beneficial uses of the land and water.
Says Andrew Mackenzie, manager, corporate environmental affairs, "This application of the risk assessment methodology does not guarantee that a closure certificate will be granted or that the company will be released from any future liabilities arising at this site. However, it provides significant additional information that will greatly assist in obtaining final site closure. The mining industry will need to continue its negotiations with government in an attempt to develop mutually acceptable mechanisms that provide an acceptable degree of risk assurance and at the same time release shareholders from indefinite future liabilities".
Click to enlarge image
Click to enlarge image
A heap leach pad is a lined facility, with an impervious base, on which a heap of gold-bearing ore is stacked. A high pH cyanide-based leaching solution is then sprayed (or dripped) over the heaped ore dissolving the precious metals as it drains down through the pile. The gold-bearing solution is then collected in drainage sumps and pumped to a gold treatment plant for further processing.
A tailings storage facility (TSF) is an engineered dam, designed and constructed as a repository for the ground rock after gold has been extracted in the treatment process. TSFs are usually associated with conven-tional gold processing plants employing carbon-in-leach, carbon-in-pulp or biox treatment processes. Although the precious metals have been removed, a concentration of other metals and salts can remain in the material.
An evapo-transpiration basin is a facility, constructed at closure, which receives water from the reclaimed heap pad or TSF. The basin is typically constructed from an existing plastic-lined pond that is then backfilled with soil and rock. The upper one metre of the basin is constructed with top soil in which a plant community is established. The plants absorb water through their roots and
'transpire' water from their leaves, thereby reducing the total volume of water on the heap pads or in the TSF during closure.