Gippsland Environment Group (GEG) recently organised a site visit to the Benambra mine tailings dam in the headwaters of the Tambo River to discuss mining company CopperChem’s proposal to re-open and massively expand the dam. The visit raised considerable alarm among all who attended. The Stockman mine has long been a controversial project and plans to re-open the operation have caused serious concerns in the community.
The following report comes from GEG:
Members of local Landcare Groups, Victorian National Parks Association, Gippsland Environment Group, Environment East Gippsland and other interested locals travelled to the Stockman Project east of Benambra to inspect the tailings dam. Staff from Earth Resources (Department of Economic Development, Jobs, Transport and Resources) and Copperchem the mining company that has applied for a licence to expand the tailings dam, were also present to answer questions.
The original Benambra copper and zinc mine was operated by Denehurst Pty Ltd from 1992 until 1996. In 1998 the company went into receivership and abandoned the site forfeiting their mine rehabilitation bond of $375,000. Denehurst left behind an unstable tailings dam containing 700,000 tonnes of toxic tailings leaking acid and heavy metals into Straight Creek which runs into the Tambo River. The tailings dam had been constructed right across the waterway over a series of groundwater springs and had been built without a spillway. Between 1999 and 2005 the EPA authorised DPI to make emergency releases of 160 megalitres of contaminated water to reduce the risk of the dam wall overtopping and undermining the dam foundations.
In 2006 DPI rehabilitated the mine site and tailings dam at a cost of $7 million to the taxpayer. In the process of remediating the tailings dam and constructing a spillway another 140 megalitres of tailings dam water was released into the waterway. The dam operates as a flow through system spilling into Straight Creek as inflows from the catchment raise the water level over the tailings. The dam water was to the mouth of the spillway at the time of the site visit.
The original tailings dam was constructed from rockfill with a concrete grout curtain at the base which has a lifespan of between two and eighty years. Due to a shortage of local clay the dam wall was lined with a geomembrane that has a lifespan of possibly another thirty years. It is impossible to replace either the concrete grout curtain or the liner as they are on the upstream side of the dam wall. An additional HDPE liner was attached to the old one when DPI raised the height of the wall in 2006. This join was inadequate, and potentially acid forming (PAF) rock was used in the embankment. The dam is currently leaking from beneath the toe of the dam at a rate of approx 86,000 litres/day.
Community members were shocked to learn that the enlarged dam will be built on top of this unstable foundation and that before work begins another three metres of contaminated water will have to be released from the tailings dam.
CopperChem plans to raise the existing dam wall another 25 metres in a series of staged lifts up to a total of 45 metres above the valley floor, which will increase the surface area from approx 7 ha to 32 has. Each raise will exacerbate the stability risks and increase the head pressure on seepage already flowing from below the dam wall. HDPE plastic liners will be used on the inside of the wall to contain the toxic tailings and polluted water. These liners have a lifespan of only 100-200 years. HDPE liners are a short-term method of dam design that does not address the long-term management requirements of such a toxic storage facility. The tailings must also remain covered with at least two metres of water forever to prevent an acid change reaction occurring. The impact of climate change on the dam water level over the next thousand years is impossible to calculate.
In December 2017 CopperChem signed a post-closure trust fund deed with the State Government to fund management of the tailings dam in perpetuity after mining ceases. The company is required to lodge a bank guarantee of only $5.7million before work begins. The post-closure trust fund does not cover residual risk and does not include any costing for the remediation of the dam wall when the HDPE liners fail or in the event of catastrophic dam failure. The company will contribute $1million towards insurance against environmental or property damage. It is not clear what amount taxpayers will also contribute.
The original Benambra mine paid no royalties during the period of operations and cost the State Government a total of $13million in financial support for the mine and rehabilitation costs after it was abandoned. Following remediation in 2006 the tailings storage facility was renamed Lake St Barbara and exempted from any future mining operations.
Earth Resources representatives on site were unable to explain why that exemption was recently revoked by Resources Minister Tim Pallas. The groups were united in their concern that any proposal to reopen and expand the tailings dam was foolhardy in the extreme and would jeopardise the health of Tambo River and Gippsland Lakes forever.
As the site visit concluded the consensus of all community members present was that the Minister for Resources Tim Pallas must reject Copperchem’s application for a mining infrastructure licence over the tailings dam.
Objections to the mining infrastructure licence application by CopperChem’s parent company WHSP Stockman Pty Ltd (Licence app no. MIN006642) may be lodged on-line at https://rram.force.com/ObjectionSubmission by April 4th.
Gippsland Environment Group
Further reading: GEG Inc objection to MIN006642, 5 March 2018