Environment

An innovative project at Cerro Vanguardia mine (CVSA) in Argentina was undertaken in 2008 to prevent the potential contamination of groundwater by low levels of cyanide.

Started in 1998, the Tailings Storage Facility (TSF) at Cerro Vanguardia is surrounded by a number of boreholes, sunk into the ground at strategic locations for the specific purpose of sampling water in the area. During routine water monitoring at the boreholes in October 2003, an isolated peak in cyanide concentration was first detected. The peak was only reflected in the samples of one particular borehole, CVM 7, and later in replacement hole CVM 10 but not at any of the other boreholes surrounding the TSF. The recorded peak in cyanide levels – a value of 0,65ppm – was above acceptable limits and management wanted to understand why this peak was occurring.

Mine management immediately stopped tailings discharge in the area, and commissioned a study to determine the source and location of the infiltration into the groundwater. Four new monitoring wells were constructed and detailed geophysical studies were conducted in the TSF area.

The study revealed that underneath the tailings dam, there runs a (previously unknown) quartz vein. This underground geological feature, known as an intrusion, creates a natural conduit for water infiltration to the groundwater system below. Running from east to west, this vein was likely to be responsible for the seepage to groundwater – and it was this leakage that was detected in monitoring borehole CVM 7 and later in CVM 10.

An engineering project led by AngloGold Ashanti hydrogeologist John de Souza and implemented by a multidisciplinary team began, addressing the way the geological intrusion was affecting the flow of water from the TSF.

The project involved scraping back the earth around the TSF (using earth-moving equipment) to expose the geological intrusion, and then placing a 1.5mm-thick high-density polyethylene (HDPE) covering layer to arrest the water seepage downward flow. This, in turn, would prevent raised cyanide levels in the groundwater. HDPE is essentially plastic sheeting, made from petroleum.

As depicted in the accompanying photos, the laying down of the HDPE sheeting began in 2007 at Cerro Vanguardia, covering an area of about 80 metres by 400 metres. Two layers of the sheeting were laid down, one a few inches below the other, to ensure that if one layer was punctured, the lining would still protect the groundwater. Electronic sensors were installed between the layers to measure and monitor the water as it seeps through and reaches the plastic lining. The installation phase came to a successful end in 2008, and regular monitoring and evaluation of the lining and the groundwater from the TSF indicate that the issue of peaking levels of cyanide has been successfully addressed.