Environment

An important part of the development of the Córrego do Sítio mine, which is located near the town of Santa Barbara in the province of Minas Gerais in south-eastern Brazil, has been the planning for and execution of the rehabilitation of land affected by mining.

After AngloGold Ashanti’s biennial environmental workshop, which was held during November 2008 in Brazil, delegates were offered the opportunity to attend training on a low cost, rapid rehabilitation performance assessment methodology by a leading Australia-based landscape ecologist at the Córrego do S?tio mine. Developed and used extensively in Australia, Landscape Function Analysis (LFA) is a field monitoring procedure that uses rapidly acquired field-assessed indicators to evaluate the bio-geochemical functioning of landscapes at a single slope level.

Says consultant David Tongway, “Monitoring a landscape’s health over time in response to environmental, management or regulatory drivers is important for land managers, ranging from individuals to governments, especially when the monitoring output has direct relevance for management decision-making. Monitoring may be seeking to look for evidence of landscape degradation or of rehabilitation progress and the procedure needs to have equal facility in dealing with these scenarios. Monitoring may also provide information for day-to-day property management.

“Evaluating and setting expectations for mine site rehabilitation needs to take into account the nature of the landscapes in the vicinity of the mine. In the case of Córrego do Sítio, the surrounding landscape for some distance around the mine site is naturally quite erodable, with numerous land slips off hillsides visible to the casual observer. These are unrelated to mining and, in some cases, the result of human land use activities. I have seen similar intrinsically unstable landscapes in Papua New Guinea and in Madagascar.”

Successful rehabilitation at the mine is further complicated by the non-cohesive soil material which is available and high levels of rainfall in the area (as much as 1500mm per annum) and consequently high levels of soil erosion.

As part of the analyses undertaken during the course, several new and prospective monitoring sites were assessed. The current rehabilitation techniques employed by the mine includes spreading a grass-litter, comprising seeds and grassy mulch, secured by a twine net over the slope to be rehabilitated. The slope itself has been covered with stockpiled topsoil. The twine for this netting is purposely non-biodegradable as it is able to hold the grassy litter in place for more than 10 years, and is not moved by overland flow.

This surface cover provides an immediate protective cover from raindrop erosion and, over time, permits the germination of grass, fern and forb species, protecting the soil from surface erosion. LFA indicators on previously rehabilitated sites show an improvement over time, indicating that the surface vegetation has played a valuable role. The LFA typically looks at three key indicators – stability, infiltration and nutrient cycling. The efficacy of this process is demonstrated in the images below, and in the table that shows the LFA values for an unrehabilitated slope, a newly rehabilitated slope, and a slope rehabilitated some 18 months ago, compared with a secondary forest area at the mine.

Fresh soil prior to being covered with a grassy net. The high basic erodability is very evident indicating that adding the net is the highest priority after land-forming.

Grassy mat now covering the entire slope.

Grassy mat has now been superseded by dense growth of grass, ferns and forb, thus providing a living cover to mitigate against soil erosion and provide a transpiration pump.

It is planned that eventually an analogue to the natural forest should be established over as much of the disturbed land as possible. Typically this occurs over decades and is virtually assured once the driving factors that the LFA measures are in place, namely stability, infiltration and nutrient cycling.