Surface of a former mine in French Guiana, due to be rehabilitated.
© Naomi Nitschke
The French overseas territory of French Guiana has geological formations rich in gold within its greenstone belts. Various extraction techniques – both legal and illegal – are used in parallel.
Illegal gold panning, which by its very nature is unregulated, involves the use of mercury to extract gold, and leads to contamination of rivers and the surrounding soil.
While legal gold mines in French Guiana are prohibited from using mercury in the gold extraction process, this does not mean that their environmental impact is negligible. Standards have been established to minimise the impact of mining, both during and after extraction. Under these, mining companies are required to rehabilitate their mining sites once operations have ceased.
However, although these techniques help to limit soil erosion, they are not sufficient to restore all of the soil’s biological functions.
Loss of soil structure and spread of pollutants
To understand the environmental impact of gold panning, it is worth beginning by pointing out that gold extraction involves removing the surface layer of soil to gain access to the gold-bearing stratum.
This soil, which then loses its structure and is left exposed, is more susceptible to erosion, which results in large quantities of particles being carried into rivers, with harmful effects on their ecosystems.
Moreover, the soil of French Guiana (and the Amazon region in general) is naturally rich in mercury, the result of atmospheric mercury accumulating in it over millions of years. This mercury is therefore also released with the soil into the rivers, where it can enter the food chain and cause health problems.
Remediation that limits erosion
With legal gold panning, remediation involves carrying out earthworks at the mining site, restoring the affected watercourse and planting trees over at least 30% of the mining site’s area.
When a remediation project is implemented, erosion is reduced by two-thirds within six months. After three years, it falls to virtually zero. Given that the vast majority of the mercury is combined with soil particles, reducing erosion also leads to a decrease in mercury being washed into the river.
This rapid stabilisation of the soil is mainly due to the spontaneous regrowth of herbaceous species on the surface of the mining site, rather than to any planting of vegetation by the mining company. This does not mean that remediation measures serve no purpose. These earthworks and the restoration of the river's natural course ensure that any flooding is contained within a defined area and help the herbaceous plants to thrive.
Growth in vegetation following the remediation of a plot of land on a mining site.
© Naomi Nitschke
Limited recovery of the soil's biological activity
However, superficial herbaceous plant cover is not enough to ensure the effective remediation of a site. The ultimate aim is to have a secondary forest.
In order to transition from a vegetative stage (herbaceous plants and shrubs) to a forested stage, the soil's functions must be restored.
Ultimately, no change in these various indicators was observed over the five-year period under study. They remained at levels far lower than those observed in the surrounding forest.
It can therefore be concluded that while current remediation techniques can successfully limit soil erosion, they are far less effective at stimulating the recovery of soil functions and re-establishing the different carbon and nitrogen cycles.
Reforestation through the "edge effect"
There is, however, a sign of hope. Although no changes over time were observed in this study, spatial differences were noted.
On the edge of the mining site, indicators of enzymatic activity and of nitrogen and carbon concentrations in the soil were higher than within the mining site. This can be explained by inputs of organic matter, micro-organisms and even seeds from the neighbouring forest during rainfall.
This phenomenon, which has been extensively studied elsewhere and is known as the "edge effect", probably results in the effective reforestation of mining sites, starting from the boundaries and working inwards.
The need to change strategy
At present, the government regulatory bodies responsible for setting remediation standards focus primarily on reducing transfers of particles into rivers. This objective has broadly been achieved.
However, close attention should also be paid to soil functions, to prevent rehabilitated sites from remaining at the vegetative stage without any real forest regrowth. There are strategies that enable soil functions to recover more quickly, thus leading to more effective remediation, but these naturally come at an additional cost.
