Spatial variation of mercury in surface water impacted by artisanal gold mining in Marmato, Colombia

Marmato, Colombia has been an important centre of gold mining since before the first Spanish colonisers arrived in 1536. The district is currently experiencing a surge in both major mining projects and artisanal mining, driven by the high price of gold. Ore from small scale and artisanal mines is processed in numerous small mills or ‘entables’, which impact surface water quality through the discharge of milled waste rock slurry and highly alkaline cyanide-treated effluent. A pressing environmental concern is the use of mercury to amalgamate gold in concentrates from the small mills, which is widespread in other gold producing regions of Colombia. To investigate the spatial distribution of mercury and other potentially toxic elements, surface water samples were collected around Marmato and analysed for dissolved Hg using atomic fluorescence spectroscopy (AFS), and for other trace metals using ICP-MS. The total dissolved Hg in stream water ranged from below detection limits to 142 ng L-1, despite the deposit having a low Hg signature. Other potentially toxic metals, including Pb, Sb, Cd and As were also elevated, but were attributed to the sulphide mineralogy in the deposit. The spatial relationship between dissolved Hg and possible point sources of pollution was analysed using ArcGIS software. The highest values of dissolved Hg are around the Marmato gold deposit where there is a concentration of small mines and mills. Since this is a low Hg deposit, a natural source for Hg can be eliminated, confirming that dissolved mercury reflects sporadic use by artisanal miners