Metals in water used by artisanal and small-scale gold miners for gold-mercury amalgamation in Western Kenya

BACKGROUND: Workers in artisanal and small-scale gold mining (ASGM) are exposed to metals and metalloids (referred to here as "metals") while mixing milled ore and elemental mercury to produce a gold-mercury amalgam. Although concentrations of metals in surface waters near ASGM activities have been described, little is known about concentrations of metals in the water-ore-mercury slurry with which workers have extensive dermal contact. We sought to characterize those concentrations. METHODS: Water samples (n = 76) were collected from amalgamation basins and milled ore washing ponds at 13 ASGM sites in Western Kenya. Samples were filtered and metals in the filtrate and metals retained on filters were analyzed for trace elements by inductively coupled plasma mass spectrometry (ICP-MS). Based upon the volume filtered and ICP-MS results, total metal concentrations in the original samples (pre-filtration), were calculated. RESULTS: Concentrations of metals in the amalgamation basins were high. The median concentrations of arsenic (240.23 µg/L), chromium (312.97 µg/L) and total mercury (3.52 µg/L) all exceeded Kenya's drinking water standard by several fold. Only 3.38% of arsenic, 0.28% of chromium, 40.51% of manganese, 0.22% of mercury and 0.01% of lead mass were in filtrate, with the remainder of the metal mass retained on filters. CONCLUSIONS: Concentrations of arsenic, chromium, manganese and lead to which ASGM workers are exposed in the amalgamation process were approximately 5-100-fold higher concentrations than reported in prior studies of metals in surface waters near ASGM sites. These findings should be useful in assessments of exposure and health risk of the many thousands ASGM workers who amalgamate milled ore. The high concentrations of As, Mn and Hg put at risk the health of children who live near or work at ASGM sites. Policy measures and changes in occupational practices are urgently needed to reduce Hg use in ASGM and to protect individuals from metals present in milled ore.