Reconnaissance study of groundwater quality in the artisanal gold mining districts of Migori County, Kenya

Information on groundwater quality in areas of artisanal and small-scale gold mining assists in the sustainable development, use and protection of groundwater resources. Groundwater contamination associated with mining may occur for different reasons: from pumping of mineralised and acidic mine water due to oxidation of previously reduced ore; from point or diffuse sources of contamination, mostly represented by leaks and runoff from mine tailings; and from leaks of spent water from processing plants. The extent of groundwater contamination is also influenced by many variables besides mining practices, including climatic conditions, geology and geomorphology, and their interaction. The British Geological Survey (BGS) collaborated with the University of Nairobi to carry out a groundwater quality reconnaissance and mine drainage characterisation in Migori County, Kenya. Migori County is a major artisanal and small-scale gold mining (ASGM) centre located near Lake Victoria, in southwest Kenya, close to the Tanzanian border. Gold is produced from quartz– carbonate veins within the meta-volcano-sedimentary rocks of the Precambrian Migori Greenstone Belt. This report describes the sampling and chemical analysis results of baseline water samples collected from twenty-one boreholes, dug wells or springs and seven mine shafts and panning ponds during the rainy season in November 2019. Samples were analysed for inorganic chemistry composition. The analysed inorganic constituents of waters from the boreholes, dug wells, and springs reveal that most parameters are within the acceptable limits of the World Health Organization drinking water standards (WHO, 2017). Nevertheless, there are a number of WHO exceedances of the permissible drinking water limits. These are summarised below. The nutrient nitrate (NO3) content is generally low and only found above the WHO drinking water standard of 50 mg/l in one dug well and one spring, which may be impacted by contamination from domestic wastewaters, household septic tanks and pit latrines. The fluoride (F) content is generally less than the WHO drinking water standard of 1.5 mg/l, with three exceedances out of twenty-one groundwater samples, all deep (>50 m) groundwater and with F less than 2 mg/l. Arsenic (As) shows more exceedances of the WHO drinking water quality guideline of 10 μg/l, in five of the twenty-one groundwater samples (values from 13.7 μg/l to 129 μg/l). Although high arsenic in groundwater has not previously been recorded in the area, these new findings are comparable to values already known from neighbouring mining areas in the Lake Victoria Basin in Tanzania. The primary source of As is likely to be the As mineral arsenopyrite (FeAsS), which constitutes an important part of the ore assemblage in the gold-quartz veins. The relative importance of anthropogenic and geogenic sources and the processes affecting high concentrations of arsenic in groundwater require further research. Other trace elements (e.g. Cd, Co, Cu, Ni, Pb, Sb, Zn) also potentially linked to the quartz-gold vein ore mined in the area are found in low concentrations and, for those elements of healthbased concerns, are below the relevant drinking water quality standards in all the groundwater samples. Mercury (Hg) data have been reported, although all values should be taken as a minimum due to a problem with field preservation during sampling. Mercury presence in ASGM areas is due to its use to extract gold from ore as an amalgam, and is a major human health concern. Provisional total mercury concentrations were well below the WHO standard for drinking water of 6 μg/l (for inorganic mercury) in all groundwater samples, with a median value of 0.016 μg/l. A relatively high value of dissolved Hg of 0.16 μg/l was found in a dug well water in the proximity of an ASGM site. At this site gold is extracted from tailings via tank leaching using cyanide solution and spent tailings heaps accumulate. This single sample may indicate the potential environmental threat that the cyanide gold leaching practice of mercury-laden tailings poses to groundwater, due to the formation of water-soluble cyanide-mercury complexes which enhance mercury mobility. Additional sampling and analysis of mine waters was done, as they are a potential source of contamination and also, in some cases, may be used for domestic water supply. The mine water was sampled from shafts at four sites where gold-quartz veins are mined. The mine waters, sampled from shafts mining the gold-quartz veins at four different sites, show wide variation in the concentrations of total dissolved elements, but none of them are acid, with a range of nearneutral pH values from 6.19 to 7.75. They are all bicarbonate waters and their conductivity (SEC) increases with mine shaft depth (SEC 164 to 1733 μS/cm). Fluoride and arsenic are the only elements that exceed WHO drinking water standards. Fluoride is above the WHO of 1.5 mg/l in one deep mine. Arsenic is above the WHO standard of 10 μg/l in all four mine waters and greatly exceeds the limit in two of them (178 and 190 μg/l). The chemistry of water from two panning ponds and tailings leachate confirms previous findings, being highly mineralised, with sulphate as major ion. There is variable composition in terms of potential hazardous elements, which is likely to reflect the different ore type processed at each mine: i) very acid (pH 3) metal-rich waters at Macalder, associated with the reprocessing of past tailings from mining of the Macalder poly-metallic Volcanogenic Massive Sulphide ore deposit; and ii) pH neutral waters with low hazardous element content at Osiri mine. Mercury was detected at concentration of 1 μg/l in the Osiri mine pond.