MINERAL SOLUBILITY CONTROLS ON DRINKING WATER QUALITY IN THE AREAS OF GOLD MINING IN GEITA AND MARA REGIONS OF NORTHERN TANZANIA

Access to safe drinking water is a prerequisite for human health and for a sustainable socio-economic development in global perspectives. In Tanzania, only 57% of rural communities have access to improved drinking water services. Recent studies have shown that contamination of surface and groundwater is occurring in gold mining areas within the Lake Victoria Basin in Northern Tanzania. Our study aims to assess the quality of some drinking water sources in Geita and Mara regions and to understand the solubility controls on arsenic due to the toxic and cancerogenic effects of ingesting As in drinking water. The dominant geology formations in the area under study are greenstone belts, where gold sulfide deposits have developed along the contact zones with granitoid and migmatite. Water samples were collected from shallow wells (n=12), deep boreholes (n=19), springs (n=8) and rivers (n=15) used as source of drinking water in proximity of gold mining activities. The samples were analyzed for major ions, dissolved organic carbon (DOC), total arsenic and other trace elements. Bivariate plots, stiff diagrams and [HCO3-]/[SiO2] ratio reveal a substantial difference in the dominant solubility controls between Geita and Mara regions: silicate weathering in the first one, and carbonate dissolution in Mara region. In total, 53% of analyzed samples exceeded the WHO guideline value for As (10 µg/L). Arsenic concentrations were lowest in Geita region (30% > 10 µg/L) and highest in samples collected in Mara region (73% > 10 µg/L), suggesting an As-rich gold mineralization. Because of high oxidizing conditions (Eh>400mV) of the sampled drinking water, the mobilization of As is presumed to be oxidation of As bearing sulfide minerals. The geochemical modelling reveals that once As has been released into the aquatic environment, its mobility is drastically reduced by co-precipitation with Al and Fe hydroxides and by adsorption on clay minerals.