Hydrogeochemistry of arsenic, fluoride, and other trace elements in groundwater in northern Tanzania : Occurrence, distribution, and impacts on drinking water quality

Sammanfattning: The occurrence of arsenic (As), fluoride (F-), and other trace elements (TEs) with potential toxicity in groundwater is a global environmental concern. According to the National water resource mapping database, about 76 % of the drinking water supply in Tanzania is abstracted from groundwater sources. In northern Tanzania groundwater accounts for more than 80 % of the drinking water supply. Albeit the importance of groundwater in northern Tanzania, its quality concerning geogenic contaminants is not optimally understood. The present study focused on estimating the groundwater quality using analytical, geochemical modeling, statistical, and GIS techniques which were carried out to better understand the chemical quality of groundwater in the framework of safe drinking water supply in Tanzania. Fluoride was established as the principal geogenic groundwater contaminant in northern Tanzania, particularly in the Pangani and internal drainage basins, parts of the East Africa Rift Valley (EARV) system. About 8 % and 57.6 % of the water samples collected from Geita and Sanya alluvial plain have indicated F- concentration above the WHO guidelines value (1.5 mg/L) for drinking water. The high F- concentration in groundwater originates from F- rich minerals and ash deposits from the granitic and alkaline volcanic parent rocks. The consumption of elevated concentrations of F- in groundwater has been responsible for dental, skeletal, and crippling fluorosis. In the Lake Victoria basin (LVB) goldfields, As was reported to be the main geogenic contaminant of health concern. The concentration of As in 50 % and 82 % of the groundwater samples from Tarime and Geita respectively exceeded the WHO guidelines (10 µg/L). The high concentration of As in groundwater from the LVB was highly associated with natural geochemical processes as well as gold mining activities. Results from geochemical modeling revealed that As mobilization was influenced by oxidative and/or reductive dissolution of As-containing sulfide and iron minerals, respectively from the parent rocks. The concentrations of other TEs (aluminum, chromium, cobalt, copper, iron, manganese, nickel, and zinc) were observed below the WHO drinking water guideline. The spatial variability of As and F- concentration from drinking groundwater sources was associated with the different groundwater levels, screened depths, and contact time of groundwater interaction with the As and F-- containing rock minerals in the aquifers. Furthermore, irrigation water sourced from the F- contaminated boreholes poses risks of contaminating the shallow aquifers that are easily accessible for human and domestic animals’ consumption. The current WHO recommended guidelines of 10 µg As/L and 1.5 mg F-/L intake for humans are open to further epidemiological scrutiny, especially considering chronic/long-term exposure in the region. The present study highlights the need for large-scale hydrogeochemical and human health risk investigations concerning geogenic contaminants in the region. The findings contribute to the local, regional, and global initiatives toward sustainable exploitation of groundwater resources in conformity with the Sustainable Development Goal (SDG) 6 for universal access to safe water and sanitation for all.