GEOCHEMICAL PROCESSES CONTROLLING THE GROUNDWATER CHEMISTRY AND FLUORIDE CONTAMINATION IN THE AQUIFER SYSTEMS ON THE EASTERN, WESTERN AND NORTHERN FLANKS OF MOUNT MERU, TANZANIA

In the Arusha volcanic region in northern Tanzania, within the eastern branch of the East African Rift, water shortage is common and much of the surface water carries unacceptable levels of dissolved fluoride; hence groundwater is the main source of drinking water. Unfortunately, the quality of groundwater in this region is also very poor due to a high fluoride (F⁻) content because of natural contamination from the surrounding geological environment. Consumption of this water leads to dental and skeletal fluorosis among the local population.

Existing borehole completion reports and field stratigraphic description of sub-surface sediments during the digging of new hand dug wells were used to characterise the geology of aquifers. On the western flank, the main aquifer is a fractured and weathered basalt while on the south-western and north-eastern flanks the aquifers are composed of volcanic deposits (ash, tuffs, pumice, lapilli) and fractured and weathered basalt.

A total of 158 groundwater samples from 113 water points (61 hand-dug wells and 52 springs) were collected for chemical analysis. Laboratory results show Sodium (Na⁺) and bicarbonate (HCO₃⁻) are the dominant ions hence the main water type in this area is Sodium bicarbonate (NaHCO₃). Preliminary results show strong contrasts in pH, EC, TDS and F⁻ concentration across the study area, with some spatial patterns for water samples from springs (on the north-eastern flank) where pH, EC, TDS and F⁻ concentration increase with a decrease in elevation.

In all 158 groundwater samples, 91% (143 samples) have F⁻ concentration higher than the WHO limit (1.5 mg/l) recommended for drinking water. The 9% (15 samples) with F⁻ concentration lower than the WHO limit are water from the springs found in the recharge area at higher elevation on the eastern flanks of Mount Meru inside the Arusha National Park.

The chemistry of the groundwater shows that weathering and dissolution of silicate minerals (especially chemical weathering of Na-K-rich volcanic rocks), dissolution of fluoride-rich minerals and calcite precipitation play great roles on the chemical evolution of groundwater in the study area while long residence time and nature of the geological formations (including degree of weathering) play great roles in the groundwater fluoride contamination in the area.