Natural occurrence of potentially harmful fluoride contamination in groundwater: an example from Nakuru County, the Kenyan Rift Valley

High fluoride concentrations have been known to have several health implications on humans and animal’s health. In this paper, we present a case study dealing with high occurrences and distribution of fluoride in different aquifers in Nakuru County, Kenyan Rift Valley. Water quality data from 32 boreholes and hydrogeological data were acquired from the Catholic Diocese of Nakuru Water program and from literature, respectively. Results show that more than 87% of the boreholes are characterized by fluoride levels higher than the limit recommended by the World Health Organization for safe drinking water. Fluoride levels range from 0.5 to 72 mg/l, with a mean of 11.08 mg/l, and show a strong positive correlation with the dominant physicochemical parameters and a negative correlation with calcium, borehole depth, and water hardness. The correlations suggest that minerals dissolution and evaporative enrichment might be the main processes of fluoride release and concentration in groundwater. Spatial analysis showed that fluoride concentrations were not confined to the type of aquifers, but rather to their locations. Low-fluoride aquifers were located close to the rift’s Bahati and Mau escarpments, while high fluoride aquifers were in the rift floor, where the fluoride hot spot was located. Dilution of groundwater by high rainfall and little residence time in the escarpments recharge zones lead to low fluoride concentrations. Accumulation of dissolved solutes in the rift floor aquifers as groundwater flows from the escarpments and evaporative enrichments from high temperature lead to high fluoride concentrations. These high concentrations of fluoride in water are known to cause serious health issues on animals and humans.