Fluoride enrichment mechanism and geospatial distribution in the volcanic aquifers of the Middle Awash basin, Northern Main Ethiopian Rift

Considering the anomalous concentration of fluoride in the ground waters of the Middle Awash basin, a comprehensive survey of the enrichment mechanism as well as its association with common hydrochemical variables was conducted using multivariate statistical methods, Hierarchal Cluster Analysis (HCA) and Principal Component Analysis (PCA). The HCA results indicate a marked heterogeneous spatial distribution of the fluoride concentration, the magnitude of which varies more gradually in the SSW–NNE direction along the Wonji Fault Belt (WFB) than it does in the E-W direction away from this belt. This is strongly associated with the geothermal anomaly that is prominent in the basin interior area. Furthermore, the PCA results show that the magnitude of the fluoride concentration is higher in the groundwater derived from non-calcium bearing aquifers, which are widely distributed in the rift floor. Hydrochemical processes involving a cation exchange reaction cause a systematic Ca2+ removal from solution from highland towards the rift floor. This geochemical reaction enhances the fluoride enrichment of groundwater along the same flow direction. In this regard, the geothermal belt of the basin interior area is a hydrogeological block marked by high loads of fluoride whereas the plateau areas are low-fluoride zones. The geospatial distribution of fluoride at the basin scale was estimated using the kriging procedure. Appropriate discrimination between local and regional aquifers is important in order to secure low-fluoride water supply for the community in the basin.

► Groundwater circulating in the Middle Awash basin interior has high load of fluoride much higher than the permitted standard that can cause serious fluoride intoxication.
► Multivariate PCA and stable isotopes indicate removal of Ca2+ from solution in the flow direction, which leads to progressive enrichment of fluoride along flow direction towards rift floor.
► Multivariate HCA and geospatial kriging of fluoride data shows a trend of high fluoride in groundwater constrained within the active tectonic zone of the Wonji Fault Belt.
► Groundwaters marked by high-temperature and pH contain high level of fluoride.
► Measuring pH and water temperature provides information on fluoride content of groundwater. This can be cost-effective in-situ groundwater quality assurance before well construction and commissioning.