Spatial analysis of arsenic and fluoride mobilization in groundwater around Mount Meru, northern Tanzania

Sammanfattning: In Tanzania, approximately 76% of the drinking water supplies is dependent on groundwater sources. However, the groundwater in north Tanzania is susceptible to contamination by arsenic and fluoride. This study examines the spatial analysis of arsenic and fluoride mobilization in groundwater around Mount Meru, located in northern Tanzania. The study area covers approximately 1,450 Km2 around Mount Meru in Arusha region, Tanzania. The statistical analysis was performed in IBM SPSS Statistics version 28.0.0.0 to interpret data and unveil the correlation and trends between chemical elements in 11 springs and in 17 wells.  The geochemical modelling software, Visual MINTEQ ver.3.1. have been used to simulate chemical elements in springs and wells in the study area to predict the chemical species, saturation indices, and adsorption efficiency of arsenic to goethite and of fluoride to gibbsite. Moreover, the spatial analysis was done using ArcGIS Desktop 10.8.2 and Geoda 1.20.0.22 software to identify the hotspots areas and to connect spatial patterns of arsenic and fluoride distribution within the study area. The statistical analysis revealed a positive correlation between arsenic and pH in springs. The Na-Cl-HCO3 water type was found in springs, with increased F- concentration in groundwater. The results of geochemical modelling confirmed that more undersaturated values for F- minerals were observed in springs than in wells, which were likely influenced by the fluoride adsorption to gibbsite surface. It was observed that initial concentration and increase of adsorbent dose promoted adsorption efficiency of arsenic and fluoride on goethite and gibbsite, respectively, between pH 4 and pH 6. However, the increase in pH value hinders adsorption efficiency. Moreover, the highest adsorption efficiency was observed at adsorbent dose of 0.05 g/L of goethite for arsenic adsorption, and at adsorbent dose 6.6 g/L and 10 g/L of gibbsite for fluoride adsorption. The spatial analysis results showed that the concentration of fluoride was high in the northeast and eastern part of the study area, where the low elevation area is composed of Tertiary and Quaternary volcanic rocks and debris avalanche deposits. In contrast, the southwestern part of the study area had comparatively low fluoride concentrations, which could be attributed to the less permeable nature of the Tertiary and Quaternary unconsolidated soil. The geological map revealed that the study area is composed of pyroclastic rocks and basic igneous rocks, which predict the presence of fluoride rich minerals such as apatite and fluorapatite. The spatial distribution of arsenic and fluoride in springs and wells revealed that the hotspots groundwater sources such as S5, S6, S7and W3, W4, W5, W6 were in low elevated areas that accumulated from the mobilization of dissolved arsenic and fluoride species from high recharge altitude, groundwater interaction with arsenic and fluoride bearing minerals and agricultural areas. The findings of this study can be useful to the future researchers and local authorities to know about high risky sources of groundwater for better managing and mitigating the risks associated with arsenic and fluoride contamination in Mount Meru groundwater aquifers.