Geochemistry and sources of fluoride and nitrate contamination of groundwater in six districts of the Dodoma region in Tanzania.

Abstract

Fluoride (F−) and nitrate (NO3−) are common groundwater contaminants. In high concentrations, F− and NO3− cause skeletal fluorosis and methemoglobinemia in children respectively. Therefore protection of public health necessitates the evaluation of groundwater for F− and NO3− contamination. The goal of this study was to assess the geochemistry and spatial distribution of F− and NO3− in six districts in the Dodoma region. Results show that greater than 98% of samples were found to contain F− below 1.50 mgl−1 (WHO 2017). F− Levels exist in the gradient and concentrated to the north of the region. The highest and lowest levels of F− were found in Chemba district, and Mpwapwa district. In the Chemba district, values for F− are (max= 7.30 mgl−1, min = 0.11 mgl−1, mean and SD = 0.87±1.67). In the Mpwapwa district, values for F− are (max= 0.80 mgl−1, min = 0.06 mgl−1, mean and SD= 0.36±0.23). Positive correlation between F− and HCO3− / Ca2+ +Mg+2 and negative correlation between F− and TH, F− and Ca2++Mg2+, and F− and Ca2++Mg2+/Na++K+ suggests HCO3− promote dissolution of F containing rocks while Ca2++Mg2+ removes F− through precipitation reactions. Elevated levels of NO3− were found throughout the districts. The order of districts with NO3− levels above 45.0 mgl−1 was found to be Kongwa (94.74%) > Chemba (69.23%) > Dodoma urban (34.80%) > Chamwino (28.57%) > Mpwapwa (6.60%) and Kondoa (0.00%). Kongwa and Kondoa districts are located south and north of the region. The values in Kongwa are; (max= 196.8 mgl−1, min = 12.5 mgl−1, mean and SD= 137.02±48.47). Values in Kondoa are; (max= 28.00 mgl−1, min = 12.00 mgl−1, mean and SD= 19.97±4.61). Conclusions; groundwater in the Dodoma region safe for drinking with respect to F− and unsafe for drinking with respect to NO3− without remediation.