Groundwater Suitability for Domestic, Irrigation, and Livestock Uses in and Around Dodoma City, Tanzania

Abstract

This paper presents a comprehensive assessment of the physicochemical groundwater quality parameters in and around Dodoma City, the capital of Tanzania. To achieve this, a combination of field measurements, laboratory analyses, and geostatistical assessments was conducted. The primary objective was to evaluate the suitability of the groundwater for various purposes, including domestic consumption, livestock watering, irrigation, transportation, storage in metallic facilities, and industrial use. The results indicate that the major anions and cations followed the order of abundance: HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ for anions and Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ for cations. The average concentrations were as follows: 292.4, 170, 90, and 49.5 ppm for HCO₃⁻, Cl⁻, SO₄²⁻, and NO₃⁻, respectively, while Na⁺, Ca²⁺ Mg²⁺, and K⁺ had mean concentrations of 139, 74, 40, and 10 ppm, respectively. Areas with high population density, particularly the city center and agricultural zones, exhibited elevated levels of NO₃⁻ and Cl⁻. The spatial coexistence of these contaminants suggests a common source, primarily human-related pollution. In contrast, less populated areas demonstrated better groundwater quality and were deemed suitable for most uses. Due to the high NO₃⁻ levels, approximately 38% of groundwater sources are unsuitable for drinking purposes and/or require close monitoring of humans and livestock to detect symptoms of poisoning. Moreover, this study highlights the NO₃⁻ issue that is spreading to previously safe areas, indicating the need to review current aquifer management strategies to address the evolving challenges. Groundwater recharge sites generally exhibited better water quality compared to discharge areas with heavy groundwater abstraction, including the city center, Hombolo, Nzuguni, and the Makutupora wellfield. While there were spatial variations in groundwater suitability indices, the majority of groundwater sources were generally suitable for most uses, except for extreme cases involving high salinity and NO₃⁻. By employing modified water suitability indices and grouping groundwater quality parameters, a clearer zonation pattern was established, facilitating effective groundwater and land use planning. However, it is important to note that 99.5% of groundwater sources may cause soil and plant growth problems when used for irrigation, based on the sodium absorption ratio (SAR), permeability index (PI), and Kelly's Ratio (KR) indices. Furthermore, assessing the corrosivity ratio (CR) revealed that 97% of water points cannot be transported or stored in metal facilities due to prevalent groundwater infrastructure corrosion, necessitating the use of PVC instead. Last, this study identified shallow-dug wells contaminated by potentially toxic blue-green algae blooms, which were found in 43% of the 1248 surveyed sources. These wells are considered unsuitable for human and livestock use, yet they are frequently utilised due to the lack of alternative water sources. Although this study may not be exhaustive, it provides a crucial management tool for groundwater and land use planning in the city, offering valuable insights for sustainable water resource management and infrastructure development.