Hydrogeochemical characteristics and spatial distribution of groundwater quality in Arusha well fields, Northern Tanzania

Abstract

Arusha aquifers have been exploited intensively serving as the main source of domestic water supply in the city. But the quality of groundwater is not clearly documented for future planning and management. Hydrogeochemical assessment was carried out to establish groundwater quality and its spatial distribution with the aid of geostatistical techniques. Groundwater samples were collected and analyzed for major cations and anions using conventional methods of water analysis. Well lithology and geological map were considered for hydrogeological interpretation of the area. The results of piper diagram revealed Na-K-HCO 3 water type with sodium and bicarbonate ions dominating in all samples. High fluoride concentrations and general groundwater chemistry are mainly controlled by aquifer lithology than anthropogenic activities. The levels of anthropogenic pollution indicators such as nitrate, chloride and sulfate in deep wells are generally low and most likely coming from natural sources. The geological sections indicate two potential aquifers (volcanic sediment and weathered/ fractured formation) both yield water containing significant concentration of fluoride. Fluoride concentrations were higher than WHO guidelines (1.5 mg/l) and Tanzanian standards (4.0 mg/l) by 82 and 36% of the analyzed groundwater samples, respectively. The southern part of the study area yields groundwater of better quality for human consumption than northern zones which is at high elevation on the foot of Mt. Meru. With exception of fluoride, the quality of groundwater in the study area is generally suitable for drinking purpose and other socioeconomic uses.

Impacts of Emerging Agricultural Practices on Groundwater Quality in Kahe Catchment, Tanzania

“…Generally, groundwater chemistry in the catchment is characterized by magnesium-sodium-bicarbonate water type. Similar findings have been reported the dominance of the sodium and magnesium in groundwater [44,60,61], as well as groundwater type in the Kilimanjaro aquifer [24], and Arusha aquifer [44] in Tanzania. purposes, but a value above 200 mg/L is unsuitable for drinking water [58,59].…”

“…This correlation probably occurs through ionic exchange reaction and precipitation, when calcium and magnesium ion removed from the groundwater system additional K + and F − ions are released from minerals such as nepheline in the aquifer materials. This study argues with several scholars who reported the relation between fluoride and alkali earth metals [62,63], as well as fluoride with bicarbonate (weak acid) [44].…”

“…through ionic exchange reaction and precipitation, when calcium and magnesium ion removed from the groundwater system additional K + and F − ions are released from minerals such as nepheline in the aquifer materials. This study argues with several scholars who reported the relation between fluoride and alkali earth metals [62,63], as well as fluoride with bicarbonate (weak acid) [44]. The spatial distribution of fluoride concentrations indicates high fluoride concentration in shallow wells, i.e., at a depth 7-16 mn (cluster B and C), and lower values in deep boreholes (at a depth >100 m) for cluster A, but not significantly different with r (25) = 0.627, F = 1.215, p = 0.548 in both seasons in the area.…”

“…Figure 2b shows that its formation was associated with limited faults which makes the basement rocks impermeable for a groundwater flow system [29]. Also, volcanic ashes and sedimentary formations, particularly fine-grained alluvium deposits, are characterized by low transmissivity which becomes practically impervious [44]. Groundwater recharge mainly takes place in high elevation on the slopes of Mt.…”

“…The fluoride concentration varied from 0.20 mg/L to 1.61 mg/L and from 0.06 to 1.54 mg/L during the wet and dry season, respectively (Table 1). Its variation from one site to another is possibly associated with the geological formation (Figure 2a), which may have different dissociation rate, rainfall intensity, and cation exchange process in the aquifer system [44]. There was a significant positive relationship between F − and K + , r (25) = 0.57, p < 0.01) (Appendix 2A), and negative correlation with alkaline earth elements (Ca 2+ and Mg 2+ ) (Appendix 2B).…”