Abstract
Nitrate isotopic values are often used as a tool to identify sources of nitrate in order to effectively manage ground water quality. In this study, the concentrations of NO3−, NO2−, and NH4+ from 50 boreholes and shallow wells in the Singida and Manyoni Districts were analyzed during the dry and wet seasons, followed by identification of nitrate sources using the hydrochemical method (NO3−/Cl−) and stable isotope (δ15N and δ18O) techniques. Results showed that NO2− and NH4+ concentrations were very low in both seasons due to the nitrification process. The concentrations of NO3− ranged from 2.4 ppm to 929.6 ppm with mean values of 118.5 ppm ± 118.5 ppm , during the dry season and from 2.4 ppm to 1620.0 ppm with mean values of 171.6 ppm ± 312.3 ppm , during the wet season. The higher NO3− contamination observed in the wet season could be due to rainfall which accelerated the surface runoff that collects different materials from various settings into the ground water sources. Nitrate source identification through hydrochemical technique revealed that most nitrates originated from sewage effluents and/or organic wastes such as manure. Likewise, the mean values of δ15N-NO3− ( + 20.90 ‰ ± 5.17 ‰ and + 18.30 ‰ ± 6.33 ‰ ) and the mean values of δ18O-NO3−( + 13.86 ‰ ± 3.18 ‰ and + 13.69 ‰ ± 3.97 ‰ ) suggest that 80% of boreholes and 52% of shallow wells were dominated with nitrate from sewage effluents and/or manure as most ground water sources were situated in densely populated areas with congested and poorly constructed onsite sanitation facilities such as pit latrines and manure. Therefore, to reduce nitrate pollution in the study area, a central sewer must be constructed to treat the discharged wastes. Also, groundwater harvesting should consider the proper principles for groundwater harvesting recommended by the respective authority to minimize chances of contamination and hence prevention of health risk.
Highlights
Ground water is an important source of drinking water in arid and semiarid areas because of the limited availability of surface water [1,2,3,4,5,6]
Results for 50 ground water samples collected in Singida Urban and Manyoni Districts during the dry and wet seasons showed that most of the urban boreholes and shallow wells are contaminated with domestic sewage effluents infiltrated from old and poorly constructed onsite sanitation facilities, while contamination in the peri-urban sources was mostly caused by animal manure that has probably been carried to water systems during the rainy season
More than 59–64% of all sources had higher nitrate concentrations above the World Health Organization (WHO) recommended standards that implies about 60% of the Singida population are prone to unacceptable nitrate contamination which may lead to a serious health risk to the community in the area
Summary
Ground water is an important source of drinking water in arid and semiarid areas because of the limited availability of surface water [1,2,3,4,5,6]. The quality of ground water is increasingly deteriorating due to natural and anthropogenic contamination from various sources including domestic waste from onsite sanitation facilities such as pit latrines, septic tanks, [8,9,10], agricultural runoff [11,12,13], and mining and industrial waste [14,15,16,17]. Nitrate is among the common contaminants in ground water and surface water [18, 19] It is extremely soluble in water and tends to be adsorbed in soils or leaches and elevates its concentrations into ground water sources [20,21,22]. The elevated nitrate concentrations in ground water are a significant threat facing water resources as they compromise water quality [19, 23]
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