Occurrence and delineation of high nitrate contamination in the groundwater of Mithi sub-district, Thar Desert, Pakistan

Abstract

An extensive study has been carried out to evaluate the occurrence and spatial distribution of high nitrate (NO3 −) concentrations in the groundwater of Mithi sub-district of district, Tharparkar, Sindh Province, Pakistan. Groundwater is the only consistent source of drinking water in the study area which is available at a depth between 3 and 125 m and is fetched by domestic animals from hand-dug wells. A total of 243 groundwater sample were collected to analyze for nitrate ion along with other physicochemical parameters. Overall groundwater quality reflects the influence of evaporation and silicate mineral weathering. Nitrate concentrations range up to 1610 mg/L with mean and median values of 240 and 162 mg/L, respectively, and almost 88.5% (n = 215) of the groundwater samples contain nitrate concentrations higher than the limit of 50 mg/L set by WHO for drinking water. The variations in nitrate contents in the groundwater are illustrated on regional map, showing high concentrations across the entire study area. High nitrate concentrations have been positively correlated with potassium, sodium and sulfate ions, indicating their possible genetic affiliations. The strength of association of nitrate is moderate with respect to potassium (r = .416, p = 1.3E−11) and sulfate (r = .419, p = 9.2E−12) and low with respect to sodium (r = .293, p = 3.5E−06). Agriculture is very limited in the study area due to the limited availability of water and is unlikely to be the source of the nitrate. Livestock manure is an important and most noticeable source of nitrate as most of the dug-wells serve as common water source for humans and livestock. Furthermore, the direct inclusion of manure-contaminated soil in water source also contributes to further exacerbating the problem.