Water pollution by nitrate in a weathered/fractured basement rock aquifer: the case of Offa area, West Central Nigeria

摘要

The study area generally falls within the broad region of Nigeria known as the grass plains, where the land is heavily farmed with limited area of forest reserve. Offa, the headquarters of the present Offa Local Government has long been the largest commercial centre in the entire Oyun region and next in size and population to the capital city of Kwara State in Nigeria. A survey was made of nitrate concentrations in potable groundwater sources in this area. Analytical results reveal that 53% of the water samples had nitrate concentration above 45 mg/L (the international drinking water standard). In virtually all the sampling campaigns up to 33% of the water samples were recorded with concentrations between 120-225 mg NO_3~-/L. Unlined pit toilets and indiscriminate disposal of human and animal wastes characterize the study area. Measurements in over 48 wells mostly completed within the weathered regolith and fractured basement aquifers around the township showed nitrate enrichment up to 225 mg/L, indicating groundwater pollution by nitrate. This situation is bound to constitute a major quality problem to the potable water sources in the area. However, the measured nitrate values generally do show some depth control and were observed to be less for non-residential areas and boreholes in residential buildings only within the Government Reserved Areas (GRA). Moreover, nitrate showed good correlation with electrical conductivity, dissolved oxygen, total hardness, bicarbonate alkalinity and sulphate concentrations. The implication of these results to water protection and management in the area is also discussed.