Seasonal variation of nitrate-N in surface and ground-waters of South Western Nigeria: a preliminary survey

Two surveys of nitrate-N concentrations in surface and ground water in Ogun and parts of Lagos and Oyo States of south-western Nigeria were undertaken between October 1997 and December 1998, and between July 2000 and May 2001 (covering both dry and rainy seasons). The study was conducted to ascertain the extent of nitrate-N pollution of the surface/ground water in some parts of the three states. Seventy-two water points (33 rivers/streams, 21 wells and 18 boreholes) were sampled three times during each of the two seasons. Nitrate-N was detected in all the river/stream samples analysed with concentrations ranging from 0.5 to 15.3 mg/l and 1.0 to 7.7 mg/l during the rainy and dry seasons, respectively (average value 2.7 and 2.4 mg/l for rainy and dry season, respectively). 98.5% of the rivers/streams sampling points contained nitrate-N in amounts equal to or less than 10 mg/l. 88.1% of wells and 97.2% of boreholes had a nitrate-N content less than 5 mg/l. Nitrate-N concentrations in the sampled wells ranged from 1.4 to 7.4 mg/l and 1.1 to 6.0 mg/l during the rainy and dry seasons, respectively (average value 2.6 and 2.2 mg/l, respectively). The sampled boreholes had a lower nitrate-N concentration ranging from 0.1 to 5.2 mg/l and from below the detection limit to 5.0 mg/l during the rainy and dry seasons, respectively (average value 1.8 and 1.6 mg/l, respectively). Nitrate-N concentrations in boreholes were below detection limit in 22% of the samples. Generally, for most of the sampling points, nitrate-N concentrations were slightly higher during the rainy season compared with the dry season. The present results indicated that nitrate-N concentrations in all the water points (except one) were below the World Health Organization (WHO) maximum acceptable limit in potable waters (10 mg/l). Therefore, contamination of the surface and ground water of the study area is not indicated. This study provides background data against which future changes in nitrate-N concentrations of surface and ground waters in the study area can be measured.