Abstract

Shallow borehole and dug well data are used in describing groundwater conditions in the vicinity of Abraka in the Nigerian coastal plain. Drill cuttings from ten boreholes show that the Abraka area is underlain by reddish brown unconsolidated sands, followed by a succession of grey- off-white medium grained sands of the Benin Formation. Hydraulic conductivity estimated from grain size analysis of cuttings obtained from typical borehole screened horizons range from 0.12 to 0.19 msec^-1. Regional groundwater flow is from north east to southwest with local distortions on this regional trend resulting from ground water abstraction in densely populated areas. Maximum TDS in ground water was recorded at 28 and 85 mg/l from dug wells. The trilinear plots of major ions in water indicate a mixing of mainly sodium chloride and calcium chloride water types. The stiff diagrams are also suggestive of possible stratification of water chemistry with depth. Borehole water quality is well within WHO and Nigerian drinking water quality standards while that from dug wells contains minimal levels of fecal coliform. Ground water and surface water are determined to be suitable for irrigation. It is also shown that the quality of water in the River Ethiope, TDS 6.6 to 8.09 mg/l, Escherichia coli occurrence at less than an average of 50 cfu/100 ml from selected recreation sites meets WHO standards for body contact recreation.

Key words: Benin Formation, River Ethiope, Abraka, Niger Delta, groundwater, water quality.