Abstract

Ten (10) water boreholes for water level monitoring and sub-soil analysis were carried out in the Alakiri Field, Niger Delta, Nigeria. The positions of the boreholes were chosen on the basis of grid sampling to reflect a good representation of the entire Alakiri field. Boring of the drill holes to 20 m each was carried out with the use of a light cable percussion rig. Stratigraphic synopses of the boreholes display considerable variations and heterogeneity in the soil types, indicating that the soils were deposited under varying energy environmental conditions of river (fluviatile) and tidal channel (marine) activities. Grain size analysis results show a range of well graded (poorly sorted) aquiferous sands with very little fines (1.60 to 3.10% finer than No.200 sieve) to uniformly graded (well sorted) clayey sands, sandy and silty clays with moderate to very high fines or silt/clay fractions (17.41 to 94.60% passing 0.075 mm sieve). Permeability tests of the samples vary from low of 6.40 x 10^-3 to 7.82 x 10^-3 cm/s for the silty clayey mud to sandy clay, to fairly high of 7.20 x 10^-1 to 1.01 cm/s for the clayey sand to sand. Coefficient of permeability increases with increasing size of voids, which in turn increases with increasing grain size. Soil consistency results show a low plasticity range (2.35 to 13.40%), with the predominantly sandy soils being non-plastic. The low value of plasticity indices of the plastic soils is an indication of low water retaining capacity. The soils fall within the soil classification groups of SC, SW, CL, OL and PT under the Unified Soil Classification System (U.S.C) scheme. Resulting from almost saturated state of the soils, water boreholes sited in the area, for domestic and industrial utilization will have to go considerable depths (over 200 m) to obtain uncontaminated water in case of pollution and because of the prevalent saline water.

Key words: Environmental assessment, ecological management, soils, geotechnical engineering, Niger Delta