GEOENVIRONMENTAL ASSESMENT AND STRATIGRAPHIC CHARACTERISATION OF A CONTAMINATED SITE IN BARUWA, LAGOS STATE.

Abstract

Stratigraphic characterization through geotechnical and geophysical exploration were performed for the petroleum hydrocarbon contaminated Baruwa site in Lagos Nigeria. Soil stratigraphy was obtained from borehole data. Interpolation between boreholes was performed by projecting borehole data on cross-sectional planes, by using Computer Aided Drafting (CAD), together with manual interpolation between the boreholes. Detailed laboratory geotechnical analysis of subsurface soil was employed and twenty-seven soil samples (undisturbed and disturbed) were taken altogether. The laboratory tests that were carried out on these soil samples include index properties of soil, chemical analysis, permeability and clay mineralogy. Coefficient of permeability for Baruwa soil samples ranges from 1.72- 6.36x104cm/s. Soil specific gravity was between 2.64 and 2.66. The extent of contamination was studied using geophysical survey techniques, to measure the geophysical property of resistivity for the subsurface. Electrical resistivity soundings were also used to study the various stratigraphic units in the Baruwa subsurface. The determination of the stratigraphy / soil subsurface profile for Baruwa site using geophysical and geotechnical methods has revealed the role of unconsolidated lateritic soils, granular backfill and sand lenses in vertical contaminant migration for the subsurface, especially in the unsaturated soil moisture zone, and this has hastened the pace of the leaked hydrocarbon through the thick surficial aquitard (Average aquitard thickness was 23.65m) in Baruwa. The topography, stratigraphy and hydraulic gradient of the area has aided the movement of the leakage from the source into the surrounding wells at various degrees. The severity of the pollution was also discovered to be based on the heterogeneous nature of the material immediately around the depth of burial of the source (2.0m), the direction of groundwater flow and the topography of the study area. On a regional basis the groundwater moves from north to south, while on local basis it moves from east to west. From the suspected source of leakage the hydrocarbon product migrates and causes a thickness of 0.72metres on top of groundwater at well 20, 0.65m at well 15,0.35mat well 41and 0.12mat well 69. This also explains why contamination in well 30east of the source is not severe but has a O.2metreshydrocarbon level