Subsurface characterization using electrical resistivity and MASW techniques for suitable municipal solid waste disposal site

Abstract

Investigation of the subsurface becomes a compulsion before waste disposal site is developed. The content of this paper presents integrated geophysical methods in a basement complex terrain, southwestern Nigeria. The dumpsite is located at Oke Saje, Abeokuta, in Ogun state. This was done to investigate the leachate curtailment capacity (LCC) of the existing dumpsite by characterizing the subsurface. A clay overburden thickness of about 15 m which acts as leachate filter is a standard for determining the LCC of a dumpsite. Prior to this work, there is no documented record that such investigation had been done to reveal the LCC before establishment of the dumpsite. Vertical electrical sounding (VES), 2D electrical resistivity tomography (2D-ERT) and multi-channel analysis of surface waves techniques were adopted for the investigation on the existing dumpsite. Eighteen VES was carried out using Schlumberger configuration at 5–10-m intervals to obtain resistivity values and the respective lithological units of the subsurface and three electrical resistivity imaging using dipole–dipole array. MASW techniques were later carried out to ascertain the underlain materials by determining seismic wave arrival times which were processed to obtain the shear wave velocities (SWVs) of the subsurface. The 2D-ERT revealed clearer features of the subsurface as topsoil, weathered, fractured and fresh basement. The result of the VES revealed topsoil with resistivity values ranging from 18 to 1381 Ωm, weathered layer (clay) with resistivity values 15–84 Ωm, fresh basement and fractured layer with resistivity values 192/366–5585 Ωm. The thickness of the clay deposition ranges from 1.8 to 18.5 m. The MASW result using SWVs revealed topsoil, sandy clay, clayey sand and clay with rigidity modulus ranging between 42.4 and 160 N/m2 at the depth of 30 m. The combined methods carried out at the dumpsites revealed the same lithological units. The waste disposal site is said to have a good LCC due to the presence of clay overburden thickness but not up to 15 m in some parts of the dumpsite.