Engineering geological assessment of lateritic soils as landfill liners in Ekiti State, Southwestern Nigeria.

Abstract

The quantity of waste generated in Ekiti state, southwestern Nigeria, has been increasing dramatically due to a rapid urban expansion resulting from population growth and associated economic activities. However, these increasing wastes produced are disposed of into large open dumpsites located in the premises of urban and peri-urban centres, which lack engineered control systems designed to prevent environmental degradation, especially contamination of groundwater and the ecosystem. Thus, there is a need for an affordable and viable long-term solution to protect the environment, including the groundwater system. Landfilling of waste in engineered facilities has gained a wide acceptance especially in developing countries, where it is almost the only viable way of solid waste disposal. Landfills usually require the emplacement of liners, which are constructed with geosynthetic materials and or the use of compacted natural soils. Due to the low cost, good contaminant attenuation capacity and ease of use, compacted soil liners (CSLs) are more attractive options than Geosynthetic Clay Liners (GCLs). However, the unavailability of clay soils for the construction of CSLs has necessitated the search for alternative materials such as lateritic soils. Lateritic soils are the most common construction soils in tropical environments where their formation and nature are determined by different environmental factors. These soils are very diverse in their physicochemical properties and it is essential that their suitability as potential CSLs must be carefully evaluated. The aims of this research are to evaluate the suitability of lateritic soils from Ekiti State, southwestern Nigeria for use as landfill liners, and examine the influence of various parent rocks on the geotechnical and geochemical properties of these soils. Field observations, petrographic, geochemical and mineralogical analyses of the various rock types were undertaken to ascertain the different parent rocks that are present in the study area. Geochemical and geotechnical analyses as well as batch equilibrium tests were conducted on representative soil samples derived from the different parent rock types in order to determine the clay mineralogy of the different soil types, their index and geotechnical properties, and their contaminant attenuation capacities. Petrographic and geochemical analyses show that the dominant parent rocks present on site are porphyritic granite, charnockite, migmatite, granite-gneiss, schist and quartzite. The dominant clay minerals present in the lateritic soils are kaolinite and dickite, implying low desiccation cracking and low shrink-swell behaviour. The tested soils have cation exchange capacities that range from 10.3 to 15.9 meq/100g, indicating good contaminant attenuation potential. The batch equilibrium sorption analysis undertaken to assess the competitive sorption of trace metals, namely, Cd, Pb, Cr, Mn and Cu found commonly in landfill leachate of the study area showed that the lateritic soils derived from granite-gneiss and charnockite exhibit better sorption potential than those derived from schist and quartzite as indicated by their maximum adsorption capacity, which is closely related to the goethite content in the soils. The sorption isotherms of these trace metals onto the lateritic soils are described by the Langmuir equation and these isotherms deviate from the corresponding desorption isotherms to different degrees which is indicative of various extents of hysteresis. The sorption hysteresis indices for these trace metals range from 0.63 to 0.99 and imply that the trace metals may be re-released and leached to the surrounding soils and groundwater. The geotechnical index test results indicate that the lateritic soils are well graded and could be classified as inorganic clays and silts of low to high plasticity. Furthermore, the lateritic soils derived from porphyritic granite possess the lowest average Maximum Dry Density (MDD) of 1.45 mg/m3, while the highest MDD of 1.61 mg/m3 was obtained for lateritic soils derived from granite-gneiss. Lateritic soils derived from charnockite, migmatite and quartzite are found to have MDD of 1.48 mg/m3, 1.50 mg/m3 and 1.51 mg/m3, respectively. The total shear strength parameters indicate that the average values of the total cohesion of the soils derived from porphyritic granite, charnockite, migmatite, granite-gneiss and quartzite are 45.6 kPa, 44.7 kPa, 40.3 kPa, 42.7 kPa and 35.2 kPa, respectively. The average total angle of internal friction obtained for the lateritic soils derived from porphyritic granite, charnockite, migmatite, granite-gneiss and quartzite friction are 10.8º, 10.9º, 11.3º, 11.1º and 9.6º, respectively. These shear strength parameters indicate that the soils derived from the various parent rocks have adequate shear strength as they display results above the required minimum for landfill liners. The measured coefficient of permeability (K) values of the lateritic soils tested range from 1.1x10-8 m/s for soils derived from porphyritic granite to 1.3×10-8 m/s for soils derived from quartzite. Although, these K values indicate very low degree of permeability, they do not conform to the required limit (≤1×10-9 m/s) for landfill liners. The most converging findings of this study are that most of the lateritic soils in Ekiti state in southwestern Nigeria meet the index and engineering quality requirements stipulated by the USEPA (2021) for soil liner materials except for the hydraulic conductivity. The coefficient of permeability observed in some lateritic soils was significantly reduced by mixing of lateritic soils of various provenances having low coefficient of permeability. Furthermore, the significant adsorption capacity of the lateritic soils for Cd and Pb indicates the remarkable efficiency of the soils to sorb these trace metals if it comes in contact with leachate. However, the sorption hysteresis indices of the soils are very high and close to unity for some soils, indicating a high reversibility of the sorption process. Hence, it is recommended that landfills that utilize these lateritic soils adopt a composite containment design by integrating liners and leachate collection systems coupled with routine monitoring.