The effect of leachates on the physical and engineering properties of subsoil varies across different rock types and depth layers

Abstract

Waste disposal on land is a major environmental issue that affects groundwater through soil. The aim of this study is to evaluate and compare the physical and engineering properties of subsoil at varying depths from different dumpsites and to determine the effects of leachates and parent rock types on these soil properties. Thirty-six subsoil samples were obtained from twelve trial pit profiles at depths of 0.5 m, 1.0 m and 1.5 m respectively. These soil samples were subjected to both geotechnical index and strength tests. Grain size analysis, linear shrinkage, CBR and compaction tests revealed that many of the soil samples from dumpsites and their environs met the criteria of a good landfill material. All the soil samples have low permeability that ranges from 2.07 × 10–6 to 1.49 × 10–4. Liquid limit, plasticity index and MDD values were higher in the control samples while the dumpsite soils became more permeable. The results of grain size analysis, linear shrinkage and CBR revealed that there was no significant difference in the properties of dumpsite and control soils. All the soil samples have TDS and EC values below 1000 ppm and 1000 µS/cm, except for trial pit 1, suggesting that the soil has a low risk of leaching contaminants into groundwater. For analysis of variance and Pearson’s correlation coefficient, the P values of some parameters such as pH, EC, TDS, CBR, OMC, MDD, SG, LS, PI and LL were significant at a 0.05 level of significance. The following pairwise parameters: pH-MDD, TDS-CBR, EC-CBR, OMC-MDD and TDS-EC recorded strong positive correlation values for the three dumpsites. The statistical analyses reveal that the soil's properties were only slightly influenced by parent rock types and can be used to limit contaminant flow into the groundwater in the short term.