The Effect of Variability in Hydraulic Conductivity on Contaminant Transport through Soil–Bentonite Cutoff Walls

Abstract

Statistical analyses of data sets from five case histories indicate that soil–bentonite hydraulic conductivity is distributed log normally. The advection–diffusion equation was used to investigate the impact of log-normal variation in hydraulic conductivity on both steady-state and transient contaminant flux through a cutoff wall with idealized initial and boundary conditions. The results demonstrate that contaminant flux through cutoff walls increases as the variability in hydraulic conductivity increases while all other variables are held constant, including the area-weighted average conductivity. The effect of variability is most pronounced when advective transport and diffusive transport act in opposite directions, as occurs for circumferential cutoff walls that are operated with inward-directed hydraulic gradients to contain contaminated ground water. In this case, the increase in total outward flux due to variability of hydraulic conductivity occurs because the increase in inward advective flux in areas where the seepage velocity is higher than average is more than offset by the increase in outward diffusive flux in areas where the seepage velocity is lower than average.