ABSTRACT
It is well recognized that in the design of systems where there will be gradients and hence flow out
of the waste disposal facility, the bulk hydraulic conductivity of the barrier and/or aquitard will be a key factor affecting potential impact of the landfill on an underlying aquifer. For situations involving outward gradients as shown in Figure 16.1, and all other things being equal, the greater the bulk hydraulic conductivity, the greater will be the outward advective flow, and hence the greater the potential impact on the aquifer. However, when dealing with inward gradient designs (i.e., hydraulic traps) as shown in Figure 16.2, since the inward advection from the aquifer to the landfill resists outward diffusion of contaminants, greater potential impact on the aquifer will occur for lower bulk hydraulic conductivity. Thus, although the hydrogeology may be exactly the same for Figures 16.1 and 16.2, with the only difference being the depth of the base contours adopted in the engineering design, the effect of uncertainty associated with the hydraulic conductivity of both the natural and engineered components of the barrier system, and hence what constitutes a "conservative" estimate of hydraulic conductivity, is quite different.
