Causes of High Internal Pore Pressure in a Downward-Draining MSW Landfill

A two-phase liquid/gas flow numerical model has been used to investigate the presence of elevated pore water pressures in a 20-m-deep municipal solid wastes (MSW) landfill underlain by a fully drained leachate collection layer. Monitoring of leachate levels in the landfill using piezometers located at different discrete levels found water table type conditions to within 10 m of the surface and established a downward hydraulic gradient at an infiltration rate of $\sim 400\mathrm{mm}/\mathrm{year}$. Short-duration falling-head piezometer tests indicated landfill hydraulic conductivities ($K_h$) between $1\times {10}^{-4}$ and $1\times {10}^{-5}\mathrm{m}/\mathrm{s}$, with a general reduction in $K$ with depth. Several different hypotheses to explain the high pore water pressures in the landfill were investigated using a one-dimensional configuration of the landfill degradation and transport model LDAT. It was assumed that the unsaturated properties of the landfilled wastes can be bounded by two sets of van Genuchten parameters. Comparing the values of $K_v$ required to create a match between observed and modeled leachate heads with the measured $K_h$ values at the site led to a tentative conclusion that landfill-scale anisotropy could be as high as $\sim 1:\mathrm{1,000}$. The introduction of a distributed landfill gas (LFG) source term into LDAT at a rate of $0.61{\mathrm{m}}_{\mathrm{LFG}}^3{t}_w^{-1}{\mathrm{year}}^{-1}$, similar to the gassing rate at the site, increased the adopted saturated permeability relationship in LDAT by a factor of between $\sim 3$ and $\sim 7.5$ compared with a no-gassing scenario. Introducing even moderate gas generation rates ($5.6{\mathrm{m}}_{\mathrm{LFG}}^3{\mathrm{t}}_w^{-1}{\mathrm{year}}^{-1}$) into models simulating low infiltration rates of $50\mathrm{mm}/\mathrm{year}$ can result in a significant depth of waste where pore water pressures are more than 1 kPa (10-cm water head). This results in apparent below-water-table type conditions because water will enter piezometers installed into such wastes, even though the gassing reduces the degree of saturation to below one.