Causes of high internal pore-pressures in a downward-draining MSW landfill

A two-phase liquid/gas flow numerical model has been used to investigate the presence of elevated porewater pressures in a 20 metre deep landfill underlain by a fully drained leachate collection layer. Monitoring of leachate levels in the landfill using piezometers located at different discrete levels within the landfill found water table type conditions to within 10 metres of the surface and a strong downward hydraulic gradient at an infiltration rate of 400 mm/year. Short duration falling head tests in piezometers indicated landfill hydraulic conductivities (K) between 1x10-4 and 1x10-5 m/s, with a general reduction in K with depth. Several different hypotheses that could explain the high porewater pressures in the landfill were investigated using a one-dimensional configuration of the landfill degradation and transport numerical 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 Kv required to create a match between observed and modelled leachate heads with the measured Kh values at the site, leads to a tentative conclusion that landfill scale anisotropy could be as high as ~1:1000 at the study site (i.e. Kh approximately 3 orders of magnitude higher than Kv). The introduction of a distributed landfill gas (LFG) source term into LDAT at a rate of 0.61 m3LFG tw-1 yr-1, similar to the gassing rate at the site, increased the adopted permeability relationship in LDAT by a factor of between 3 and ~7.5 compared with a no gassing scenario. Other scenarios of differing gas generation rates (up to 36.5 m3LFG tw-1 yr-1) and a reduced infiltration of 50 mm/ year were investigated. Introducing even modest gas generation rates into the model can result in a significant depth of waste where porewater pressures are more than 1 kPa (10 cm water head). This results in apparent below water table type conditions as water will enter piezometers installed into such wastes, even though the gassing reduces the degree of saturation to below 1