The US EPA HELP model is a tool for simulating landfill hydrology and elaborating environmentally safe landfill designs that is widely used by consultants and regulators in the US. The HELP model has been extensively tested by its developers and users but its results can be, at times, surprising and appear seemingly unintuitive to the specialist.

Results of two different studies were analyzed to identify the origins of these surprising results.

In the first study, the effect of different HELP parameters on the volume of landfill leachate has been studied for two different climates. Among surface runoff parameters, evapotranspiration and parameters of the upper lowpermeable capping layer, field capacity and wilting point of the upper layer have the greatest impact on leachate production. Small variation of these parameters (a factor of 2) may result in leachate estimate variations of 8-9 times. The sensitivity of leaching to these two parameters is several times higher for the Fraserburg site (South Africa) with an arid climate than for the Kingston Site (Canada) with a humid climate. Mechanisms providing the observed results are explained.

In the second study, the effect of the evaporative zone depth on lateral drainage has been studied for an arid climate conditions. It has been found that the volume of the lateral drainage has a parabolic-shape dependency on the evaporative zone depth. This phenomenon is explained by a combined effect of profile structure and layer parameters.

HELP is a well-developed and information-balanced civil engineering model which may produce seemingly unintuitive results at first glance, but in fact provides reliable results in a wide range of climate and soil-physical conditions.