Modeling lateral gas transport in soil adjacent to an old landfill

Lateral migration of landfill gases in soils surrounding old (closed) municipal landfills can lead to explosion hazards and damage to vegetation. Landfill gas production and migration is controlled by microbial activity and soil physical properties such as gas (air) permeability, gas diffusivity, and atmospheric pressure variations.

Gas diffusivity and air permeability were measured on undisturbed samples collected at an agricultural field adjacent to an old Danish municipal landfill. Empirical expressions for predicting gas diffusivity and air permeability from soil-water content were fitted to the data. An empirical expression for calculating methane oxidation rate as a function of soil temperature was developed using site-specific measurements of methane oxidation rate.

The transport and degradation parameter expressions were incorporated into a numerical model for simulating landfill gas transport, using soil-water content, temperature, and soil organic matter content as measured input parameters (functions of time and soil depth). Previous measurements of landfill gas (CO2 and CH4) concentrations and fluxes in the agricultural field soil conducted over a period of one year were used to calibrate the gas transport model using methane oxidation rate, landfill gas pressure, and wind-induced dispersion as fitting parameters.

The model was subsequently tested against independent concentration and flux data (not used in the model calibration). This yielded a prediction accuracy similar to that found during the calibration. The model was used to evaluate the sensitivity of landfill gas concentration and flux with respect to a set of governing parameters.

The results of the sensitivity analysis indicated that landfill gas migration in the field soil was most sensitive to air permeability and soil-water content. Methane oxidation rate and atmospheric pressure variations had some effects, especially on the gas flux, whereas gas diffusion was not important.