Dynamics of landfill gas migration in unconsolidated sands

A gas plume from the Foxhall landfill in Suffolk, UK contains elevated conc entrations of methane and carbon dioxide in unsaturated unconsolidated sand s which comprise Pleistocene "Red Crag" deposits. The plume emanates from a relatively restricted zone along one side of the landfill, but extends ove r 100 m from the site boundary. The reduction in the methane to carbon diox ide ratio with distance and the systematic changes in their carbon and hydr ogen stable isotope ratios are evidence of microbially mediated methane oxi dation. A one-dimensional advection-diffusion model was used to describe th e combined concentration of methane and carbon dioxide in the plume. Diffus ion alone underestimated the concentration profile, but a good fit to the d ata was achieved with an advective flux of 4.5 m yr(-1), indicating that ad vection due to a pressure gradient from the landfill as well as diffusion s hould be considered in gas migration modelling. The kinetics of methane oxi dation was studied by parameter-fitting a reaction rate into the advection- diffusion equation with first-order decay. A decay constant of -0.063 yr(-1 ) (half-life 11 yr) produced a poor fit to the methane profile, suggesting that oxidation may not be constant throughout the plume. However, the stabl e isotope data allowed two rates of oxidation to be inferred. A slow rate o f oxidation with a half-life of the order of 4.3 to 7.6 yr was inferred in the centre of the plume where oxygen was absent. A much faster rate with a half-life no longer than 0.76 to 1.21 yr occurred beyond 60 m of the landfi ll and around the top fringe of the plume where oxygen was present. These r ates are considered to reflect the difference between aerobic and anaerobic oxidation, the latter utilizing iron(III) in the sediment as an electron a cceptor. The shape of the plume is asymmetrical, indicating a geological co ntrol on gas migration. In a two-dimensional model a poor fit to the observ ed data was obtained when the sand was assumed to be homogeneous and where the gas entered from a restricted part of the landfill boundary. However, a better model was produced by varying the diffusion coefficient in the sand s over the range 5 x 10(-7) to 2 X 10(-6) m(2) s(-1) without the need to re strict the zone of gas release along the landfill boundary. Such a range in transport properties could be accounted for by normal variability in the p orosity, tortuosity and water content of the sand. The long-term dissipatio n of the plume assuming only diffusion was predicted to take up to 30 yr fo r the gas concentration to reduce to 10% of its initial value. However, the plume disappeared within a year after pumping from gas wells in the landfi ll, indicating chat advection under an imposed pressure gradient was a majo r control on remediation. This study shows that models can be used to expla in landfill gas migration and to infer oxidation rates which can be used to predict gas migration at other sites. However, the need to obtain field da ta on gas permeabilities and diffusivities will always be a major limitatio n in predicting gas migration in permeable formations.