Temporal variability of soil gas composition in landfill covers

In order to assess the temporal variability of the conditions for the microbial oxidation of methane in landfill cover soils and their driving variables, gas composition at non-emissive and strongly emissive locations (hotspots) was monitored on a seasonal, daily and hourly time scale on an old, unlined landfill in northern Germany. Our study showed that the impact of the various environmental factors varied with the mode of gas transport and with the time scale considered. At non-emissive sites, governed by diffusive gas transport, soil gas composition was subject to a pronounced seasonal variation. A high extent of aeration, low methane concentrations and a high ratio of CO₂ to CH₄ were found across the entire depth of the soil cover during the warm and dry period, whereas in the cool and moist period aeration was less and landfill gas migrated further upward. Statistically, variation in soil gas composition was best explained by the variation in soil temperature. At locations dominated by advective gas transport and showing considerable emissions of methane, this pattern was far less pronounced with only little increase in the extent of aeration during drier periods. Here, the change of barometric pressure was found to impact soil gas composition. On a daily scale under constant conditions of temperature, gas transport at both types of locations was strongly impacted by the change in soil moisture. On an hourly scale, under constant conditions of temperature and moisture, gas migration was impacted most by the change in barometric pressure. It was shown that at diffusion-dominated sites complete methane oxidation was achieved even under adverse wintry conditions, whereas at hotspots, even under favorable dry and warm conditions, aerobic biological activity can be limited to the upper crust of the soil.