Variability of methane emissions from an old landfill over different time-scales

Methane (CH4) emission patterns were investigated at an old landfill in northern Germany during a 2-year campaign over three different time-scales (seasonal, daily and diurnal) using modified static chambers. Emissions were not uniformly distributed over the cover soil, but occurred through localized preferential pathways (hotspots). The range of emissions from the 14 investigated hotspots at any one measurement event was large, reflecting large spatial variability of emissions from the landfill. In addition, the individual hotspots showed a large temporal variability of emissions (09.7 mol CH4 day1 over a range of 20 months). Environmental variables effectively modified either the biological process of methane oxidation in the soil (temperature) or the physical gas transport process (moisture and atmospheric pressure). During the seasonal campaign, emissions generally were greater in winter (moister and cooler) and smaller during the summer period (dryer and warmer). Under dry conditions, emissions were independent of soil moisture but correlated negatively with soil temperature. A positive correlation of emissions with moisture was found for moisture levels relating to a matric potential of 30 to 6 kPa, reflecting impeded diffusive oxygen ingress and hence reduced methanotrophic activity. Under very moist conditions, more than 6 kPa, emissions were negatively correlated with moisture content because of decreased gas permeability in the soil. In addition, a pressure decrease preceding the measurement was at all time-scales often related to large emissions, whereas constant or rising pressure prior to measurement resulted in small emissions. The particular importance of the different effects depends on the type of gas transport at the specific emission location and whether it is advective or diffusive gas transport or a mixture of both, and varies at the temporal scale both between and within locations.