Decomposition of organic waste deposited in landfills is a global source of anthropogenic greenhouse gases (GHG). Landfills contribute circa 8% of worldwide CH4 (up to 20% in the United States and Europe). Landfill gas (LFG) emission reporting under the United Nations Framework Convention on Climate Change is largely based on decay modelling related to the input of organic waste into landfills. In many older landfills large uncertainty exists about the type and amounts of waste deposited historically. In these cases, modelling emissions is best constrained by a set of parameters describing a landfill’s CH4 mass balance:
CH4(emitted) = CH4(generated) - CH4(recovered) - CH4(oxidized) -CH4(migrated) - dCH4(stored)
These parameters are dependent on site-specific conditions (waste composition, geomorphology, climate, type of oxidation cover, type of barriers used, etc.), which are often poorly documented and costly to characterize. Therefore, estimates of CH4 and CO2 emissions from most landfills are fraught with high uncertainty. Direct emission measurements, in turn, are challenged by a large spatial and temporal variability of instantaneous emissions rates, which makes it difficult to estimate annual emissions. The techniques with highest spatial resolution (vertical soil gas concentration profiles, surface flux chambers) are very time-consuming and difficult to scale up to the whole site, while techniques integrating fluxes over large areas (e.g., eddy covariance, aerial measurements) are challenged by variable topography, interfering CH4 sources, and rely on accurate micrometeorological models often not available at many sites.
Stable isotope ratios may be used as a tool for understanding landfill processes. The approach is based on exploiting the difference in mass between isotopes of the same element. Many common elements have two or more naturally occurring stable isotopes (e.g. 1H, 2H, 12C, 13C). Molecules containing these elements have the same chemical properties but may behave differently due to relative differences in mass. This leads to isotopic fractionation, enriching or depleting substrates and products for the rare heavier isotopes. The resulting isotopic natural abundance signatures of CH4 and CO2 can thus, together with other parameters, be used to give insight into the extent and duration of processes forming and consuming LFG. Variations in isotopic ratios of CH4 isotopocules have been documented in many landfills and interpreted in terms of methanogenesis, transportation and oxidation. However, using this approach at a landfill scale suffers from the same challenges as quantifying emission directly, and more work is needed to improve the applicability of this technique.
This PhD project aims to advance possibilities to estimate LFG production, emissions pathways and remaining degradation/production potential in landfills by combining recent developments in field measuring techniques (rapid soil flux measurements using high-sensitivity laser-based spectrometers) with stable isotope techniques. Time series data will be produced at selected landfills over the course of two or more years, including stable isotope data on carbon and hydrogen in methane and CO2. The new data will be interpreted in terms of landfill degradation status, gas production and migration processes. The specific objectives are:
To establish stable isotope markers in landfill efflux as proxies for estimating the remaining production/degradation potential
To develop a methodology for rapidly quantifying landfill emissions and cover soil CH4 oxidation efficiencies at landfill scale
To use the improved understanding of landfill processes together with the improved methodology for surface mapping to develop a practical methodology for estimating the remaining degradation potential of a landfill site
In the course of this work, novel insights into the seasonal effects of freeze/thaw and drying/rewetting cycles on landfill gas emissions and oxidation efficiency in Nordic climates will be obtained.
As of December 2022, progress has been made on several of these specific objectives. A methodology for mapping landfill surface emissions rates has been tested at several field sites and shows increased data quality and decreased measurement times relative to currently used measurement techniques. In addition, a time series was commenced in December 2021 from 11 different gas collection wells at the Brånåsdalen landfill, with gas being collected weekly for composition and isotopic determination. The initial results from the first several weeks of gas collection indicate spatial differences in isotope ratios. The differences are maintained over many months and are statistically significant, and the isotope ratios in CO2 and CH4 show a strong correlation to the degradation state of the landfilled waste.