USING ISOTOPIC AND MOLECULAR-DATA TO MODEL LANDFILL GAS PROCESSES

Using a large data set, a preliminary investigation has been made to e valuate the usefulness of stable isotope ratios for improving our unde rstanding of methane and carbon dioxide generation in landfills. Inclu ded are approximately 130 landfill gas samples from across the U.S.A., and 18 recent samples from: (1) an Argonne Laboratory study area in t he Brea-Olinda Landfill, Orange County, California (U.S.A); and (2) se veral Los Angeles County landfills, California (U.S.A). The following isotope ratios were examined: delta(13)C for methane, delta(13)C for c arbon dioxide and delta D for methane. Using simple ratio plots supple mented by mass-balance calculations, these data show promise for indic ating the relative contributions of the four major carbon cycle proces ses in landfills, namely: (1) direct oxidation of organic material to carbon dioxide; (2) methane generation from fermentation (acetate clea vage); (3) methane generation from carbon dioxide reduction; and (4) m ethane oxidation to carbon dioxide by methanotrophic bacteria. Both th e methane generation and oxidation reactions are central to an explana tion of the trends discussed herein. The data also suggest that direct oxidation of organic matter in the refuse may be contributing to the observed isotopic ratios in some cases. The trends observed at the Bre a-Olinda site were similar to trends using the large U.S. database, su ggesting that isotopic techniques may be useful to better constrain ca rbon cycle processes common to all landfill settings. (C) 1996 ISWA