|APARTMENT COMPLEX BUILT ON FORMER LANDFILL|
The contour map of methane concentrations (ppmv) shows three distinct plumes containing significantly high concentrations of methane. Two plumes on the north boundary appear to continue to the north. However, it may be the edge of a plume that is migrating onto the site from adjoining property to the north since this was also a landfill area. Since no soil gas samples were collected off the property, the northern extent of the plume is undefined. A large north-south area of methane concentrations was also observed in the central part of the apartment complex from the vicinity of buildings 5 to 19. The 50,000 ppmv contour line defines the area of the survey, whithin which concentrations of methane were greater than 5 %, which is considered to be the lower limit of methane concentration needed for an explosive condition.
extremely high concentrations of methane suggest a biogenic source of
generation as the apartment complex was built over a landfill. In a reducing
environment, biogenic methane is generated in large quantities in the
subsurface as a result of the action of bacteria on the organic matter
disposed of in a landfill. The overlay of the isopach of the solid waste
coincides extremely well with maximum concentrations of methane. The methane
plume becomes narrow in the vicinity of building 20. This could be due
to subsurface drainage extending from east to west. This drainage feature
either vents the gases in this area or interrupts the flow of gases across
Top of limestone depths were taken from the Engineering borings drilled on the site in 1984. Surface elevations for each boring were obtained from a pre-1984 (pre-construction) topographic contour map. The resultant top of limestone map (datum: sea level) probably represents the floor of the old gravel quarry that occupied the site before landfill operations started in 1966. Areas of low or gentle gradient on the top of limestone map coincide well with areas of thick solid waste accumulation.
Anomalously high levels of ethane, propane, and N-butane also exist in different areas of the apartment complex. Concentrations of these heavier hydrocarbon components, associated together, suggest the presence of a natural gas leak. Since no natural gas pipelines exist in the immediate vicinity of the site, then the presence of the heavier components could be explained as resulting from either dumping of bottled gases or disposal of other types of petroleum products in the landfill. The sources of the contaminants can not be determined based on the scope of work performed to date. It is possible that substances containing hydrocarbon compounds have been included in the landfill underlying the apartment complex. The disposal of used refined petroleum products in portions of the landfill can also account for anomalous concentrations of the heavier components (ethane, propane, butane).
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