A geochemical soil gas survey consisting of 62 sampling sites was conducted on June, 1992. The soil gas samples were collected from a 0.5 inch probe at four foot depths unless soil conditions necessitated collection from a lesser depth. The scope of the survey was to determine the nature and the areal extent of possible methane gas generation from biogenic sources and/or hydrocarbon contamination in the near surface soils and/or groundwater. All of the soil gas samples were taken in the ground. No penetrations had to be made in the paved parking areas or sidewalks. The samples were analyzed for methane, ethane, propane, iso-butane, and n-butane.

Methane Concentrations Contour Map (ppmv)

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.

Solid Waste Isopach

The 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 the barrier.
Intervals of solid waste encountered in the borings were isopached to illustrate the areal and vertical distribution over the site. Maximum thicknesses of 13.5 feet of solid waste were encountered in borings 15, 16 and 31. This observation differs considerably from the thickness of 30 to 50 feet of solid waste under the site reported by other site investigators. Areas of thick solid waste agree well with areas of high methane concentrations.

Top of Limestone Contour Map (Dessau Chalk)

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.

Ethane, Propane, and N-Butane Concentrations Contour Map

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).

Chronology of Events

Major incidents which led to closing of complex by City of Austin on 07/25/92