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Re: another question ...landfill gases

  Perhaps what Jon read was RACHEL's summary of TOXIC GASES EMITTED FROM
  LANDFILLS, http://www.envirolink.org/pubs/rachel/rhwn226.htm
  <<The California Air Resources Board (CARB) selected 10 toxic gases for
  measurement; they selected these particular gases because they are 
  known to have ill effects (particularly cancer) on humans who are 
  exposed for extended periods. The ten toxic gases they tested for are:
  vinyl chloride, benzene, ethylene dibromide, ethylene dichloride,
  methylene chloride, perchloroethylene, carbon tetrachloride,
  1,1,1-trichloroethane (methyl chloroform), trichloroethylene, and
  chloroform. In addition, landfill gas samples were also analyzed for
  oxygen, nitrogen, methane, and carbon dioxide.
  The CARB summarized their findings this way: 
  1) One or more of the 10 toxic chemicals could be measured in gases
  emitted from 240 out of 356 landfills tested; in other words, 67%
  of the tested landfills emitted one or more of the toxic gases. 
  2) Hazardous waste landfills and municipal solid waste landfills
  appeared to be similar in their ability to produce toxic gases. 
  3) In many cases, but not all, toxic gases escaping from landfills could
  be measured at the property line, the legal boundary of the landfill.  
  4) Methane at concentrations greater than the regulatory limit of 5% was
  found to be migrating offsite underground at approximately
  20% of the landfills. Methane is a naturally-occurring gas created by
  the decay of organic matter inside a landfill. As methane is formed,
  it builds up pressure and then begins to move through the soil,
  following the path of least resistance; often it moves sideways for a
  time before breaking through to the surface of the ground. Methane is
  lighter than air and is flammable. If it enters a closed building and
  the concentration builds up to about 15% in the air, a spark or a flame
  is likely to cause a serious explosion. For this reason, landfill
  designers sometimes install a set of pipes full of holes like a swiss
  cheese to provide a known pathway for the methane to escape
  through; such systems are sometimes successful and sometimes not. 
  The new California study does not go into great detail, but it certainly
  provides evidence that toxic gases are likely to be measurable in
  the air near landfills. For example, of 340 California landfills
  studied, more than half had measurable airborne releases of benzene
  (average: 2.5 parts per million [ppm]), methylene chloride (average: 4.8
  ppm), perchloroethylene (average: 1.1 ppm),
  1,1,1-trichloroethane (average 650 parts per billion [ppb]), and
  trichloroethylene (average: 840 ppb). Nearly half had releases of vinyl
  chloride (average: 2.2 ppm). Methane was found at three quarters of all
  landfills tested. At half of these, the concentration was 10% or
  less. In the other half, the concentration varied from 11% to 73%. These
  were measurements at the ground surface of the cap of the landfill. 
  Another set of measurements was taken at the property boundary of each
  of 288 landfills, to see if toxic gases could be detected in the
  "ambient" outdoor air. At 57% of these landfills, 1,1,1-trichloroethane
  was detected (maximum: 51 ppb); at 49%, perchloroethylene was
  detected (maximum: 269 ppb); at 45%, methylene chloride (maximum: 1.3
  ppm); at 40%, benzene (maximum: 500 ppb); at 32%,
  trichloroethylene (maximum: 130 ppb); at 22%, carbon tetrachloride
  (maximum: 15 ppb); at 13%, chloroform (maximum: 32 ppb). 
  In all, off-site migration of gases, including methane, was detected at
  83% of all the 288 landfills.>>
  Another earlier study found vinyl chloride in landfill leachate. 
  However, landfill gases were not mentioned.  The study  "An Estimation
  of the Risk Associated with the Organic Constituents of Hazardous and
  Municipal Waste Landfill Leachates," appeared in the journal, HAZARDOUS
  WASTES AND HAZARDOUS MATERIALS, Vol. 5, No. 1 (Spring, 1988), pgs.
  Peter Montague summarized this study in RACHEL's Hazardous Waste New #90
  at http://www.envirolink.org/pubs/rachel/rhwn090a.htm
  Susan Snow