During the fourth CERCLA statutory five year review process, EPA became aware of the release or threat of release of toxic, asphyxiating, and potentially explosive gases at the delisted Superfund Site. Gases detected at levels at or above their Immediately Dangerous to Life and Health (IDLH) and Short Term Exposure Limit (STEL) and at or above their Lower Explosive Limit (LEL) include Hydrogen Cyanide (HCN), Hydrogen Sulfide (H2S), Carbon Dioxide (CO2) and methane. Phosphine is also expected to be present based on the chemical consituents of the waste materials in the landfills. The conditions at the Site may constitute an imminent and substantial endangerment to public health or welfare or the environment because of the actual or threatened release of hazardous substances from the Site. Immediate response action to sample and analyze the threat or potential threat release at the Site to protect human health and the environment is needed. Additional response action may be needed once the threat has been assessed.