Final Site Information Package for National Remedy Review Board ...

10. Principal Threat Materials
The NCP has established an expectation that USEPA will use treatment to address the
principal threats posed by a site wherever practicable (NCP §300.430(a)(1)(iii)(A)). Where
USEPA determines that it is not practicable to use treatment to address principal threat
materials (PTM), they may be transported offsite, consistent with the Off-Site Disposal Rule,
40 CFR 300.440, or managed safely onsite, consistent with all ARARs. This may include
containment and consolidation in a PTM cell that has a secure liner system.
PTM are those source materials considered to be highly toxic or highly mobile that generally
cannot be reliably contained and/or would present a significant risk to human health or the
environment if exposure were to occur (USEPA, 1991b). Additional information for defining
PTM can be found in A Guide to Principal Threat and Low Level Threat Wastes (USEPA, 1991b).
The guidance notes that identification of PTM is made on a site-specific basis and is
intended to help streamline and focus the remedy selection process.
As noted in the 2002 ROD for OU 3 (USEPA, 2002), it is not believed that PTM will be
encountered during cleanup conducted in the Upper Basin. The following concentrations
were used to define PTM in the Bunker Hill Box (USEPA, 1992) and OU 3 (USEPA, 2002):
PTM Concentrations
Parameter (parts per million [ppm])
Antimony 127,000 ppm
Arsenic 15,000 ppm
Cadmium 71,000 ppm
Lead 84,600 ppm
Mercury 33,000 ppm
The 1996 ROD Amendment for OU 2 (USEPA, 1996b) required that all PTM be placed in a
high-density polyethylene (HDPE) bottom-lined and three-ply copolymer top-lined
monocell. The PTM monocell is contained within the larger SCA and under the SCA’s
HDPE cap, affording an additional layer of protection for the PTM monocell. Because of the
mobility and toxicity of mercury, free mercury PTM were stabilized using a specific concrete
mix developed as a result of analysis and bench testing prior to placement in the PTM
monocell. Other materials classified as PTM due to the presence of one or more other
threshold metals (antimony, arsenic, cadmium, and lead) were placed in the monocell
without treatment because they were determined to be stable. A time-critical removal action
was conducted in 1999 to address all known surface contamination associated with rail
transport along the Wallace-Mullan branch of the Union Pacific Railroad.
While additional PTM are not expected, if additional concentrates or other materials that
meet the definition of PTM are encountered during remedy implementation, these materials
would be managed in a manner that is protective of human health and the environment and
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