Re: A "novel" soil standard for TCE

["Bill Smith" <WJASmith@aol.com>]

In general I think standards based on soil gas concentrations for mobile
volatiles such as TCE are more protective than standards based on soil
concentrations. Its much easier to detect TCE in soil gas than to find an
isolated pocket in a clay lense with soil sampling. While with a major
government environmental contractor, I helped develop soil gas models using
Henry's Law.

However, cleanup standards based on either gas or soil concentrations both
have major drawbacks as the distribution of contaminants in the 
subsurface is
not uniform, which is the unstated assumption behind all cleanup standards
based on concentrations. Far better to follow the lead of air pollution
regulations and to use best available control technology (BACT) as the basis
for standards. To apply BACT standards, though, each site would have 
to be
well characterized and understood - which could be more expensive in the
short term than a force fit one size fits all concentration or risk
assessment method.

At many sites a complete characterization (e.g. the location of every
contaminant pocket) may not be necessary to apply BACT standards. A more
productive approach to cleanup at these sites is to base closure 
standards on
fate, not arbitrary concentrations. If it can be demonstrated that the
material is not spreading or reaching receptors and that nature is
remediating the contaminant within a reasonable time, close the site and
impose monitoring restrictions. Otherwise, leave the site open until action
has been taken that will allow nature to restore itself. That may be years
or decades - but that type of site is important to keep track of.

Please keep me posted on this novel soil standard for TCE.