Cleanup Standards versus risk

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

Regarding the discussion on relative risk, the possible fates of contaminants
are frequently ignored in conventional risk assessments. For example, 2800
ppb of TCE found in a Silicon Valley Well, could, under the right conditions,
transform into several hundred ppb vinyl chloride with accompanying risks
that are orders of magnitude higher than those assigne to TCE.

Regarding TPH cleanup standards, in many places where large quantities of
fuel have spilled into the subsurface, there are large subsurface fuel
pockets that are difficult to detect with soil sampling and conventional soil
gas sampling. I've been on several sites where soil and soil gas has tested
clean, but high methane levels and low oxygen levels indicated the presence
of pockets of TPH somewhere. At one Air Force Base, these pockets supported a
soil vapor extraction system for several years without TPH ever being
detected in soil samples and TPH being detected in soil gas only after
several hours of vapor extraction. Yet methane concentrations were high and
oxygen concentrations low were found in the original samples of undisturbed
soil gas.

Most investigations don't look for methane or measure oxygen concentrations
in the soil gas. In the San Francisco Bay area some environmentalists and
citizens are beginning to insist that these measurements be made before they
will agree to "no action." The methane at Naval Air Station in Alameda, CA,
constitutes 60% of the soil gas under parking lots and constitutes a very
real explosive hazard if it leaks into utility trenches or buildings built
later at the site. Yet methane is not regulated by federal or state toxics
laws, nor accounted for in quantitative risk assessments. Quantitative risk
assessments have a very narrow scope and qualitative risks such as
transformation into more toxic products and explosive hazards should be taken
into account.

In general, if oxygen is present in the subsurface where hydrocarbons are
found, some many Bay Area citizens are more comfortable with higher TPH
cleanup levels.

William J. Smith, Ph.D., P.E.

Allied Technology Group, Inc.
47475 Fremont Blvd,
Fremont, CA 94538

Phone: (510)490-3008
Fax: (510)651-3731
 WJASmith@AOL.com