Environmental Analysis
Evaluations of the need for cleanup of contamination at Superfund
sites now require quantitative risk assessments. A 1996 paper titled
Quantitative
Uncertainty Analysis of Superfund Residential Risk Pathway Models
for Soil and Groundwater: White Paper by E.A. Dawoud and
S.T. Purucker discusses quantitative risk assessments in great detail.
The EPA has produced handbooks that specify the equations and parameter
values to be used for such assessments. The values are point estimates
and are generally chosen conservatively (i.e., they err on the side
of safety).
The EPA's methods advocate a tiered approach, beginning with point
estimates of risk, and progressing to probabilistic estimates that
characterize variability and uncertainty in risk using Monte Carlo
analysis. In the past few years, the EPA has worked to provide technical
guidance on the application of probabilistic methods to human health
and ecological risk assessment, in particular for Monte Carlo methods
(see Risk
Assessment Guidance for Superfund Volume 3 Part A: Process for Conducting
Probabilistic Risk Assessment RAGS 3A - DRAFT for more information).
"Without a quantitative assessment of uncertainty, millions of
dollars may be misallocated to clean up contamination that only
appears to present an unacceptable risk due to the hidden effect
of compound conservatism in the EPA risk calculations." In actuality,
such contamination might present a health risk that can be considered
trivial.
In his risk assessments, Dr. F. Owen Hoffman, an environmental
scientist previously at Oak Ridge National Laboratory and a longtime
proponent of the use of uncertainty analysis, uses the EPA approach
only as an initial screening calculation. Such screening calculation
is useful in identifying low risk exposure pathways and contaminants
that can be designated clearly as low priority for further investigation.
For the remaining contaminants and exposure pathways, he uses Crystal
Ball to translate his estimates of uncertainty in each of the parameters
used in the risk assessment equation into a confidence interval
about the final risk estimate.
Dr. Hoffman found that Crystal Ball not only helped to determine
the need for cleanup more appropriately but also helped in setting
priorities for collection of better data. "Crystal Ball really helps
focus our risk assessment," he said. "By taking into account
the range of possibilities, I was immediately able to see what type
of action was required."
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