Environmental Cleanup at Navy Facilities Risk-Based Methods (1999) / Chapter Skim
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4 Uncertainty in Risk-Based Methodologies
4 Uncertainty in Risk-Based Methodologies
Pages 83-110
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From page 83... ...
A case study illustrates the trade-offs of doing an uncertainty analysis versus using conservative cleanup goals. The chapter closes with an examination of the uncertainties associated with remedial options, including both technical and non-technical solutions.
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From page 84... ...
Recent trends indicate that uncertainty analyses may become more commonplace during risk assessment and management (Finkel, 1990; Morgan and Henrion, 1990; NRC, 1994a, 1996; Browner, 1995~. The first half of this chapter discusses the major uncertainties to consider when conducting an uncertainty analysis during risk assessment.
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From page 85... ...
For example, delineating a primary or secondary source in three dimensions requires extensive sampling efforts, and a potential source of uncertainty lies in possible errors associated with the location, density, and handling of samples. Fortunately, these sources of uncertainty may be assessed (or even reduced through improved techniques)
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From page 86... ...
Pathway Characterization Pathway characterization with contaminant fate-and-transport models is an essential ingredient of risk assessment and management. The purpose of fateand-transport modeling is to determine contaminant concentrations at human or ecological receptors as a function of time, given some measured or assumed source concentration.
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From page 87... ...
unconfined Single vs. multiple aquifers Location of bedrock Hydrologic boundaries Rivers, lakes, water table · Recharge spatial end temporal variability · Source/plume delineation · Field measurements hydraulic heads, solute concentrations · Inferred parameters calculated from field observations Hydraulic conductivity Specific storativity Effective porosity Dispersivity Matrix diffusion Distribution coefficient Chemical and biological degradation rates Reaction rates, stoichiometry
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From page 88... ...
Variability in the hydraulic conductivity of subsurface materials directly affects the dispersive properties of an aquifer (i.e., how the aquifer material causes a contaminant to mix with the native ground water)
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From page 89... ...
The parameters of ground water flow and solute transport models are hydraulic conductivity, specific storativity, effective porosity, dispersivity, matrix diffusion, distribution coefficients, decay coefficients, and reaction rates. These parameters are inferred by fitting mathematical models to measured head and solute concentration data.
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From page 90... ...
If contaminants have traveled through the subsurface, variable aquifer parameters, such as hydraulic conductivity, will create variable exposure patterns for receptors. This variability can be eliminated only if toxicants are measured directly at the point and time of exposure.
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From page 91... ...
For reasons stated above, risk assessment models that consider only adult receptors may underestimate the impact of any particular waste site on children. This problem can be overcome by replacing single exposure models based on adult males with a series of models that
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From page 92... ...
Many factors influence the dose received by an organism. The mobility of organisms and the temporal and spatial variability of toxicants produce considerable uncertainty regarding the amount of a specific toxicant absorbed and its resulting internal concentration in target organs.
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From page 93... ...
It might appear from the above discussion that successful management of ecological risks is an impossible task. However, over the past several decades, ecological risk assessors have developed a variety of strategies for identifying and dealing with these uncertainties, such as site-specific studies of impacted Western gulls nesting on a landfill at the Alameda Naval Air Station.
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From page 94... ...
The committee strongly supports the use of a properly designed and implemented long-term monitoring program to reduce uncertainty at contaminated sites. Long-term monitoring provides data that can be used to evaluate assumptions made during fate-and-transport modeling, and these data can be used to improve the estimates of various model parameters, thereby leading to a reduction in modeling uncertainty.
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From page 95... ...
Second, the case study provides a clear example of how variability in the hydraulic conductivity of natural materials and variability in human receptors can be quantitatively incorporated into risk assessment calculations. The same method could be expanded to include uncertainties in other parameters.
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From page 100... ...
After examining 77 active pump-and-treat systems in detail, it was determined that only 8 of the systems had achieved ground water cleanup goals. Whether the remaining systems would achieve cleanup goals was unlikely at 34 of the 77 sites.
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From page 101... ...
Navy. recognized, these systems are still the most common treatment technology at large contaminated sites (used at 93 percent of CERCLA sites where ground water restoration is under way, according to the most recent available EPA data [NRC, 19971~.
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From page 102... ...
At complex sites make greater use of expert panels to review cleanup plans and the feasibility of achieving cleanup goals. A panel of independent reviewers could provide guidance on the site conceptual model, design of the cleanup system, probability of achieving cleanup standards, and the need for a containment system when achievement of cleanup goals is not feasible.
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From page 103... ...
. Another common institutional control is to limit access to ground water by shutting down drinking water wells and restricting new well drilling.
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From page 104... ...
Enforcement of legal institutional controls at active military bases is even more difficult, since there is no property deed, and local governments have no zoning authority. The military has a poor record for keeping track of toxic hazards because it is not subject to local building codes.
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From page 105... ...
Local reuse authorities, developers, and owners have virtually no interest in fencing or patrolling potentially hazardous property. Even when an institutional control has been agreed to by all relevant parties, there may be unforeseen future changes in land use that negate the effectiveness of the control.
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From page 106... ...
This could be done by the EPA' s Federal Facilities Enforcement Office. The EPA should develop criteria for determining whether institutional controls are functioning properly and under what conditions controls can be relaxed.
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From page 108... ...
1994. Variability of hydraulic conductivity and sorption in a heterogeneous aquifer.
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From page 109... ...
1998. Uncertainty Analyses of Fuel Hydrocarbon Biodegradation Signatures in Ground Water by Probabilistic Modeling.
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From page 110... ...
1991. Spatial Variability of Strontium Distribution Coefficients and Their Hydraulic Conductivity in the Canadian Forces Base Borden Aquifer.
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