Risk-Based Waste Classification in California (1999) / Chapter Skim
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3 Scenario Selection and Modeling
3 Scenario Selection and Modeling
Pages 55-76
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From page 55... ...
. A framework for evaluation of the exposure scenarios and aspects of the models that DISC used are discussed.
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From page 56... ...
As emphasized in Chapter 2, the risk-management goals drive the selection of the exposure scenarios; and an adequate selection of scenarios is essential for the validity of a risk assessment. Even if the best possible models of environmental fate and transport were to be used, a poor selection of exposure scenarios might invalidate any conclusions drawn from the assessment.
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From page 57... ...
The vapor might be inhaled, and the particles might be inhaled or deposited on backyard soil and vegetation. Residents might come into direct contact with resulting contaminated soil and they might consume foods contaminated by chemicals taken up from the contaminated soil or deposited dust particles.
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From page 58... ...
The residents have attributes and exposure pathways similar to those in the adjacent resident scenario, but are additionally assumed to eat fish from a surface water body adjacent to the land on which the material was applied. Ecological Effects Scenario Ecological effects were addressed using a 2-step approach.
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From page 59... ...
59 ~5 o ._ 4 ._ o ._ a' s 1 o ._ a' Vet au s 4= JO a, V)
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No implementations equivalent to the spreadsheets used for human receptors in the exposure scenarios were applied for ecological receptors. ANALYSIS OF SCENARIOS AND MODELING Because of the large number of assumptions that went into the exposure scenarios, it is not feasible to address them all in this report.
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From page 61... ...
Without any discussion of the policies that the DTSC wishes to implement, the committee found great difficulty in assessing the appropriateness of the exposure scenarios selected by DTSC. The DTSC in written and oral responses (DTSC, personal common., October 9, 199S, see Appendix C, No.
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From page 62... ...
There is no documentation that such an action was ruled out as a policy; but if not, then the waste worker scenario selected might be completely inappropriate a better approach might be to select acceptable waste concentrations based on other criteria, and then regulate the behavior of the landfill worker, as is done for hazardous wastes. Many of the details of the adjacent resident scenario were heavily criticized in the public comments.
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From page 63... ...
However, any correspondence between the scenarios is not documented, and there appear to be significant differences for example the raininess implicit in the SERT scenario appears to contradict the dustiness of the adjacent resident scenario. Once again, the major problem appears to be poorly defined policy goals which, according to oral presentations by DTSC (DTSC, personal commun., September 10, 1998)
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From page 64... ...
Without some discussion of the totality of situations that the DTSC expects to regulate, there is no way to judge the completeness of the scenario selection or the adequacy of the few scenarios selected to take account of other situations. For example, the committee was unable to discern whether DTSC intended to evaluate potentially contaminated soil removed from one site and emplaced on another.
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From page 65... ...
To that end, the correct models have to be selected to represent the physical processes involved in the scenarios; and parameter values appropriate to the scenario must be used. For example, in the landed} scenarios, little consideration has been given to the groundwater pathway (although the groundwater pathway is discussed separately in the SERT derivations, it should also be incorporated in any multipathway assessment)
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From page 66... ...
~. Mathematical Models and Their Implementation Estimation of environmental concentrations resulting from particular physical processes is generally carried out by using mathematical models.
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From page 67... ...
Scenarios and Models: Parameter Values Even if adequate mathematical models are selected to match the physical processes occurring in each scenario, the evaluations can be invalidated by selection of incorrect parameter values for use in those models. Chapter 4 discusses many cases of incorrect or unjustified parameter values.
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From page 68... ...
A box model might be adequate in both contexts; but the parameter values are critical and must be based on physical processes, not arbitrarily selected. In the original context of the PEA model, an urban garden of a dimension about 22 m, a box height of 2 m might be appropriate; but it is certainly not appropriate for DTSC's dispersion modeling of a landfill, where the dimension of the landfill is approximately 670 m.
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From page 69... ...
Variability is defined as the individual-to-individual differences in quantities associated with predicted risk such as in measures of or parameters used to model ambient concentration, uptake or exposure per unit ambient concentration, biologically effective does per unit exposure, and increased risk per unit effective dose. Uncertainty is defined as the lack of precise knowledge as to what the truth is, whether qualitative or quantitative.
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From page 70... ...
Some attempt needs to be made to verify that the distribution of results is not too affected by the assumption that all input distributions are lognormal, particularly where very few distributions contribute the majority of the variability in the output. Models: Sensitivity Analyses Sensitivity analysis is a procedure for determining how sensitive the results of a complex mode!
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From page 71... ...
For example, DTSC staff responsible for some of the analyses indicated that the sensitivity analyses demonstrated that the model's results were insensitive to assumptions about how much clothing a landfill operator was wearing because dermal exposures were not responsible for significant amounts of risk. Assuming that this interpretation is correct, communicating it clearly to the public would have saved significant amounts of controversy.2 Another method of assessing the sensitivity and quality of the models is to evaluate intermediate results within the calculation.
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From page 72... ...
Although such subfunctions are quite useful for a scoping analysis, a more thorough treatment of sensitivity analysis is required. In particular, the only parameter values for which sensitivity analyses were performed were those assigned distributions in the analyses.
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From page 73... ...
The only difference between numerical results produced by the implementation and those produced by the simplified mathematical model, for any allowed input values, should be due to the finite arithmetic precision of the practical computational devices. Good practice should be applied to minimize any such differences.
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From page 74... ...
for ecological receptors would be less than the exit level for humans using the HWIR scenarios and analyses. The methods applied by DTSC to derive TTLCs that are specific for wildlife did not include wildlife-specific exposure scenarios, but rather only applied wildlifespecific threshold values for the hazard of chemicals.
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From page 75... ...
Based on this line of reasoning, the use of human exposure scenarios is inappropriate for wildlife. Of the 36 chemicals listed by DTSC, 28 have both HWIR human or HWIR ecological exit levels3 (DTSC 199Sa, p.
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From page 76... ...
For those chemicals for which the minimum dataset cannot be satisfied, the Agency should clearly indicate that the exit criteria are based solely on human health considerations. The exit criteria should be re-evaluated, however, when and if additional data on ecological effects become available.
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