Site Specific Development of Risk Based Cleanup Levels

Each sediment sample consists of a different composition of cPAHs that results in a correspondingly different probability of risk. For example, two sediment samples may have equivalent total cPAH concentrations. However, the one sample that has a greater percentage of ben-zo[a]pyrene, one of the most potent cPAHs, will pose the greater probability of risk. To illustrate this relationship among the sediment samples, a log-log plot of the probability of risk versus the total cPAH concentration was developed (Figure 1). Cleanup levels corresponding to

Total Carcinogenic PAHs (mg/Kg) Figure 1 Risks to children associated with total cPAHs resulting from exposure to pond sediments.
Table 4 Total cPAH Concentrations and Their Corresponding Target Risk Levels

Total cPAH concentration

Risk level

(mg/kg)

10E-4

280

10E~5

28

10E-6

2.8

target risk levels can be determined by simply locating them on the regression line. For example, a target risk level of 10-5 corresponds to an average total cPAH concentration of 28 mg/kg (Table 4).

This pond has only four of its 27 samples with total cPAH concentrations exceeding a concentration of 30 mg/kg (approximately the target risk of 10~5). Remediating areas with elevated levels of total cPAH to this concentration or "pickup level" would reduce the average total cPAH concentration, or cleanup level, in the pond and consequently reduce the associated risk. Table 5 presents various pickup levels and the risks that correspond to the cleanup levels for children who wade in the pond. A pickup level of 30 mg/kg results in a reduction of the cleanup level for total cPAH concentration from 20 mg/kg to 12 mg/kg with a corresponding reduction in risk from 7.3 X 10~6 to 4.3 X 10~6. This approach results in limiting the remediation to the heavily impacted areas and minimizing unnecessary cleanup.

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