Cleanup Levels for Potentially Carcinogenic Constituents

Results of the risk assessment indicated that potential risks for exposure of on-site workers and construction workers to surface and subsurface soils were above the U.S. EPA's target risk range of 10~4-10-6 [7]; therefore, risk-based levels were derived for these potential human receptor groups. Constituents evaluated in the risk assessment included PAHs, pentachlorophenol, volatile organic compounds (VOCs), and metals. Cleanup levels can be developed for all of the evaluated constituents of interest (COIs), but this is not necessary if the potential risks from the individual COIs are insignificant. Following EPA Region III guidelines, cleanup levels were developed for only those constituents that contribute to 99% of the total potential risk. Tables 6 and 7 present a summary of the potential risks and the percent contribution of the individual constituents to the total risk for the evaluated human receptor groups. As can be seen from these tables, PAHs, pentachlorophenol, and arsenic were the COIs responsible for 99% of

Table 6 Summary of Potential Risks of Surface Soils to On-Site Workers

Constituent

Area A

Area B

Area C

Area D

Area E

Potential risk

PAHs

4.7E-04

4.4E-03

1.7E-03

7.8E-05

1.5E-03

Pentachlorophenol

8.6E-06

1.4E-06

1.4E-04

8.8E-06

1.5E-06

Benzene

O.OE+OO

1.5E-10

2.3E-09

4.6E-09

O.OE+OO

Styrene

0.0E+00

7.3E-11

5.5E-10

8.9E-10

O.OE+OO

Arsenic

2.8E-05

I.0E-04

0.0E+00

5.8E-05

1.1E-05

Chromium

0.0E+00

O.OE+OO

O.OE+OO

3. IE—07

3.5E-07

Total potential risk

5.IE—04

4.5E-03

1.9E-03

8.5E-04

1.6E-03

Percent contribution to total potential risk

PAHs

92.75%

97.73%

92.66%

92.04%

99.14%

Pentachlorophenol

1.69%

0.03%

7.34%

1.03% .

0.10%

Benzene

0.00%

0.00%

0.00%

0.00%

0.00%

Styrene

0.00%

0.00%

0.00%

0.00%

0.00%

Chromium

0.00%

0.00%

0.00%

0.04%

0.02%

Arsenic

5.56%

2.23%

0.00%

6.89%

0.74%

Table 7 Summary of Potential Risks of Surface and Subsurface Soils to Future Construction Workers

Constituent

Area A

Area B

Area C

Area D

Area E

Potential risk

PAHs

2.8E-05

4.6E-04

2.3E-05

1.8E-05

6.2E-05

Pentachlorophenol

1.4E-07

1.8E-08

1.7E-06

1.6E-07

1.0E-07

Benzene

0.0E+00

7.9E-12

3.8E-11

4.7E-11

2.2E-10

Styrene

4.6E-13

1.6E-12

1.4E-09

9.3E-12

1.9E-11

Arsenic

7.0E-07

2.8E-06

0.0E+00

1.6E-06

3.8E-06

Chromium

4.2E-08

4.2E-07

O.OE+OO

5.0E-08

8.0E-08

Total potential risk

2.9E-05

4.6E-04

2.5E-05

1.9E-05

6.6E-05

Percent contribution to total potential risk

PAHs

96.99%

99.30%

93.25%

90.48%

93.98%

Pentachlorophenol

0.47%

0.00%

6.74%

0.85%

0.16%

Benzene

0.00%

0.00%

0.00%

0.00%

0.00%

Styrene

0.00%

0.00%

0.01%

0.00%

0.00%

Arsenic

2.40%

0.61%

0.00%

8.42%

5.75%

Chromium

0.14%

0.09%

0.00%

0.26%

0.12%

100.00%

100.00%

100.00%

100.00%

100.00%

the potential risk to on-site workers and construction workers; thus cleanup levels were derived only for these COIs.

Potential risks were estimated in the risk assessment by evaluating exposures to site surface and subsurface soils via dermal contact, incidental ingestion, and inhalation exposure pathways. Applying the 99% contribution criteria mentioned above to each exposure pathway indicated that cleanup levels should be derived using each of the evaluated exposure pathways.

Risk-based cleanup levels were derived using the procedures and exposures assumptions presented in the risk assessment for estimating potential risk. Instead of using the sampled COI concentration in soil and solving the risk equations for potential risk, a target risk level was assumed and the risk equations were solved for the concentration of the COI in soil.

The following equation was used in the risk assessment to estimate potential risk (PR):

where Cs is concentration of COI, CSF is cancer slope factor, and EXP is the product of intake parameters used to establish long-term daily intake of COI. EXP is defined as

where CR is contact rate, EF is exposure frequency, ED is exposure duration, BW is average body weight, and AT is averaging time.

Solving Equation (3) to yield a soil concentration results in

Soil cleanup levels were derived for target risk levels of 10 4, 10 5, and 10 6 as required by the U.S. EPA.

Using the above method and the same intake assumptions as were used in the risk assessment, cleanup levels corresponding to 10~4, 10-5, and 10~6 were calculated for pentachlo-rophenol and arsenic and are presented in Table 8.

A different method was used for deriving cleanup levels for PAHs. PAHs comprise several constituents, and it is more desirable to express a cleanup level for PAHs as total PAHs than to derive cleanup levels for each individual constituent. First, the potential risk resulting from exposure to only PAHs for on-site workers and construction workers was calculated at each soil-sampling location. Second, the concentrations of total PAHs were plotted against the corresponding potential risks as shown in Figures 2 and 3. Having statistically significant correlations (R2), the cleanup goals corresponding to 10~4, 10~5, and 10~6 risk levels were located on the plot and are presented in Table 8. The cleanup levels presented in Table 8 are appropriate for those areas of the site that contain only PAHs, pentachlorophenol, or arsenic and not a combination of the three COIs.

Table 8 Cleanup Levels (mg/kg) with No Apportioning of Target Risk Levels

Target risk level

Constituent

On-site workers

Pentachlorophenol

Arsenic

Total PAHs

700 70 7

314 31 3

229 16 1

Construction workers

Pentachlorophenol

Arsenic

Total PAHs

30,500 3050 305 6,620 662 66

20,000 1300 85

1.0E-02

1.0E-03

to CC

1.0E-05

1.0E-06

1 10 100 1,000 10,000

Total PAHs (mg/Kg)

Figure 2 Potential risk vs. total PAHs—on-site workers.

1.0E-03

to CC

1.0E-05

1 10 100 1,000 10,000

Total PAHs (mg/Kg)

Figure 2 Potential risk vs. total PAHs—on-site workers.

CC c

1.0E-03

1.0E-04

1.0E-05

1.0E-06

1.0E-07

Figure 3 Potential risk vs. total PAHs—construction workers.

1.0E-03

1.0E-04

1.0E-05

1.0E-06

1.0E-07

1.0E-08

100 1,000 Total PAHs (mg/Kg)

10,000

100,000

Figure 3 Potential risk vs. total PAHs—construction workers.

Table 9 Distribution of Risk to On-site Workers from Exposure to Surface Soils

Constituent

Arithmetic mean

Standard deviation

Minimum

Maximum

Median

PAHs

Pentachlorophenol

Arsenic

Total

PAHs

Pentachlorophenol Arsenic

Potential risk

2.0E-06 O.OE+OO 1.2E-06 8.0E-06 Percent contribution

Since the total potential risk for most areas of the site is the result of exposure to several COIs, the target risk levels must be apportioned among the COIs. The target risk level was apportioned based on the contributions of the risk levels of the individual COIs. For example, if the site average potential risk level is 1 x 10~4 and the site average potential risk from PAHs is 7.5 x ](T5, it would be assumed that (7.5 x 10~5)/(1 x 1(T4), or 75%, of the target risk level would be apportioned among the PAHs. This is the preferred method for apportioning the target risk level among the COIs of the site, since potential risk is based on both the toxicity and the concentration of the COI.

Polycyclic aromatic hydrocarbons, pentachlorophenol, and arsenic were identified above as the major contributors to the potential risk at the site. Eighty-seven surface samples and 65 subsurface samples were collected at the site and analyzed for PAHs and pentachlorophenol, but only 32 of the surface soil samples and 27 of the subsurface samples were analyzed for arsenic. Only those soil samples analyzed for all three COIs were used in determining the apportionment of the risk levels.

The potential risk of exposure to PAHs, pentachlorophenol, and arsenic for on-site workers and construction workers was calculated for each of the selected soil samples. The average contribution of each COI to the total risk for on-site workers is presented in Table 9. Table 9 illustrates that 65% of the total risk for on-site workers is due to PAHs, 33% is due to arsenic, and 2% is due to pentachlorophenol. Table 10 shows that 75% of the potential risk for construction workers is due to PAHs, 23% is due to arsenic, and 1.4% is due to pentachlorophenol. The target risk levels were apportioned according to these distributions. As an example, assuming a target risk level of 1 X 10~4 for on-site workers, cleanup levels would be derived for PAHs corresponding to 65% of the 1 x 10"4, or a 6.5 x 10-5 target risk level. A 3.3 x 10~5 target risk level would be used for arsenic, and a 2 x 10-6 target risk level for pentachlorophenol.

Cleanup levels for on-site workers and construction workers based on the apportioning method described above are presented in Table 11. As can be seen from the table, cleanup goals for on-site workers are lower than the corresponding goals for construction workers.

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