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Consulting • Engineering • Remediation March 13, 1996 35 Magog Part Acton. MA 01 720 FAX (508) 635-91 80 ENSRRef. No: 7218-001 ENSRDoc. No: 55-DPG-552 Ms. Caroline Kwan Emergency and Remedial Response Division U.S. Environmental Protection Agency 26 Federal Plaza New York, NY 10278 RE: Comments on Draft Final FS Tutu Wells Site, St. Thomas, U.S. Virgin Islands Dear Ms. Kwan: At the request of Western Auto's attorneys, this letter provides comments on the Draft Final Feasibility Study (FS). » The use of Ease Tutu SSL values at Western Auto is inconsistent with the available geologic Information. On page 2-15, paragraph 1 it is stated that the SSL values developed for Tutu Esso are used for all properties in the Tutu Site region, except those where site specific SSLs were developed, because '...the subsurface conditions at the Esso Tutu Service Station are representative of conditions throughout the Tutu Valley*. This means that the SSL values developed for Esso Tutu have been used for the former underground storage tank (UST) area behind Western Auto. However, the geologic conditions behind Western Auto are not similar to those at Esso Tutu, or elsewhere in Tutu Valley. This is, in fact, stated in the FS on page 2-5 (paragraph 1, last sentence) as follows... * An exception to this condition was observed at Western Auto, where alluvial deposits were only saturated in a perched zone within a gravel layer overlying a day layer". The hydraulic conductivity of the clay at Western Auto is extremely low with measurements values of 1.2 x 10* and 4.3 x 10* (see enclosed Stephens and Associates report). The very low hydraulic conductivity and the continuous extent of this day layer creates a condition unlike that elsewhere in Tutu Valley. These geologic conditions effectively isolate the overlying alluvial material from the groundwater. Since the intent of the SSL values is apparently to provide soil cleanup values that are protective of groundwater and appropriate for site specific conditions, it is dear that the Esso Tutu SSL values are inappropriate for Western Auto. Site-specific cleanup values that consider the extremely low transport in the day layer at Western Auto should be calculated. , i 4i-;,v..f. •!>.«„• Si- TUT 05O7 *65039* 65039 Ms. Caroline Kwan March 13, 1996 • SSL values for benzene should not be applied for other BTEX parameters. It is apparent from FS Table 2-4 that the same SSL values have been used for all of the BTEX parameters. Based on Table 1 included with EPA's comments on the draft FS, it appears as if the SSL values were determined from an analysis of benzene. Applying SSL values derived for benzene for the other BTEX parameters (toluene, ethylbenzene, and xylenes) is inappropriate and overly conservative. The drinking water standard or Maximum Contaminant Level (MCL) value for benzene is 5 ppb, while MCL values for toluene, ethylbenzene, and xylenes are 1000 ppb, TOO ppb, and 10,000 ppb respectively. Since soil cleanup levels are a function of toxicity and mobility, the SSLs for BTEX parameters other than benzene are much too stringent. Based on toxicity alone, the SSLs for xylenes are at least 2000 times too stringent. • The proposed soil excavation at Western Auto is based on Inappropriate data. The basis for the proposed soil excavation at Western Auto is presumably the comparison between the soil contaminant concentration data and SSL values presented In Table 2-4. The Western Auto soil contaminant concentration data is not appropriate for this comparison because the soil samples used in the comparison are from soil that was excavated and removed from the site. Also, as pointed out in our comments on the draft FS, the sample with the highest BTEX concentrations (SS-1) is not a soil sample. Sample SS-1 is presented as a soil sample (page 2-19; paragraph 3) but is actually a sample of product that leaked out of the waste oil vent pipe after the pipe was inadvertently ruptured during the tank removal. in addition to the inappropriate soil data that has been used in the comparison, the SSL values are also inappropriate. As discussed above, SSL values derived for Esso Tutu are used for Western Auto, even though the geologic conditions that would determine site-specific deanup values are significantly different for the two sites. As also discussed above, SSL values derived for benzene have been used for the other BTEX parameters, even though SSLs for the other BTEX parameters should be orders of magnitude higher than those for benzene. • The extent of the proposed excavation for Western Auto is not Justified. A soil remediation option for Western Auto consisting of excavation is discussed In Section 4.6.2.5. A soil excavation volume of 181 cubic yards for Western Auto is developed in Table 4-5 and the estimated cost for the excavation option is developed in Table 5-6. We have several concerns with this item. First, the amount of soil to be excavated should be justified. We made this comment on the draft FS and EPA apparently agreed to our comment TUT OO8 OS 03 Ms. Caroline Kwan March 13,1996 since In their response to our comment they stated"... the FS should dearly explain how affected areas and depths were obtained for all sites.* Table 4-5 provides the input values for the volume calculation, consisting of estimated depths and areas of impacted soils; however, no source or justification for these input parameters has been provided. Second, the draft FS proposed a soil removal volume of 133 cubic yards and this value has somehow been increased to 181 cubic yards in the draft final FS. No justification was provided for the previous value 'm the draft FS, and no justification has been provided for the change in the value in the current version of the FS. Finally, it is not clear that any soil at all should be removed from the site given the inappropriate soil contaminant concentration data and SSL values (as discussed above) used in the comparison that is the basis for the proposed excavation. • The value of the soil remediation credit is Incorrect and the methodology of application of the credit is confusing. A "credit" for 35 cubic yards of soil previously removed and replaced with clean backfill has been included as part of the soil excavation option for Western Auto. This is stated on page 4-51 and reflected in the amount of soil to be disposed of, as indicated on Table 5-6. The 35 cubic yard value is apparently incorrect. During soil removal associated with UST closure, 85 cubic yards of material were excavated and removed from the area. This is discussed in the ENSR June 1994 report on UST closure. In addition, Geraghty and Miller removed substantial amounts of material during the installation of monitoring well MW-24. The source of the incorrect 35 cubic yard value is not provided in the FS. Also, the methodology for application of the credit is confusing. The proposed area of excavation at Western Auto shown on FS Rgure 4-5 is largely encompassed by the area of soil removed during the tank closure activities. Figure 3 of the June 1994 UST Closure Report shows this area. Since the FS apparently proposes excavation of the same area as that previously remediated, it is not dear what is meant by a "credit" and how the credit is to be applied. • Discussion of remediation of the 4-inch PVC pipe should be Included in an FS section on Four Winds, not Western Auto. Remediation of the 4-Inch FVC pipe is discussed in Section 4.6.2.5, the Western Auto soil remediation section. It is stated that the "...FVC pipe is not related to Western Auto operations. However, the further investigation and possible remediation of the FVC pipe should be coordinated with Western Auto due to the proximity of the FVC pipe and the former Western Auto USTs." Ms. Caroline Kwan March 13, 1996 As we have pointed out previously, the 4-inch PVC pipe has already been removed from the ground, not only in the vicinity of the former USTs but up to the edge of the Four Winds building where it is capped. The pipe is still intact under the Four Winds building. Remediation of the pipe should be evaluated but this effort is dearly associated with the Four Winds property, not Western Auto. Including a discussion of this effort in the Western Auto section gives the impression that it is the responsibility of Western Auto, even though a statement to the contrary has been induded. • Capping is the most appropriate soil remediation technique for Western Auto. Section 4.6.2.5 discusses the two retained Western Auto soil remediation options, excavation and capping. Table 5-4 sets forth estimated capital costs for capping 117 square yards behind Western Auto. Section 4.6.2.5 only briefly mentions capping but has a relatively extensive discussion of excavation, giving the impression that excavation is the preferred alternative. However, given the previously performed soil removal, and the impermeable nature of the day soil in the region combined with related lack of a transport mechanism for any remaining low levels of contaminants in soil to reach groundwater, capping provides the best remedial option for this area. Furthermore, Four Winds has already capped the vast majority of this area with a thick layer of concrete. The only portion remaining to be capped is a narrow band associated with the trench left over from removal of the 4-inch pipe. Also, the source and justification for the value of 117 square yards associated with the capping option in Table 5-4 are not provided. • The Gore-Sorber study results indicate that both diesel and heavy range hydrocarbons, unrelated to Western Auto operations, are present upgradlent under the Four Winds building. On page 2-18, paragraph 2 of the final FS it is stated that"... at the Four Winds Plaza, potential impacts to soil cannot be ruled out due to the detection of elevated soil gas concentrations of diesel components/ The Gore-Sorber data indeed indicate the presence of diesel range petroleum hydrocarbons beneath the Four Winds building. However, the Gore-Sorber results also indicate the presence of heavy range petroleum hydrocarbons under the building, with the highest concentrations centered under the Cost-U-Less store. The maximum concentration of C18 compounds in the Gore-Sorber study was 2.2 jig/sorber, while the highest concentration of tridecane was 1.0 -1.2 jig/sorber. Tridecane is representative of diesel range petroleum hydrocarbons. C18 compounds are at the upper weight range of the compounds detected by Gore-Sorbers and therefore are representative of heavy range petroleum hydrocarbons. Higher weight compounds are less volatile than lower weight compounds (such as tridecane) and thus are less amenable to soil vapor detection. Based on this, it is expected Ms. Caroline Kwan March 13, 1996 Pages that C18 measurements would be much lower than tridecane measurements, unless there were much greater concentrations of heavier weight petroleum hydrocarbons present Since C18 compounds were measured at higher concentrations than tridecane, there must be significant concentrations of heavy weight petroleum hydrocarbons present under the Four Winds building. In summary, the Gore-Sorber data indicate that both diesel and heavy weight petroleum hydrocarbons are present under the Four Winds building. This data and the other available information also indicate that the peak concentrations of this contamination are located under the building, upgradient of the former Western Auto UST location. This petroleum contamination was apparently transported into the former UST location via the 4-inch PVC pipe and the gravel layer, which acted as transport mechanisms. - In sum, the objective data pertinent to the soil condition behind Western Auto conclusively indicates that any contamination behind the Four Winds Plaza is an isolated condition unrelated to the Tutu Aquifer. The scientific data which has been gathered and analyzed pertaining to the area behind Four Winds Plaza points to the conclusion that any contamination present in the soil remained in the soil and could not have been transported to the Tutu Aquifer. The data which leads to this conclusion may be summarized as follows: - The hydraulic conductivity of the clay soils in which the USTs were located has been measured at values of 1.2 x 10"9 and 4.3 x 10*, values which virtually preclude any transport. The lack of transport is demonstrated by the documented perched water table condition existing behind the Four Winds building. - Groundwater testing and data from MW-24 conclusively demonstrates that there has been no contribution of contaminants to groundwater directly beneath the location of the former USTs. - The area has historically been paved and, in fact, was recently largely capped with a thick layer of concrete. The only area remaining uncapped is a strip directly above the path of the 4-inch PVC pipe which was removed and capped at approximately the time of the tank removal. - Western Auto has removed and disposed of 85 cubic yards of contaminated material near the location of the former USTs. This area was then lined with a plastic liner and backfilled with dean soil. Geraghty and Miller removed additional soil during the installation of MW-24. Ms. Caroline Kwan March 13, 1996 In addition, it should be noted that the most signfficant contamination concern at the Tutu site is the chlorinated VOC contamination of groundwater. Essentially all the discussion with respect to potential contamination at Western Auto is related to petroleum constituents. Chlorinated VOCs and, in particular, chlorinated VOC contamination of groundwater are not even an issue for Western Auto. We appreciate the opportunity to provide EPA with comments on the FS. Please feel free to contact us with any questions that you may have on our comments. Sincerely, Donald P. Galya, P.E. Program Director John Bierschenk, P.G. Senior Remediation Geologist Eric Butler, Ph.D. Senior Consulting Chemist TOTAL P.07