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SENT BY:VIDPNR/DEP-watergut : 2-27- 2 : 3:48PM ; VlDPNR/DEP-ft'atergui- 1212 637 4284:* 5/10 ^^v^tosv UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGIONS 290 BROADWAY ( ^n.-c n t NEW YORK. NY 10007-1666 \ SDMS Document '^>.p(«ff^ 115618 APR I 9 200i Express Mail- Return Receipt Requested HoUis Griffin, Director Division of Environmental Protection .Department of Planning and Natural Resources Division cf Environmental Protection Bldg. Ill-Apt. 14A Water Gut Homes-118 Christiansted, St. Croix USVI 00820-5065 Re; Comments on the Final Draft Feasibility Study Report. Virgin Islands Chemical Superfund Site, St. Croix. USVI Dear Mr. Griffin; Enclosed for your review and comment is the draft Proposed Plan for the Virgin Islands Chemical Superfund Site. The proposed remedy for the Site is soil vapor extraction/air sparging for the lank fann area and monitored natural attenuation, with a contingency pump and treat system, for the former process pit area. This and other alternate remedies, as well as the supporting documentation, have been discussed with your staff and consultant in various meetings and conference calls. Prior to the preparation ofthe Proposed Plan, several reports were submitted to your office for review and comment, including a Draft Feasibility Study in July 2000, Draft Soil Vapor Exhaction/Air Sparging Pilot Study Report in October 2000, and the Final Draft Feasibility Study (FS) report in November 2000. To date , we have received commcnls from your office Internet Addrass (URL) • htt|>.7/www.i»pa.gov R*cycl«d/R«oycU.bla • Prtnlad wKh V6g»Uibl» Oil 3ased Inks on Hocyded Pap«r (Minimum 30% PosJeonsumef) 400194 SENT BY:VIDPNR/DEP-watergui ; 2-27- 2 : 3:49PM : VIDPNR/DEP-Waiergui- 1212 637 4284;# 6/10 only on the Final Draft Feasibility Study. Tiiese comments, which addressed the site hydrogeology, were discussed wilh your staff and consultant via teleconference on January 25, 2001 and are further addressed in the enclosed response. Should you have any further comments on these reports, please submit them with your response letter on the Proposed Plan. Please submit your comments on the Proposed Plan by May 11,2001. Please call me if you have any questions. Sincerely, ^Richard Caspe, Director Q Emergency Response and Remedial Division Enclosure 4 0 0 1 9 5 SENT BY:VIDPNR/DEP-waiergut : 2-27- 2 : 3:49PM ; VIDPNTl/DEP-Waiergut- 1212 637 4284:# 7/10 Enclosure 1. Pafic 1. "Estimated Cleanup Times." general - In response to DPNR's request for further clarification, a brief discussion will be added to the FS report regarding the uncertainly associated with cleanup lime estimates. The technical basis for cleanup time estimates corresponding to various remedial allematives is detailed in the Final Draft FS Report. Cleanup time was assessed for MNA alternatives using the EPA BIOSCREEN model; electronic files were also attached for independent evaluation. Pump-and-treat cleanup time estimates were completed using the standard pore volume flush approach. SVE/AS cleanup time estimates were based upon the results ofthe SVE/AS pilot study imd engineering judgement corresponding lo general performance of SVE/AS systems lor similar applications. Overall, the cleanup time e.slimates are consistent with industry practice for FS evaluations and are sufficient lo support remedy selection forthe range of allematives evaluated. 'ITic FS evaluations also included conventional remedial altemalives, such as SVE/AS and gi'Oimdwater extraction/treatment, which are capable of actively controlling contaminant migration and removing contaminant mass from the environment. The fundamental assumption is that these active treatment technologies will operate until: 1) the cleanup goals are achieved, or 2) their effectiveness decreases to a level that is comparable to the erfeclivcncss of monitored natural attenuation. Standard industry practice dictates that all conclusions and decisions regarding treatment system shut down, tlie effectiveness of treatment system performance and natuml attenuation, and achievement of cleanup goals be based upon groundwater and soil diita obtained from long-term moniloring and confirmatory sampling. 2. Pa^es 2 and 3. "geo-hvdrologv" - We do noi believe that the Final Draft FS Report is "flawed based upon the misinterpretation of site and area geo-hydrology," as indicated by DPNR. The source for the background hydrogeologic information presented in the Final Draft FS Report is the Final Remedial Investigation (Rl) Report (M/H 2000), which was previously reviewed and accepted by both EPA and DPNR. USGS hydrogeologic publications for St. Croix were consulted during the RI, were referenced in the Final RI Report, and are indirectly referenced in the Final Draft FS Report. DPNR's comments are based upon information obtained from USGS publications on Ihe regional hydrogeology. The comments do not adequately consider the accepted findings ofthe RI or site-specific information in general, such as boring logs, continuous and discrete rounds of water level measurements, and groundwater data. For example, the continuous waier level measurements reported in the Final Draft FS Report, wliich were taken during the SVE/AS pilot study, clearly support that the shallowest portion ofthe alluvial iiquifer (i.e., approximately 15 to 40 feci below ground surface (bgs)); daily water 4 0 0 1 9 6 SENT BY:VlDPNT^/DEP-watergut : 2-27- 2 : 3:49PM ; VIDPNR/DEP-WatergjJt- 1212 637 4284:# 8/10 level variations due to offsite pumping not apparent) is not in direct communication with Ihe deeper portion ofthe alluvial aquifer (i.e., deeper than 40 feet bgs; daily water level variations due to offsite pumping are apparent). Hence, semi-confined conditions exist at the site on a localized scale within the subject portion of the alluvial aquifer. In addition, the historic groundwater data support that groundwater contamination exceeding MCLs remains localized, both hori'/.ontally and vertically, within Ihe upper portion ofthe alluvial aquifer at the AST and FPP areas. Finally, the Kingshill formation is deeper at the site than indicated by DPNR; it is not pertinent to FS evaluations. 400197