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TEXACO CARIBBEAN INC. us. VIRGIN ISLANDS PRODUCTS YOU CAN TRUST RECEIVED FH: , SKI A. 1 <JRR CDM-FPC, NYC The Honorable Alan D. Smith C o m m i o O i o n c r o f Planning a n d M u C u r A l R a o o u r c o o 179 Alcona & Welgunst St. T h o m a o . USUI 00807 Dear Sir: Please find attached revised pages to—b-c -incxHrtcd tnte the Final Report on Results of Soil Gas Survey, Tutu, St. Thomas, U.S.V.I., which was submitted to the Department of Planning and Natural Resources and the U. S. Environmental Protection Agency by Texaco Caribbean Inc. These pages incorporate changes agreed upon by representatives of the Department of Planning and Natural Resources, U. S. Environmental Protection Agency and Texaco Caribbean Inc. during a meeting held on January 12, 1988 in St. Thomas. The changes made also include aii the pending points addressed ITT Ms. Caroline Kwan's letter of February 1, 1988. Very truly yours, William Hroch Representative A t t a o h m d n t C cc: Ms. Caroline Kwan - USEPA Region II w/2 attachments TU" P,O. BOX 1458, FREDERIKSTEO, ST. CROIX, V.I, OOS40 (S09) 772-0104 P.O. BOX 3740, CHARLOTTE AMALIE, ST. THOMAS. V.I. OOS01 («09) 77-VlOjl *64457* 64457 FINAL REPORT ON RESULTS OF SOIL GAS SURVEY, TUTU, ST. THOMAS, U. S. VIRGIN ISLANDS December 18. 1987 Revised January 25, 1988 Prepared for: TEXACO CARIBBEAN. INC. Prepared by: GEOSCIENCE CONSULTANTS, LTD. HEADQUARTERS 500 Copper Avenue, NW Suite 200 Albuquerque, New Mexico 87102 (505) 842-0001 FAX (505) 842-0595 WESTERN REGION OFFICE 5000 Birch Street West Tower, Suite 3000 Newport Beach. CA 92660 (714) 476-3650 EASTERN REGIONAL OFFICE 1109 Spring Street Suite 706 Silver Spring, Maryland 20910 (301) 587-2088 FAX (301) 587-2086 1247 Caribbean concurred with the request, and discussions were held with chemists from Tracer and from the ERA oversight contractor to identify an appropriate column. The Carbopak AT-1000 column, a much slower column than the OV-101, was selected. During calibration of the system, it became apparent that the full-length AT-1000 column was unsuitable for field use because of its excessive retention time. The AT-1000 column required as much as an hour to analyze one sample, as compared with about 5 minutes for the OV-101. After a few attempts, the column was cut to a length of 1.5 feet (from its original 6-foot length), which shortened the run time to about 15 minutes, and was used to analyze a single sample taken from the vicinity of the underground storage tanks (USTs) at the Tutu service station. The column proved capable of quantifying benzene at this location, but toluene, ethyl benzene and xylene, if present, were still undeterminable due to interference by other peaks on the chromatogram. Due to the excessive run times required to obtain results that were little better than those obtained with the OV-101, no further attempts were made to use the AT-1000 column. The soil gas survey is intended as a preliminary screening tool to define the approximate extent of any hydrocarbons identified in the soil zone. The use of total hydrocarbon values is adequate to perform that task. Although chlorinated compounds in significant quantities were considered unlikely to have resulted from activities at the Tutu service station, their presence in ground water is a matter.-of concern. GCL therefore conducted a limited soil gas sampling program which was intended to determine whether these compounds could have come from the Texaco station. Sampling for 6 chlorinated hydrocarbons was performed at 9 points near the Tutu station and upgradient from the Tillett water well, in which some of the compounds have been detected. Analyses were performed using the electron capture detector. The soil gas sample collection procedure was identical to that used in sampling for petroleum components. 10 Page Revised 1/25/88 are entirely separate, because lithologic and hydrologic conditions in the Intervening area prevented adequate soil gas analysis (see Section 4.1.3). 4.1.1 Service Station Site Soil gas hydrocarbon values measured on and immediately adjacent to the Tutu service station site are shown in Figure 4-1. In cases where a hole was sampled at two different depths, both concentration values are shown, but the value from the greater depth was used in mapping the concentra- tion contours. In all holes except B-2, the deeper point yielded the higher value; in B-2 the values determined at 5 feet and 8.5 feet were similar. A zone of hydrocarbon concentrations significantly higher than background levels is centered southwest of the underground storage tanks and extends southward and westward beneath the highways adjacent to the station property (Figure 4-2; Zone I, Figure 4-3). Elevated hydrocarbon con- centrations were found consistently at depths of about 5 feet and at depths of about 8 feet (maximum depth of probe penetration) in the central part of this area; however, at some points along the station boundaries (A-4, A-2A, C-4) hydrocarbon concentrations appear to increase significantly with depth between 4 feet and 8 feet. There was no evidence of significant hydrocarbon presence along the northern and eastern boundaries of the station site. The value of 1,700 ug/1 recorded in hole C-4 near the north boundary of the station is probably a result of hydrocarbon vapors migrating from the adjacent oil separator pit. Movement of hydrocarbons east of the station area may be inhibited by the steep southwestward topographic and water table gra- dients. 4.1.2 Other Areas With Hydrocarbon Concentration Above Background Levels An attempt was made to trace the zone of elevated hydrocarbon concentra- tions to the south across Highway 38 from the Tutu station. However, no 16 Page Revised 1/25/88 003 124V 5.0 PROBABLE MIGRATION PATHWAYS 5.1 GROUND-WATER DATA The water table in the upper Turpentine Run basin is generally within 30 feet or less of the ground surface, and is influenced by the local topography. During November 1987 the static water levels were measured by GCL in the Tillett well and the eastern Four Winds well. The western Four Winds well was not measurable because it lacked an adequate port for insertion of the water level probe. Water levels were 23.2 feet below top of casing in the Tillett well, and 14.0 feet below top of casing in the eastern Four Winds well. Top of casing at the Tillett well is about 1 foot above the ground surface, while that of the Four Winds well is even with the ground surface. Taking into account the difference in well head elevations (approximately 28 feet, as measured by tape and Brunton compass), this indicates a difference of about 19 feet in water-table elevations at the two wells, and a water-table gradient of about 0.076, if the Four Winds well is assumed to be directly downgradient from the Tillett well. The gradient along the valley axis may be less than this figure, whereas that on the eastern slope may be greater. It is not known whether the Tillett and Four Winds wells intersect the same fracture zone, or zones which are hydraulically connected. 5.2 BEDROCK FRACTURE SYSTEMS Most ground water in the upper Turpentine Run basin is contained within fracture systems of the Louisenhoj bedrock. Only minimal flow is expected in the matrix of this fine-grained volcanic unit. Observations of the soil profile, where it is exposed in road cuts and construction excavations, indicated that the zone of significant weathering extends to only about 6 feet below the surface in most areas, with fractured bedrock below that level. These observations were confirmed by the difficulty encountered in driving soil gas probes deeper than about 5 or 6 feet in most of the study area. Consequently, the direction of local ground- water flow will be strongly influenced by the orientation of the bedrock fracture systems through which the water is moving. 25 Page Revised 1/25/88 f\J •-4 SIIIU AMD DIP OF WUOI fHACIUM JI5ICH ii»ut *«o oir of yitsioiMT ruciuic mutt illlK Of »IMIICAI FUCIUM S1SKK uf Slum siuit CUIlAt Mil Of VAIlil I NODTH SCALC too FIGURE 5-1 ORIENTATIONS OF MAJOR FRACTURE SYSTEMS IN THE TUTU AREA, UPPER TURPENTINE RUN BASIN.