ARCHIVED REPORTS XR0010082 - Laserfiche WebLink

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i The local groundwater gradient can be established from January, 1991 monitoring data from 8181 E. Eleventh Street (Genstar West- ern Stone), 400 S. Tracy Blvd. (Tracy Estates), 6700 GrLntline Road O.B. Terminal). The site locations and gradient directions relative to the subject site are shown on Figure 3. The most recent groundwater gradient map and well logs for each site are included in Appendix V. All the gradients are essentially in a northerly direction. There are nr known surface or subsurface oCcurrenres which would effect a significant influence on the direction of groundwater flow. Therefore, even though the direc- tion of flow cannot be specifically determined from on-site wells, the direction can be safely assumed to be consistent with the nearby sites. MW2 is appropriately ]located to monitor groundwater downgradient form the excavation and MW3 is adequate �. for upgradient observations. z The water samples collected from MW2 and MW3 were analyzed for — BTEX and TPH as diesel by two separate certified labo-atories. No detectable petroleum constituents were present in azy of the samples. Results are summarized on Table 2. laborazoi y data sheets, chain-of-custody forms and QA/QC documentatior. are in- cluded in Appendix IV. a 4.0 CONCLUSIONS AND RECOMMENDATIONS Although elevated concentrations of petroleum constituents were and are still present in the vadose soil, the downg.adient well WW2) did not detect hydrocarbons above method detection limits. OF Natural biodegradation was apparently successful in mitigating the contaminants in contact with groundwater even before excava- tion of most of the contaminated soils. Since the area has been newly capped with asphalt and most of the contawinated soil has been removed, remaining contaminated vadose soils do not present a significant threat to groundwater. There- fore, additional excavation is not warranted, particularly con- sidering the physical restrictions involved. _ Contaminated soil below the groundwater table appeared to be undergoing biodegradation (discoloration, cementation) and does not appear to be causing significant groundwater contamination. Therefore quarterly monitoring of MW2 and MW3 is proposed until completion of a full year of sampling and analysis indicating no detectable petroleum hydrocarbon constituents present in ground- water. Samples will be analyzed for BTEX and TPHD. Groundwater gradient and direction will be determined quarterly at the nearby sites discussed above, or other acceptable sites in the vicinity. 6