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- I Additional Vapor and Groundwater Monitoring Well Work Plan �?14l Raymond Investment Corporation �• �1� f August 4,2009 25 r 9 pOM a,/age 4 45 feet for comparison to data previously collected from SB-3 and SB-11. Other soil samples may be substituted for analysis by the supervising geologist based upon field observations. (fb q� MW-6 will be installed 30 feet southwest of -4 and�in"line with the former tank location. MW-6 will be constructed with a screen depth of 30 to 4 .feet bgs. The purpose of MW-6 will be to provide data on the horizontal distribution of sorbed and aqueous phase hydrocarbons southwest of MW-4. Soil samples will be collected from the soil boring made for the installation of MW-6 at the 25- foot, 35-foot, and 40- foot intervals. The samples will be analyzed by the laboratory, as described below, for comparison to data previously collected from SB-9 5 q _3L 04, MW-7 will be installed 60 feet southwe t .�-4 and in line with the former tank location. MW-7 will L be constructed with a screen depth of 30 to,4 feet bgs. The purpose of MW-7 will be to provide data on the horizontal distribution of sorbed and aqueous phase hydrocarbons southwest of MW-4. Soil samples will be collected from the soil boring made for the installation of MW-7 at 10 foot, 25 foot, 35 foot, and 40-foot intervals. The samples will be analyzed by the laboratory, as described below, for comparison to data'p eviously collected from SB-5 rJ_,!! 1b 3 ��,+ _ tjI Three soil vapor monitoring wells will be installed near the wall of the building to collect samples suitable for evaluation of the potential impact of soil vapor on indoor air quality. The SJCEHD letter dated May 12, 2009 indicates, and regulatory notes on GeoTracker state that, "'Potential vapor intrusion threat from the remaining contamination in soil and groundwater must be further evaluated." Long-term soil vapor monitoring wells are proposed based upon United States Environmental Protection Agency (USEPA) guidance On the Computation of a 95% Upper Confidence Limit of the Unknown Population Mean Based Upon Data sets with Below Detection Limit Observations, EPA/600/R-061022, page 91 which suggests that the number of analytical results should not be small (e.g., n<10-15). Previous soil vapor samples must be supplemented with more data and with higher quality data objectives (DQO). Long-term soil vapor monitoring wells can be used cast-effectively to collect samples in the future and their superior construction should yield data of higher quality (e.g. lower detection limits). Additional monitoring wells will be installed if laboratory analytical results,from.MW=5,identify,and. quantify"hydrocarbons for two consecutive quarterly..monitoring events, or if the installation of:MW-5 fails. Three additiorial monitoring�wells,MW-8, MW-9, and MW-10,majbe advanced in the sidewalk or - parking,.lane�on the west side of Grant Street and the south side of_Channel Street to a depth of approximately 40;feet}MW-8, MW-9,and MW-10 would be constructed with a screen depth of 30.to 40 feet.bgs. No soil samples will be collected from the soil borings made for the installation of monitoring wells MW-8, MW-9, and MW-10 because sorbed phase soil contamination is considered Fto Abe - _ = is=-- . r improbable at such a great distance a.=from the source area.. The monitor�win:...g well locations arc approximately 130 feet down gradient of MW-4 and are positioned outside of the building due to structural considerations including interior walls and a basement with very limited access. The proposed boring and well locations are shown on Figure 3,Appendix A. 4.0 BORING AND WELL INSTALLATION FIELD PROCEDURES 4.1 SITE PREPARATION Condor will prepare street and/or sidewalk closure, easement, and monitoring well installation permit applications for submittal to the SJCEHD and City of Stockton. Condor will receive the permits prior to initiating any fieldwork. The proposed monitoring well locations will be marked with white paint and j 4M �� CONDOR