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I Revised Additional vapor and Groundwater Monitoring Well Work Plan Raymond Investment Corporation i S October 30,2009 Page 4 Monitoringwell -5 will be lac d northeast of MW-4. MW-5 will be constructed with a screen we _ w p interval of 2�5 to 45.ft from the surfa e. The purpose of MW-5 will be to provide data on the horizontal distribution sorbed and aqueous p ase hydrocarbons northwest of MW-4. Soil samples will be collected from.the soil-boring made for he installation of MW-5 at 5-foot intervals. The samples will be analyzed by the laboratory, as-described elow, from the depths of 5 feet, 15 feet,:25 feet,"35 feet, and 45,� �— feet for comparison to data previously} ollected.from,SB-3_fandSB-11..,�-Other soil samples may be substituted for analysis by the supervising 6ologist based upon field-observations. E MW-6 will be installed30 feet southwes�tf MW-4 and in line with the former tank location. MW-6 will be constructed with a screen depth of 25 to 45 fee. gs. 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 25469t, 35-foot, and" t 40-foot intervals. ThA' mples will be analyzed by the laboratory, as described below, for comparison to data previously collected from SB-9. Soil borings SB-12, SB-13, and SB-14 will be installed approximately 75 feet southwest of MW-4 and near the south end of the former tank location. The Iocation of SB-12 is Iocated near the location where Sri diesel and motor oil were detected during the underground storage tank removal in April 2003. The purpose of SB-12 will be to provide data on the vertical distribution of sorbed and aqueous phase ,t� hydrocarbons south of SB-5 and the former tank area. The locations of SB-13 and SB-14 will be on they east and west sides of SB-12 to provide lateral delineation related to SB-12. Soil sam les will be collected from the soil borings made for the installation of SB-12, SB-13, and SB-14 t 8 fee , 10 feet, 15 feet, 25 feet, and 35 feet bgs. A groundwater grab; sample will be collected from t e 30-,oto 34-foot interval or at the uppermost groundwater. The samples will be analyzed by the laborato , as describ ld below, for comparison to data prdviously collected from SB-5. A generalized soil bor' g constructio diagram is included as Figure 7, Appendix A. f 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 posted on the State Water Resources Control Board UST site database, 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-06/022, page 91, which suggests that the number 1•� 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 cost-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 either laboratory analytical results from MW-5 identify and quantify the presence hydrocarbons, or field observations made during the installation of MW-5 indicate the presence of hydrocarbons in groundwateri. Three additional monitoring wells, MW-7, MW-8, and MW-9, may be 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 45 feet. These three additional wells would be constructed with a screen depth Gf 2` 5 to 45 feet b s.Ido soil samples` will be collected from the soil r; is CONDOR t "