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' 9s-0&200n Gee-Sok LLC-H dro e®! is investigation #or Nunan Pug2 7 a1 00 The model demonstrates that the concentration of benzene will decrease to levels below laborato detection limits before it ever reaches groundwater. 1 In addition,from a strictly qualitative and hypothetical standpoint, since the original date of the gasoline release occurred no more recently than 1991 when the UST system was removed, the time it took the benzene to travel from some point ' within the upper ten (10) feet of soil to a depth of approximately 40 feet bgs in the year 2001, is approximately 10 years. Therefore, it should take the leachable benzene, still residing at 40 feet bgs, at least another ten (10) years to migrate to 70 to 80 feet bgs. This is also corroborated by the fact that TCLP testing in soil boring B-7 revealed that the residual benzene in soil was no longer leachable to depths greater than 59 feet bgs where the level of benzene had dissipated with depth to a concentration of 0.011 ppm and the benzene was leachable to non-detectable levels. (See Appendix V for Laboratory Data Sheets of Soil Physical C harocaarlafts, Calculations for the Vertical Attenuation of Renzeno Dn Soil) (See Table OV IaT Physical Testing of Soil) THE WPACT OF GROUNDWATER VELOCITY ON THE TRANSPORT OF DISSOLVED CONTAMINANTS FROM THE SITE TO HEAR13Y WATER SUPPLY VVELLS Considering eho fact that most perched agu fern only supply sough wager fforr Nousehold use (C.W. Fehr,Applied Hydrogeology, Third Fd., page T 11, paragraph 1),that this perched aquifer is at most only six(6) feet thick, that the site is located on a commercial property which cannot be dependent on only enough water for household use, and that the site has had, and will in the future, a connection to municipal water supply, the question remains as to whether or not the contamination from the site could reach nearby groundwater supply welis 19 *n reached -groundwater. Although the evidence accumulated to date demonstrates that no significant levels of benzene can reach the groundwater of the perched aquifer, the characteristics of the hydrogeologic regime show that dissolved benzene cannot reach the nearby water supply wells, identified in this investigation, even if it could reach the groundwater. The closest water supply well is the Lockeford School well located approximately 650 feet cross gradient form the site. The maintenance supervisor at the school, Randy Bermar [(209) 331-7184], stated on 4-30-01, that the well has, and is, used for irrigation only and is slated to be abandoned. The next closest water supply well is the Lockeford Water Tower well (See Appendix G for Well Information on 9ho Water Tower well obtained from T. Monseur of DHS (P [209] 948.3879) which extracts groundwater from a screened interval between 126 feet and 254 feet bgs, well below the perched aquifer which lies between 80 and 86 feet bgs. Since relatively dry soils were identified in MW-1 from the 86 feet to 120 feet bgs below the perched aquifer, it indicates that the screened interval of the Water Tower well i% not hydraulically connected with the perched zone beneath the subject site. Further evidence to bolster this supposition is the fact that depth of groundwater first encountered, in State Water Supply well No. 43054 (See Appendix G for Well for Water Well Drillers Report), was 126 feet bgs and the potentiometric surface stabilized at 99 feet bgs, indicating a confined condition. 05-03-2008 Geo.Soiv, LLC-hydrogeologic Investigation for Nunan Page 7 of 10