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T'IreadwelEPtollo <br /> A LANGAN COMPANY <br /> Request for Low-Threat Case Closure 6 June 2013 <br /> RWQCB case#391149 and#391162 Page 14 <br /> 420 and 423 N Madison Street <br /> Stockton,Califomia <br /> Project.,731589001 <br /> Guideline Justification for Site Closure(423 N Madison) <br /> Media-specific Criteria: Groundwater <br /> Is the contaminant plume that exceeds water BTEX and MTBE have not been detected at <br /> quality objectives stable or decreasing in areal sampling locations as close as 45 feet down- <br /> extent? gradient. <br /> Does the contaminant plume that exceeds water The contaminant plume meets groundwater criteria <br /> quality objectives meet all of the additional class number one: a)The contaminant plume that <br /> characteristics of one of the five classes of sites? exceeds WQOs is less than 100 feet in length; b) <br /> There is no free product; and c)The nearest <br /> existing water supply well or surface water body is <br /> greater than 250 feet from the defined plume <br /> boundary. <br /> a) The contaminant plume that exceeds TPHd and TPHmo were detected at low <br /> WQOs is less than 100 feet in length. concentrations (190 pg/L and 480 pg/L, <br /> respectively) only at boring 15B-8-GW, located 45 <br /> feet down-gradient of the former B-6 UST. BTEX <br /> and MTBE have not been detected at this or other <br /> sampling locations. <br /> b) There is no free product. Free product has not been observed at this Site <br /> during soil or groundwater sampling, UST removal, <br /> or remedial excavation. <br /> c) The nearest existing water supply well or The nearest surface water body is McCleod Lake, <br /> surface water body is greater than 250 feet located approximately 400 feet to the southeast <br /> from the from the defined plume boundary. and upgradient from the location of the former B-6 <br /> UST. The nearest existing water supply well is <br /> greater than 250 feet away from the defined plume <br /> boundary. <br /> SUMMARY AND CASE CLOSURE JUSTIFICATIONS <br /> • The Sites have been remediated to remove primary and secondary sources of contaminants of <br /> concern and remaining impacts to soil and groundwater at these sites are limited. <br /> • Free product has not been observed at the Sites. <br /> • Biodegradation is likely occurring in groundwater based on geochemical and microbiological data <br /> and observed decreasing benzene trends in monitoring wells at 423 N Madison. <br />