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T'ireadwell&Rollo A LAN6AN COMPANY Request for Low-Threat Case Closure 6 June 2013 RWQCB case#391149 and#391162 Page 11 420 and 423 N Madison Street Stockton,California Project:731589001 LTCP Guideline Justification for Site Closure(Carando) Secondary source has been removed to the extent Secondary sources include residual impacts to soil practicable. and shallow groundwater. The over-excavation around three USTs and the pump island areas in 2004 and 2005 removed the majority of the secondary source in soil. Only low concentrations of BTEX were detected in groundwater within the UST excavation. Oxygen release compound was added to the groundwater within the excavation in 2005 to accelerate biodegradation of hydrocarbons. Soil and groundwater have been tested for MTBE, Soil and groundwater have been tested for MTBE and results have been reported in accordance with which has not been detected, or detected only at Health and Safety Code section 25296.15 (indicates very low concentrations(Attachment 1). that results of MTBE tests are known to the Regional Water Board). Nuisance as defined by Water Code section 13050 Groundwater impacts are not considered to be a does not exist at the site (indicates no nuisance nuisance due to the lack of contact with human or odors or threat to public health and safety). other ecological receptors. Depth to groundwater has been between approximately 16 to 18 feet bgs historically, which is deeper than potential future trenching work. Media-specific Criteria: Groundwater Is the contaminant plume that exceeds water Petroleum hydrocarbons in groundwater at the quality objectives stable or decreasing in areal adjacent 423 N Madison site are limited in extent extent? and similar geochemical conditions at this site are likely to also limit the extent and persistence of hydrocarbons at Carando. While time series data are not available for this site, groundwater monitoring data and microbial data from adjacent 423 N Madison indicate favorable conditions for biodegredation and limited plume extent. Does the contaminant plume that exceeds water The BioScreen modeling scenarios provide a quality objectives meet all of the additional reasonable estimate of the benzene plume length characteristics of one of the five classes of sites? of 140 feet down-gradient of the source area. The site generally meets groundwater criteria class number two with the exception that there is a surface water body closer than 1,000 feet from the estimated plume, McCleod Lake. Historical data show shallow groundwater flows consistently to the north-northwest awa from McCleod Lake. Other