ARCHIVED REPORTS_XR0007133 - Laserfiche WebLink

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Site History 14000 Highway 88 Lockeford CA Time Oil Property No 04-049 concentrations beneath the site are generally stable and that hydrocarbon migration does not appear to be occurring A corrective action plan for the site was generated in August 1998 This report summarizes site-specific data collected to date, contains a Risk-Based Corrective Action (RBCA) evaluation to determine alternative cleanup soil and groundwater cleanup levels, compares remediation alternatives for the site, and contains a conceptual design of the recommended remediation alternative Extent of Hydrocarbon Impacts Soil data collected to date indicate that a well-graded sand is present across the site from ground surface to approximately 50 feet to 55 feet below ground surface (bgs) This strata contains intermittent laterally discontinuous interbedded clay and silt layers ranging from 4 feet to 14 feet thick Deeper soils generally consist of gravels with varying amounts of sand and clay Gravel content decreases and clay content Increases with depth TPHg concentrations in soil which exceed 1,000 ppm are generally limited to within 20 feet of the former tank excavation and dispensers, with maximum concentrations occurring at approximately 30 feet bgs SECOR estimates that the general area of significantly impacted soils is approximately 8,500 square feet and that between 12,000 pounds and 36,000 pounds of hydrocarbons are present in soils beneath the site Groundwater depth beneath the site has ranged from 66 feet to 87 feet bgs Flow direction is generally to the south with an average gradient of 0 04 ft/ft Historically, hydrocarbon concentrations in groundwater have been highest in MW-1, located adjacent to the pump islands SECOR estimates that between 19 pounds to . 56 pounds of dissolved hydrocarbons are present in groundwater beneath the subject site Sensitive Receptor Survey A review of Division of Water Resources (DWR) records conducted in June 1997 indicated that one irrigation well and two municipal wells are present within a 2,000-foot radius of the site One municipal well Is located approximately 900 feet northeast of the subject site and the other is located approximately 1,500 feet southwest of the site The closest well Is screened below the Interval of impacted groundwater The site is located approximately 0 75 miles south of the Mokelumne River A creek located approximately 600 feet to the northwest is the closest surface water body to the site A drainage pond for runoff during rainfall events is present on the south corner of the site, this pond Is generally dry RBCA Evaluation SECOR performed a RBCA evaluation to determine if concentrations of soil and groundwater contaminants at the subject site pose any risk to potentially exposed populations This risk analysis was performed in accordance with ASTM RBCA guidelines SECOR's evaluation included an identification of contaminants of concern at the subject site, a discussion of potential exposure routes, an evaluation of receptors that may be exposed to the contaminants of concern, and an evaluation of the risk posed to receptors by the contaminants of concern A Tier 1 analysis was first performed in order to compare maximum contaminant concentrations associated with the subject site with risk-based screening levels (RSBLs) which are primarily derived from ASTM's default assumptions Because onsite contaminant concentrations exceeded several Tier 1 RSBLs, a Tier 2 evaluation was also performed to evaluate site-specific target levels (SSTLs) for the various chemicals of potential concern at the subject site Site-specific Information used in the Tier 2 analysis included the actual area of impacted soil, length of affected soil parallel to the wind direction, vadose zone thickness and depth to water, depth to the top of Impacted subsurface sods, and a more representative soil density value for 1a