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Larry's Auto -2 - 19 April 2017 <br />308 North Grant Street <br />Stockton, San Joaquin County <br />Central Valley Water Board staff have the following comments on the status of the assessment <br />and cleanup of the petroleum release, and the remaining impediments to closure: <br />1. The Site does not meet the State Water Resource Control Board's Low Threat <br />Underground Storage Tank Case Closure Policy (LTCP) Groundwater Specific criteria <br />as the current maximum benzene concentration in groundwater is greater than 3,000 <br />ug/L (25,300 ug/L in MW-5). Additionally, there are two water supply wells located <br />approximately 500 and 900 feet to the northeast. <br />AGE has identified six (6) hydrologic units (H Us) at the Site. Below is a table with HU <br />information and January 2016 (most recent) groundwater data. <br />Table 1: January 2016 Groundwater Data <br />Unit Depth <br />(ft bgs) <br />No. of <br />Wells <br />Well <br />Screens <br />(ft bgs) <br />Max TPHg <br />(ug/L) <br />Max <br />Benzene <br />(ug/L) <br />Max MTBE <br />(ug/L) <br />HU1 WT-55 8 26-55 32,300 25,300 <25 <br />HU2 55-85 6 54-85 10,100 588 <10 <br />HU3 85-110 0 -- 7,200A 980A <br />HU4 110-135 2 110-121 8,310 3.9 2.1 A <br />HU5 135-156 0 -- 45,0008 3605 <br />HU6 156-200 0 -- 120c 2.0c <1.0c <br />'NT = water able, ft bgs = feet below ground surface, ug/L. = micrograms per liter <br />A=Grab groundwater sample from CPT-2, CPT-3, CPT-5, CPT-8 (2001-2005). <br />B=Grab groundwater samples from CPT-2, CPT-3, CPT-7 (2001-2005). <br />C=Grab groundwater samples from CPT-3, CPT-7, CPT-9 (2001-2005). <br />Recent depth to water in all Site wells is approximately 32-33 feet below top of casing <br />(TOC). <br />Delineation of hydrocarbon impacts to groundwater does not appear complete. The <br />maximum TPHg and maximum benzene detections noted for HU1 in the table above are <br />from MW-5. MW-5 is located approximately 50 feet down-gradient (northeast) of Tank <br />Area 2, and is screened in HUI, from 26 to 46 feet below ground surface (ft bgs). As a <br />result, additional lateral and vertical delineation is needed in the vicinity of and down- <br />gradient of MW-5. <br />However, petroleum hydrocarbon concentrations in MW-5 are significantly higher than <br />those in wells MW-4 and MW-8, which are located immediately adjacent to Tank Area 2. <br />These data suggest that the petroleum hydrocarbons detected in well MW-5 may come <br />from an alternate source. As such, additional investigation is needed to determine if <br />petroleum impact in MW-5 is a result of the Larry's Auto release. Any additional <br />delineation of petroleum impact to groundwater downgradient of MW-5 should be <br />deferred until it is determined whether MW-5 impacts are from the Site release (See <br />Comment 2 below). <br />2. To identify the source of impact to groundwater at MW-5, Central Valley Water Board <br />staff requests that you collect shallow soil samples in the vicinity of MW-5. By <br />26 May 2017, please submit a Work Plan which includes a proposal for the collection of