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e OF CAL1FO;* From: Daniel Villanueva <dvillanueva@advgeoenv.com> Sent: Wednesday, October 17, 2018 3:57 PM To: Buehler, Alan@Waterboards <Alan.Buehler@Waterboards.ca.gov> Cc: Robert Marty <rmarty@advgeoenv.com>; Brian W. Millman <bmillman@advgeoenv.com> Subject: RE: Ralph's Square Mr. Buehler, Advanced Geo Environmental Inc. (AGE) has reviewed the Central Valley Regional Water Quality Control Board (Water Board) letter dated 06 August 2018, for the property located at 2122 South Airport Way, Stockton, California (herein referred to as site). In the letter, Water Board staff requests that the responsible party install two (2) groundwater monitoring wells downgradient of monitoring well MW -5 to further assess historically elevated methyl tertiary butyl ether (MTBE) and recently elevated tertiary butyl alcohol (TBA) concentrations from an unauthorized release from the former underground storage tank (UST) system at the site. AGE provides the following responses to items referenced in the letter: A total of seven (7) groundwater monitoring events have been performed at downgradient shallow (MW -14, MW -16 and MW -17) and intermediate wells (MW -12, MW -13 and MW -15) since being installed in January 2015. These downgradient groundwater monitoring wells have been monitored over a period of over 3 years and have been non -detect for constituents of concern. In the Regional Board letter referenced above, two additional wells, upgradient to wells MW -12 through MW17 are being requested. Based on drilling accessibility (residential housing east of the site), and City of Stockton requirements, potential well locations would need to be proposed in the sidewalk areas west of the existing shallow and intermediate wells. The potential well locations have been illustrated in the attached figure. Based on the location, drilling accessibility and City requirements, well locations would be placed at an estimated distance of 30 and 70 feet from existing wells. Considering that the potential locations are upgradient of wells that were installed for the purpose of defining the downgradient extent of the dissolved hydrocarbon plume, it does not appear necessary or reasonable to install additional groundwater monitoring wells in these locations. Installation of additional wells is not cost effective, nor would it provide additional data that is necessary to determine if the site meets the criteria established in the Low -Threat UST Case Closure Policy (LTCP). AGE has conducted an additional review of the site as it relates to the LTCP. Evaluating all current and historical data, the site meets Scenario 4 of the groundwater specific criteria. Assuming that the plume has reached the currently installed downgradient wells, the plume would: 1) remain less than 1,000 feet in length, 2) there is no free product, 3) the nearest existing water supply well or surface water body is greater than 1,000 feet from the defined