SITE INFORMATION AND CORRESPONDENCE

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a ST Services Stockton Termin - 2 - 26 March 2008 Mr. Joe Aldridge The Report states that these actions will be effective at achieving the interim remedial action objectives (IRAOs) for the Site, which Nustar proposed in the 5 January 2006 Cleanup Plan. The IRAOs are (1) continue to verify that land and water use in the vicinity of the [Site] remains unchanged and (2) ensure continuing destruction of petroleum hydrocarbons in groundwater. Nustar recognizes that the long-term RAO is to meet water quality objectives (WQOs). ST Services has conducted monitoring at the Site since 1992. In 2002, gasoline releases occurred from aboveground storage tanks (ALTs) G 3301 , G 3302, and D 1503. Groundwater monitoring at the Site is conducted pursuant to Monitoring and Reporting Program (MRP) No. R5-2007-0818. Regional Water Board staff reviewed the 30 January 2008 Groundwater Monitoring Report, Fourth Quarter and Annual 2007 (Annual Report). The principal contaminants of concern (COCs) include total petroleum hydrocarbons as gasoline (TPHg), TPH as diesel (TPHd), benzene, toluene, ethylbenzene, and xylenes (BTEX), tertiary amyl methyl ether (TAME), MTBE, tertiary butyl alcohol (TBA), and di-isopropyl ether (DIPE). Groundwater bearing units underlying the Site are subdivided into four progressively deeper units designated Zones "A" through "D". Over the third and fourth quarters of 2007, depth to groundwater ranged from about 6 to 11 feet below ground surface (bgs). Ash Creek calculated that horizontal gradients ranged from 0.001 feet per foot in Zone D to 0.0009 feet per foot in Zone A in an east-northeasterly direction. Downward vertical gradients were dominant between the water bearing zones during the two quarters and ranged from -0.146 to -0.001 feet per foot. Historically, downward vertical gradients have persisted between the water bearing units. The source area monitoring well, ST/MW-14, has historically contained SPH and therefore was not usually sampled until August 2005, when no SPH was observed. During 2007, ST/MW-14 generally contained the highest concentrations of dissolved phase petroleum hydrocarbons during the third quarter. Maximum TPHg and TPHd concentrations in this well during the third quarter included 280,000 micrograms per liter (Ng/L) and 11,000 pg/L, respectively. Maximum BTEX concentrations were 22,000 pg/L, 15,000 pg/L, 2,700 pg/L, and 13,000 pg/L, respectively. Maximum MTBE, TBA and TAME concentrations were 220,000 pg/L, 160 pg/L and 1,200 pg/L, respectively. ST Services did not sample monitoring well UP/MW-1 during the fourth quarter, as required by MRP No. R5-2007-0818. Our comments are presented below. 1 . We do not concur that the data presented in the Report justify the selection of MNA as the most effective groundwater remedy for the Site. Our 13 August 2007 letter provided the results of simple linear regression (SLR) models using historical data from 1992 to 2007, which indicated that the dissolved concentrations of TPHg, TPHd, benzene, and MTBE in monitoring well ST/MW-1 are increasing and that decay is not occurring in the source area. NuStar submitted responses to the August 2007 letter in a 29 February 2008 letter (Response Letter). The Response Letter states that the increasing petroleum hydrocarbon trends in ST/MW-1 described in the August 2007 letter were the result of plotting data from 1992 to present. NuStar utilized a 2002 U.S. EPA document entitled Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies (EPA Guideline) to calculate a first-order attenuation rate constant for ST/MW-1 by plotting data from only