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Wendy Cohen,SLIC Unit � -2- 1 February 1996 <br /> In an 8 December 1995 letter from State Water Board Executive Director Walt Pettit to Regional <br /> Water Board Chairpersons and Executive Officers and Underground Tank Local Oversight <br /> Program Agency Directors, direction was given for petroleum hydrocarbon releases that"[f)or <br /> cases affecting low risk groundwater(for instance, shallow groundwater with maximum depth to <br /> water Ness than 50 feet and no drinking water wells screened in the shallow groundwater zone <br /> within 250 feet of the leak) we recommend that active remediation be replaced with monitoring to <br /> detenrune if the fuel leak plume is stable." Since shallow ground water has been polluted <br /> (constituents of concern in excess of applicable water quality objectives) at this site and no one is <br /> likely to be using shallow ground water within 250 feet of the pipeline (this needs to be <br /> confirmed), this site would appear to fit within that definition of"low risk groundwater." <br /> Monitoring has not conclusively demonstrated plume stability downgradient of MW 13. <br /> Therefore, additional monitoring would appear to be necessary. <br /> DISCUSSION <br /> The report includes ground water monitoring data collected from several monitoring wells along <br /> and adjacent (in the downgradient direction) to the former pipelines. Levels of petroleum <br /> hydrocarbons were measured as: <br /> TRPH—total recoverable petroleum hydrocarbons by USEPA Method 418.1; <br /> TPPH—total purgeable petroleum hydrocarbons by purge & trap followed by GC/FID and <br /> limited to hydrocarbons with 5 to 12 carbon atoms (C5 through C12); <br /> TEPH—total extractable petroleum hydrocarbons by solvent extraction followed by GC/FID <br /> and limited to hydrocarbons in the C.through C36 range; <br /> FNAs—polynuclear aromatic hydrocarbons by USEPA Method 8310; and <br /> BTEX—benzene, toluene,ethylbenzene, and xylenes by USEPA Method 8020. <br /> TRPH data are normally of limited utility, since they cover such a broad range of chemicals of <br /> varying, carbon chain lengths, mobilities, and water quality impacts. TPPH data represent what is <br /> normaliy referred to as gasoline-range hydrocarbons. Gasoline-range hydrocarbons can have <br /> significant subsurface mobility via both vapor and water-borne mechanisms and can pose toxicity <br /> as well as organoleptic effects. TEPH data represent hydrocarbons of the diesel and higher ranges. <br /> Up to about C20 or so, diesel-range hydrocarbons can also be significantly mobile in the <br /> subsurface. <br /> Benefu-ial Use-Protective Water Quality Criteria <br /> Several numerical limits are available from the literature to assess beneficial use impacts to <br /> ground water from petroleum hydrocarbon mixtures and constituents. The State Water Board <br /> publication Water Quality Criteria cites an organoleptic threshold for gasoline in water at about <br /> 5 µg/1. USEPA has published an organoleptic threshold for diesel in water at 100µg/1. Both of <br /> these organoleptic thresholds would be relevant to determining compliance with the narrative taste <br /> and odor water quality objective in the Basin Plan. In 1992, preliminary heath-based criteria were <br /> calculated for fuel mixtures by USEPA, Office of Research &Development. These criteria are <br />