SITE INFORMATION AND CORRESPONDENCE

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Sent By: RwOCe SACRAMENTO- 916 255 3015; Apr-6-99 8:37; Page 6/7 -4- 2 The limited verticai extent of affected soil was re-confined in Jams of i with the analyses of soil samples from a boring drilled to a depth of 90 feet. The data�w that only trace"hits~ of toluene(18 ppb) and xylenes(17 ppb)are present at a depth of 70 feet. Soil samples collected from the depths of 80 and 90 feet nested`hon detect" for all residual petroleum constituents, including MTBE. The limited vertical extent of affected groundwater was also aff need in February of 1999. Two wells were constructed near the source of the release and screened at 50 to 60 feet(MW-10)and 80 to 90 feet(NP-1)below site grade. Analysis of the initial groundwater sample from well Mw 10 produced concentrations of petroleum constituents comparable to bistoric concentrations detected in an adjacent well(MW-2),which is screened from 20 to 50 fed below grade. Analysis of the initial groundwater sample from well NP-1 indioatcdTM&and benzene concentrations of 6,000 ppb and 214 ppb,respectively. While it be ed v definition of petroleum hydrocarbons in groundwater is grill undefined,these constant cancers ons arc an artifact a ewe on process to love or � 7 7 ,•nometeet,co orfs>n mbseqpent wow sample analyw& . Basin Plan Water Oaft Objectives(WQ9sl ` Dissolved phase petroleum hydrocarbon constituents in groundwater exceed WQOs in the vicinity of the release and within an estimated 100 to 120 feet down-gradient. Water totality analyses also show that background concentrations of nitrate(45 to 100 ppm),Total Dissolved Solids(700 to 1,500 ppm), and in some instances chloride(470 ppm),also exceed WQOL The plume of dissolved phase petroleum hydrocarbons is demonstrably stable and attenuating as these constituents undergo continuing active biodegradation. Meanwhile,a widespread`plume" of inorganic contamination is presort m shallow groundwater beneath much of ft City of Stockton. and is migrating eastward at an estimated rate of 30 feet per year- Clearly.even with complete removal of every trace of the limited residual dissolved petroleum hydrocarbon constituents in shallow groundwater at the site,this groundwater would not meat Basin Plan Objectives. Consequently,anyone considering the construction of a water supply well in the City of Stockton should be advised by the permitting agency that poor quality Sroundwater is present to a depth of at least 100 feet, and that appropriate measures must be takers. Specifically, i seals production or the migration of poor quality water to deeper production aquifers. 'Analysis of a water sam c collected from this well on March 12, 1999.showed that tine conccntiation of 8TEX W decreased by about 40 percent from the initial sample colacenvatxm California Envirotrntental Protection Agency