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• Monitoring Well and Sparge Point Installation Report-Former Unocal Service Station 40187
<br /> July 15,2002
<br /> only benzene was reported at a concentration of 1 9 parts per billion (ppb) Again, there is a significant
<br /> drop in concentrations in Zone B from the source area to downgradient wells U-13, U-18, and U-20 In
<br /> Zone C, dissolved hydrocarbons were reported in wells U-10, NP-I-111, NP-2-116, U-12, U-19, U-21,
<br /> and U-25 Trace concentrations of benzene were reported in wells NP-I-111, U-17, and U-25 at 0 73,
<br /> 13, and 1 I ppb, respectively Total Petroleum Hydrocarbons as gasoline (TPHg) and benzene
<br /> concentrations detected in Zone C wells range from 170 to 280 ppb, and 0 73 to 67 ppb, respectively,with
<br /> concentrations diminishing toward the perimeter of the plume Well locations are shown on Figure 3
<br /> Groundwater Monitoring and Sampling
<br /> Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994 During
<br /> the most recent sampling event on February 25 through 27, 2002, TPHg range hydrocarbons were
<br /> reported at concentrations ranging from 61 to 32,000 ppb, and benzene concentrations ranged from 0 73
<br /> to 1,100 ppb MtBE was reported in wells U-12, U-24, U-25, NP-1-22, and NP-2-22 at concentrations
<br /> ranging from 12 to 2 1 ppb by EPA Method 8260B The highest concentrations of TPHg range
<br /> hydrocarbons were reported in well NP-1-65, and highest benzene concentration was reported to well
<br /> NP-2-60 Depth to groundwater ranged from 23 72 to 28 73 feet below top of casing, and the
<br /> groundwater flow direction was in a northeasterly direction at a gradient ranging from 0 001 to 0 2
<br /> Quarterly sampling and monitoring of the wells is performed by GR Historical groundwater chemical
<br /> analytical data are summarized in the most recent GR report (Groundwater Monitoring and Sampling
<br /> Report First Quarter-Event of February 25, 26 and 27, 2002, dated April 12, 2002)
<br /> Remedial Actions
<br /> GR personnel performed a five-day vapor extraction (VE)pilot test at the site from November 27 through
<br /> December 1, 2000 The VE system operated for a total of 40 hours, and soil vapors were extracted from
<br /> wells UV-1, UV-2, and UV-3 The purpose of the VE pilot test was to evaluate the feasibility of the VE
<br /> technology and collect needed site-specific data for the design of a VE system if deemed necessary The
<br /> conclusion of the VE pilot test was that VE technology is not appropriate for this site
<br /> From December 2000 through July 2001, GR performed an interim remedial action test of the existing
<br /> ozone microsparging system The system was operated continuously over this period of time and
<br /> groundwater samples were collected from surrounding groundwater monitoring wells to evaluate the
<br /> effectiveness of the system Groundwater samples were collected weekly for the first three months, then
<br /> every two weeks through the end of the test The results of the test show that the system was successful
<br /> in significantly reducing dissolved hydrocarbon concentrations in the groundwater beneath the site, and
<br /> there were some indications that the treatment system might be reducing concentrations in downgradient
<br /> wells Based on these results, the system continues to operate
<br /> FIELD ACTIVITIES
<br /> To further delineate the extent of dissolved petroleum hydrocarbons in groundwater downgradient,
<br /> monitoring well U-26 was installed southeast of the subject site (Figure 3) To expand the existing ozone
<br /> microsparge remedial system, two additional ozone microsparge wells (SP-1C and SP-3C) were
<br /> installed on-site in the vicinity of the former gasoline USTs, and six ozone microsparge wells (SP-5A,
<br /> SP-5B, SP-5C, SP-6A, SP-6B, and SP-6C) were installed across California Street, east of the subject site
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