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CLEARWATER
<br /> C, R 0 L !' l \ C
<br /> • Environmental Se,v,,os
<br /> operation, influent TPHg levels only slightly exceeded laboratory detection limits Calculations
<br /> indicated the SVE/AS system recovered approximately 7,377 lbs (or 1,209 gals) of gasoline from the
<br /> subsurface by the end of September 1999 The amount of contamination recovered from the subsurface
<br /> compared well to that calculated to be in the subsurface prior to remediation, although slightly higher,
<br /> indicating that the system had removed the vast maloirty of contaminants beneath the site
<br /> Remediation efforts were also reflected by a marked decrease, over two orders of magnitude, in
<br /> groundwater contaminants in the source area Prior to remediation, maximum concentrations of
<br /> TPHg, benzene, and MTBE in groundwater had been 150,000 jig/L, 9,400 fig/L, and 3,800 pg/L,
<br /> respectively Following shut-down of the remediation system, maximum TPHg, benzene, and MTBE
<br /> concentrations in groundwater were only 990 pg/L, 13 pg/L, and 150 pg/L, respectively, in November
<br /> 1999 Clearwater recommended shut-down of the SVE/AS system at the end of September 1999 because
<br /> operational data indicated that it had reached a plateau contaminant recovery The SJCPHS/EHD
<br /> approved system shut-down in a letter dated September 20, 1999 In that letter, the SJCPHS/EHD
<br /> indicated that groundwater beneath the site should continue to be monitored for several quarters to
<br /> evaluate post-remediation trends in residual groundwater contaminants and that confirmation soil
<br /> borings would be required in the future to demonstrate rernediation of the vadose zone The mobile
<br /> catalytic oxidizer was removed from the site in October 1999 However, all vapor extraction piping
<br /> remains in the ground, and the above ground manifold was secured (Figure 2)
<br /> ® In February2000, Clearwater supervised the drilling of four confirmation soil borings (CB-1 through
<br /> r �
<br /> CB-4) The borings were drilled in areas of known soil contamination prior to remediation
<br /> Comparison of pre- and post-remediation soil samples indicated that the SVE/AS system had
<br /> effectively reduced gasoline contaminants in the vadose zone Based on the results of the confirmation
<br /> soil samples, it appears that the last of the residual soil contamination exists at the bottom of the former
<br /> south underground storage tank The 10 foot depth level in CB-1 corresponds to this location At this
<br /> location, pre-remediation levels of gasoline in soil were X10,000 mg/kg The results from the samples
<br /> in CB-1 indicate a post-remediation level of 230 mg/kg at this 10 foot depth level At the same depth, at
<br /> the base of the former north underground storage tank pit, pre-remediation levels of gasoline in soil
<br /> were also >10,000 mg/kg Results from the sample at 10 feet bgs in CB-2 indicate a post-remediation
<br /> level of gasoline in soil of <1 mg/kg Prior to remediation the mass of TPHg in the vadose zone was
<br /> estimated to be 5,050 lbs (842 gallons), however, based on confirmation sample results, post-remediation
<br /> mass of TPHg in the vadose is estimated to be 37 lbs (6 gallons) This indicates that the SVE/AS system
<br /> was effective at removing more than 99% of the gasoline from the soil column beneath the site
<br /> Additionally, ongoing groundwater monitoring results indicated the limited residual dissolved-phase
<br /> contaminant concentrations continued decrease following remediation Results of confirmation soil
<br /> sampling were presented in Clearwater's Confirmation Soil Sampling, Groundwater Monitoring - First
<br /> Quarter 2000, and Closure Request dated March 27, 2000
<br /> Based on confirmation sampling results, Clearwater requested regulatory case closure for the site
<br /> Following discussion with Ms Lori Duncan of the SJCPHS/EHD, it was agreed that closure would be
<br /> ZB107C GWMR 3Q00 2 August 3,2000
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