ARCHIVED REPORTS XR0006544 - Laserfiche WebLink

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CLEARWATER C, R0UP ERVIra-mental Services averaging approximately 2 lbs/hr during the first five months of operation However, by July 1999, contaminant recovery rates had decreased to approximately 0 02 lbs/hr For the last three months of operation, influent TPHg levels only slightly exceeded laboratory detection limits Calculations indicated the SVE/AS system recovered approximately 7,377 lbs (or 1,209 gals) of gasoline from the subsurface by the end of September 1999 The amount of contamination recovered from the subsurface compared well to that calculated to be in the subsurface prior to remediation, although slightly higher, indicating that the system had removed the vast majority of contaminants beneath the site Remediation efforts were also reflected by a marked decrease, over two orders of magnitude, in groundwater contaminants in the source area Prior to remediation, maximum concentrations of TPHg, benzene, and MTBE in groundwater had been 150,000 gg/L, 9,400 pg/L, and 3,800 pg/L, respectively Following shut-down of the remediation system, maximum TPHg, benzene, and MTBE concentrations in groundwater were only 990 pg/L, 13 pg/L, and 150 pg/L, respectively, in November 1999 Clearwater recommended shut-down of the SVE/AS system at the end of September 1999 because operational data indicated that it had reached a plateau in contaminant recovery The SJCPHS/EHD approved system shut-down in a letter dated September 20, 1999 In that letter, the SJCPHS/EHD indicated that groundwater beneath the site should continue to be monitored for several quarters to evaluate post-remediation trends in residual groundwater contaminants and that confirmation soil borings would be required in the future to demonstrate remediation of the vadose zone The mobile catalytic oxidizer was removed from the site in October 1999 However, all vapor extraction piping remains in the ground, and the above ground manifold was secured (Figure 2) In February 2000, Clearwater supervised the drilling of four confirmation soil borings (CB-1 through CB-4) The borings were drilled in areas of known soil contamination prior to remediation Comparison of pre- and post-remediation soil samples indicated that the SVE/AS system had effectively reduced gasolene contaminants in the vadose zone Based on the results of the confirmation soil samples, it appears that the last of the residual soil contamination exists at the bottom of the former south underground storage tank The 10 foot depth level in CB-1 corresponds to this location At this location, pre-remediation levels of gasoline in soil were X10,000 mg/kg The results from the samples in CB-1 indicate a past-remediation level of 230 mg/kg at this 10 foot depth level At the same depth, at the base of the former north underground storage tank pit, pre-remediation levels of gasoline in soil were also >10,000 mg/kg Results from the sample at 10 feet bgs in CB-2 indicate a post-remediation level of gasoline in soil of <1 mg/kg Prior to remediation the mass of TPHg in the vadose zone was estimated to be 5,050 lbs (842 gallons), however, based on confirmation sample results, post-remediation mass of TPHg in the vadose is estimated to be 37 lbs (6 gallons) This indicates that the SVE/AS system was effective at removing more than 99% of the gasoline from the soil column beneath the site Additionally, ongoing groundwater monitoring results indicated the limited residual dissolved-phase contaminant concentrations continued decrease following remediation Results of confirmation soil sampling were presented in Clearwater's Confirmation Soil Sampling, Groundwater Monitoring - First Quarter 2000, and Closure Request dated March 27, 2000 The SJCPHS/EHD responded to the closure ZB107C GWMR 2Q01 2 April 9 2001