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CLEAR-WATER
<br /> c r. o ` P. i ti C.
<br /> y` y ErtvrrvnmPnW Services
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<br /> 1.0 INTRODUCTION
<br /> This report, submitted by Clearwater Group, Inc. (Clearwater)on behalf of Ruiz Grocery, pertains to activities conducted
<br /> at the Ruiz Grocery located at 4491 West Durham Ferry Road in Tracy, California(Figure 1). The report summarizes the
<br /> results of quarterly groundwater monitoring for the fourth quarter 1999, and also presents a workplan for proposed
<br /> confirmation borings to evaluate the effectiveness of the soil vapor extraction (SVE) /air sparge (AS) system operated
<br /> from October 1998 to September 1999.
<br /> 1.1 Site Background
<br /> Two gasoline USTs and dispensers were removed from the site in 1988. Seven monitoring wells (MW-1 through MW-
<br /> 7), one groundwater recovery well (RW-1), four soil vapor extraction wells (VW-1 through VW-4). and two air sparge
<br /> wells (SW-1 and SW-2) have been installed at the site. A domestic well (DW), screened in a deeper water bearing zon(,.
<br /> also exists at the site (Table 1), he extent of soil and water contamination has been delineatedQ�
<br /> CT )'Iased on historical S�anCt_
<br /> investigation data, it was calculated that approximately 5,050 lbs (842 gals) of gasoline contaminants remained in the
<br /> vadose zone. Additionally, pilot testing for groundwater extraction, soil vapor extraction, and air sparging were
<br /> competed. Based on cumulative site data, Clearwater recommended construction of a remediation system consisting of
<br /> combined groundwater and soil vapor extraction with air sparging. The results of this work were presented in
<br /> Clearwater's Problem Assessment Report, Feasibility Study, and Remedial Action Plan dated February 21, 1996.
<br /> The SJCPHS/EHD did not fully concur with the recommended plan for remediation. Following further discussion,
<br /> Clearwater submitted a Revised Remedial Action Plan dated November 12, 1996, which recommended remediation by
<br /> only soil vapor extraction and air sparging. The SJCPHS/EHD approved this plan in a letter dated November 6, 1996.
<br /> In late summer 1998. a SVE/AS remediation system was installed at the site. Wells RW-1, MW-1, MW-2 and VW-1
<br /> through VW-4 were used to extract soil vapor§,, which were treated by an electric catalytic oxidizer. An air blower was
<br /> used to deliver air to the subsurface via wells SW-1 and SW-2. The system was operational from October 1998 to the
<br /> end of September 1999, which resulted in 6,638 hours of total system operation. Contaminant recovery from the
<br /> subsurface was dramatic at first,averaging approximately 2 lbs/hr during the first five months of operation. However,by
<br /> the July 1999, contaminant recovery rates had decreased dramatically to approximately 0.02 lbs/hr. For the last three
<br /> months of operation, influent TPHg levels only slightly exceeded laboratory detection limits. Calculations indicate the
<br /> SVE/AS system recovered a total of 7,377 lbs (or 1,209 gals) of gasoline from the subsurface by the end of September
<br /> 1999. The amount of contamination recovered from the subsurface compared well to that calculated to be in the
<br /> subsurface prior to remediation,indicating that the system had removed the vast majority of contaminants beneath the site.
<br /> Remediation efforts were also reflected by a marked decrease, over two orders of magnitude, in groundwater
<br /> contaminants in the source area. Prior to remediation, maximum concentrations of TPHg and benzene in groundwater
<br /> had been 150,000 gg/L and 3,400 pg/L,respectively. Following shut-down of the remediation system,maximum TPHg
<br /> and benzene concentrations in groundwater were only 990 µg/L and 1.3 pg/L, respectively, in November 1999.
<br /> Clearwater recommended shut-down of the SVE/AS system at the end of September 1999 because operational data
<br /> indicated that it had reached a plateau contaminant recovery. The SJCPHS/EHD approved system shut-down in a letter
<br /> dated September 20, 1999. In that letter,the SJCPHS/EHD indicated that groundwater beneath the site should continue
<br /> D-107 4Q99 GW&WP Confirmation Borings I November 15, 1999
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