|
S E C 0 R
<br /> ConocoPhilllps
<br /> Former 76 Service Station No 4409
<br /> • September 30, 2003
<br /> Page 2
<br /> the former station canopy, canopy footings and kiosk, additional over-excavation was completed in
<br /> the dispenser area Over-excavation was terminated when obviously impacted soil was removed, the
<br /> structural integrity of the sidewalk was endangered, or groundwater was encountered A total of
<br /> approximately 1,707 tons of soil was transported off-site for disposal at the Forward Landfill in
<br /> Manteca, California Historical maps of soil sample locations and areas of over-excavation are
<br /> Included in Attachment 2 and historical soil analytical data Is included in Table 1
<br /> On December 9 and 10, 1999, four groundwater monitoring wells (MW-1 through MW-4) were
<br /> Installed to investigate the extent of petroleum hydrocarbons in soil and groundwater beneath the
<br /> site Well MW-1 was completed at 50 feet bgs, and wells MW-2 through MW-4 were completed at
<br /> 45 feet bgs Petroleum hydrocarbons were not detected In any soil samples from beneath the site
<br /> Groundwater sampling from the new wells was Initiated on January 5, 2000 Concentrations of total
<br /> petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX),
<br /> and methyl tertiary butyl ether (MtBE) were generally not detected in the wells, with the exception of
<br /> minor concentrations of benzene and/or MtBE (EPA Method 8260) detected in wells MW-3 and MW-
<br /> 4 Initial depth to water was approximately 28 to 29 feet bgs The direction of groundwater flow was
<br /> established in the easterly direction, at an approximate gradient of 0 003 foot/foot (IT, February 25,
<br /> 2000) Historical soil analytical data Is included in Table 1, soil boring and well completion details are
<br /> included in Table 2, and boring logs are included as Attachment 3
<br /> On July 20 and 21, 2000, four Geoprobe borings (GP-1 through GP-4) were completed by IT to a
<br /> depth of 60 feet bgs to investigate the vertical extent of petroleum hydrocarbons in soil and
<br /> groundwater Petroleum hydrocarbon-impacted soil and groundwater were encountered at depths of
<br /> approximately 50 to 60 feet bgs Based on the results of this Investigation, IT recommended an
<br /> additional Investigation to determine the vertical extent of petroleum hydrocarbons in soil beyond 60
<br /> feet bgs, and the Installation of additional groundwater monitoring wells for assessment of deeper
<br /> zone groundwater Impacts (IT, August 30, 2000) 4
<br /> On January 22, 2001, three deep on-site groundwater monitoring wells (MW-5, MW-6, and MW-7)
<br /> were installed by IT to depths of 80 feet bgs to delineate the vertical extent of petroleum
<br /> hydrocarbons in groundwater The highest concentrations of petroleum hydrocarbons were detected
<br /> in soil samples collected between 60 and 70 feet bgs, at concentrations of TPHg, and benzene up to
<br /> 4,780 parts per million (ppm) and 55 3 ppm (MW-60360') Petroleum hydrocarbons were detected in
<br /> groundwater samples collected from all three wells Maximum concentrations of TPHg, benzene,
<br /> and MtBE In groundwater were reported at 2,300 ppb (MW-6), 22 ppb (MW-6), and 16 ppb (MtBE,
<br /> EPA Method 8260) Based on the results of the Investigation, IT concluded that the vertical extent of
<br /> petroleum hydrocarbons In soil were generally defined to a depth of 80 feet bgs IT further
<br /> recommended that additional assessment activities be performed to evaluate the lateral extent of ,
<br /> dissolved petroleum hydrocarbon impacts between approximately 50 and 80 feet bgs (IT, April 16,
<br /> 2001)
<br /> a
<br /> On August 30, 31, and September 5, 2001, IT installed three additional on-site deep groundwater
<br /> monitoring wells (MW-8 through MW-10) at locations upgradient of the UST complex, in the vicinity of
<br /> the pump islands and downgradient of the pump Islands Wells were completed at depths of 79 feet
<br /> (MW-8) and 80 feet bgs (MW-9 and MW-10) Petroleum hydrocarbons were present In soil samples
<br /> collected between 60 and 80 feet bgs, with concentrations of TPHg and benzene reported up to
<br /> 4,400 mg/kg and 77 mg/kg, respectively TPHg was detected In wells MW-8 through MW-10 at
<br /> concentrations up to 22,000 ug/l, and benzene was detected in wells MW-9 and MW-10 at
<br /> concentrations up to 600 ug/I Fuel oxygenates were not detected in any soil and groundwater
<br /> samples collected from the site Based on the results of this Investigation, SECOR recommended L
<br /> the installation of additional off-site deep zone groundwater monitoring wells for further delineation of al
<br /> the dissolved petroleum hydrocarbon plume between approximately 60 feet and 80 feet bgs
<br /> �} (SECOR, May 29, 2002)
<br /> 5
<br /> 1\Conoco Phi Ilips\Tosco Marketing144091Reports\Site Conceptual Model doc
<br />
|