|
URS
<br /> I
<br /> • Mr. Jeffrey Wong
<br /> San Joaquin County Environmental Health Department
<br /> October 4, 2002
<br /> Page 2
<br /> BC sampled well MW-1 on July 30, 1996, November 15, 1996, and March 19, 1997. Laboratory
<br /> analysis reported concentrations of methyl tert-butyl ether (MtBE) of 35 micrograms per liter(gglL), 8
<br /> pg/L, and 19 µglL, respectively, per sampling event. TPHg and BTEX were not detected in any of the me
<br /> three sampling events. Depth to groundwater ranged from 8.89 feet to 12.37 feet bgs. -\,-o V
<br /> 5A\kt
<br /> 0' �&0
<br /> On August 4, 1997, the California Regional Water Quality Control Board --- Central Valley Region ,q
<br /> (CRWQCB-CVR) issued a "No Further Action Required"status for the site. 1
<br /> Between 1997 and 1999, well MW-1 was abandoned. A well abandonment report was not provided to
<br /> SECOR; however, on November 12, 2001, SECOR personnel visually confirmed the abandonment of
<br /> well MW-1.
<br /> On November 24, 1999, Delta Environmental Consultants, Incorporated (Delta) performed oversight
<br /> and soil sampling during facility product piping and dispenser upgrade activities. Ten soil samples
<br /> were collected beneath the product piping lines and eight soil samples were collected beneath the
<br /> product dispensers. In soil sample PL-9-5.5, laboratory analysis reported concentrations of toluene,
<br /> ethylbenzene, xylenes, and MtBE of 0.0436 mg/kg, 0.0124 mg/kg, 0.0925 mg/kg, and 0.201 mg/kg,
<br /> respectively. Xylene concentrations of 0.0448 and 0.0109 mg/kg were also reported in soil samples PL-
<br /> 44.0 and DP84.5, respectively. PL-9 and PL4 samples were taken from east of the dispensers.
<br /> Neither hydrocarbons nor volatile organic compounds (VOCs) were detected in any other soil sample
<br /> collected.
<br /> On February 21, 2002, three soil borings (SB-1, SB-2, and SB-3)were completed at the site by Delta. The
<br /> soil borings were advanced to a depth of approximately 20 feet bgs. Grab groundwater samples were
<br /> collected from each of the borings. Analytical results for these samples indicated that TPHg was not
<br /> detected in any of the samples; however, MtBE was detected in all three samples at concentrations
<br /> ranging from 3.7 µg/L in SB-2 to 68 gg/L in SB-1.
<br /> In a letter from SJCEHD dated April 18, 2002, an additional investigation was requested to assess impacts
<br /> to groundwater. On July 5, 2002, URS submitted a workplan to install three groundwater monitoring
<br /> wells to comply with the SJCEHD request.
<br /> ASSESSMENT ACTIVITIES AND FINDINGS
<br /> On August 28, 2002, three groundwater monitoring wells were installed at the site to monitor MtBE in
<br /> groundwater. Presented below is a discussion of subsurface conditions, soil sampling and analysis,
<br /> groundwater monitoring well development, sampling and surveying, groundwater analytical data,
<br /> groundwater flow direction and gradient, and a conduit and well survey performed at the site.
<br /> Subsurface Conditions
<br /> Soil encountered in each of the borings consisted primarily of silty to sandy clay. A thin layer of fine-
<br /> grained silty sand approximately 1 foot thick was present at MW-2 and MW-3 at approximately 5 feet
<br /> bgs. A layer of soft, plastic clay was observed between 12 to 19 feet bgs at MW4. First groundwater
<br /> was encountered in silty clay between 16 and 19 feet bgs and rose to approximately 10 feet bgs in the
<br /> wells. Boring logs are presented as Attachment II.
<br /> 7G600-022-02/Site Investigation ReportK:U5 BP ARCO\05450LSite Investigation Report.doc
<br />
|