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' Mr. M. Scott Mansholt—CEMC
<br /> Fourth Quarter 2006 Groundwater Monitoring and Sampling Report and
<br /> Annual Summary Report January 12, 2007
<br /> ' Vernalis Pump Station Page 4 of 7
<br /> Arcos Property
<br /> ' 2003 Site Investigations.' In October of 2003, Geomatrix conducted a preliminary soil and groundwater
<br /> investigation at the Arcos property, which was part of the Former Vernalis Pump Station,but was not
<br /> under environmental investigation prior to 2003. The preliminary site investigation included review of
<br /> public records, historical maps, aerial photographs,the geophysical survey, evaluation of current property
<br /> conditions, and subsurface investigation. Between October 20 and 28, 2003, Gregg Drilling,under the
<br /> supervision of Geomatrix, advanced 12 soil borings,ASB-1 through ASB-12, on and around the Arcos
<br /> ' property. TPHc were detected in borings ASB-1,ASB-2, and ASB-4 at a maximum concentration of
<br /> 17,000 mg/kg. TPHc were not detected in any other borings. Low concentrations of BTEX and PAHs
<br /> were sporadically detected in several soil samples. Geomatrix collected grab groundwater samples from
<br /> ' soil borings ASB-1,ASB-5 through ASB-8,ASB-10,ASB-11, and ASB-12. TPHd were detected in grab
<br /> groundwater samples collected from borings ASB-1,ASB-5, ASB-6, ASB-7, and ASB-11, at a maximum
<br /> concentration of 2,900 pg/L.
<br /> ' PETROLEUM DISTRIBUTION IN GROUNDWATER
<br /> The RWQCB water quality objectives (WQOs) include TPH measurements for diesel products,but not
<br /> for crude oil/Bunker C fuel oil. According to the Technical Approach to Site Evaluation and Decision-
<br /> ' Making for the Historical Pipeline Portfolio (referred to as the Consistent Technical Approach[CTA]),
<br /> diesel products are not associated with the Historical Pipeline Portfolio(HPP) sites and typically are not
<br /> present at these sites.' However, WQOs are applied to"diesel-range hydrocarbons"(extractable TPH,
<br /> ' C,o—C25) found in groundwater samples from HPP sites because no WQOs are established for crude
<br /> oil/Bunker C fuel oil,and historical data indicates that diesel-range hydrocarbons are often detected at
<br /> sites affected by crude oil/Bunker C fuel oil. Per the CTA, groundwater samples are quantified against a
<br /> diesel standard so that the comparison to the WQOs is as direct as possible.
<br /> Based on the previous soil and groundwater investigations,petroleum-affected soil and groundwater have
<br /> been delineated vertically to a maximum depth of approximately 80 feet bgs. The RWQCB accepted the
<br /> ' delineation results in their letter dated March 19,2004. Based on the 2005 site investigation, it appears
<br /> that affected soil is laterally delineated north-northwest,north-northeast, south-southwest,and south-
<br /> southeast of the former boiler house and pump house,and affected groundwater is delineated northwest
<br /> ' and west of the former pump house and boiler house. Groundwater monitoring wells MW-5,MW-6, and
<br /> MW-7 were installed during the May 2005 investigation. A report of this investigation was prepared and
<br /> submitted to the RWQCB in August 2006. A sheen of SPO has been consistently observed in well MW-
<br /> 1. TPHd have been consistently detected in one well,MW-7, at concentrations ranging from 52 to 710
<br /> pg/L. No TPHg or BTEX have been detected in the groundwater from the site wells. Low levels of
<br /> PAHs have been detected in groundwater from MW-2,MW-3,MW-5,MW-6, and MW-7.
<br /> ' GROUNDWATER MONITORING AND SAMPLING
<br /> MONITORING
<br /> ' On November 7,2006, depth to groundwater was measured in wells MW-1 through MW-4, MW-6, and
<br /> MW-7 using a hydrocarbon/water interface probe. Depth to groundwater was not measured in MW-5
<br /> because it was not accessible during the monitoring event. Depth to groundwater in these wells ranged
<br /> ' from 33.46 feet below top of casing(btoc) in MW-6 to 37.96 feet btoc in MW-4. Calculated groundwater
<br /> ' 7. Geomatrix Consultants,Inc.,2004. Report of Soil and Groundwater Investigations,Former Roberts Property, Vernalis,
<br /> California. January.
<br /> 8. Geomatrix Consultants,Inc.,2005. Technical Approach to Site Evaluation and Decision-Making,Historical Pipeline
<br /> Portfolio, Central Valley Region. February.
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