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Site History
<br /> 14000 Highway 88, Lockeford CA
<br /> Time Oil Property No 04-049
<br /> Additional Site Investigation and Third Quarter 1996 Groundwater Samplina Results - October 1996
<br /> On July 15 through 18 and July 31, 1996, six groundwater monitoring wells were installed onsite and
<br /> offsite to assist in defining the extent of hydrocarbon impacts As in previous site activities, soils
<br /> encountered during drilling consisted of fine silty sands to approximately 15 feet below ground surface
<br /> (bgs), this strata was underlain by fine to coarse sands to approximately 30 feet bgs Clayey silt and silty
<br /> clay were generally encountered at 30 feet to 35 feet while fine to coarse sands and gravelly sands were
<br /> encountered to 70 feet bgs Silty clay with sandy and gravelly stringers was encountered from 70 feet
<br /> bgs to the extent of each boring Groundwater was first encountered at depths of approximately 75 feet
<br /> to 80 feet bgs
<br /> Each of the wells was installed as follows
<br /> Casing Total Screened
<br /> Well No Diameter(in ) Depth (ft ) Interval (ft Purpose
<br /> MW-5 2 100 35 HC delineation to the NW of former tanks(sidegradient)
<br /> MW-6 2 100 35 HC delineation to the NE of former tanks(upgradient)
<br /> MW-7 2 100 35 HC delineation to the NE of former tanks(up/sidegradient)
<br /> MW-8 2 100 35 HC delineation to the E of former tanks(sidegradient)
<br /> MW-9 2 100 35 HC delineation to the SE of former tanks(sidegradient)
<br /> MW-1 0 2 975 35 HC delineation to the SW of former tanks(downgradient)
<br /> The casing diameter, well depth, and screened interval of each well was selected by a representative of
<br />• SECOR based on the proposed well use and the estimated depth to groundwater at each drilling location
<br /> Each of the wells was constructed with 0 020-inch slotted casing
<br /> Soil samples collected during the drilling of each of the wells were submitted to an analytical laboratory
<br /> Where possible, capillary fringe samples were submitted, although shallower soil samples were also
<br /> analyzed Although the sample collected from MW-8 at a depth of 75 feet logs contained trace
<br /> concentrations of hydrocarbons (0 064 ppb benzene, 0 005 ppb toluene, and 0 007 ppb xylenes), none of
<br /> the other soil samples contained detectable concentrations of hydrocarbons
<br /> On July 19, 1996, samples were collected from MW-1 through MW-9 by a representative of SECOR
<br /> MW-10 was sampled on August 1, 1996 Pnor to purging, the depth to water in each well was measured
<br /> to evaluate elevation, flow direction, gradient, and free product thickness Groundwater with a
<br /> southwesterly flow direction and a gradient of 0 03 ft/ft were encountered in the wells at depths of 67 7
<br /> feet to 84 07 feet below ground surface Free product was not encountered in any of the wells
<br /> Since the previous sampling event, groundwater elevation decreased in MW-3 by 0 07 feet and
<br /> increased in wells MW-1, ASW-2, and MW-4 by 0 15 feet to 0 33 feet, resulting in an overall average
<br /> increase in groundwater elevation of 018 feet across the site Groundwater flow direction changed from
<br /> southeast to southwest and gradient decreased from 0 15 ft/ft 0 03 ft/ft The variations in groundwater
<br /> elevation, flow direction, and gradient could have been a result of additional data points provided by the
<br /> new wells, or they could have been due to seasonal variation
<br /> Analyses Identified elevated concentrations of hydrocarbons in the samples collected from MW-1
<br /> (13,600 ppb gasoline, 1,130 ppb benzene, 4,100 ppb toluene, 482 ppb ethylbenzene, and 2,710 ppb
<br /> xylenes), MW-3 (2,700 ppb gasoline, 288 ppb benzene, 242 ppb toluene, 133 ppb ethylbenzene, and 280
<br /> ppb xylenes), and MW-6 (1,370 ppb gasoline, 142 ppb benzene, 292 ppb toluene, 81 ppb ethylbenzene,
<br /> and 495 ppb xylenes) Trace concentrations of hydrocarbons were found in the samples collected from
<br />. MW-4 (83 5 ppb gasoline, and 1 4 ppb benzene), MW-5 (64 ppb gasoline, 15 3 ppb benzene, 0 7 ppb
<br /> toluene, 2 5 ppb ethylbenzene, and 1 4 ppb xylenes), MW-8 (5 8 ppb benzene and 1 0 ppb xylenes), and
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