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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 Sampling 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.1 Depth (ft.) IntervalIft.) 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-10 2 97.5 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 bgs 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. Prior 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, MW-2, and MW-4 by 0.15 feet to 0.33 feel, resulting in an overall average
<br /> increase in groundwater elevation of 0.18 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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