|
Site History
<br /> 4000 Highway 88,Lockeford, CA
<br /> Time Oil Property No.04-049
<br /> MW-9 (1.4 ppb benzene). Hydrocarbons were not present at detectable concentrations in the samples
<br /> collected from MW-2, MW-7, and MW-10.
<br /> Based on the results of the site investigation it appears as though soil impacts remain limited to the
<br /> immediate vicinity of the former tank system. Although low concentrations of hydrocarbons appear to be
<br /> migrating offsite to the east (MW-8, sidegradient) and west (MW-5, sidegradient), groundwater impacts
<br /> appear to be fairly well defined in except to the northwest (MW-6, upgradient).
<br /> Since the May 1996 quarterly sampling event hydrocarbon concentrations decreased in MW-2 and MW-4
<br /> and increased in MW-1 and MW-3; however, the changes were not very great in any of the wells. The
<br /> observed fluctuations in hydrocarbon concentrations may be a result of a changes in groundwater
<br /> elevation; however, not enough data has been collected to evaluate their significance. Additional
<br /> groundwater data will be collected to further evaluate the extent of the dissolved hydrocarbon plume.
<br /> The results of the most recent site investigation suggest that the extent of impacted soil has been
<br /> defined in all directions, and that the extent of groundwater impacts is fairly well defined except to the
<br /> northeast (upgradient) of the former tank system. Because the extent of impacts is nearly defined, and it
<br /> may be difficult to obtain access to install additional upgradient monitoring wells, Time proposed that
<br /> efforts be focused on interim remediation rather than further investigation.
<br /> Time believes that the installation and operation of an interim remediation system, such as a vapor
<br /> extraction system, may mitigate the need for additional investigation in the upgradient direction, as well
<br /> as prevent additional groundwater impacts. During the operation of the interim remediation system,
<br /> groundwater monitoring would be continued to confirm that additional contaminant migration is not
<br /> occurring, to monitor remediation efforts, and to assist in identifying source areas which may require the
<br /> w.r implementation of additional remediation technologies.
<br /> Time requested the PHS/EHD's concurrence that additional investigative efforts will not be required at
<br /> this time and that interim remediation efforts would be appropriate. A copy of SECOR's "Environmental
<br /> Investigation and Quarterly Summary Report" dated October 9, 1996 was forwarded to the approporate
<br /> agencies on October 21, 1996.
<br /> Fourth Quarter 1996 Groundwater Sampling Results - December 1996
<br /> On December 19, 1996, samples were collected from each of the groundwater monitoring wells (MW-1
<br /> through MW-10) at the subject site by a representative of SECOR. Prior to purging, the depth to water in
<br /> each well was measured to evaluate elevation, flow direction, and gradient. Groundwater with a
<br /> southwesterly flow at a rate of 0.02 ft./ft. was encountered in the wells at depths of 66.90 feet to 79.92
<br /> feel below ground surface. Free product was not encountered in any of the wells.
<br /> Since the Third Quarter 1996 sampling event, groundwater elevation decreased in MW-2, MW-4, MW-7
<br /> and MW-8 by 0.05 feet to 0.53 feet, and groundwater elevation increased in wells MW-1, MW-3, MW-5,
<br /> MW-6, MW-9, and MW-10 by 0.09 feet to 4.55 feet. These changes resulted in an overall average
<br /> increase in groundwater elevation of 0.63 feet across the site and are most likely a result of seasonal
<br /> variation. Groundwater flow direction and gradient remained relatively unchanged from the previous
<br /> quarter.
<br /> Analyses identified elevated concentrations of hydrocarbons in the samples collected from MW-1
<br /> (21,000 ppb gasoline, 1,500 ppb benzene, 5,230 ppb toluene, 742 ppb ethylbenzene, and 3,590 ppb
<br /> xylenes), MW-3 (3,510 ppb gasoline, 522 ppb benzene, 184 ppb toluene, 191 ppb ethylbenzene, and 349
<br /> ppb xylenes), MW-6 (515 ppb gasoline, 45.5 ppb benzene, 45.2 ppb toluene, 7.5 ppb ethylbenzene, and
<br /> 124 ppb xylenes), and MW-9 (2,400 ppb gasoline, 140 ppb benzene, 90 ppb toluene, 88 ppb
<br /> ethylbenzene, and 173 ppb xylenes). Lower concentrations of hydrocarbons were found in the samples
<br /> 10
<br />
|