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Work Plan for Additional Subsurface Investigation @ Former Unocal Station 110187 November 12, 1999 contain detectable concentrations of the oxygenate compounds with the exception ofthe samples from wells U-4 (MtBE @ 2.8 ppb) and U-8 (DIPE @ 2.2 ppb). However, as requested by SJCPHS, Unocal will continue to sample the wells on a quarterly basis and has authorized GR to analyze for the oxygenate compounds in the upcoming fourth quarter monitoring and sampling event which is scheduled for late November 1999. To further evaluate the horizontal and vertical extent of petroleum hydrocarbons downgradient of the subject site,GR proposes to install four groundwater monitoring wells. Three of these wells will be constructed to depths that coincide with the screen intervals of piezometers NP-1 and NP-2. In addition,an offsite 2-inch diameter well will be installed adjacent to the nested piezometers NP-2. This well will be screened from approximately 110 to 120 feet bsg. This well will be used to compare the analytical results from the deep nested piezometer NP-2. To implement this scope of work, GR proposes the following six tasks: Task 1. Pre-field Activities GR will update the site-specific safety plan,and will obtain the necessary monitoring well installation permits from the SCPHS. Unocal will obtain the necessary right-of-entry from the property owner. Underground Service Alert(USA)will be notified at least 48 hours prior to initiating field work. A private line locator will be contracted to clear the monitoring well locations. Task 2. Well Installation GR will install four groundwater monitoring wells at the locations shown on Figure 3. Drilling and well construction will be performed by a California licensed driller. Three of these well borings will be drilled with 8-inch diameter hollow-stem augers to depths of approximately 30, 70, and 120 feet bgs. Because three of the wells will be installed side by side, soil samples for description and possible chemical analysis will only be obtained from the deepest well boring at five-foot intervals. A GR geologist will monitor the drilling activities and prepare a log of the deepest boring only. Soil samples collected at 10 feet bgs,and every 10 feet in depth thereafter, from the deepest boring will be submitted for chemical analysis as described in Task 5. The boring installed adjacent to piezometer NP-2 will not be logged or samples collected for analyses. The soil samples from piezometer NP-2 delineated soil vertically. Soil from each sampled interval will be screened in the field for the presence of volatile organic compounds using a photoionization detector(PID). These data will be collected for reconnaissance purposes only, and will not be used as verification of the presence or absence of petroleum hydrocarbons. Screening data will be recorded on the boring logs. The groundwater monitoring wells will be constructed with 2-inch diameter Schedule 40 polyvinyl chloride(PVC)well casing and 0.02-inch machine slotted well screen. One well will be constructed with a screened interval of between approximately 20 and 30 feet bgs, one will be screened from approximately 60 to 70 feet bgs,and two will be screened from approximately 110 to 120 feet bgs. The actual screen interval will depend on the groundwater depth and lithologic conditions 240042.05 5