WORK PLANS_FILE 1 - Laserfiche WebLink

Home Browse Search

09/12.'97 16:42 GR/GSI—SACTC -+ 12094640138 goo Work Plan For Soil Boring and Monitoring Well Insla9ap0a-Unocal Station No. 6981 September 12, 1997 In February 1996, wells MW-3, MW-10, MW-12, and MW-13 were abandoned at the request of the city of Stockton. This work was performed to facilitate reconstruction of the intersection and widening of Pacific Avenue. Quarterly Monitoring Data A total of 29 monitoring events have been performed sine; initiation of quarterly monitoring in 1990. Groundwater in the vicinity of the site has risen from depths of approximately 58 feet below ground surface (bgs) in 1990 to approximately 32 to 35 Feet bgs in 1997, a rise of about 25 feet. These data indicate that groundwater beneath the site initially flowed toward the east-northeast, but has gradually shitted and now appears to flow toward the north-northeast. Gradients during this time were relatively flat, fluctuating from 0.003 to 0.007. The downgradient (northeastern) edge of the dissolved hydrocarbon plume is undefined at this gime(third quarter 1997 data). The plume was previously defined in this area by wells MW-12 and MW--13, which have been abandoned. Currently, the plume is defined to the north, west-northwest, and south. SCOPE OF WORK lb delineate the dissolved hydrocarbon plume downgradient of the site, GSI proposes to install one groundwater monitoring well between the former locations of wells MW 12 and MW-13. This location is downgradient of wells MW-2 and VW-4, which currently and historically have contained the highest dissolved concentrations. This well will be constructed with 30 feet of well screen (25 to 55 feet bgs), which extends across must of the range of groundwater fluctuation while leaving some room to accommodate an additional rise in groundwater levels. GSI will also drill one soil boring into groundwater (approximately 40 feet bgs) adjacent to vapor extraction well VW-2. The highest residual hydrocimbon concentrations (ranging from 3,900 to 5,300 ppm) were detected in samples from boring VW-2. Soil samples collected from this boring will be analyzed to evaluate the effectiveness of the treatment system in removing residual hydrocarbons from tine subsurface. In addition to these activities, GSl will continue to operate the VPS, and MPDS will continue to monitor the wells and collect groundwater samples on a quarterly basis. GSI will attempt to locate well MW 11 in time for inclusion in the next sampling event. ORC will be added to well MW-2 to enhance the naturii attenuation of the dissolved hydrocarbon plume. 11 0 1`t 7ass.zt-1 3 x.25 �5 30° 3 ID qLA OD -Z S 15"