SITE INFORMATION AND CORRESPONDENCE_FILE 3

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Stantec ADDENDUM TO JULY 8,2010 WORK PLAN FOR MONITORING WELL DESTRUCTION AND REPLACEMENT AND ADDITIONAL PATH FORWARD Former Thrifty No. 172/ConocoPhillips No.5449 September 23,2010 boxes will be removed. Well construction details and destruction/preservation rationales are included in Table 1A. 1.1.5 Well Protections In order to protect remaining undestroyed monitoring wells (MW-3, MW-7, and MW-12R) during scheduled redevelopment activities, Stantec will contract a C-57 California licensed drilling company to air-knife five feet below ground surface at each of the well locations. The grout collars will be removed and the casings will be cut five feet below ground surface. Heavy duty Morrison boxes will be set around the wells and filled with sand. The areas above the well boxes will be backfilled with the loose soil previously removed. Construction activities will take place over the well locations. After construction activities, a private utility locator or surveyor will be hired to locate the buried well boxes. Once located, the driller will saw-cut a 3' X 3' box around each location and excavate via vac-truck down around the buried well boxes. The heavy duty Morrison boxes will be removed, the casings will be extended, and the locations will be backfilled with cement grout. New well boxes will be installed flush to grade. 1.1.6 Proposed Shallow Groundwater Monitoring Wells Three onsite shallow groundwater monitoring wells (MW-4R, MW-14, and MW-15), one onsite deep groundwater monitoring well (MW-13), and one offsite shallow groundwater monitoring well (MW-16) will be constructed using HSA (Figure 2). Offsite well MW-16 will be installed during the well destruction/protection mobilization. Onsite wells MW-4R and MW-13 through MW-15 will be installed after completion of the redevelopment activities. MW-13 and MW-14 will be located near former well MW-11 R, MW-15 will be located near deeper well MW-3, and MW-16 will be located near MW-6 to allow vertical gradient evaluation. The depth to groundwater is anticipated to be between 32 and 36 feet bgs. The borings for the shallow wells will be advanced to a depth estimated to be no greater than 45 feet bgs, depending on the subsurface materials encountered and the depths of nearby companion wells. The boring for the deeper well (MW-13) will be advanced to a depth estimated to be no greater than 60 feet bgs. Upon completion of drilling, groundwater monitoring wells will be installed in the borings. Groundwater monitoring wells MW-13 through MW-15 will be constructed of four-inch diameter schedule 40 PVC casings with factory-slotted 0.010" well screens (MW-13 may be constructed with 0.020" well screen if the materials encountered within the screened interval are primarily coarse-grained), and wells MW-4R and MW-16 will be constructed of two-inch diameter schedule 40 PVC casings with factory slotted 0.010" well screens. The wells will have locking caps and be completed at the surface with traffic rated, bolted well boxes. 1.1.7 Soil Sample Collection and Analysis Soil samples will be collected from the borings for the new wells at approximate 5-foot depths and at changes in soil stratigraphy beginning at six feet deep using a California-Modified split spoon sampler. The samples will be described in accordance with the Unified Soil Classification System (USCS) by a Stantec scientist under the direction of a California Professional Geologist, will be examined for odors and visible signs of petroleum hydrocarbon impact, and will be screened for hydrocarbon vapors using a photo-ionization detector(PID). Soil samples showing positive readings on a PID will be analyzed for TPPH, BTEX, 1,2-DCA, DIPE, EDB, ETBE, ethanol, MTBE, TAME, and TBA by EPA Method 8260. Soil samples will be sent to a California state-certified laboratory under chain-of-custody protocol.