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Stantec April 28,2009 Mr. Bob Aragon Page 2 of 3 Re: Response to DTSC Comments—Well Abandonment Work Plan the RWQCB's Central Lodi Plume data which indicates that the first encountered groundwater zone in the vicinity of the site is at a depth of approximately 57 feet(MW-24A, 5/9/07). Our assumption is that the wells were installed in 1913 at the same time the MGP facility was constructed since they are shown on the historical MGP drawings and water was a necessary component of the gas manufacturing process. The City of Lodi's 2005 Urban Water Management Plan reported "the average annual decrease in groundwater levels from 1927 to 2004 has been 0.39 feet per year." Historical water level data for Lodi shows a decrease of 30 feet in 77 years. If the water table was at least 30 feet higher when the wells were installed in 1913, the first encountered water would have been at 27 feet below ground surface. Therefore, based on historical water levels and present-day groundwater analytical results, it is unlikely that these wells act as conduits to deeper groundwater zones. It is also unlikely that the wells were completed at a depth of 50 feet or greater with an excess of 25 feet of screen;however, the since the actual depth of the casing is unknown, an option for sampling of groundwater in the vicinity of the wells will be included as part of the well destruction work plan. PG&E proposes to measure the static water level in the well at the conclusion of water rotary drilling operations. The following day, prior to video logging, the static water level will be measured again. If water is present in the well casing it will be purged. After purging, the well will be videoed with a downhole camera. The video log will allow PG&E to evaluate the condition of the well casing and well screen. The static water level will be measured again after completion of the video logging. Because the fill material in the well casing may contain waste materials, any water sample from inside the well casing likely will not be indicative of groundwater chemical concentrations. If the well casing is found to extend below the current water table depth, PG&E will determine, in consultation with DISC, an appropriate method of collecting a representative groundwater sample by drilling a boring adjacent to the abandoned wells and collecting a grab groundwater sample. 2)The annulus between blank casings and the borehole walls above groundwater may not be or are not likely properly sealed since these wells are very old. Properly sealing the vadose zone is important to prevent a conduit for contamination to flow to groundwater along the annulus between the well casing and the borehole wall. Recommendation: Perforate casing above groundwater prior to pressure grouting to ensure that a conduit is not present that could allow contamination to flow to groundwater. Perforating casing could consist of four one inch cuts around a casing for every one foot. Pressure grouting should be performed to ensure that an adequate amount of grout is placed into the space between the casing and borehole walls. Response: Wells will be perforated at four points for every one foot starting above groundwater, if encountered, to the top of the well casing. If groundwater is not encountered, wells will be perforated at four points for every one foot from the bottom to the top of the well casing. 3)Since the wells are old it is possible that screened portions of the wells are filled with sediment or other material. If the well screens are long then it is possible that they cross aquitards. Recommendation: Perforate screened portions of the well casings prior to pressure grouting to ensure that a sufficient seal is placed between the screened sections of the wells and the borehole walls. Response: Wells will be perforated at four points for every one foot starting above groundwater, if encountered, to the top of the well casing. If groundwater is not encountered, wells will be perforated at four points for every one foot from the bottom to the top of the well casing.