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Well Abandonment Work Plan -2- 16 December 1991 <br /> DDRW, Tracy <br /> Page 2-3. The Work Plan discusses the location of Well 1 with respect to monitor <br /> wells in the Middle and Lower Horizons, but does not discuss the location <br /> with respect to monitor wells in the Upper Horizon. Well 1 is <br /> approximately 200 feet to the northwest of LM-25. LM-25 has historically <br /> had the highest concentrations of trichloroethylene (TCE) . The current <br /> concentrations of TCE in LM-25 are 268 micrograms per liter (µg/1 ) (August <br /> 1991 Quarter) . <br /> Page 2-5. The Plan states that it was decided to abandon LM-60 due to the damage <br /> caused by the agricultural equipment. The Plan does not state, but should <br /> have, whether the damage was too severe to attempt repair. Unless the <br /> monitor well is beyond repair, it should not be abandoned, but rather <br /> repaired because it provides a useful monitoring point. <br /> Page 2-5. Monitor well LM-60 is screened in the Middle Horizon. Samples collected <br /> from this well prior to the damage have been non-detect (ND) . LM-59 is one <br /> of the other wells in the cluster with LM-60 and is also screened in the <br /> Middle Horizon but above LM-60. The concentrations of TCE and <br /> tetrachloroethylene (PCE) in samples from LM-59 have historically been and <br /> are currently below the Primary Maximum Contaminant Level (MCL) of 5.0 µg/1 <br /> for both constituents. Therefore, LM-60 does not need to be replaced if it <br /> is abandoned. However, if the VOC concentrations in LM-59 are found to <br /> rise significantly above the Primary MCL, then a deeper monitor well may be <br /> needed to define the vertical extent of contamination. <br /> Page 3-1 . The abandonment proposals for LM-60 are the removal of the well casing and <br /> well construction materials including the cement, bentonite and sand pack. <br /> I do not concur with the abandonment plan for LM-60. Because the well was <br /> constructed for monitoring purposes, it has an annular seal down to a depth <br /> of about 75 feet below ground surface (bgs) . Therefore, the annular space <br /> should not serve as a conduit for cross-contamination of lower water <br /> bearing zones. In addition, concentrations of VOCs have not been found in <br /> samples from this monitor well . <br /> It seems excessive and unnecessary to remove the well casing and <br /> construction materials for abandonment. This method of abandonment would <br /> only be necessary if there were some question as to the integrity of the <br /> original well construction. Therefore, for abandonment of LM-60, the <br /> cement slurry should be pumped into the well under enough pressure to force <br /> the cement into the filter pack through the screen and then fill the casing <br /> from the bottom up to the ground surface with the remaining slurry. This <br /> method should reduce the cost of abandonment. <br /> Page 3-2. The Plan states that a 14 inch bit will be used to drill out a 14 inch <br /> inside diameter well casing for Wells 1 and 2. Based on this proposal , <br /> there may be inadequate space between the bit and the inside diameter of <br /> the casing to allow drilling without significant damage to the casing. <br /> This size bit may be too large because of scaling, deviations or areas of <br /> cave-ins in the casing. A smaller diameter bit should be used to drill out <br /> the cement plug and sand in the supply wells. <br /> Page 3-3. The Work Plan contains proposals for sampling of the two supply wells to be <br /> abandoned and the two active supply wells. However, the Plan does not <br />