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STATE&*F CALIFORNIA--CALIFORNW ENVIRONMENTAL PROTECTION AGENCY PETE WILSON, <br /> DEPARTMENT OF TOXIC SUBSTANCES CONTROL <br /> REGION 1 <br /> 10151 CROYDON WAY,SUITE 3 <br /> SACRAMENTO,CA 95827-2106 <br /> 916-255-3666 <br /> MEMORANDUM <br /> TO: OxyChem-Lathrop File <br /> FROM: Marvin Woods <br /> Hazardous Substances Engineering Geologist <br /> Sacramento Responsible Party Unit <br /> Northern California-Central Cleanup Operations Branch <br /> DATE: September 18, 1996 <br /> SUBJECT: Permanent Monitoring Well Replacement Workplan — Occidental <br /> Chemical Corporation, Groundwater Remedial Program, Lathrop, <br /> California <br /> I have reviewed the subject workplan(TreaTek, August 22, 1996)and have the following <br /> comments. <br /> 1 Several monitoring wells need to be replaced because they routinely pump dry during <br /> purging for sampling, and recover either very slowly or not at all. I note in the <br /> description of well construction(page 4, paragraph 2)that no transition seal was installed <br /> between the filter pack and the annular grout seal. Inspection of well completion logs <br /> (Appendix A) confirms that no proper transition seals were used(a few logs indicate that <br /> the gravel pack of#21 sand was topped by a one-foot layer of Monterey #3 sand). Lack <br /> of a transition seal probably permitted immediate damage to the filter pack as the grout <br /> for the annular seal was delivered(i.e.. grout infiltration of the filter pack). The result of <br /> such damage would presumably be a significantly shortened useful life of the wells. <br /> Fortunately, the proposed construction of the replacement wells includes transition seals, <br /> consisting of at least two feet of bentonite. <br /> 2 The workplan indicates that, for each well, "the pilot hole for each well will be logged by <br /> a registered geologist,"and that the depth of the five-foot well screen will be adjusted, as <br /> necessary, to ensure that the well is completed across the most permeable zone. <br /> Unfortunately, the workplan is not explicit regarding the nature of the type of well log to <br /> be produced. If it based solely on cuttings delivered by the mud-rotary drilling method, <br /> the precision of the lithologic data produced probably will be insufficient to optimize the <br /> depth of such a short well screen. I recommend that, in addition to logging the cuttings, <br /> the borings should be logged using standard geophysical methods(electric log), so as to <br /> better ensure optimal vertical position of well screens. <br />