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i <br /> i <br /> i <br /> Baxter will construct additional wells to provide the neces- <br /> sary delineation of onsite groundwater contamination. For <br /> example, if contaminated groundwater is encountered in <br /> DSW-4D and in DSW-6C, it may be necessary to install a <br /> D zone well at the DSW-6 location. <br /> Construction <br /> I <br /> All wells will be constructed in boreholes drilled by mud <br /> rotary with stainless steel well screen and steel casing. <br /> The annular space surrounding the well screen will be filled <br /> with a sand-gravel <br /> g pack. Sand-gravel pack and screen slot Ii <br /> sizes will be designed on the basis of sieve analyses of <br /> samples from the strata to be screened. The annular space <br /> above the gravel pack will be sealed with a cement-bentonite � 1 <br /> grout. The well construction procedures will conform with L <br /> the California Site Mitigation Decision Tree guidelines and <br /> accepted hazardous waste industry practices; these proce- <br /> dures are explained in detail in Appendix A. Figure 4-4 <br /> illustrates the anticipated general well construction <br /> detail. <br /> TASK 2--OFFSITE WELLS <br /> Purpose <br /> i <br /> Offsite wells are needed to define the horizontal and verti- <br /> cal extent of groundwater contamination, to the extent nec- <br /> essary to evaluate the threat to public health and the <br /> environment, and to evaluate and select remedial alterna- <br /> tives. <br /> Background <br /> Results of depth sampling during drilling at three offsite <br /> locations have indicated the presence of groundwater contam- <br /> ination in the C zone at the OS-3 location, and possibly in <br /> the B zone at the OS-1 location (Figure 4-1) . Groundwater <br /> samples from the offsite E zone wells (OS-lE, OS-2E, and <br /> OS-3E) have not revealed detectable amounts of PAHs or II <br /> pentachlorophenol contamination. <br /> Scope of Work <br /> The complete scope of work necessary to determine the areal <br /> and vertical extent of offsite groundwater contamination <br /> cannot be fully defined from available data. McCormick and <br /> Baxter is, however, committed to defining the areal and ver- <br /> tical extent of offsite groundwater contamination to the <br /> precision necessary to develop, evaluate, and implement I ' <br /> appropriate remedial actions. Such a definition will likely <br /> require more wells than described in this report. The loca- <br /> tions and depths of additional wells will be based on the <br /> 4-4 <br />