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Mr. Robert C. Dos aP.E. • <br /> Page 3 <br /> June 29, 1990 <br /> Ground water has been shown to be contaminated with arsenic, <br /> volatile ;organic compounds, naphthalene, and acenaphthene. <br /> Further wells -both onsite and offsite will be needed to monitor <br /> ground water contaminant levels and flow direction. PG&E should <br /> determine the factors controlling the hydrology in the vicinity <br /> of the site and account for the observed ground water gradient in <br /> the vicinity of the site. (The RI Report established a southerly <br /> gradient when the regional gradient is easterly. ) <br /> PG&E should perform a well survey within one mile of the site and <br /> analyze any domestic wells that may be affected by contamination <br /> from the site. The wells inventoried should be evaluated for <br /> their effect on the observed gradient in the vicinity of the <br /> site. <br /> Any information regarding the abandoned onsite well (next to <br /> DB-1) such as construction details, depth, former use, <br /> abandonment procedure(s) , and date of abandonment should be <br /> provided. This well must not provide an avenue for ground water <br /> contamination. <br /> The four shallow wells and one deep well completed by PG&E onsite <br /> were constructed with stainless steel casings and 20 foot slotted <br /> stainless steel screens. PG&E should consider spiral wound <br /> stainless steel screens over slotted screens since they provide <br /> much greater open area per screen section at comparable cost. <br /> The Department does not recommend screens longer than 20 feet <br /> when installed across the water table. Monitoring wells <br /> completed below the free water table should have screens no <br /> longer than 10 feet and only screen the most permeable zone. <br /> Excessively long screens create the possibility for vertical <br /> migration of contaminants, loss of depth discrete equi-potential <br /> data, and dilution of water quality samples. <br /> Part of Appendix A (Geotechnical Report) described the well <br /> drilling and construction techniques used by your subcontractor, <br /> Geo/Resource Consultants, Inc. The wells completed were drilled <br /> with, and constructed through, a hollow stem auger. This <br /> practice is acceptable, except, for wells installed below the <br /> free water table which should be drilled with mud rotary <br /> equipment. This technique circumvents the possibility of the <br /> formation "caving in" as occurred at deep well MW-1D while the <br /> auger was retrieved and possible vertical migration of <br /> contaminants. Moreover, future deep wells (below the water <br /> table) should never be left open overnight as was done while deep <br /> well MW-1D was being constructed. Wells drilled using mud rotary <br /> equipment should be constructed with centralizers placed at the <br /> top and bottom of the screened section and at 40 foot intervals <br /> on blank casing. <br />