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i i i l To properly decommission a well, borehole, or lost drive casing in 4 an open borehole, several considerations and complementing technical criteria should be addressed. It is important to note that each well decommissioning should be approached as a unique situation on a site-specific and case-specific basis. Also, techniques used to decommission wells with extensive filter packs are usually always time consuming and expensive if adequate results are to be achieved. Decommissioning these types of wells can cost more than the well originally cost to install. In addition, wells with extensive filter packs are usually more subject to encounter ! field problems during the physical decommissioning process because i the complexity of activities is always greater and more iterative. A sampling of the most important criteria that need to be the focus of decommissioning efforts are discussed in this paper. However, it is noted that it is not possible to provide a complete listing of all techniques and technical considerations that could be used in decommissioning activities. SUMMARY OF DECOMMISSIONING ACTIVITIES Overview Without exception, the best technique for decommissioning a well is to remove all well construction materials by over-drilling to create a slightly larger borehole than previously existed. The newly created borehole should then be sealed with impermeable grouting material to ground surface. During initial planning to decommission a well, over-drilling may seem impractical, too expensive, and technically unwarranted. However, the integrity of this procedure to unquestionably prevent future cross contamination of water-bearing zones may provide justification for the technique. Also, if unforseen technical difficulties are considered, which almost always occur and generate increased and unbudgeted costs, over-drilling should not be automatically dismissed as an unjustifiable technique. If well construction materials are not removed by over-drilling, there are two important considerations for adequately decommissioning a well: 1) define subsurface geology penetrated by the well; and, 2) define well construction specifications. Of these, subsurface geology is probably the most important criteria. Available geologic logs of borehole lithology and construction specifications should be thoroughly reviewed. Sources of geologic logs and construction diagrams include the California Department of Water Resources (DWR) , county and city governments, and local drilling and consulting companies. In a sedimentary environment, essentially all emphasis of a decommissioning procedure should be placed on locating and sealing aquitards to prevent cross contamination in the subsurface. Sealing permeable geologic materials should not be the focus of 2