1993-11-24_REVISION - M1988112 (2)

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' As the dual-wall drill string cases off the formation, the wells were completed by emplacing 4- ' inch, inside diameter, Schedule 40 PVC casing and 10-slot screen inside the 6-inch opening of the center string. The drill string was pulled up concurrently as the annular space between the PVC casing and borehole was backfilled with select ]0-20 mesh Colorado silica sand. This technique assured that the wells were completed in the designated intervals and to total depth, ' without the concern of formation caving or sloughing into the borehole. The sGleci sand pack was brought up to a level, above the uppermost screen, where fast water was encountered. In ' this way, the entire uppermost saturated interval of the Santa Fe Formation is contributing flow into the wellbore. A bentonite pellet seal was then emplaced above the select sand pack and the remainder of the annular space was filled with bentonite grout. Bentonite grout was chosen over cement grout due to the initially high pH readings of the water samples in well M-13. Steel ' protective casing and locking cap enclosed the PVC casing at the surface and a concrete pad, sloping away from the well, was poured around the protective steel cover. The construction ' details for wells M-13R and M-14 are presented in Figures 2 and 3, respectively, For reference and completeness, the well completion details for all the monitoring wells shown in Figure 1 are ' listed in Table 1. Initially, the replacement well for M-13 was drilled at a location approximately 37 feet west of existing well M-13. The borehole was advanced to a total depth (TD) of 177 feet, Based on ' the static water level in adjacent well M-l3, first water was anticipated in the vicinity of 140 feet. During drilling, potential formation water entering the borehole was monitored at the ]55 and 165 levels. No water had entered the borehole at either Level. Throughout the anticipated saturated section, the return sample cuttings were essentially dry with the ezcepiion of a 2 feet ' interval at approximately 160 feet. After reaching the total depth, the drill string was extracted in approximate 10 feet intervals to allow formation water to enter the borehole. The lowermost ' two intervals, 162 ft. to TD and 153 ft. to TD were dry. The casing was then extracted to 146 feet and TD was measured to be 171 feet below ground level, indicating that the lowermost 6 ' feet of the borehole had caved in. This 25 feet interval of open borehole produced water at the ' rate of 0.4 gallons per minute (gpm). Due to this low recovery rate and its ramifications on adequate ground water monitoring, it was decided to abandon this borehole and redrill the ' -4- I~