SITE INFORMATION AND CORRESPONDENCE_1997

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tevine4ficke-fle(gIn ENGINEERS.HYMOGEOLOCISTS A AppLIEdE Based on these data, we may decide to construct a multiple-completion well cluster in the borehole to facilitate monitoring of each of the major permeable layers in the B-zone. At a minimum, we are Proposing to construct new monitoring well which would screen the lowest permeable zone within the B-zone aquifer. The final evaluation of the integrity and appropriateness of MW-21 would be performed after completion of this well installation. Based upon LFR's success in collecting groundwater samples from the B-zone aquifer during the Plume perimeter pilot boring and monitoring discrete depth groundwater samples in well installation program, LFR recommends that the vicinity of MW-21 be obtained from a mud-rotary pilot boring using a PIPP sampler. LFR does not plan to use cone penetrometer testing (CPT)/PIPP technology, as recommended by LPL in its March 31, 1997 letter, for the following reasons: • While CPT/PIPP sampling using a direct-push CPT rig may be capable of reaching the upper water-yielding unit in the B-zone aquifer(approximately 90 to 95 feet bgs in the MW-21 boring), neither LFR nor the CPT subcontractor, Gregg In Situ, Inc., believe that a CPT rig will be capable of reaching the bottom water-yielding unit in the B-zone aquifer (approximately 135 to 138 feet bgs in the MW-21 boring). • MW-21 is located in an area where A-zone groundwater contains high concentrations (exceeding 2,500 micrograms per liter) of tetrachloroethylene. The ability to completely grout the CPT/PIPP borings to 140 feet (the total depth of the borings) is uncertain. This uncertainty exists because the CPT rods must fust be completely removed from the hole and a set of grouting rods must be pushed down the same hole. The resulting open hole could potentially collapse or permit cross contamination between the A- and B-zone aquifers. • Mud rotary drilling combined with the installation of a permanent conductor casing through the overlaying A-zone is considered the optimum method to minimize the potential for cross contamination between the A- and B-zones. Conductor casing would facilitate installation of a replacement monitoring well, if necessary. The proposed field program to evaluate the groundwater quality distribution in the vicinity of MW-21 is designed to minimize the disruption to the operations of Lincoln Center. The proposed program would also minimize the quantity of waste generated, reducing the costs associated with disposal of contaminated drilling mud. To meet these goals, a single conductor casing would be installed for the boring through the A zone so that a range of monitoring well completions could be constructed in the B zone. Constructing either a single well or, if appropriate, multiple completion B-zone wells in this manner would greatly reduce the time typically required to install multiple wells in separate boreholes. Our recommended method for drilling the B-zone boring and collecting PIPP groundwater samples, well construction methods, a proposed sampling schedule, and a traffic control plan for the work are presented below. Background and data evaluation discussions related to this investigation were presented in LFR's March 18, 1997 letter. LCM W21-3.LTR:SRL 2