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contaminated by groundwater originating from the 25-50 region of the SB 1 borehole due <br /> to failure of the hydropunch equipment to collect a discrete groundwater sample. <br /> If the three hydropunch samples had produced negative results, it could be reasonably <br /> inferred, without further testing, that vertical migration of groundwater contaminants had <br /> not extended below 50 feet. In this case, the positive results from the hydropunch <br /> samples will require the installation of groundwater monitor wells that are screened <br /> below 50 feet to confirm the hydropunch results. These monitor wells should be <br /> completed so as to collect discrete water samples from the aquifers that were encountered <br /> below 50 feet in SB 1. There are three sandy intervals (potential aquifers) between 50 and <br /> 80 feet bgs that are not currently monitored by the existing groundwater monitoring <br /> wells: 52.5-56 feet, 60-64 feet, and 78-80 feet. A new monitor well should be completed <br /> to collect water samples from the lowermost aquifer that begins at 78 feet bgs. During <br /> the drilling of the monitor well boring, soil samples should be collected continuously <br /> below 78 feet bgs to determine the base of the aquifer. The well should be bottomed in <br /> the next aquitard that is encountered and slotted casing installed between 75 feet and the <br /> base of the well. If repeated sampling of this monitor well shows the groundwater at this <br /> depth to be contaminated, it could then be assumed that the positive results from the <br /> hydropunch samples collected at 60-62 feet bgs and 70-72 feet bgs also represent actual <br /> vertical migration of the contaminants. If repeated sampling of the new well shows no <br /> contamination, it may be necessary to complete another monitor well in the next aquifer <br /> (60-64 feet bgs) to confirm the extent of vertical contaminant migration. This <br /> information can be used to determine the total depth of future monitor wells necessary to <br /> assess the offsite migration of the contaminant plume. <br /> RECOMMENDATIONS <br /> • Install monitor well MW4, as specified in the September 28, 2000 workplan, to the <br /> first aquitard below 80 feet bgs in the immediate vicinity of SB I and complete with <br /> slotted casing interval between 75 feet and the base of the boring. <br /> • Collect groundwater samples from MW4 for at least two quarters along with <br /> groundwater samples from existing wells and analyze for TPH-G, BTEX, and MTBE <br /> to confirm or disprove the existence of groundwater contamination at depths below <br /> 50 feet. <br /> • Proceed with the installation of offsite groundwater monitoring wells as specified in <br /> the September 28, 2000 workplan with depths and zones of completion based on the <br /> results from MW4. <br /> ARC SRPT 3/2/01 6 <br />