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therefore be approximately 50-100 feet north of the 100 ug/I isopleth. Alt interpretation of the plume shape is presented in Figure 2 based on these considerations. One important observation based on comparison of the plume geometry in 1992 and 1995 is that , although the area within the 50 ug/I isopleth is larger, the maximum concentrations are much less. Both the 500 and 1000 ug/1 isopleths are gone by 1995. This dissipation is attributed to pumping- aeration, and/or degradation (natural or bioremediation). 2.7 Extent of the Chloroform Plume Triple C wishes to delay any characterization work of the extent of the chloroform plume north of monitoring well MW-7. Tlie chloroform concentration detected in MW-7 is still within the range of chloroform from other potential sources in the area. 1. Swimming pool water sampled for the Final Report of the AB-1803 Follow-up Program dated December, 1987 (excerpts from that report are included in Appendix E) indicate that chloroform content in swimming pool water across the street from MW-7 was 15.9 ppb (sample No. 611 MM-PP). 2. As of 1987 there are no fewer than eight swimming pools in the Fairoaks area within 2,000 feet of MW-7. In addition, groundwater extraction efforts proposed in this work plan are designed to capture any flow away from MW-7 which may have occurred since the observed elevated samples have been reported. Design modeling of the pumping for containment of the plume described below predicts a now stagnation point will occur several hundred feet north of MW-7. 3. Groundwater pumping for containment of plume Alternatives for containment of the plume by groundwater pumping include a well or wells capable of intercepting the plume and extracting chloroform-containing groundwater. Alternatives include: • pumping from existing wells, • constructing a new well near MW-6 designed for extraction, and • constructing additional pumping wells elsewhere. "Triple E proposes to combine pumping from a new well with pumping from existing wells. Triple E will continue pumping from well MW-4A at approximately 5 gpm and will initiate pumping from a new well in the vicinity of MW-6 at a rate of approximately 20 gpm. Zile approximate capture zone from pumping in this configuration is shown on Figure 2. Tile capture zone was estimated using computer modeling described below. Sustained pumping at a rate of 25 gpm exceeds by a factor of 19 the calculated flow discharge for the aquifer draining from the plume south of MW-6 (1.3 gpm). This fact, coupled with the knowledge that the gradient is significantly flatter to the north of MW-6 suggests that sustained pumping from MW-6 at 25 gpm would contain groundwater flowing from the source area. L� 8 CONDOR