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Table 3. Mean Chloroform Concentration in Four Monitoring Wells Before and After Bioremediation Activities Before Bioremediation: April `90 - Sept. `93 After Bioremediation: Sept. `93 - present Well Number of Samples Mean CI IC13 Number of samples Mean ClIC13 Number _ __u I --- u MW-2 14 102 6 152 Mw- 14 1071 24 599 4A MW-6 10 83 6 173 —MW-7 5 10 6 — --- 18 2.6 Chloroform Plume Distribution/Dissipation The gradient calculations in Section 2.2 indicate a change in regime north of MW-6, where gradients decrease by an order of magnitude and azimuths fluctuate over 107 degrees. This points to MW-6 as a crucial position in the subsurface flow field where groundwater flowing from the plume area spills out into a flat basin. In the South, groundwater flows rapidly northwestward towards MW-6 in gravel layers-of high hydraulic conductivity. North of MW-6 groundwater tends to slow significantly and disperse laterally. The chloroform plume was interpreted from chemical data in a report to the Regional Board in 1992. A copy of Figure 4 from that report is included in Appendix C. The heart of the plume exceeded 1000 ug/1 chloroform in the vicinity of MW-4A. Chemical sampling of chloroform in groundwater has been performed quarterly since April 1990. Additional samples were taken during certain phases of bioremediation. -I1re data for each well are charted and presented in Appendix D. The most recent sampling data for chloroform are displayed at each well location on Figure 2. Recent chemical data was contoured based on a chemical gradient interpreted from the shape of the plume concentration through time at MW-2 and MW-6 (Appendix D), in conjunction with the calculated linear velocity from Section 2.3. It was assumed that the 100 ug/1 isopleth arrived at MW-6 in February of 1992; this is conservative because levels returned to background for more than a year after that time. At a linear velocity north of MW-6 of 4.7 ft/yr the 100 ug/l isopleth should have migrated approximately 20 feet past MW-6 by 1995. Assuming an uncertainty factor of 20 in the calculation due to aquifer heterogeneities, the 100 ug/I isopleth may be within approximately 400 feet of MW-6 to the north. The 50 ug/1 isopleth appears to be advancing about a year ahead of the 100 ug/I isopleth at MW-2, and six months ahead of the 100 ug/I isopleth at MW-6. 11uese travel patterns are consistent with the observed flattening of the gradient north of MW-6. The 50 ug/I isopletlu arrives at MW-6 first and is slowed while the 100 ug/1 isopleth behind it is still traveling at the faster linear velocity consistent with the steeper gradient to the south of MW-6. In this way it tends to catch up. The 50 ug/1 isopleth would `� 7 CONDOR 1