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Mr. Philip S. Isoreo • <br />March 10, 1999 <br />Page 2 <br />level switch that actuates the pump motor and a low-level switch that turns it off. By <br />adjusting the depths of the switches and/or their vertical separation, the pump on/off <br />cycling rate and the well drawdown can be controlled. Parsons plans to measure water <br />levels periodically in the surrounding wells and construct groundwater elevation maps to <br />confirm that target drawdowns, relative to wells outside the capture zone, are being <br />achieved (i.e. that a sufficient cone of depression is being maintained). Initially, water <br />level measurements will be frequent (e.g. weekly) until the success of the system is <br />confirmed. Seasonal fluctuations in groundwater level will be compensated by manually <br />adjusting the placement of the high- and low-level switches. It is expected that higher <br />extraction rates may be required in winter in order to compensate for the recharging water <br />table. <br />RWQCB Comments on Fourth Quarter Monitoring Report <br />Response to Item 1. We agree that the trend of increasing concentration of VOCs in well <br />MW -5R should not be characterized as "within the normal range of variation". The trend <br />of increases over the previous four samplings was reversed in the most recent quarterly <br />monitoring event conducted in January 1999 when the sample from MW -5R had - a <br />concentration of 4.3 gg/L total VOCs (Parsons, report in progress). Per your request, <br />future quarterly reports will depict current chemical concentrations on a map as was done <br />in the RI reports. <br />Response to Item 2. We concur that natural attenuation screening data collected in the <br />Second Quarter and Fourth Quarter of 1998 indicate a groundwater geochemical <br />environment that is not conducive to natural biodegradation of chlorinated compounds <br />especially in the source zone wells. Limited positive evidence that was found (e.g. <br />seasonally lowjD.O. readings and trace detections of methane, ethane, and ethene in <br />certain wells such as MW -10 and MW -7) suggest that if biodegradation is occurring it is <br />probably too localized and sporadic to contribute strongly to natural attenuation. Other <br />remedial alternatives for achieving groundwater beneficial use protective levels will be <br />evaluated. <br />Proposed Program to Address Downgradient Chemical Migration <br />Parsons will address the two areas of offsite contamination discussed in the referenced <br />RWQCB letter: (1) the area south and east of well MW -5R which exhibited increasing <br />chemical concentrations in summer and fall 1998 and (2) the area offsite to the northeast <br />of the AASC property. A three -fold study and analysis approach is proposed to address <br />concern about these areas. The components of the program would be groundwater <br />HydropunchTM sampling and analyses, contaminant fate and transport modeling, <br />feasibility level cost comparisons, and evaluation of effectiveness of remedial <br />alternatives. <br />Sampling on 3235 Zephyr Court Property. MW -5R was constructed as a replacement for <br />the abandoned MW -5 which was sampled for five consecutive quarters and yielded one <br />RESP_RWQ <br />