SITE INFORMATION AND CORRESPONDENCE_1987-1992

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Antonia K. J. Vorster 6 December 18, 1989 In addition, it is uncertain to what degree the San Joaquin County distance limitations referred to in the revised workplan (pertaining to the minimal distance of a well to a sewer line, and the depth of a well 's sanitary seal ) has influenced the consultant's shallow zone ground water monitoring well design. These limitations are not necessarily applicable to a ground water monitoring well . The location of a monitoring well should be based upon the proximity to a source and the ground water flow direction. Since, in this investigation, the source is likely a sewer line it is imperative that samples of ground water be taken as near the source as possible. Also, the screened interval for wells in the uppermost water bearing formation should be consistent. Monitoring well #4 is the only existing monitoring well that is screened below 50 feet and it does not bridge the water table surface. Monitoring wells #1, #2, and #3 are all screened to bridge the water table. The location and screened interval depths for the new shallow zone wells should be reconsidered based on definable objectives of the investigation (i .e. , source monitoring and consistency) and not on the county's distance restrictions. The rationale for the monitoring well placements and their screened interval depths should be submitted to us for review. Therefore, the approval of the shallow zone monitoring well locations and design are withheld until we can review the water level data requested in Comment 1 above, and until we can review the well placement and screening rationale requested above. 4) The vertical extent of contamination in the vicinity of the LVMD wells must be determined. Well clusters will be required to determine the extent of the contamination, both areally and vertically, and to determine the magnitude and direction of the vertical hydraulic gradients. Depending upon the lithology of the site (see Comment 5 below) , it may be necessary to install wells between the uppermost water bearing zone and the zone intercepted by the LVMD wells. The rationale for the well clusters' placements and their screened interval depths should be submitted to us for review. 5) The lithological logging of the monitoring wells will be crucial to the understanding of the site's geology, to establishing the hydraulic connection between the uppermost water bearing zone and the deeper aquifer, and to the design of the well clusters, specifically their screened interval depths. Therefore, I recommend that for hollow stem auger drilling that soil samples be taken every five feet or at a noticeable change in the formation (based upon cuttings and/or drilling progress); for mud rotary drilling that geophysical logs be taken including long and short normal electrical resistivity, spontaneous potential , and caliper; and for air percussion/casing hammer drilling that logging be accomplished by the continuous monitoring of the cuttings from the cyclone. The revised workplan should contain the proposed drilling and lithological logging techniques.