SITE INFORMATION AND CORRESPONDENCE

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Mr. Richard Callistro January 21, 1993 San Joaquin County .Capital Projects Page 2 3. WESTON originally proposed sampling only the five of the seven monitoring wells at the site because MW-2 and MW-3 have been dry for some time. However, recent rains may have raised groundwater elevations sufficiently that samples could be collected from wells MW-2 and MW-3. Because these wells will provide useful monitoring data (if active), these wells will be checked during each monitoring event and added to the monitoring program if they can be sampled. The potential additional labor and chemical analysis cost are noted in the cost estimate section of this proposal. 4. With regard to purge water produced during the monitoring program, there are several storage/disposal options available to the County. One option is to store the water in barrels onsite until after the monitor well chemical analyses are available, then either discharge the water to the sanitary sewer or dispose of the water in some other manner (i.e., recycling, etc.) if chemical concentrations exceed the maximum limit that can be discharged to the sewer system. Another option is to purchase a 500-700 gallon plastic tank which can be used to store the purge water from multiple sampling events. The water could then be characterized as a single volume and disposed of through the sanitary sewer or by other methods, depending on the hydrocarbon concentrations in the tank. By combining the water into a single volume, any elevated hydrocarbon concentrations produced by any individual well are likely to be diluted to within a concentration range that can be discharged to the sewer. WESTON recommends the latter approach as the most convenient and cost effective. Additionally, the water currently being stored in barrels near the site can be placed in the tank during the first sampling round. 5. Because the groundwater flow direction beneath the site has not been adequately defined, groundwater elevation data will be collected during each monitoring event and will be presented in the form of a groundwater surface contour map in each quarterly report. While preparation of these maps was not included in the original monitoring proposal, WESTON recommends the addition of this task because it is in the County's best interest to determine a consistent groundwater flow direction in order to establish the hydrocar- bon source area. At the conclusion of the previous work, this issue was unresolved and is perhaps the most important question to be answered by the quarterly monitoring program because it may indicate sources other than the Human Services property are contributing to the groundwater impacts. As noted in the above discussion., the proposed quarterly monitoring program is comprised of a base program for each event plus potential add-ons for additional well monitoring (MW 2 and MW-3) and additional chemical analysis if required by the EHD. S C ItoP A NC1 fto.WD