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Canepa's Car Wash Work Plan — Groundwater Mounding Study August 15, 2001 Page -2- t Mounding Study Condor will conduct a study to evaluate possible sources of the mounding in the vicinity of MW-2. The study will include both a hydrogeological assessment and an investigation of potential surface and subsurface sources. The hydrogeological assessment will evaluate the flow rates through the subsurface at and below the groundwater table. The hydrogeological assessment will include a review of the existing groundwater elevations in monitoring wells in the vicinity of the site. The existing groundwater elevations at nearby sites will provide data on the potential size of the mound (whether it is confined to the site or extends off- site). The goal is to determine the radius of influence of the recharge causing the mound observed at site wells. The hydrogeological assessment will also include slug testing of wells yet to be constructed as part of a different work plan. Depending on the results of CPT sampling, up to six wells, three screened at deep zones (to 120 feet bgs) and three at the current water table (35 feet bgs) will be completed. These wells will be slug tested to calculate hydraulic properties of the geologic materials at these location and depths. The data from the slug tests of the new wells will be combined with data from the 24-hour pumping test of MWA , conducted in 1994 when the water table elevation was approximately 20 feet lower, to evaluate the average hydraulic conductivity of geologic materials beneath the site. The radius of influence and average hydraulic conductivity will be used in an equilibrium flow analysis (e.g. Bouwer, Gromidwaer Hydrology, McGraw Hill, 1978: p.69) to provide an estimate of the inflow necessary to maintain the mound. The mounding investigation will include a site reconnaissance to evaluate the potential for surface infiltration. Condor will also review City of Stockton utility plans to identify any current or historic water, wastewater, or storm drain piping traversing the site. The subsurface investigation will include a review of any readily available site construction drawings indicating subsurface piping. Report Condor will prepare a report providing the results of the mounding study. The report will include an estimate of the inflow necessary to maintain the mound, and any identified potential sources. The report will also include any recommendations developed by Condor for additional investigation activities indicated such as pipeline inspections, dye studies, flow monitoring, or excavation for direct observation. Schedule Prior to implementation of this work plan, Condor will install additional monitoring wells under another scope of work. As part of that scope, all site wells will be resurveyed and depth to water measured to confirm the existence of the mound. After the completion of the wells, upon approval of this work plan by the PHS/EHD, pre-approval from the California UST Clean-up Fund, and notification from Canepa's Car Wash to proceed, Condor will conduct the study described in this work plan within four weeks. If you have any questions or require additional information, please phone us at (209) 234-0518 . Respectfully submitted, CONDOR EARTH TECHNOLOGIES, INC. Jeff Senior oject Manager P\1707 Canepa-Pacific\Cort pundcnc x -15-01 J-Won&doc *1 % • CONDOR