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i <br /> rSTATE OF CALIFORNIA GEORGE DEUKMEJIAN,Governor. <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD— <br /> CENTRAL VALLEY REGION <br /> 3443 ROUTIER ROAD <br /> SACRAMENTO,CA 95827-3098 <br /> 17 May 1989 <br /> Ms. Diane Hinson <br /> San Joaquin Local Health District <br /> P.O. Box 2009 <br /> Stockton, CA 95201 <br /> COMMENTS FROM REVIEWS OF THE OCTOBER 1988 AND FEBRUARY 1989 PROJECT UPDATE <br /> REPORTS AND APRIL 1989 WORK PLAN FOR 7-ELEVEN STORE AT 9110 THORNTON ROAD, <br /> STOCKTON, SAN JOAQUIN COUNTY <br /> I have reviewed the October 1988 and February 1989 Project Update Reports and <br /> I agree there are major problems at this site. I have the following general <br /> comments: <br /> 1. The leaking water pipe behind the store should be fixed as soon as <br /> possible. The ground water mound caused by this pipe is causing clean <br /> water to infiltrate down to the contaminated water allowing for dilution <br /> and changes in the ground water gradient beneath the site. <br /> 2. The ground water plume needs to be redefined. The ground water and <br /> concentration contour maps presented in these reports are confusing and <br /> in cases optimistic. The only thing clearly illustrated is that <br /> Groundwater Technology, Incorporated after three years still has not <br /> defined the problem. The plume is shown to be very dynamic by <br /> contracting and expanding hundreds of feet between monthly sampling <br /> events. The "zero line" needs to be determined so a remedial system can <br /> be designed which will completely capture the plume. <br /> 3. Hydraulic control of the contaminant plume is doubtful with the current <br /> system. Pumping wells PW-1 and RW-1 do not appear to be creating large <br /> enough cones of depression to capture the entire plume. In addition, <br /> the leaking water pipe has caused the ground water elevation in PW-1 to <br /> be higher than in monitoring well MW-11 . PW-1 is not functioning <br /> adequately as a recovery well . <br /> 4. Once the plume has been delineated, additional recovery wells may be <br /> needed to cause sufficient cones of depression to capture the entire <br /> plume. <br /> 5. The vertical distribution of hydrocarbons needs to be investigated. <br /> Frequently, samples from MW-1 show high levels of contaminants while <br /> recovery well RW-1, 10 to 15 feet downgradient, has non-detectable <br /> levels. MW-1 is screened to a depth of 45 feet and draws from the upper <br /> I to 2 feet of the water table. RW-1 is screened to a depth of 57 feet, <br />