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e ,moi e. Mr. Gordon Lee Boggs April 25, 1989 Page 2 2. The Regional Board requested soil samples be collected for laboratory analyses every 5-foot interval. ARCO agrees to do this. 3. The Regional Board requested vapor meter readings on the continuous core samples. ARCO and Brown and Caldwell do not think this is necessary for the following reasons: (a) Vapor meter data does not correlate reliably with quantified laboratory analytical data. (b) To take vapor meter data on nine 50-foot deep, continuously cored = boreholes would be extremely time-c©nsuming.-- (c) Laboratory analytical results at 5-foot intervals with the field geologists logged observations of hydrocarbkon odors and visual evidence of hydrocarbons is sufficient. 4. The Regional Board requested that slant boring depths be converted trigonometri- cally to vertical depths and that lithoiogia cross sections showing this and other information be prepared. AIthough not stated in the work plan, it is Brown and Caldwell's intention to do this. Proposed Modification to Soil Characterization 'Work. ARCO is concerned with the timely remediation of this site. The Regional Board has delayed the groundwater extraction well installation pending results of additional off-site monitoring well installation. In addition, the Regional Board's and U.S. Environmental Protection Agency's approval of ARCO's National Pollutant Discharge Elimination System (NPDES) permit may take some time. It is thought that hydrocarbons in the vadose. zone are a continuing source of hydrocarbons to the groundwater. For all these reasons, ARCO considers it important to poceed expeditiously with the soil remediation. Rather than backfilling all the soil boreholes with a cement/bentonite grout,_ ARCO y proposes to install 52-foot deep vapor extraction wells in those boreholes which appear, based on field evidence, to have intercepted zones of hydrocarbon contamination. These wells will be used to exhaust hydrocarbon vapors in a soil venting system. Construction of wells will be completed with approximately 45 feet of 3-inch-diameter, polyvinyl chloride (PVC) .020-inch slotted, flush-threaded casing. Blank casing will extend approximately 7 feet from the top of the perforated section to ground surface. A PVC cap will be installed at the bottom of each well. A capped PVC T joint will be installed at the top of each well. The wells will be packed with No. 1/20 sand from the bottom of the screen to approximately 2 feet above the top of the screen. A 2- to 3-foot thick layer of bentonite pellets will be placed on top of the sand pack. A cement seal will be placed on top of the bentonite. The wells will be completed at grade using vault boxes. Typical well construction is illustrated on Figure 1. BROWN AND CALDWELL 723 S STREET SACRAMENTO,CALIFORNIA 95814-7092•(916)444-0123