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1 <br /> Southland/Stockton 10 <br /> March 1991 <br /> 1 <br /> feet below grade One sample, collected from about 40-feet below grade in well 17A was reported to <br />' contain 100 parts-per-million (ppm) total petroleum hydrocarbons-as-gasoline The soil analyses results <br /> from the boring for MW-17A suggest that the soil-vent system has been effective at remediating the soils <br />' above and below the depth of 40 feet The presence of concentrations of TPH in the sample collected <br /> from the 40-feet depth suggests that there may be some lithologic differences such as higher clay <br />' content at that depth and location The sod vent system was operated for another twelve months after <br /> the soil samples were collected from the boring for well 17A, and subsequent reductions in the <br /> concentrations of TPH have likely occurred in that interim <br /> PROPOSED CLOSURE APPROACH <br />' Based on the observed reductions in concentrations of TPH inroundwater and soil, <br /> , <br />' Groundwater Technology is proposing a closure program which uses a decision tree approach by which <br /> site activities can be reduced and eventually terminated, while ensuring that there is protection of human <br />'. health and the environment The decision tree approach will require Implementation of work steps and <br /> the definition of "trigger" points at which either, (1) further reductions in sampling and monitoring <br /> frequency and/or volume occur, (2) the sampling and monitoring program is confirmed to be adequate <br /> for another period of time, (3) reactivation of the soil venting and/or hydraulic control systems occurs, or <br /> (4) site activities cease <br />' The work steps associated with the proposed site closure include the following <br /> • Removal of the existing underground storage tanks and associated piping <br /> f • The installation of three monitoring wells to confirm water quality data <br /> • The collection of sod samples on-site to confirm the effectiveness of the soil-vent system <br /> in removing soil contamination <br /> • Sampling andmonitoring of monitoring wells and domestic wells at a frequency <br /> y <br /> Iappropriate for evaluating site conditions <br /> I <br /> C� GROUNDWATER <br /> TECHNOLOGY, INC <br />