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Work Plan for Additional Ac_c_es=_ment S E C O R ARCO Service Station 2130 Stockton, California Page 5 o 15 the vertical extent of the hydrocarbon plume one deep well is proposed to be screened from 90 to 100 feet bgs (MW-20) . Locations of proposed wells may require modification due to field conditions (obstructions) . It is also possible that the locations of some wells may be moved for space constraints and/or to accommodate drilling equipment. Each boring will be advanced using a hollow-stem auger drill rig equipped with 8- inch diameter continuous flight augers. During advancement, soil samples for chemical analyses will be collected at five-foot intervals using a California-modified split-spoon sampler driven approximately 18 inches using a 1406pound hammer falling 30 inches (blow count) . Soil contained in the sampler will also be classified according to the Unified Soil Classification System (USCS) by a scientist under the direction of a State of California Professional Geologist (PG) or Professional Engineer (PE) . Soil will be screened in the field for the presence of volatile organic compounds (VOCs) using a photoionization detector (PID) . Soil classifications, PID measurements, sample intervals and other pertinent geologic information will be recorded on boring logs. All down- hole drilling equipment is steam-cleaned following the completion of each soil boring. Down -hole sampling equipment is washed in an Alconox solution between samples. At the terminal depth of each boring , a groundwater monitoring well will be installed within the annulus of the auger. Each well will be constructed using 2- inch diameter, flush- threaded, schedule 40 polyvinyl chloride (PVC) well casing with approximately 20 feet of 0. 020-inch factory- slotted well screen . The new wells will be constructed as follows: • MW- 14 through MW- 17 will be screened from 30 to 50 feet bgs, • MW- 18 and MW- 19 will be screened from 60 to 75 feet bgs, and • MW-20 will be screened from 90 to 100 feet bgs. A filter pack consisting of sand will be placed in the annular space surrounding the entire screened interval, and will extend approximately one foot above the top of the screen interval. A bentonite pellet annular seal will be placed above the filter pack, and each well will be completed with cement and/or grout from the top of the seal to the ground surface. The well will be secured with a locking well cap, and completed with a watertight traffic- rated well box installed at grade . Task 3 — Laboratory Analyses Soil samples collected will be selected for chemical analyses based on visual observations and PID measurements . A minimum of two soil samples will be analyzed from each boring. Soil samples selected for chemical analyses will be contained in 2%diameter by 6" -long brass or stainless steel sleeves, capped at each end with Teflon® sheets and plastic end caps, and placed on ice in an insulated cooler until relinquished under chain-of- custody to a certified analytical laboratory. Selected soil samples will be analyzed for the presence of TPHd (C9-C24) according to Environmental Protection Agency (EPA) Method 8015 ; and for TPHg (GRO C&C12) , BTEX, fuel oxygenates (including MtBE) , and 1 ,2- DCA according to EPA Method 82608. In addition , one soil sample from the capillary and one grab groundwater sample will be collected from borings MW-14, MW- 15, and MW- 16 and sent to the laboratory for oxygen treatment analysis. Once at the laboratory the samples will be prepared for treatment by mixing the required mass of soil and groundwater and adding the appropriate mass of MtBE gasoline to each sample. The samples will be homogenized to provide adequate mixing. The groundwater samples will be spiked with MtBE gasoline to ensure that adequate BTEX, TPH , and MtBE mass are present to observe the desired reactions. A description of the oxygen treatment analysis is included in Attachment B.