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:I Work Plan Frontier Transportation ., Page 3 First subsurface water was encountered at depths from J1' to 12' bg during tank removal -, and monitoring well installation at this site in 1995 and 1996 . Since then, shallow subsurface Jwater has been encountered between 6' and 1 �`bg. The flow of groundwater is generally in a western direction beneath the site (See Figure 3 , Groundwater Gradient Map). 4.0) PROPOSED SCOPE OF WORK ' The extant of the MTBE contaminant plume is not clearly defined to the west of the shop based on current data so BEII proposes the installation of 8 additional wells five (5) shallow and three (3 ) deep monitoring wells (See Figure 4 for proposed well location) . BEII feels that the i analytical data from the soil and groundwater samples at the new wells will be invaluable in defining the boundary of the contaminant plume. 5.0) GROUNDWATER WELL INSTALLATION _ A hollow stem auger drill rig will be used to provide the soil borings for well installation. 1 Drillers will hold a California State C-57 contractor's license. Typical auger size for the well installation described below is 10-inch outside diameter and 6-inch inside diameter. The borings for shallow well installation will be advanced to 20 ' bg and for the deep wells to a depth of 40 ' �1 bg. Each well will basically consist of a central section composed of riser and screen. The riser and screen will be constructed of four-inch inside diameter, schedule 40 polyvinyl chloride (PVC) pipe. The proposed shallow wells will consist of slotted PVC extend from approximately 20 ' bg to 5 ' bg and it will contain three rows of 0.010-inch slots . The annular space around the i_ d screen will be filled with sand from 20 ' bg to 4.5' bg to form a filter pack. This will be overlain with a 3 ' seal of bentonite pellets, then grouted to near ground surface with a 5% bentonite mixture with cement and water. The proposed deep wells will consiso lttePVC extend from approximately 40 ' bg to 30 ' bg and it will contain three rows of 0. 1-tuic slsofs. The /off annular space around the screen will be filled with sand from 40 ' bg to 28 ' bg to forma filter I pack. This will be overlain with a 5' seal of bentonite pellets, then grouted to near ground surface J with a 5% bentonite mixture with cement and water. Ends of the screen and riser sections will be _ constructed with flush joining threaded fittings to ensure leak-proof unions. Vwt ii -zgl0' A filter pack of sand will be placed around the screen to retain the finer grained fractions of formation material and to create a zone of high permeability that allows free entry of water in J the well. The pack will consist of clean sand (#2/l The bentonite and annular seals are necessary to prevent the annulus from serving as a vertical conduit for potential contaminants. i Following placement of the annular seal, a large-diameter flush-mount protective casing will be installed around the well heads . The casing will be firmly seated in concrete and supplied with L I