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pushed or driven to a depth below the base of the temporary drill casing in order to access undisturbed formation water. After advancement,the tool is retracted to expose a screen which allows groundwater to enter the tool but larger sediment to be blocked by the screen. Note:Sample volume requirement(for polycyclic aromatic hydrocarbon analysis)is 1 liter per sample. At one sample location volume must be double for collection of a field duplicate sample. The Contractor will be responsible for groundwater sample retrieval from the boring and in sufficient quantity,while the on-site Government geologist or engineer will fill sample bottles and package and ship samples to the lab. Laboratory results utilizing express overnight shipment of samples and 24-hour turn-around- time will dictate where additional optional borings and/or wells are located. 5.5. Task 4—Install and Develop Groundwater Monitoring Wells. Under this task the Contractor is to construct and install two 4-inch diameter groundwater monitoring wells inside nominal 8-inch diameter,temporarily cased borings. The boring depth will be up to 300 feet;therefore,the well, including all pertinent backfill materials,will be up to 300 feet as well. The on-site Government geologist or engineer logging the lithology of the boring will determine the final screen depth and backfill configuration; however,general backfill sequencing may include bentonite backfill below the base of the screen,and will include filter pack sand backfill within the screened interval of the well,a bentonite seal above the sand pack,and grout from above the bentonite seal to approximate land surface. Grout mix design shall be Type 1 Portland cement,5% dry bentonite per 94-]b sack of dry cement,and maximum 6 to 7 gallons water per sack cement. A steel flush-mount protective monument will be installed to protect the well casing from damage. Well screens shall be flush joint 4-inch nominal internal diameter,Schedule 80 PVC,with 0.010-inch screen slot size,ten(10)feet in length. No tail pipe is to be installed in the monitoring wells,just a threaded end plug at the base of the well screen. Riser pipe shall be of like Schedule 80 PVC. Final stick-up of the PVC casing/riser shall be approximately 0.5 feet below land surface. Centralizers shall be placed above the well screen and every 40 feet to the top of the riser pipe. If the depth of well screen placement is not within two feet of the bottom of the boring,bentonite backfill shall be required from the bottom of the boring to within two feet of the base of the screen. Silica sand filter pack(#20-40 mesh)shall be placed in the annulus from two feet below the base of the well screen to three feet above the screen. The well screen shall be surged after initial filter pack placement and before bentonite seal placement to ensure any settlement occurs at that time. A five-foot thick bentonite seal shall be placed above the filter pack and hydrated for one hour before placing cement grout to the top of the well casing. Well surface completion shall consist of a minimum 8-inch protective steel monument flush mount designed for water well usage,and traffic rated,around the PVC well casing/riser to be very slightly above land surface and sloped away from the flush mount to shed rainwater away from the flush mount. The lid opening of the flush mount shall have a bolt-down cover. The protective flush mount shall be seated into the cement grout annular seal. The PVC well casing,completed to be inside the flush mount,shall be equipped with a locking cap. The Government will furnish the padlocks for each well. A metal well identification tag shall be stamped with the site well ID and state identification/permit number,and shall be affixed to the top of the well casing inside the flush mount. Well development shall be considered an integral component of each well installation. The purpose of well development is to remove sediment and turbidity from the well in order for future groundwater samples collected from the well to be representative of the native aquifer formation.Wells shall be sufficiently developed until very fine-grained sediment in the filter pack and nearby formation has been removed so that ground water samples are not highly turbid,and so that silting of the well will not occur. Wells shall be developed by mechanical surging, pumping,and bailing. In addition to development after the well is completed,pre-development shall be conducted by surging inside the well screen after filter pack placement but before bentonite seal and grout placement in order to settle filter pack. Bailers may be used to remove higher volumes of sediment from the well prior to use of the pump,and as a final step in development to ensure the bottom of the well is free of sediment accumulation. The surge block will have a diameter slightly less(approximately 1/8-inch)than the inside diameter of the well casing. Wells shall be developed no sooner than 48 hours after cement grout emplacement and not later than 7 days after cement grout emplacement. Development shall continue for a minimum of four hours or until the USACE field geologist or engineer determines that the well is adequately developed for sampling purposes. Assume maximum