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f designed to address groundwater containing nitrate (NO,), radium 226, total uranium, and chloride in aquifers at depths of over 200 feet below grade (California Regional Water Quality Control Board, Central Valley Region, 1983). 1.5 Regional Geology and Hydrogeology The site is situated in the Central Valley,which comprises the lowlands between the Sierra Nevada to the east and the coastal range to the west. As a structural trough, the Central Valley received sediment from the adjacent mountains under both marine and continental conditions. Continual uplift during the Pleistocene resulted in formation of the Sierra Mountains and accelerated deposition of silt,sand,and gravel in alluvial fans along the valley margins. The topography of the site vicinity slopes gently to the west toward the San Joaquin River, which is 1.5 miles to the west of the site (Page,Ben M., 1966)_ 2.0 FIELD METHODS The procedures and methods used during field activities are described in the following sections. 2.1 Drilling and Soil Sampling Before drilling, well installation permits were acquired from the SJCEHD, and encroachment permits were acquired from the California Department of Transportation and the City of -{' Lathrop. Copies of the permits are presented in Appendix B. On August 12, 1998 Borings MW-8 through MW-12 were drilled at the site. Drilling was performed by V &W Drilling,Rio Vista, California, using a Longyear B-81 drill rig equipped with 8-inch-diameter,hollow-stem augers under contract to Alisto. The borings were drilled to depths ranging from 15 to 16.5 feet_ Drilling and soil sampling procedures are presented in Appendix C. Soil samples were described in accordance with the Unified Soils Classification System, including color, moisture, and consistency. Additionally, each soil sample collected was field screened with a Thermo Model 580B photo-ionizing detector,which was calibrated on a daily basis. The boring logs,which include field screening measurement, are presented in Appendix D. 2.2 Monitoring Well Installation and Construction On August 12, 1998, Soil Borings MW-8 through MW-12 were completed as Monitoring Wells MW-8 through MW-12 in accordance with the procedures for groundwater monitoring well installation presented in Appendix C. Monitoring Wells MW-8 through MW-12 were constructed using 2-inch-diameter, flush-threaded, Schedule 40, polyvinyl chloride blank casing and 0.010-inch slotted casing. The slotted casing was generally installed from 4 to 13 feet below grade. Well construction details are included on the boring logs presented in Appendix D. After analysis of the composited soil sample, the drill cuttings were transported to a designated disposal facility. 4