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i Ms. Laurie Cotulla July 25, 1989 Page 8 Brown and Caldwell recommended quarterly monitoring and sampling. of the groundwater at the site. The purpose of that`monitoring and sampling was to identify whether the plume moves off site, and to document the natural:degradation of the dissolved constituents. The i quarterly monitoring and sampling was authorized by ARCO in Task Order 2093-3, dated December 10, 1987. CURRENT ACTIVITY The scope of Brown- and Caldwell's April 1987 site investigation was to determine the presence or absence of hydrocarbons in the soil and groundwater. The investigation did not define the extent of hydrocarbons in soil or groundwater at this site. - The 'RWQCB directed i ARCO to define the extent of hydrocarbons by use of exploratory borings -with additional monitoring wells .installed. Brown and Caldwell was authorized by ARCO on Task Order Number 2093-88-2A, dated September 21,- 1988,'to. conduct an additional site investigation which included the installation of three additional on'-site groundwater monitoring wells. This investigation was limited to the ARCO 2093 property. Brown and Caldwell submitted a work plan to the RWQCB and the County for this additional work at the site on ,October 10, 1988 (included as Appendix A in this report). On February 28, 1989, the RWQCB approved Brown and Caldwell's October 10, 1988, work plan and authorized the County to issue drilling permits. Field Methods This section describes the field methods for, the current .Brown and Caldwell investigation. ` During the week of March S, 1989, Brown and Caldwell supervised the drilling of three boreholes with the installation of monitoring wells in each of the boreholes. Figure 2 shows the locations of .the four previously installed and three new monitoring wells. Soil. The boreholes were drilled by PC Exploration, Inc. (PC Exploration), with a truck- mounted drilling rig.equipped with 6-inch-inside-diameter and 12-inch-outside-diameter hollow- stem augers. PC Exploration has a C-57 contractor's license and has insurance information L on file with the County. i The boreholes were advanced to depths of approximately 30 feet below ground level or 15,to 20 feet`below the groundwater surface. Soil samples were collected continuously from ground surface to the groundwater surface :for lithologic identification using a 24-inch-long by 2- inch-diameter California Modified;-split-spoon drive sampler (drive sampler. The drive sampler was driven ahead of the auger into undisturbed soil to obtain.all soil samples. One soil I; sample was selected from each borehole for laboratory analysis. The drive.sampler was lined with four 6-inch-long brass tubes when the sample to be analyzed was to be collected. The 4 sample tubes were then removed from the drive sampler, the ends visually inspected in the field to identify lithology; and capped with aluminum foil, plastic caps, and duct tape t provide i an airtight seal. The lithology,was recorded on fieldlogs which are included as Appendix B BROWN AND CALDWELL. ' L 723 S STREET SACRAMENTO,CALIFORNIA 95814-7092•`(916)444-0123