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screening and headspace vapor screening are more sensitive methods that have been effective in detecting contamination at concentrations less than regulatory cleanup guidelines. Sheen screening involves placing soil in a pan of water and observing the water surface for signs of sheen. Sheen classifications are as follows: ■ No Sheen (NS): No visible sheen on water surface. ■ Slight Sheen (SS): Light, colorless, dull sheen; spread is irregular, not rapid; sheen dissipates rapidly. ■ Moderate Sheen (MS): Light to heavy sheen, may have some color/iridescence; spread is irregular to flowing;few remaining areas of no sheen on water surface. ■ Heavy Sheen (HS): Heavy sheen with color/iridescence;spread is rapid;entire water surface may be covered with sheen. Headspace vapor screening involves placing a soil sample in a plastic sample bag. Air is captured in the bag and the bag is shaken to expose the soil to the air trapped in the bag.The probe of a PID is inserted in the bag and the instrument measures the concentration of combustible vapor in the air removed from the sample headspace. The PID measures concentrations in ppm (parts per million) and is calibrated to isobutylene. The PID is designed to quantify combustible gas and organic vapor concentrations up to 2,500 ppm.A lower threshold of significance of 1 ppm was used in this application. Field screening results are site-specific and vary with soil type, soil moisture content,temperature and type of contaminant. Groundwater Monitoring Well Sampling Groundwater samples were obtained from the monitoring wells for compliance monitoring purposes. Groundwater samples were collected by low-flow methods using new disposable tubing and a peristaltic or submersible pump. A portion of each groundwater sample collected was field filtered using a single-use 0.45 micron in-line filter prior to collection in laboratory provided container. Containers with field filtered samples were labeled as field filtered. Each groundwater sample obtained duringthe investigation was identified by a unique sample designation. The sample designation was documented in the field report and included on the sample container label and laboratory chain-of-custody. The soil sample designation scheme is as follows: ■ Monitoring well number MW-1 etc., followed by the date of collection using two-digit month, day and year. For example, MW-1-070920 is from monitoring well MW-1 and sampled on July 9, 2020. Groundwater samples were placed in a cooler with ice for transport to the laboratory within proper hold- times. Standard chain-of-custody procedures were followed in transporting the samples to the laboratory. Copies of the laboratory reports are provided in Appendix C. GEoENGINEER� January 24,2022 Page B-2 File No.0661-108-03