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GR Field Methods and Procedures C. presence or absence of discoloration or staining f. presence or absence of obvious gasoline hydrocarbon odors g. presence or absence of organic vapors detected by headspace analysis Field Screening of Soil Samples A PID is used to perform head -space analysis in the field for the presence of organic vapors from the soil sample. This test procedure involves removing some soil from one of the sample tubes not retained for chemical analysis and immediately covering the end of the tube with a plastic cap. The PID probe is inserted into the headspace inside the tube through a hole in the plastic cap. Iead-space screening results are recorded on the boring log. Head -space screening procedures are performed and results recorded as reconnaissance data. GR does not consider field screening techniques to be verification of the presence or absence of hydrocarbons. Construction of Monitoring Wells Monitoring wells are constructed in the exploratory soil borings with Schedule 40 polyvinyl chloride (PVC) casing. All joints are thread joined; no glues, cements, or solvents are used in well construction. The screened interval is constructed of machine -slotted PVC well screen which generally extends from the total well depth to a point above the groundwater. An appropriately -sized sorted sand is placed in the annular space adjacent to the entire screened interval. A bentonite transition seal is placed in the annular space above the sand, and the remaining annular space is sealed with neat cement or cement grout. Wellheads are protected with water-resistant traffic -rated vault boxes placed flush with the ground surface. The top of the well casing is sealed with a locking waterproof cap. A lock is placed on the well cap to prevent vandalism and unintentional introduction of materials into the well. Measurement of Water Levels The top of the newly -installed well casing is surveyed by a California -licensed Land Surveyor to mean sea level (MSL). Depth -to -groundwater in the well is measured from the top of the well casing with an electronic water -level indicator. Depth -to -groundwater is measured to the nearest 0.01 -foot, and referenced to MSL. A-2