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• Monitoring Well Surveying - The top of casing of the new wells were surveyed to ensure a comparable basis for groundwater contouring. Groundwater was encountered in the borehole for TW-5 at a depth of approximately 8.5 feet bgs. Based on previously observed groundwater elevations at the Site, the depth to groundwater was consistent with other wells at the Site. 3.2.4.2 Groundwater Sampling Procedures All groundwater sampling activities were conducted in conformance with the Cal/EPA, Guidance Manual for Groundwater Investigations, Representative Sampling of Groundwater for Hazardous Substances (DTSC, 2006). Groundwater sampling was conducted on wells TW-1 through TW-5. Prior to removing water from the monitoring wells, an electronic water-level indicator was used to measure the static water level in each well to the nearest 0.01 feet. Additionally, the depth to the bottom of each well was measured by slowly lowering the water-level probe to the bottom and taking the measurement to the nearest 0.01 foot. A submersible bladder pump and low-flow procedures were used for purging the monitoring wells. During purging, temperature, pH, dissolved oxygen, oxidation reduction potential, conductivity, and turbidity of the groundwater were measured. Purging continued until measured groundwater parameters were within 10 percent between readings collected three minutes apart, thus indicating that water in the well was fresh aquifer groundwater. All purged water was placed into DOT-approved 55-gallon drums, which were left on-site for later management off-site. After purging requirements were met, the pump discharge was reduced to a minimal flow rate and samples were collected. The water was carefully poured down the inner walls of the laboratory supplied sample containers to minimize aeration of the sample. The project laboratory added any necessary chemical preservatives to sample containers prior to shipment. Sample containers for VOC analysis were completely filled so that no air space remained in the container. Samples were labeled and completed with appropriate information, such as date and time of collection, project name and number, sample number, sampler's name, and analyses requested. The sample containers were placed inside padded plastic bags and stored in a cooler with ice for preservation at a maximum 4°C prior to shipment to the analytical laboratory. A completed COC form accompanied the samples to the laboratory. All personnel that handled the groundwater samples wore nitrile gloves. Trip blanks (supplied by the laboratory) accompanied each cooler shipment, and equipment blanks were collected at the rate of one per day. Information generated during sampling activities was recorded on groundwater sampling logs, which are presented in Appendix I1. It should be noted that, for groundwater samples collected during the SSI (i.e., follow-up samples), the pump used to purge and sample the groundwater from monitoring well TW-5 was placed in the upper three feet of the water column as requested by DTSC. Results for these samples are discussed in Section 4.3. PARSONS 26 FINAL RI REPORT-FORMER TRACY MGP APRIL 2010