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• � II City of Stockton TL 394-0074-01 May 4, 1994 Page 12 Groundwater elevations will be calculated by subtracting measured water depths from surveyed casing rim (measuring point) elevations. The direction of groundwater flow and gradient will be calculated using the groundwater elevation data. A-5 Decontamination. Decontamination procedures will be implemented to reduce the potential for cross-contamination between soil borings. Before, during, and following drilling operations, the augers and associated drilling and sampling equipment will be thoroughly cleaned using a high pressure hot water (steam) washer. The hot water washer has a pressure of approximately 1,200 pounds per square inch and a water temperature of approximately 100 degrees Centigrade (°C). The sampler and brass sleeves will be washed in a AlconoxO detergent solution, rinsed with clean tap water, and air-dried prior to initial use and between each sampling event. The augers will be steam cleaned prior to the drilling of each soil boring. Prior to construction of monitoring wells, all casings will be cleaned in the same manner as l the drilling equipment. A-6 Sample Handling and Chain-of-Custody. Records will be developed for soil and groundwater samples including: sampling date, sample type, location, depth, job number, name of sampling personnel, and method of preservation. Each sample container will be labelled immediately following collection in the field. Chain-of-custody protocol, described in United States Environmental Protection Agency (U.S. EPA), 1986, Test Methods for Evaluating Solid Waste, SW-846, 3rd edition, will be followed. Samples will be placed in an ice chest and maintained at approximately 4°C until they are delivered to the laboratory. The temperature within the ice chest will be monitored and recorded during the course of transport and delivery. Samples will be accompanied by proper chain-of-custody documentation. Upon arrival at the laboratory, the samples will be preserved for analysis in accordance with U.S. EPA and state of California requirements. Samples will be delivered to Twining's chemistry laboratory in Fresno, California. Analytical methods with corresponding method detection limits and copies of the chain-of-custody records will be provided in the technical report. A-7 Laboratory Quality Assurance/Quality Control (OA/OC). This section describes the laboratory QA/QC procedures which will be followed during the analyses of the soil and groundwater samples by Twining's analytical chemistry laboratory. The QA/QC procedures will be developed to minimize outside interferences during analyses for each sample. The laboratory manager will be in charge of creating and maintaining the program. In general, QA/QC proposed for this investigation are summarized as follows: