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Mr. Dan Johnson May 31, 2007 Page 2 percent by volume (0 to 100 percent) in the gas sampled. The detection limit for this instrument is approximately 0.10 percent by volume. Gas monitoring well testing procedures are as follows: • Connect sample tubing, open test valve, observe pressure and record data. • For probes 20 feet deep or less,turn on sample pump, observe and record methane, oxygen, and carbon dioxide gas concentrations,when readings have stabilized for 30 seconds. • For probes more than 20 feet deep,turn on sample pump, extract a minimum of one probe volume, observed and record methane, oxygen, and carbon dioxide gas concentrations,when readings have stabilized for 30 seconds. ® Close test valve, disconnect sample instruments and secure sample location. SCS monitored structure vent risers as directed by the County. Other required monitoring documentation (e.g., sampling personnel, date, time and meteorological conditions) is shown on the attached testing data tables. Instrument calibration logs are attached to this report. Gas Monitoring Well Testing In accordance with the approved testing schedule, all perimeter GW probes are to be tested at a minimum, on a quarterly basis. On April 12, 2007, all compliance perimeter monitoring probes defined as GW Nos. 1 through 12 were tested. Results of GW probe testing(shown on Table 1) indicated that all GW probes exhibited methane gas concentrations below the LEL (5 percent by volume) or no methane gas was detected. SCS will continue to monitor all probes quarterly as directed by the County. SCS is scheduled to perform Third quarterly monitoring of 2007 (July through September) of all the GW probes before the end of the month of September 2007. Structure Vent Riser Testing In accordance with the approved testing schedule, the structure vent risers are to be tested on a quarterly basis. On April 12, 2007, all vent risers were tested for the presence of methane gas. Test results (shown on Table 2) indicated no methane gas was detected at any structure vent riser location tested. SCS is scheduled to perform third quarterly monitoring of 2007 (July through September) of the structure vent risers before the end of the month of September 2007.