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Mr. Gabriel Ka m, P.E. Project 939-01.09 <br /> November , 1 <br /> Page 4 <br /> The soil-gas pressures measured in the probe casings varied significantly <br /> between the two monitoring dates. The pressures measured on the first <br /> date (October 11) were typical of gas pressures at other landfills, and <br /> ranged from -0.24 to +0.74 inch water column gauge (in. w.c.). The pres- <br /> sures measured on the second date (October 18) were much lower than <br /> the first set, and ranged from -2.43 to +0.06 in. w.c. The deeper probe <br /> casings typically showed the largest decreases in gas pressure. <br /> Decreases of gas pressure of this magnitude could be caused by a large <br /> short-term variation in barometric pressure (not detected) or a rapid draw- <br /> down of the ground-water table underlying the site. <br /> Time-based variations of gas concentrations and pressures in soil-gas <br /> probes are characteristic in landfills, especially during the initial monitoring <br /> period. Continued long-term, periodic monitoring of probes should provide <br /> a more consistent definition of trends in gas migration at the site. <br /> CONCLUSIONS <br /> The monitoring data and analytical results indicate that landfill gas is <br /> present at the property line along the north, east, and west edges of the <br /> landfill. The measured methane concentrations appear to exceed the LEL <br /> in all gas wells along each of these boundade The full lateral extent of <br /> gas migration cannot be determined with the available data. <br /> The CIWMB and the local enforcement agency (LEA) should be notified as <br /> required by applicable regulations. A program of periodic monitoring <br /> should be developed and implemented to track the long term trends in gas <br /> migration at the site. The monitoring frequency should be a minimum of <br /> quarterly, as required by the closure/postclosure requirements. <br /> Developed areas adjacent to the landfill site should be monitored for <br /> methane accumulations and inspected for potential gas migration <br /> pathways. Based on the monitoring results, the County should prepare an <br /> action program in coordination with the CIWMB and the LEA, pursuant to <br /> Title 14 regulations. <br /> PJ9 9390109.GOW <br />