To establish an effective leachate recirculation process, the moisture distribution within a landfill must first be estimated so that leachate can be added appropriately to create optimal and homogeneous moisture conditions throughout the waste mass. Several seismic surveys were conducted at a bioreactor landfill in Ste-Sophie, Quebec, Canada with the objective of mapping the moisture distribution using seismic data collected atop of the landfill. Two different sources of signal were tested: a seismic hammer and a more powerful seismic gun. Arrivals were recorded using a 24-channel seismograph instrumented with single-component vertical geophones and three-component geophones. Analysis of the travel times of direct and refracted seismic waves established that the landfill consists of a 4.5m upper layer of loose waste underlain by a 25.5m layer of compacted waste with average compressional wave (P-wave) interval velocities of 280m/s and 380m/s, respectively. Seismic velocity analyses indicated that an increase in moisture content, caused by leachate injection, increased the P-wave velocity of waste. More specifically, the injection of approximately 1m3 of leachate per meter length of trench through two recirculation trenches caused an increase on the order of 22m/s (7%) in the stacking velocity of the P-waves reflected off the landfill bottom. The lateral radius of influence of the injected leachate was 5-10m and the velocity results reached steady state conditions approximately 3 hours after the injection ceased.
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