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A new in-situ method for directly measuring the collection efficiency of a gas extraction well was developed. This method requires the injection of an inert tracer gas, a system for measuring this tracer gas in the gas collection well, and accurate measurement of the volumetric flow rate of gas in the gas extraction well during the test. A key to the success of the technique was the use of an on-site photoacoustic field gas monitor to measure the tracer gas in real time.

Thirteen gas tracer tests were conducted within the region of influence (ROI) of a gas extraction well at Yolo County Central Landfill. For 12 tests the gas collection was excellent, always exceeding 70% with eight of the 12 tests showing a collection efficiency exceeding 90%. Here, gas collection efficiency is defined for the point where the tracer gas was injected. Injection points varied from 5 to 15ft in depth for radial distances between 8 and 24ft from the extraction well. These distances are close to the extraction well (D23), since our focus was the development of the gas tracer technology and test durations were shorter if the injection points were near D23. To create more severe conditions for landfill gas (LFG) collection, gas flow from D23 was adjusted downward to decrease the effectiveness of the gas well within the ROI. Even when gas pressures were atmospheric or slightly above atmospheric at the point of tracer injection, gas collection efficiency was very good.

For Test 11 gas collection efficiency was poor – only 7%. Here, the poor efficiency was associated with water-saturated refuse or refuse/ soil located between the point of tracer injection at 5ft depth (MW1-5) and the well screen for D23. Although there was a measureable effect of D23 on gas pressure at MW1-5, the travel path from this point to the gas collection well was likely long and tortuous. This tracer injection point was located within the ROI of D23. Thus, even within an ROI, gas collection efficiency might be poor if gas flow is inhibited, here because of the presence of liquid water. This highlights the need for care when operating landfills as bioreactors, as the addition of liquid or recirculation of leachate may lead to water-saturated conditions in some portions of the landfill.

In addition to conducting gas tracer tests, gas pressures were also measured at all monitoring wells to assess the gas pressure field created by D23. Under ideal conditions, this field is symmetric around the gas collection well. For these field tests, the gas pressure field was not symmetric, with gas pressures along the southern transect much more responsive to D23 than along other transects. Locations with poor gas suction from D23 coincided with the location of Test 11, where tracer recovery was also poor. Thus, measuring the gas pressure field in refuse appears valid for assessing regions of the landfill that might have good or poor LFG collection.

The objective of this project was the development of a gas tracer technology for assessing landfill gas capture efficiency at different points within a landfill. The technology appears best suited for assessing alternative well designs and management practices on landfill gas collection. It may be used to quantify the landfill gas capture efficiency for an entire landfill cell, although the viability of this approach should be tested in future work.