The Utility of Purge and Recovery Testing in Ground-Gas Flux Risk Assessment

Purge and recovery testing is a novel method for measuring ground-gas flux. This method has been tested with Gasclam in boreholes at two landfill sites. The recovery rate of individual gases was used to predict when and at what concentration they would reach the unmonitored receptors. For example, at site 1 in borehole 2, CH4 and CO2 had concentrations of 22 % and 4 % respectively before purging in September 2010 and, 23 % and 4.4 % respectively in May 2011 before purging. While 22 % of CH4 and 4 % of CO2 were recovered in 32 hrs and 14 hrs respectively after purging, 23 % of CH4 and 4.4 % of CO2 were recovered in 52 hrs and 27 hrs respectively after purging. This implies that CH4 and CO2 concentrations recovery in this borehole was faster in September 2010 than in May 2011. The recovery time of the investigated gases [CH4, CO2 and Volatile Organic Compounds (VOC)] varied from the length of site visit to over a day. It also varied with season and site. Soil permeability and gas production rate are suspected to be responsible for variations in gas recharge rate. In general, the recovery profiles of VOC were very different from those of CH4 and CO2. VOC tended to recover in no time, indicating that they could get to the receptor much faster compared to other gases, thereby posing more danger. The reproducibility of the gases was not consistent due to their low concentrations.