Techniques For Establishing Landfill Gas (rather Than Liquid) As The Source of VOC in Groundwater Sa
Several techniques have been developed for evaluating whether landfill gas (rather than leachate) is the source of volatile organic compounds (VOCs) in groundwater samples. Leachate water frequently has elevated levels of tritium compared to background groundwater and a leachate (water) release would increase tritium levels in affected groundwater samples, while landfill gas has been shown not to do so. Although landfill gas components can react with minerals and alter inorganic constituents present in groundwater samples such as alkalinity, calcium, and magnesium, a frequent major leachate constituent, chloride, can be used to evaluate whether leachate has affected the sample.
Highly soluble VOCs, such as MtBE, diethyl ether, and tetrahydrofuran, are evidence of leachate effects, since they are too water-soluble to migrate in landfill gas. The presence of highly soluble semi-volatile organic compounds, such as phenols, are also consistent with leachate effects on the sample. Elevated concentrations of dissolved CO2 have been shown to be a symptom of landfill gas effects--this is because not all of the CO2 in landfill gas reacts immediately with aquifer minerals, while such reactions are complete in leachate due to the presence of soils as daily cover in the waste. To assess whether VOCs are partitioning into groundwater in a specific location, such as a monitoring well, the headspace gas and dissolved VOC concentrations can be compared. If the Henry's Law constant multiplied by the water concentration is significantly less than the measured gas concentration, the data are consistent with VOCs partitioning from landfill gas into the groundwater.
| Typical landfill gas composition | %(dry volume basis)a |
|---|---|
| Methane, CH4 | 45-60 |
| Carbon dioxide, CO2 | 40-60 |
| Nitrogen, N2 | 2-5 |
| Oxygen, O2 | 0.1-1.0 |
| Sulphides, disulphides, mercaptans etc. | 0-1.0 |
| Ammonia, NH3 | 0.1-1.0 |
| hydrogen, H2 | 0-0.2 |
| carbon monoxide, CO | 0-0.2 |
| Trace constituents | 0.01-0.6 |
aExact percentage distribution will vary with the age of the landfill
Read more about this topic: Landfill Gas Monitoring
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