Measurement of advective soil gas flux: results of field and laboratory experiments with CO₂

Abstract

A multi-channel, steady-state flow-through (SSFT), soil-CO₂ flux monitoring system was modified to include a larger-diameter vent tube and an array of inexpensive pyroelectric non-dispersive infrared detectors for full-range (0–100 %) coverage of CO₂ concentrations without dilution. Field testing of this system was then conducted from late July to mid-September 2010 at the Zero Emissions Research and Technology project site located in Bozeman, Montana, USA. Subsequently, laboratory testing was conducted at the Pacific Northwest National Laboratory in Richland, WA, USA using a flux bucket filled with dry sand. In the field, an array of 25 SSFT and 3 non-steady-state (NSS) flux chambers was installed in a 10 × 4 m area, the long boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. Two additional chambers (one SSFT and one NSS) were installed 10 m from the well for background measurements. Volumetric soil moisture sensors were installed at each SSFT chamber to measure mean moisture levels in the top 0.15 m of soil. A total flux of 52 kg CO₂ day⁻¹ was injected into the well for 27 days and the efflux from the soil was monitored by the chambers before, during, and for 27 days after the injection. Overall, the results were consistent with those from previous years, showing a radial efflux pattern centered on a known “hot spot”, rapid responses to changes in injection rate and wind power, evidence for movement of the CO₂ plume during the injection, and nominal flux levels from the SSFT chambers that were up to sevenfold higher than those measured by adjacent NSS chambers. Soil moisture levels varied during the experiment from moderate to near saturation with the highest levels occurring consistently at the hot spot. The effects of wind on measured flux were complex and decreased as soil moisture content increased. In the laboratory, flux-bucket testing with the SSFT chamber showed large measured-flux enhancement due to the Venturi effect on the chamber vent, but an overall decrease in measured flux when wind also reached the sand surface. Flux-bucket tests at a high flux (comparable to that at the hot spot) also showed that the measured flux levels increase linearly with the chamber-flushing rate until the actual level is reached. At the SSFT chamber-flushing rate used in the field experiment, the measured flux in the laboratory was only about a third of the actual flux. The ratio of measured to actual flux increased logarithmically as flux decreased, and reached parity at low levels typical of diffusive-flux systems. Taken together, the results suggest that values for advective CO₂ flux measured by SSFT and NSS chamber systems are likely to be significantly lower than the actual values due to back pressure developed in the chamber that diverts flux from entering the chamber. Chamber designs that counteract the back pressure and also avoid large Venturi effects associated with vent tubes, such as the SSFT with a narrow vent tube operated at a high chamber-flushing rate, are likely to yield flux measurements closer to the true values.