This study reports the first field test of a multichannel, auto-dilution, steady-state, soil-CO 2 flux monitoring system being developed to help understand the pathways by which fugitive CO 2 from a geologic sequestration site migrates to the surface. The test was conducted from late August through mid-October 2008 at the Zero Emissions Research and Technology project site located in Bozeman, MT. Twenty steady-state and five non-steadystate flux chambers were installed in a 10 9 15 m area, one boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. A total flux of 52 kg CO 2 day -1 was injected into the well for 13 days and the efflux from the soil was monitored by the chambers before, during, and for 33 days after the injection. The results showed a rapid increase in soil efflux once injection started, with maximal values reached within 3-7 days in most chambers. Efflux returned to background levels within a similar time period after injection ceased. A radial efflux pattern was observed to at least 2 m from the injection well, and evidence for movement of the CO 2 plume during the injection, presumably due to groundwater flow, was seen. The steadystate chambers yielded very stable data, but threefold to fivefold higher fluxes than the non-steady-state chambers. The higher fluxes were attributed to vacuum induced in the steady-state chambers by narrow vent tubes. High winds resulted in significant decreases in measured soil CO 2 efflux, presumably by enhancing efflux from soil outside the chambers.

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SummaryGeological sequestration has the potential capacity and longevity to significantly decrease the amount of anthropogenic CO 2 introduced into the atmosphere by combustion of fossil fuels such as coal. Effective sequestration, however, requires the ability to verify the integrity of the reservoir and ensure that potential leakage rates are kept to a minimum. Moreover, understanding the pathways by which CO 2 migrates to the surface is critical to assessing the risks and developing remediation approaches. Field experiments, such as those conducted at the Zero Emissions Research and Technology (ZERT) project test site in Bozeman, Montana, require a flexible CO 2 monitoring system that can accurately and continuously measure soil-surface CO 2 fluxes for multiple sampling points at concentrations ranging from background levels to several tens of percent. To meet this need, researchers at Pacific Northwest National Laboratory (PNNL) are developing a multi-port battery-operated system capable of both spatial and temporal monitoring of CO 2 at concentrations from ambient to at least 150,000 ppmv.The system consists of soil-gas sampling chambers and a sensing unit based on an infrared gas analyzer (IRGA). Headspace gas from the chambers is continuously pumped through a relay-driven manifold that directs gas from individual chambers into the IRGA. A unique feature of the system is its ability, based on feedback from the IRGA, to automatically dilute the sample gas stream with N 2 using a network of gas-flow controllers, thus allowing measurement of CO 2 levels beyond the normal IRGA limit of 3000 ppmv. The entire system is controlled by a programmable datalogger that also stores the output from the IRGA and gas-flow controllers. The system consists of 27 sampling chambers, thus allowing detailed spatial monitoring of a moderately sized test area (~100 m 2 ). Power can be provided either by an AC source or by an off-grid power-generation system consisting of six deep-cycle batteries charged by both wind and photovoltaic power sources. Dilution gas is provided by a liquid N 2 dewar containing the equivalent of 135,000 L of N 2 (g), enough for 4 weeks of continuous operation. The system is remotely controlled and monitored by connecting the datalogger communication port to a cellular-based modem and antenna.On 27 August 2008, a shallow test injection of CO 2 was started at the ZERT site using a horizontal well located about 2 m below the surface and 1 m below the water table. The PNNL team set up a 27-chamber sampling grid directly above and extending northwest of the injection well. Concentrations were monitored continuously in twenty of these steady-state flux chambers during 13 days of injection (308 h) and for 33 days post injection. These concentration data can be converted directly to flux data using a flow-rate specific constant.With the 20-chamber setup, data were collected on a 2.7-h cycle continuously for nearly 7 weeks. We centered the southeastern side of the sampling grid on a known leakage spot, and this ...

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