Using a laser-based CO2 carbon isotope analyser to investigate gas transfer in geological media

Abstract

International audienceCO 2 stable carbon isotopes are very attractive in environmental research to investigate both natural and an-thropogenic carbon sources. Laser-based CO 2 carbon isotope analysis provides continuous measurement at high temporal resolution and is a promising alternative to isotope ratio mass spectrometry (IRMS). We performed a thorough assessment of a commercially available CO 2 Carbon Isotope Analyser (CCIA DLT-100, Los Gatos Research) that allows in situ measurement of δ 13 C in CO 2. Using a set of reference gases of known CO 2 concentration and carbon isotopic composition, we evaluated the precision, long-term stability , temperature sensitivity and concentration dependence of the analyser. Despite good precision calculated from Allan variance (5.0 ppm for CO 2 concentration, and 0.05 h for δ 13 C at 60 s averaging), real performances are altered by two main sources of error: temperature sensitivity and dependence of δ 13 C on CO 2 concentration. Data processing is required to correct for these errors. Following application of these corrections, we achieve an accuracy of 8.7 ppm for CO 2 concentration and 1.3 h for δ 13 C, which is worse compared to mass spectrometry performance, but still allowing field applications. With this portable analyser we measured CO 2 flux degassed from rock in an underground tunnel. The obtained carbon isotopic composition agrees with IRMS measurement, and can be used to identify the carbon source. S. Guillon () · E. Pili CEA, DAM

Monitoring CO2 concentration and δ13C in an underground cavity using a commercial isotope ratio infrared spectrometer

“…These values are 5-10 times lower than the one of 0.11 g m −2 day −1 previously obtained in a borehole (Perm 4) that laterally extends 2.3 m into the host rock from the tunnel wall [6]. This difference is explained by the difference of the spatial scale of the surfaces contributing to the CO 2 flux (0.06 m 2 for the borehole, 61 m 2 for the Inner Room, and 98 m 2 for the portion of tunnel where the analyzer is installed and which is delimited by plastic curtains [26] ).…”

“…is similar to the one obtained from other tracers ( 222 Rn and SF 6 ) for the same area of the tunnel [24]. A modified box model can then be applied to obtain the evolution of δ 13 C in the tunnel during the decay of the CO 2 concentration: where δ i and δ ∞ are, respectively, the peak and background carbon isotope compositions of CO 2 .…”

“…The technique is described in detail in [15]. This is an enhanced performance version of the instrument presented in [6]. It is mainly characterized by a better thermal insulation, aimed at decreasing the observed large sensitivity to external temperature variations.…”

“…At mid-length of the tunnel, a side excavation of approximately 9 m length and 3 m width, called Inner Room, was roughly isolated by a steel door and sealing foam. An IRIS analyzer had already been used at this site to measure CO 2 carbon dynamics in a small borehole [6]. Here we present a long-term monitoring of CO 2 dynamics in the tunnel and in the Inner Room.…”

“…However, studies using such commercial analyzers have revealed difficulties to obtain reliable and accurate data. They also emphasized the need for users to thoroughly assess the analyzers' performance in the field and to establish calibration strategies to correct raw data for the identified biases [3,6]. Commercial IRIS instruments are often designed for atmospheric monitoring and present good accuracy at low and not strongly variable CO 2 concentration [3,7,8].…”