Monitoring crustal CO<sub>2</sub> flow: methods and their applications to the mofettes in West Bohemia

Abstract

Monitoring of CO2 degassing in seismoactive areas allows the study of correlations of gas release and 8 seismic activity. Reliable continuous monitoring of the gas flow rate in rough field conditions requires robust methods 9 capable of measuring gas flow at different types of gas outlets such as wet mofettes, mineral springs and boreholes. 10In this paper we focus on the methods and results of the long-term monitoring of CO2 degassing in the West 11 Bohemia/Vogtland region in Central Europe, which is typified by the occurrence of earthquake swarms and 12 emanations of carbon dioxide of magmatic origin. Besides direct flow measurement using flowmeters, we introduce 13 a novel indirect technique based on quantifying the gas bubble contents in a water column, which is capable of 14 functioning in severe environmental conditions. The method calculates the mean bubble fraction in a water-gas 15 mixture from the pressure difference along a fixed depth interval in a water column. Laboratory tests indicate the 16 nonlinear dependence of the bubble fraction on the flow rate, which is confirmed by empirical models found in the 17 chemical and nuclear engineering literature. Application of the method in a pilot borehole shows a high correlation 18between the bubble fraction and measured gas flow rate. This was specifically the case of two coseismic anomalies 19 in 2008 and 2014, when the flow rate rose during a seismic swarm to a multitude of the pre-seismic level for several 20 months and was followed by a long-term flow rate decline. However, three more seismic swarms occurring in the 21 same fault zone were not associated with any significant CO2 flow anomaly. We surmise that this could be related to 22 the slightly farther distance of the hypocenters of these swarms than the two ones which caused the coseismic CO2 23 flow rise. Further long-term CO2-flow monitoring is required to verify the mutual influence of CO2 degassing and 24 seismic activity in the area. 25 26 27 28

Microbial diversity and community dynamics in an active, high CO2subsurface rift ecosystem

“…The exact drilling position was selected in proximity to those of the previous two exploratory drilling campaigns (F1 and F2). The F1 drilling was conducted in 2007 [21, 36] and reached a depth of approximately 28 m below ground into a CO 2 -saturated, confined aquifer. The F2 borehole [36] was drilled in 2016 down to a depth ∼108 m. This drilling for the first time was conducted with the aim to evaluate whether the increased fluid and substrate flow can accelerate microbial life in active fault zones and CO 2 conduits [20, 22].…”

“…Periodically occurring earthquake swarms stimulate the migration of geogenic CO 2 from active magma chambers at the crust-mantle boundary and from lithospheric mantle depths of about 65 km [17][18][19] resulting in CO 2 rich sediments and the formation and accumulation of CO 2 in groundwater structures such as saline aquifers [20]. At the surface, CO 2rich gas is discharged in the form of natural cold gas exhalation systems such as mofette sites or released at mineral water springs [21][22][23][24][25][26]. The exceptional geo-and physicochemical conditions are likely to affect microbial development and activity, may foster microbial processes through elevated substrate support [27], and potentially trigger a diverse range of interesting rock-fluid interactions as part of geodynamic processes in the lithosphere [18,26,28].…”

ICDP drilling of the Eger Rift observatory: magmatic fluids driving the earthquake swarms and deep biosphere

“…1). In particular, two existing monitoring wells, F1 and F2 (Bussert et al, 2017;Fischer et al, 2020), were complemented by the F3 drill hole; these three adjacent boreholes, F1 (30 m), F2 (70 m) and F3 (230 m), provide continuous monitoring of fluids at high sampling rates to acquire fluid parameters (gas flow, water temperature and water level/pressure) as well as chemical (CO 2 , Ar, N 2 , O 2 , He, H 2 and CH 4 ) and isotopic (δ 13 C CO 2 , δ 18 O CO 2 and 222 Rn) gas content (Woith et al, 2020). Additionally, samples for laboratory analysis of He, Ne and Ar isotopes are taken repeatedly (roughly every 2 months), as theses isotopes are useful tracers for constraining the fluid origins and mixing ratios of mantle components.…”

“…Ultimately, a borehole seismometer will be installed at the bottom of F3 and will be complemented by a capillary tube to collect "fresh" gases from the CO 2 horizon at depth, directly at the point where the fluids enter the borehole to avoid possible contamination or impact from external processes. Further details on the instrumentation of this mofette field with massive CO 2 degassing (up to 97 t d −1 ) as well as the first monitoring results are summarized in Fischer et al (2020), Woith et al (2020) and Daskalopoulou et al (2021). Once the novel monitoring system is fully operational, fluid transients will be able to be observed in great detail.…”