Mapping and quantifying CO2 leakage using the Ground CO2 Mapper

The standard method for mapping and quantifying CO2 leakage flux from the ground surface to the atmosphere involves performing numerous point flux measurements using the accumulation chamber technique and then applying geostatistical interpolation to infer spatial distribution and estimate total mass transfer. Monte Carlo simulations using the program MCFlux have recently demonstrated, however, that uncertainty in the resultant estimate can be large if the chosen sample spacing is insufficient to capture the spatial complexity and size distribution of the leakage anomalies. In an effort to reduce this uncertainty we have developed a new tool, called the Ground CO2 Mapper, that rapidly measures the concentration of CO2 at the ground surface as a proxy for flux. Recently published results have illustrated the capabilities of the Mapper in terms of sensitivity and spatial resolution, as well as possible influencing parameters such as wind strength. The present work examines the potential of combining Mapper results with point flux measurements (using multivariate geostatistics) to improve data interpretation, with the MCFlux program being used once again to assess uncertainty in the final estimates

The behavior of rare soil gases in a seismically active area: the Fucino basin (central Italy)

Soil-gas (He, Rn) concentrations were performed to test their sensitivity for locating fault or fracture systems also when masked by non-cohesive lithologies, and to investigate about the gas-bearing properties of seismogenic faults. The Fucino basin (central Italy) was chosen as test site because it displays a network of surface and shallow-buried active faults within the valley floor that were partially reactivated during the 1915 Avezzano earthquake (Ms47.0). The highest radon values were found aligned along the most important faults bordering the eastern and the north-western sides of the plain. Moderately anomalous values of radon activity occur along the faults located in the depression of the historical lake. Highest helium values prevail in the western part of the plain, in correspondence of a horst structure inferred to be as the prolongation of the Vallelonga-Trasacco ridge. The study provides constraints on the spatial influence of tectonics and geology on deep-seated gas migration toward the surface

Continuous monitoring of natural CO2 emissions near Rome: lessons for low-level CO2 leakage detection

Continuous monitoring has been carried out at a fluvial flood-plain site near Rome for over a year. There is a mix of biogenic CO2 and deep geogenic CO2 at the site at relatively low concentrations and fluxes compared with other natural CO2 seepage sites studied previously. Factors such as temperature and soil moisture clearly affect the CO2 concentration and flux and seasonal and diurnal influences are apparent. Statistical approaches are being used to try to define these relationships and separate out the two gas components, which would be necessary in any quantification of leakage from CO2 storage

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

Fluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to ~1 km depth. Within such lithified successions, dissolution associated with the circulation of meteoric water results in increased permeability (K~10−1–100 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic conductivity is lower, decreasing from K~10−2 m/day at 150–400 m BGL to 10−3 m/day down-dip at ~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7–1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields

The consolidated European synthesis of CH₄ and N₂O emissions for the European Union and United Kingdom: 1990–2019

Knowledge of the spatial distribution of the fluxes of greenhouse gases (GHGs) and their temporal variability as well as flux attribution to natural and anthropogenic processes is essential to monitoring the progress in mitigating anthropogenic emissions under the Paris Agreement and to inform its global stocktake. This study provides a consolidated synthesis of CH₄ and N₂O emissions using bottom-up (BU) and top-down (TD) approaches for the European Union and UK (EU27 + UK) and updates earlier syntheses (Petrescu et al., 2020, 2021). The work integrates updated emission inventory data, process-based model results, data-driven sector model results and inverse modeling estimates, and it extends the previous period of 1990–2017 to 2019. BU and TD products are compared with European national greenhouse gas inventories (NGHGIs) reported by parties under the United Nations Framework Convention on Climate Change (UNFCCC) in 2021. Uncertainties in NGHGIs, as reported to the UNFCCC by the EU and its member states, are also included in the synthesis. Variations in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), arise from diverse sources including within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, the activities included are a key source of bias between estimates, e.g., anthropogenic and natural fluxes, which in atmospheric inversions are sensitive to the prior geospatial distribution of emissions. For CH₄ emissions, over the updated 2015–2019 period, which covers a sufficiently robust number of overlapping estimates, and most importantly the NGHGIs, the anthropogenic BU approaches are directly comparable, accounting for mean emissions of 20.5 Tg CH₄ yrc (EDGARv6.0, last year 2018) and 18.4 Tg CH₄ yr⁻¹ (GAINS, last year 2015), close to the NGHGI estimates of 17.5±2.1 Tg CH₄ yr⁻¹. TD inversion estimates give higher emission estimates, as they also detect natural emissions. Over the same period, high-resolution regional TD inversions report a mean emission of 34 Tg CH₄ yr⁻¹. Coarser-resolution global-scale TD inversions result in emission estimates of 23 and 24 Tg CH₄ yr⁻¹ inferred from GOSAT and surface (SURF) network atmospheric measurements, respectively. The magnitude of natural peatland and mineral soil emissions from the JSBACH–HIMMELI model, natural rivers, lake and reservoir emissions, geological sources, and biomass burning together could account for the gap between NGHGI and inversions and account for 8 Tg CH₄ yr⁻¹. For N₂O emissions, over the 2015–2019 period, both BU products (EDGARv6.0 and GAINS) report a mean value of anthropogenic emissions of 0.9 Tg N₂O yr⁻¹, close to the NGHGI data (0.8±55 % Tg N₂O yr⁻¹). Over the same period, the mean of TD global and regional inversions was 1.4 Tg N₂O yr⁻¹ (excluding TOMCAT, which reported no data). The TD and BU comparison method defined in this study can be operationalized for future annual updates for the calculation of CH₄ and N₂O budgets at the national and EU27 + UK scales. Future comparability will be enhanced with further steps involving analysis at finer temporal resolutions and estimation of emissions over intra-annual timescales, which is of great importance for CH₄ and N₂O, and may help identify sector contributions to divergence between prior and posterior estimates at the annual and/or inter-annual scale. Even if currently comparison between CH₄ and N₂O inversion estimates and NGHGIs is highly uncertain because of the large spread in the inversion results, TD inversions inferred from atmospheric observations represent the most independent data against which inventory totals can be compared. With anticipated improvements in atmospheric modeling and observations, as well as modeling of natural fluxes, TD inversions may arguably emerge as the most powerful tool for verifying emission inventories for CH₄, N₂O and other GHGs. The referenced datasets related to figures are visualized at https://doi.org/10.5281/zenodo.7553800 (Petrescu et al., 2023)

Development and testing of a rapid, sensitive, high-resolution tool to improve mapping of CO2 leakage at the ground surface

Locating and quantifying anomalous, deep-origin CO2 leakage from the soil to the atmosphere is typically accomplished by interpolating a dataset of point flux measurements, with overall accuracy and uncertainty strongly influenced by sample spacing relative to anomaly size and variability. To reduce this uncertainty we have developed the Ground CO2 Mapper, a low-cost complementary tool that rapidly measures, at high spatial resolution, the distribution of CO2 concentration at the ground-air contact as a proxy of CO2 flux. Laboratory tests show that the Mapper has a low noise level (2σ = 16 ppm) and fast response time (T90 = 1.55 s), while field tests at a small controlled-release site define a high level of reproducibility and sensitivity and illustrate the impact of wind and survey speed on instrument response. Modelling based on these results indicates that the Mapper has a greater than 60% probability of detecting an intersected 2 m wide anomaly having a maximum CO2 flux rate of 75 and 100 g m-2 d-1 at survey speeds of 2.5 and 4.8 km h-1, respectively, under the test conditions. Measurements in a large (4600 m2) grassland field where natural geogenic CO2 is leaking show how the Mapper can produce, in &lt;10% of the time, a more detailed map of CO2 flux distribution than a point flux survey conducted on a ca. 10 m grid spacing. Based on these results we believe the Ground CO2 Mapper can give a useful contribution to diffuse degassing studies in volcanic/geothermal areas and to monitoring of Carbon Capture and Storage (CCS) sites by reducing overall survey time, costs and uncertainty. Future work will test the Mapper’s response and capabilities under more diverse site and meteorological conditions than those examined in this study