River systems form key interfaces between terrestrial landscapes and the atmosphere, where weathering-derived and groundwater-transported carbon is transformed, retained, or released as CO₂. To investigate these dynamics in a transboundary river system, we conducted five spatially and seasonally resolved sampling campaigns in 2023 and 2024 along the Danube River, Europe´s second-longest river. We quantified the dominant dissolved inorganic carbon fraction (DIC; >90 % of total carbon), its stable isotope composition (δ13C_DIC), and calculated aqueous CO₂partial pressure (pCO_2(aq)) along the Danube´s main stem and selected major tributaries.

The upper Danube near its two springs exhibited the highest DIC concentrations (up to 5.3 mmol L^-1), indicating strong contributions from groundwater discharge and carbonate weathering. Associated δ13C_DIC values became progressively enriched downstream (from -13.3 towards -8.7 ‰), consistent with preferential loss of ¹²CO₂ via outgassing. Correspondingly, pCO_2(aq) values were predominantly oversaturated relative atmospheric CO₂ (>421 µatm), reaching a maximum of 3770 µatm, demonstrating persistent outgassing across seasons. Temporary near-equilibrium conditions occurred only at two downstream locations during summer, likely linked to enhanced photosynthesis and reduced turbulence. Overall, the Danube functions as a groundwater-influenced, year-round net source of CO₂, with limited seasonal buffering capacity. Downstream evolution of the carbon system is dominantly shaped by initial groundwater input, followed by continuous degassing. This novel high-resolution dataset provides a robust baseline for improving carbon budget estimates in Europe and other large river systems worldwide. It further supports future integrations of groundwater-river CO₂ interactions into continental-scale monitoring and modeling frameworks.

Granitic headwater streams are not only interfaces between groundwater and surface water but also represent poorly understood but highly reactive components of the terrestrial carbon cycle. Especially their low buffering capacity makes these environments particularly sensitive to short-term hydrological and biogeochemical fluctuations. This study examined how groundwater discharge and episodic high-flow events affect the hydrochemistry and CO₂ dynamics in a forested, high-gradient headwater stream (White Main, Fichtelgebirge Mountain, Bavaria). Over a sampling period of 15 months, baseflow conditions showed a downstream (spring to pond) decline in partial pressure (pCO₂), CO₂ fluxes (F_CO2), accompanied by increasing δ¹³C_DIC, reflecting rapid degassing from CO₂-rich groundwater. However, during episodic high-flow events, pCO₂ and F_CO2 rose sharply at downstream sites, coinciding with brown water coloration and distinct charge balance anomalies. These event signatures indicate an increased coupling between stream water and organic-rich soil layers. This leads to the mobilization of Fe-Al-bearing colloids, clay minerals, and natural organic matter (NOM), which temporarily modify proton balances, alkalinity, and carbonate equilibria. The resulting short-term CO₂ supersaturation demonstrates that hydrological pulses can bias apparent carbon fluxes in low-buffered headwaters. Recognizing this reactive soil-stream coupling is essential for correctly scaling CO₂ emissions from inland catchments.

Groundwater quality in Sri Lanka shows wide variations due to local and regional-scale geo-climatic changes. Here, we present groundwater hydrogeochemical and dissolved CO₂ variations in newly defined hydro-geo-climatic regions in Sri Lanka. Summary parameters with electrical conductivity (EC) and total hardness (TH) were used as primary indicators for 1318 groundwater samples from shallow aquifers spread over the country. Furthermore, we calculated dissolved CO₂ in groundwater (n = 667) with temperature, pH and alkalinity. Our analysis indicates that groundwater in the dry zone showed higher median EC and TH (740 µScm^-1 and 199 mg L^-1) values than in the intermediate (400 µScm^-1 and 120 mg L^-1) and wet zones (146 µScm^-1 and 53 mg L^-1). Groundwater in the northern sedimentary terrain showed extreme EC of up to 34000 µScm^-1 and TH of up to 3819 mg L^-1 with medians of 1006 µScm^-1 and 261 mg L^-1, respectively. Because of the low weathering rate of the basement minerals in the central part of Sri Lanka (Highland complex), groundwater showed the lowest EC and TH values compared to the other regions. Groundwater CO₂ values were varied from 0.01 to 4.82 mmol L^-1 and the wet zone-Wanni complex showed a higher median (1.06 mmol L^-1), while the dry zone-Vijayan complex showed a lower median (0.08 mmol L^-1). Future increased monitoring would also benefit groundwater management issues in relation to climate change and population growth in the country.

Mineral Trapping zur Speicherung von CO2 in tiefen geologischen Reservoiren (Carbon Capture & Storage, CCS) ist die langfristig stabilste Variante der CO2-Speicherung, im Vergleich zur Lösung in Tiefenwässern (solubility trapping) und residualer Phase superkritischen CO2s (residual trapping). Dazu müssen die Gleichgewichtsbedingungen im Tiefenwasser hin zur Ausfällung von Karbonatmineralen verschoben werden. Numerische Modelle hierzu, wie MIN3P oder PHREEQC sind dabei gut geeignet, hydrochemische Gleichgewichte unter Reservoirbedinungen von bis zu 300°C und 1000 Atmosphären zu berechnen. Am Beispiel des stillgelegten Heletz-Ölfeldes in Israel, in einer Sandsteinformation, wurde das Potential der mineralischen Abscheidung von CO2 simuliert. Dabei sind die Sättigunsindizes (SI) der Minerale langfristig in der Sequenz Ankerit > Dolomit > Calcit > Magnesit stabil, jedoch das Mineral Dawsonit nur in frühen Stadien, wenn Na+ in hoher Konzentration vorhanden ist. Die Ungewissheit der Modellvorhersagen wurde durch Hauptkomponentenanalyse und Sensitivitäts-Koeffizienten bestimmt. Eine große Variabilität im Auftreten von Dolomiten lässt sich durch Salinitätsunterschiede und die inhärenten Ungenauigkeiten der Berechnung der Aktivitätskoeffizienten zwischen den verwendeten hydrochemischen Datenbanken erklären.

Geological CO₂ sequestration in shallow and deep aquifers is governed by coupled thermo-hydro-mechanical-chemical (THMC) processes that control injectivity, plume migration, and the effectiveness of trapping and containment. This contribution summarizes recent advances in cyclic gas (CO₂) injection in low-permeability hydrocarbon reservoirs to extract transferable lessons for aquifer CO₂ storage. CO₂ injection/soaking experiments on intact and fractured mudrocks and siltstones under high-pressure (≈14 MPa) and high-temperature (≈70 °C) conditions were conducted across multiple cycles, paired with core-scale simulations. These experimental and modeling data documented fracture conductivity and compressibility hysteresis as well as phase-dependent (gas vs. liquid) fracture conductivity and relative permeability between multiple cycles.

Two findings were directly relevant to groundwater–CO₂ systems. First, matrix–fracture conductivity contrast was strongly phase-sensitive: gas-phase fracture conductivity was significantly, by up to an order of magnitude, larger than liquid-phase fracture conductivity, implying faster CO₂ pressure diffusion and preferential fracture migration during injection. Second, fracture compressibility and stress path induced dynamic aperture changes that manifested as fracture conductivity and compressibility hysteresis, possibly altering injectivity and the stability of residual CO₂ trapping through the injection/pressure-falloff sequence. Compositional data analysis across multiple cycles revealed pressure- and temperature-dependent hydrocarbon partitioning that analogously governs CO₂–brine–rock interactions in aquifers (e.g., dissolution and wettability shifts). Microscale imaging (scanning electron microscopy coupled with elemental analysis) indicated chemo-mechanical effects on wettability that, in turn, control flow pathways and effective relative permeability.

Translating these results to aquifer storage, a customized THMC modeling framework is proposed that (1) treats fracture transmissivity as stress- and phase-dependent and (2) includes hysteresis in fracture transmissivity and relative permeability for injection and pressure-falloff sequences. Insights from the core-scale CO₂ injection experiments and modeling suggest practical designs for pulse scheduling, pressure ramping, and fracture-sensitive monitoring, to optimize the injectivity–containment trade-off while enhancing residual, solubility, and (where relevant) mineral trapping.

By bridging cyclic CO₂ injection with groundwater storage physics, this contribution demonstrates how core-scale experimental and modeling data can calibrate predictive THMC models for CO₂ sequestration in aquifers, ultimately supporting safer, and more efficient carbon storage strategies.

References

Ghanizadeh et al. 2021. Experimental and computational evaluation of cyclic solvent injection in fractured tight hydrocarbon reservoirs. Nature Scientific Reports, 11 (1), 9497.

Ghanizadeh et al. 2023. Evaluation of Produced Hydrocarbons Composition During Cyclic CO₂ Injection (Huff-N-Puff) in Artificially-Fractured Shale Core Sample. SPE Canadian Energy Technology Conference and Exhibition, Calgary, Alberta, Canada, March 2023. Paper Number: SPE-212720-MS.

Ghanizadeh et al. 2025. Natural Fracture Compressibility and Permeability Hysteresis: Liquid vs. Gas. SPE Canadian Energy Technology Conference and Exhibition, Calgary, Alberta, Canada, March 2025. Paper Number: SPE-223996-MS.