p>The study was a laboratory validation exercise, not a field test or a demonstration of underground carbon storage performance. Researchers tested polished Dietfurter limestone in an apparatus measuring 1 metre by 1 metre by 2 centimetres. Spacers created adjustable apertures of 2 or 6 millimetres between the limestone and a polycarbonate plate, producing a simplified vertical fracture with one reactive surface and one inert surface. Carbon dioxide-enriched deionized water entered through two openings at the top and left through a uniformly draining outlet, where dissolved calcium was measured. The setup was designed to provide controlled flow conditions for testing a numerical model before it is applied to more complicated density-driven behavior.

p>Before running the comparison, the investigators used simulations to identify informative experimental conditions. They examined apertures from 1 to 10 millimetres, flow rates from 0.1 to 1.3 litres per minute and carbon dioxide partial pressures of 1, 0.5 and 0.02 bar. Predicted calcium concentrations remained above the 0.01 milligram-per-litre measurement threshold across the explored range, but varied substantially with aperture, residence time and carbon dioxide loading. Higher carbon dioxide pressure increased dissolution, while longer residence time allowed more contact with the reactive wall. The 0.02-bar condition, selected as representative of karst settings, produced concentrations too small for reliable measurement and was therefore excluded from the validation experiments.

p>The numerical system was newly implemented in the DuMu^x simulator and combined the Stokes equations for low-speed fluid flow with mass balances for water, total inorganic carbon and dissolved calcium. Unlike many Darcy-scale descriptions, the Stokes formulation accounts for density changes caused by dissolved species. Water properties followed IAPWS specifications, carbon dioxide properties used established equations, and calcium diffusion was calculated from its temperature-dependent molar conductivity. A two-dimensional cross-section was used because the dominant flow was expected to run perpendicular to the plates, while lateral velocities parallel to the limestone were expected to be much smaller. This approximation reduced computational cost but did not represent all possible three-dimensional instabilities.

p>Chemical speciation was solved from total inorganic carbon, electroneutrality and carbonate equilibrium relationships, allowing the model to recover pH and the relative proportions of dissolved carbon dioxide, carbonic acid, bicarbonate and carbonate. Calcite dissolution was represented through acid, neutral and carbonate mechanisms, including a backward carbonate reaction and suppression of dissolution as the mineral saturation ratio approached one. The reaction rates were temperature dependent, and their implementation was first checked against published fully mixed experiments. The simulated pH trajectories agreed closely with those literature measurements, providing a separate check on the kinetic formulation before fracture-scale transport effects were introduced.

p>The equations were discretized with a staggered-grid method that reduces pressure oscillations while retaining mass conservation. Square cells were used in the principal flow direction, with additional refinement across the narrow aperture to resolve concentration gradients beside the limestone. All equations were solved fully implicitly in time, using a Newton scheme with a maximum time step of 10 seconds. A grid-convergence analysis showed that further refinement changed the simulated concentration curves only modestly. Boundary conditions included no-slip walls, a prescribed inlet velocity during flow-through, an outlet condition and an initial one-hour closed period that reproduced the laboratory resting phase. That initialization allowed dissolution products to accumulate before circulation began.

p>Measurements and simulations followed the same qualitative pattern. After the one-hour resting phase, the outflow calcium concentration changed as accumulated material was flushed from the fracture and the system moved toward a dynamic steady state. The effect was sharper in the 2-millimetre opening, where the initial concentration formed a brief plateau before dropping toward its long-term value; the 6-millimetre case showed a weaker breakthrough. Both measurement and simulation produced higher normalized steady-state concentrations at lower flow velocities, corresponding to longer residence times. The influence of carbon dioxide pressure was comparatively small in the selected experiments. Simulations and observations generally agreed well in both transient and asymptotic portions of the curves, although measured data were somewhat scattered in several cases.

p>The agreement was somewhat better for the 6-millimetre aperture than for the 2-millimetre opening, and some discrepancies remained during the earliest transient, particularly in the narrow fracture. The investigators attributed some of this difference to measurement variability, noting that the laboratory curves showed less pronounced plug-flow-like behavior than the simulations. These deviations did not change the reproduced trends. The tests also revealed a technical limit: the maximum estimated cumulative denudation was about 13.1 micrometres, smaller than the vertical cell dimension of the finest grid, so the model treated the moving limestone boundary as effectively fixed. It also assumed a uniform, unchanged surface and did not include possible inhibitors associated with other ions.

p>The simulations further showed that dissolution was strongly transport-limited in every tested case. Researchers calculated Sherwood numbers to compare effective mass transfer with diffusion, and efficiency factors to distinguish transport control from reaction control. For the 6-millimetre fractures, the efficiency factors ranged from roughly 2 × 10⁻⁵ to 5 × 10⁻³. In the 2-millimetre fractures, they rose to about 10⁻², indicating somewhat less severe transport limitation while still showing that diffusion strongly influenced the result. The narrower gap allowed mass-transfer profiles to become nearly developed over the 1-metre path, whereas some wider-fracture cases had not reached the expected fully developed condition. These findings identify delivery of dissolved reaction products away from the reactive wall as the central bottleneck.

p>Both measurements and simulations reached a quasi-steady outflow calcium concentration after roughly one to two fracture volumes had passed through the apparatus. Normalized calcium concentration increased approximately linearly with the Damköhler number at low to moderate values before approaching a saturation limit, linking the observed behavior to the balance between reaction and residence time. The validated model provides a foundation for future work on stagnant water disturbed by density-driven carbon dioxide dissolution and calcite reaction. Its present evidence remains bounded by laboratory scale, a simplified two-dimensional geometry, selected carbon dioxide pressures and a nearly pure calcite surface; translating it directly to heterogeneous, three-dimensional carbonate aquifers will require additional validation.

Subject of Research: Validation of a coupled reactive-transport model for carbonic-acid dissolution of limestone in a laboratory fracture analog

Article Title: Validating a numerical model for calco-carbonic reactive transport in a laboratory-scale fracture analog

Article References: Wendel, K., Keim, L., Strauch, B., Zimmer, M., & Class, H. (2026). Validating a numerical model for calco-carbonic reactive transport in a laboratory-scale fracture analog. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03153-9

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03153-9

Keywords: reactive transport, limestone dissolution, karst fracture, carbonate chemistry, carbon dioxide, numerical modelling, mass-transfer limitation, laboratory experiment, Stokes flow, calcium concentration, Validating, numerical

The urgency of the work is grounded in real events. At Xocén, the roof of an underground cavern gave way suddenly in the middle of a Maya community, toppling a two-century-old Ceiba tree and opening a sinkhole roughly 57 meters in diameter and up to 36 meters deep. The researchers believe intense rainfall and the passage of heavy construction machinery in the days before the collapse triggered the failure of a cavern ceiling that had been weakening for a long time below the surface. At Saamal, an open cenote popular with visitors, partial collapse of the steep cliff walls suggests that even mature, apparently stable karst features can remain structurally active for years.

The scientific setting is extraordinary. The Yucatán’s cenote density traces back to the Chicxulub impact crater, the roughly 200-kilometer-wide scar left by the asteroid that ended the Cretaceous period. The crater’s fractured rim created a zone of enhanced permeability, the so-called Ring of Cenotes, along which carbonate dissolution concentrated over millions of years. When acidified rainwater percolates through limestone, it enlarges fractures and voids until the remaining roof can no longer support its own weight. Because karst terrains show few visible warning signs at the surface until failure occurs, they account for a disproportionate share of sudden ground-collapse hazards worldwide.

Conventional geotechnical investigation relies on drilling and excavation, methods that are expensive, slow, and impractical to deploy across many sites immediately after a collapse. ERT offers an alternative. By injecting electrical current into the ground through arrays of electrodes and measuring the resulting voltage differences, the technique produces cross-sectional images of subsurface resistivity. Competent dry limestone appears highly resistive, while water-saturated, clay-rich, or heavily weathered rock conducts electricity well. Air-filled voids also register as strong resistive anomalies. These contrasts map directly onto the mechanical properties that govern collapse susceptibility.

At Xocén, the team deployed a Syscal Pro system with Wenner and Schlumberger electrode arrays along three profiles totaling up to 110 meters each, reaching effective investigation depths of roughly 16 to 25 meters. After filtering unreliable readings and inverting the data with a smoothness-constrained least-squares algorithm, the final models achieved root-mean-square misfits between about 2 and 2.5 percent. The inverted sections revealed a consistent vertical stratification: a resistive cap of dry limestone more than 1000 ohm-meters, an intermediate transition zone of partially weathered rock, and a laterally continuous conductive interval below 250 ohm-meters interpreted as water-saturated, highly altered limestone, whose top approximates the local water table.

Most striking was a vertically persistent conductive anomaly in one profile, where resistivity dropped below 150 ohm-meters, linking the surface to the deep conductive layer. The researchers interpret this feature as a fracture-controlled infiltration corridor, a preferential pathway through which water percolates, accelerating dissolution and progressively weakening the rock mass. This geometry suggests that the Xocén collapse was not an isolated surface event but the geomorphological expression of a pre-existing weakened subsurface domain, with future instability most likely concentrated at the interfaces between the resistive cap, the transition zone, and the deeper conductive material.

At Saamal, five closely spaced profiles were acquired along the vulnerable cliff rim using a SuperSting R8 system, with electrodes just one meter apart to resolve the approximately 3-meter-thick limestone ledge involved in the recent failures. A robust, blocky inversion captured the sharp contrasts near the wall. The sections showed a thin weathered conductive veneer half a meter to a meter thick overlying competent carbonate rock, plus a very high-resistivity domain exceeding 1600 ohm-meters adjacent to the cliff, interpreted conservatively as a dry, possibly fractured carbonate block. Because air-filled cavities and dry rock yield similarly high resistivities, the authors caution that resistivity magnitude alone cannot confirm open voids. To visualize domain continuity between profiles, the team trained a neural network to interpolate the independently inverted two-dimensional sections into a three-dimensional resistivity volume, a supporting tool rather than a true 3D inversion.

A depth-of-investigation analysis following the established Oldenburg and Li method confirmed that the models are constrained by real data to mean depths of about 19.7 meters at Xocén and 2.9 meters at Saamal, guarding against over-interpretation of poorly resolved regions. The authors are candid about limitations: no borehole, geotechnical, or piezometric control was available, so the inferred domains remain hydrogeophysical rather than directly verified, and the resistivity thresholds are site-specific rather than universal. They recommend corroboration through fracture mapping, ground-penetrating radar, shallow seismic surveys, and repeat ERT monitoring after rainfall events.

The broader significance lies in translating geophysical images into operational disaster-risk decisions. The study proposes that collapse-prone sectors be identified not by a single universal signature but by site-specific combinations of shallow weathered conductive veneers, laterally connected saturated zones, fracture-controlled infiltration pathways, and sharp resistivity interfaces. Classifying ground into high, intermediate, and low susceptibility zones gives authorities a defensible basis for restricting access, prioritizing monitoring, and planning land use around cenotes in populated or heavily visited areas. As reported collapses of cenote roofs increase across the Yucatán, this rapid, affordable screening framework offers communities and tourism operators a practical first line of defense against one of nature’s most sudden geological traps.

Subject of Research: Electrical resistivity tomography screening of collapse susceptibility in karst cenote environments of the Yucatán Peninsula, Mexico

Article Title: Electrical Resistivity Tomography for collapse susceptibility screening in karst cenote environments: A case study in the Yucatán Peninsula, Mexico

Article References: Juárez, S. L., Alemán, J. C. O., Castañeda, C. C., Hernandez, J. F. E., Perez, D. A. P., & Martinez, J. M. (2026). Electrical Resistivity Tomography for collapse susceptibility screening in karst cenote environments: A case study in the Yucatán Peninsula, Mexico. Environmental Challenges, 25, Article 101642. https://doi.org/10.1016/j.envc.2026.101642

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101642

Keywords: cenotes, karst, electrical resistivity tomography, Yucatán Peninsula, sinkhole collapse, Chicxulub crater, geophysics, Xocén, Saamal, disaster risk, limestone dissolution, Ring of Cenotes