Advective Flow Research Articles

Neutrino-dominated Relativistic Viscous Accretion Flows around Rotating Black Holes with Shocks

Abstract We investigate the relativistic, viscous, advective neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes, incorporating neutrino cooling. By adopting an effective potential to describe the spacetime geometry around the rotating black holes, we self-consistently solve the governing NDAF equations to obtain global transonic accretion solutions. Our findings indicate that, depending on the model parameters, namely, energy (ε), angular momentum (λ), accretion rate ( m ̇ ), viscosity (α), and black hole spin (a k), NDAFs may harbor standing shocks where the Rankine–Hugoniot shock conditions are satisfied. Utilizing these shock-induced NDAF solutions, we compute the neutrino luminosity (L ν ) and neutrino annihilation luminosity ( L ν ν ¯ ) across a wide range of model parameters. We further calculate maximum neutrino luminosity ( L ν max ) and neutrino annihilation luminosity ( L ν ν ¯ max ), resulting in L ν max ∼ 1 0 51 − 53 erg s−1 (1048−51 erg s−1) and L ν ν ¯ max ∼ 1 0 48 − 52 erg s−1 (1042−49 erg s−1) for a k = 0.99 (0.0). These findings suggest that shocked NDAF solutions are potentially promising to explain the energy output of gamma-ray bursts (GRBs). We employ our NDAF model formalism to elucidate L ν ν ¯ obs for five GRBs with known redshifts and estimate their accretion rate ( m ̇ ) based on the spin (a k) of the central source of the GRBs studied here.

Relativistic Low Angular Momentum Advective Flows Onto Black Hole and Associated Observational Signatures

Abstract We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their (in)stability properties, taking into account various values of specific angular momentum, λ 0. Within the range 10−50R g (where R g denotes the Schwarzschild radius), shocks are discernible for λ 0 ≥ 1.75. In the special relativistic hydrodynamic simulation when λ 0 = 1.80, we find the merger of two shocks resulted in a dramatic increase in luminosity. We present the impact of external and internal flow collisions from the funnel region on luminosity. Notably, oscillatory behavior characterizes shocks within 1.70 ≤ λ 0 ≤ 1.80. Using free–free emission as a proxy for analysis, we show that the luminosity oscillations between frequencies of 0.1−10 Hz for λ 0 range as 1.7 ≤ λ 0 ≤ 1.80. These findings offer insights into quasi-periodic oscillation emissions from certain black hole X-ray binaries, exemplified by GX 339-4. Furthermore, for the supermassive black hole at the Milky Way's center, Sgr A*, oscillation frequencies between 10−6 and 10−5 Hz were observed. This frequency range, translating to one cycle every few days, aligns with observational data from X-ray telescopes such as Chandra, Swift, and XMM-Newton.

Assessment of geogenic radon potential with activation of advective soil air flow

Assessment of geogenic radon potential with activation of advective soil air flow

Different dissolved organic matter sources sustain microbial life in a sandy beach subterranean estuary – an incubation study

In subterranean estuaries (STE), fresh and saline groundwater introduce dissolved organic matter (DOM) of different origin. This DOM serves as substrate for microorganisms that thrive in the STE. In high-energy beaches with dynamic porewater advection, microbial communities face frequent changes in groundwater composition, even at several meters depth. It is unknown how DOM from deep STE groundwater (> 5 m depth) is transformed by prevailing microbial communities. To address this question, we performed sediment incubations in flow-through reactors (FTRs) with deep (6 m depth) STE groundwater of low (1.6) and high salinity (29.1). FTR setups were sampled daily for quantification of dissolved organic carbon (DOC), and at start and end (day 13) of the incubation for analysis of DOM composition, microbial cell numbers and community composition. Solid-phase extracted DOM was molecularly characterized via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry. Both groundwater types contained mainly reworked DOM. Corroborating its presumed origin, the fresh groundwater had a more terrestrial DOM signature with a higher proportion of aromatic compounds compared to the saline groundwater. Over the course of the incubation, DOC concentrations increased primarily due to leaching of sedimentary organic matter, providing an additional source of DOM. In all setups, the DOM composition changed significantly from start to end, and similarly for fresh and saline groundwater. From the ~2700 molecular formulae (MF) detected on day 0, 34-35% were removed during the incubations, demonstrating the potential of deep STE microbial communities to degrade recalcitrant DOM that is supplied with the advective porewater flow. However, a substantial portion of MF (63-64%) was retained in both groundwater types, indicating that a fraction of deep STE-DOM is resistant to removal. Properties of MF that were newly detected on day 13 (26-28%) were indicative of labile DOM. Some of these newly detected MF were also identified in sediment-leachates, suggesting that beach sediments are a source of fresh DOM for the STE microbial communities. It is likely that due to longer groundwater residence time in the STE, continuous leaching and microbial processing shift the molecular composition of released DOM from more labile to more recalcitrant DOM.

Insight-HXMT View of the Black Hole Candidate Swift J1727.8–1613 during Its Outburst in 2023

The transient Galactic black hole candidate Swift J1727.8-1613 went through an outburst for the very first time in 2023 August and lasted for almost 6 months. We study the timing and spectral properties of this source using publicly available archival Insight-HXMT data for the first 10 observation IDs that last from MJD 60181 to 60198 with a total of 92 exposures for each of the three energy bands. We have detected quasi-periodic oscillations (QPOs) in a frequency range of 0.21 ± 0.01–1.86 ± 0.01 Hz by fitting the power density spectrum. Based on the model-fitted parameters and properties of the QPOs, we classify them as type C in nature. We also conclude that the origin of the QPOs could be the shock instabilities in the transonic advective accretion flows around black holes. The spectral analysis was performed using simultaneous data from the three onboard instruments LE, ME, and HE of Insight-HXMT in the broad energy band of 2−150 keV. To achieve the best fit, spectral fitting required a combination of models, e.g., interstellar absorption, power-law, multicolor disk–blackbody continuum, Gaussian emission/absorption, and reflection by neutral material. From the spectral properties, we found that the source was in an intermediate state at the start of the analysis period and was transitioning to the softer states. The inner edge of the accretion disk moved inward in progressive days following the spectral nature. We found that the source has a high inclination of 78°−86°. The hydrogen column density from the model fitting varied in the range of (0.12 ± 0.02−0.39 ± 0.08) × 1022 cm−2.

Effects of the internal temperature on vertical mixing and on cloud structures in ultra-hot Jupiters

Context. The vertical mixing in hot-Jupiter atmospheres plays a critical role in the formation and spacial distribution of cloud particles in their atmospheres. This affects the observed spectra of a planet through cloud opacity, which can be influenced by the degree of cold trapping of refractory species in the deep atmosphere. Aims. We aim to isolate the effects of the internal temperature on the mixing efficiency in the atmospheres of ultra-hot Jupiters (UHJs) and the spacial distribution of cloud particles across the planet. Methods. We combined a simplified tracer-based cloud model, a picket fence radiative-transfer scheme, and a mixing length theory to the Exo-FMS general circulation model. We ran the model for five different internal temperatures at typical UHJ atmosphere system parameters. Results. Our results show the convective eddy diffusion coefficient remains low throughout the vast majority of the atmosphere, with mixing dominated by advective flows. However, some regions can show convective mixing in the upper atmosphere for colder interior temperatures. The vertical extent of the clouds is reduced as the internal temperature is increased. Additionally, a global cloud layer gets formed below the radiative-convective boundary (RCB) in the cooler cases. Conclusions. Convection is generally strongly inhibited in UHJ atmospheres above the RCB due to their strong irradiation. Convective mixing plays a minor role compared to advective mixing in keeping cloud particles aloft in UHJs with warm interiors. Higher vertical turbulent heat fluxes and the advection of potential temperature inhibit convection in warmer interiors. Our results suggest that isolated upper atmosphere regions above cold interiors may exhibit strong convective mixing in isolated regions around Rossby gyres, allowing aerosols to be better retained in these areas.

Boron diffusion, related isotope fractionation and the structural role of B in pegmatite forming melts

In this study, we experimentally investigate the diffusion of B in flux-rich (1.7 % Li2O, 2.5 % B2O3, 2.7 % P2O5 and 3.5 % F) pegmatite forming melts in order to assess the transport mechanisms of B in rare-element pegmatite deposits. Glasses were synthesized with either different B isotope compositions or with different B concentrations (up to 2.45 wt% B2O3). In order to explore the effect of water on transport properties, nominally dry and hydrous glasses (3.1–3.9 wt% H2O) were investigated. The produced glasses were combined to diffusion couples to study self-diffusion and chemical diffusion of B in the same chemical system. Experiments were conducted using an internally heated pressure vessel and a rapid-heat and rapid-quench cold-seal pressure vessel at 100 MPa Ar pressure in the temperature range of 850–1250 °C for 1.6–97.3 h. In chemical diffusion experiments, the flow of B is compensated by a combined opposing flow of all other cations and F, without changing their element ratios. Thus, effective binary diffusion coefficients are obtained. Similar activation energies were determined for B self-diffusion and chemical diffusion under nominally dry conditions (196 ± 6 kJ/mol and 200 ± 17kJ/mol, respectively). Many hydrous experiments show a tilted to strongly deformed interface after the run, which was probably formed by advective flow of the low-viscosity melts during heating under pressure. Based on the successful experiments, an activation energy of 138 ± 8 kJ/mol was estimated for chemical diffusion of B in hydrous melts. We show that B diffusivity correlates with the Eyring diffusivity and, therefore, with the melt viscosity. The stable isotopes of B fractionate kinetically along the diffusion profiles due to the faster diffusivity of 10B over 11B. This isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.032 ± 0.002. This effect is small, but significant and was previously not observed in experimental studies. While it is insensitive to the water contents used in our study, it seems to be strongly dependent on the experimental boundary conditions, such as the ratios of B between the diffusion couple halves and the B enrichment in the melt. Solid-state NMR experiments reveal that the majority of B is in trigonal coordination in our melts, which effectively weakens the melt structure. This further suggests that coordination differences are unlikely to be the driving force for B isotope fractionation between melt and fluid during late-stage fluid exsolution in pegmatitic systems.

Enhancement of heat transfer characteristics in wavy microchannel heat sinks with streamlined micropins within convergent-divergent flow passages

In the current study, the heat transfer characteristics of a novel design microchannel with a wavy sinusoidal convergent-divergent wall structure featuring streamlined pins have been numerically investigated under laminar flow conditions. Various amplitudes, wavelengths, pin heights, and Reynolds numbers are considered as parameters. The thermal and hydraulic characteristics are analyzed in terms of Nusselt number, Fanning friction factor, and Performance Evaluation Criteria number (PEC), which evaluates the heat transfer enhancement against the associated pressure drop. It is revealed that increasing the amplitude and decreasing wavelength, along with moderate pin heights, significantly augment heat transfer. The streamlined pins effectively direct the flow, and the resulting secondary flow and chaotic advection considerably enhance heat transfer in the designed configuration. The study demonstrates that the heat transfer can be improved by a factor of 4.79 compared to a straight microchannel. Under these geometric and flow conditions, the pressure drop increases by a factor of 9.87, and the PEC is found to be around 2.23. A multi-objective optimization study using the Non-dominated Sorting Genetic Algorithm (NSGA-II) is conducted with Genetic Aggregation Response Surface Methodology to evaluate the impact of various parameters on thermal-flow performance and to identify the optimal design with material quantity variation. According to the optimal results, a 5 % increase in material can augment thermal-flow performance by approximately 206.6 % compared to traditional microchannels. Finally, practical correlations for the Nusselt number and friction factor, accounting for various parameters, have been developed to facilitate in designing of microchannel cooling systems.

Study of relativistic hot accretion flow around Kerr-like wormhole

We investigate the structure of relativistic, low-angular momentum, inviscid advective accretion flow in a stationary axisymmetric Kerr-like wormhole (WH) spacetime, characterized by the spin parameter (a k), the dimensionless parameter (β), and the source mass (M WH). In doing so, we self-consistently solve the set of governing equations describing the relativistic accretion flow around a Kerr-like WH in the steady state, and for the first time, we obtain all possible classes of global accretion solutions for transonic as well as subsonic flows. We study the properties of dynamical and thermodynamical flow variables and examine how the nature of the accretion solutions alters due to the change of the model parameters, namely energy (ℰ), angular momentum (λ), a k, and β. Further, we separate the parameter space in λ-ℰ plane according to the nature of the flow solutions, and study the modification of the parameter space by varying a k and β. Moreover, we retrace the parameter space in a k-β plane that allows accretion solutions containing multiple critical points. Finally, we calculate the disc luminosity (L) considering free-free emissions for transonic solutions as these solutions are astrophysically relevant and discuss the implication of this model formalism in the context of astrophysical applications.

Multiphase flow modelling of gas migration from a hypothetical integrity-compromised petroleum well in the peace region of North-eastern British Columbia, Canada

Well-integrity failure occurs in a small subset of petroleum wells, resulting in release of fugitive gas into intersected geologic formations. Released fugitive gas from geoenergy systems is a growing environmental concern that can contaminate groundwater aquifers and emit greenhouse gases (GHGs) to atmosphere. Currently, the roles of well-cement quality and properties of intersected geologic formations on the environmental outcomes of well-integrity failure is poorly understood. To advance understanding, we numerically modelled a hypothetical fugitive methane release from a petroleum well intersecting the Sunset Paleovalley aquifer system in Northeast British Columbia, Canada. We simulate a 10-year release and migration of fugitive gas into a two dimensional, two-phase, two-component advective flow field with the subsurface properties informed by field and laboratory data. We evaluate the effects of cement quality, gas release depth, and geologic heterogeneity on fugitive-gas containment or emission by defining and/or evaluating three key numbers: a) emission-retention ratio (ERR), b) well integrity index (WII), and c) fugitive gas mobility ratio (MR) over relevant spatiotemporal scales. We show that ERR and WII capture the bifurcated impacts of fugitive gas from petroleum wells, including groundwater contamination and atmospheric emissions. A WII close to one reduces vertical fugitive-gas migration along the well bore, fosters lateral migration into intersected geologic materials and significantly limits GHG emissions to atmosphere. MR and ERR values show fugitive-gas migration and fate are primarily controlled by the casing annulus cement quality, particularly when fugitive gas is released at shallow depths. We conclude that the quality of petroleum-well cement is among the parameters controlling the migration pathways, impacts, and fate of fugitive-gas release.

Relative contribution of the ocean-air heat exchange and advective heat transport to the increase of the Barents Sea water temperature in the early 21st century

The annual water temperature in the major water masses of the Barents Sea (BS) has significantly increased since the early 2000s. Advective heat transport from the neighboring water areas and heat exchange through the sea surface are the major factors, which shape the hydrological conditions in the BS. The paper estimates the contributions of heat exchange at the sea-atmosphere boundary and advective heat transport to changes in the average water temperature of the BS for the entire sea area. The average annual heat balance of the BS is calculated using atmospheric and oceanic reanalysis data. The change in the average temperature of the BS water is estimated taking into account the heat consumption for ice melting. The average surface heat balance from 1993 to 2018 was negative throughout the entire sea area: –70…–100 W/m2 in the south and –10…–20 W/m2 in the north. The advective heat supply was calculated for 9 straits with neighboring water areas. The determining source of advective heat is the influx of Atlantic waters from the Norwegian Sea between Cape Nordkapp and Bear Island. An average of 40.8 TW of advective heat is supplied through this margin. The calculations showed the predominance of annual heat influx due to advection over heat loss from the sea surface. This excess heat influx resulted in an estimated increase in the water temperature of the BS from 1993 to 2018 at a rate of 0.28 °C per year (taking into account the heat consumption for ice melting). In conclusion, it can be argued that the analysis has validated the hypothesis proposed in the article about compensation of heat losses from the surface of the BS by advective heat flow. The hypothesis is quantitatively confirmed by calculations on a simple box model (with an accuracy of up to an order of magnitude) based on atmospheric and oceanic reanalysis data. The ERA5 and GLORYS12V1 reanalysis data reliably describe the basic patterns of observed variability of ocean, sea ice and atmospheric parameters in the Barents Sea.

The Not-so Dramatic Effect of Advective Flows on Gas Accretion

Super-Earths and mini-Neptunes are the most common types of exoplanets discovered, yet the physics of their formation are still debated. Standard core accretion models in gas-rich environments find that typical mini-Neptune mass planets would blow up into Jupiters before the underlying disk gas dissipates away. The injection of entropy from the protoplanetary disk into forming gaseous envelopes has recently been put forward as a mechanism to delay this runaway accretion, specifically at short orbital distances. Here, we reevaluate this line of reasoning by incorporating recycling flows of gas into a numerical one-dimensional thermodynamic model with a more realistic equation of state and opacities and the thermal state of the advective flow. At 0.1 au, we find that advective flows are only able to produce mini-Neptunes if they can penetrate below ∼0.25 of the planet’s gravitational sphere of influence. Otherwise, the gas-to-core mass ratio (GCR) reaches above ∼10%, which is too large to explain the measured properties of mini-Neptunes, necessitating other gas-limiting processes such as late-time core assembly. The effect of entropy advection on gas accretion weakens even further beyond 0.1 au. We present an updated scaling relation between GCR and the penetration depth of the advective flows, which varies nontrivially with orbital distances, core masses, and dusty versus dust-free opacity. We further demonstrate how measurements of planet mass distribution beyond ∼1 au using future instruments such as the Nancy Grace Roman Space Telescope could be used to disambiguate between different formation conditions of gas-poor planets.

Ambient-gas forced convection dictated evaporation kinetics of generic sessile droplets

We probe the temporal evolution of internal thermo-hydrodynamic features of evaporating sessile droplets under external forced convection. A transient numerical model based on Arbitrary Lagrangian-Eulerian (ALE) framework is adopted. The governing equations corresponding to the transport phenomena (mass, momentum and energy) are solved in a fully coupled manner in both the droplet (liquid) and ambient (gaseous) domains while accurately tracing the droplet interface evolution. The role of initial wetting state, external convection velocity and convective heating/cooling effects due to the air-flow are explored in details, and good agreements are noted with previous reports from literature. Results show that although the evaporation rate is augmented significantly in forced convection conditions, the same does not increase in proportion at higher Reynolds numbers. This is primarily due to the enhanced cooling effects that reduce saturated vapor concentration at droplet surface, and thereby supress the diffusion mechanism. Also, depending upon the contact angle, the evaporative cooling effects are compensated partially or completely due to convective heating. This altered thermal profile substantially influences internal hydrodynamic features (Marangoni flow and heat advection) during the transient and stable regimes of droplet evaporation. At higher degrees of convective heating, the droplet interface temperature becomes higher than its base temperature. This condition results in a completely opposite internal Marangoni vortex at stable state compared to that induced due to evaporative cooling effects. We also provide a regime map to show the role of the gas-phase Stefan number on the droplet-phase hydrodynamic regimes.

Fugacity model covering abiotic and biotic matrices to investigate the transfer and fate of perfluoroalkyl acids in a large shallow lake of eastern China

Solving the challenges faced during the measurement of the cross-interface transfer of perfluoroalkyl acids (PFAAs) in lakes is crucial for clarifying environmental behaviours of these chemicals and their efficient governance. This study developed a multimedia fugacity model based on the quantitative water–air–sediment interaction (QWASI) covering abiotic/biotic matrices to investigate the cross-interface transfer and fate of PFAAs in Luoma Lake, a typical PFAA-contaminated shallow lake in eastern China. The accuracy and reliability of the established model were confirmed using Percent bias and Monte Carlo simulation, respectively. Using the QWASI model, the multimedia transfer of the PFAAs and their accumulation and persistence in different sub-compartments were described and measured, and the differences among individual PFAAs were explored. The simulation results showed that the sedimentation and resuspension of PFAAs were the most intense cross-interfacial transfers, and the sediments served as a chemical sink in the long term. A significant negative correlation of NC-F (the number of CF bonds) with the relative outflow flux (TW·out-ct) but a positive correlation with the relative net transfer across the interface between water and aquatic plants (Tp-ct) was detected, indicating that the PFAA migration capacity decreased but the bioaccumulation potential increased with the CF bond number. The persistence in water (Pw) of individual PFAAs ranged from 19.65d (PFOA) to 32.22d (PFOS), with an average of 26.15d; their persistence in sediment (Ps) ranged from 432d (PFBA) to 3216d (PFOS), with an average of 1524d, increasing linearly with an increase in NC-F. The water advection flows into and out of the lake (QW·in and QW·out), the PFAA concentration of water inflow (CW·in), and bioconcentration factor of aquatic plants (BCFp) were the primary parameters sensitive to PFAAs in all sub-compartments, which are essential indexes for exploring promising remediation pathways for lacustrine PFAA contamination based on the fugacity model simulation.

New Heat Flow Measurements Offshore Montserrat: Advective Heat Flow Detected via MeBo Borehole Temperature Logging

AbstractNew heat flow measurements collected at the Lesser Antilles Arc using a Hybrid Lister‐Outrigger probe and a new logging‐while‐tripping MeBo70 drilling approach provide the first high‐resolution (meter‐to‐cm‐scale) temperature‐depth measurements across the Lesser Antilles volcanic arc and offer new insight into heat and fluid transfer at a convergent oceanic margins. At multiple sites where logging‐while‐tripping MeBo temperature measurements were made, temperature increases linearly with depth in shallowly buried hemipelagic sediment but is isothermal or significantly hotter in deeper, courser‐grained sediments associated with mass flows. We interpret these isothermal zones as regions where advective heat flow—perhaps caused by convection or pressure‐driven advection—dominates. The implication is that apparently conductive heat flow regimes observed in the shallowest upper 5–10 m of hemipelagic sediment across the Lesser Antilles Arc measured using standard lister‐type probes may often unknowingly be influenced by deeper, advective flow along buried mass transport deposits at this site. Since mass transport deposits are ubiquitous on convergent margins, higher permeability mass transport deposits may play a fundamental and previously unrecognized role in fluid and heat transport.

The dynamic mechanism and optimal parameters of hydraulic jump induced disk floatation

Abstract The hydraulic jump induced disk floatation is one of the important and complicated kinetic issues. Based on the observation of the hydraulic jump phenomenon and disk floatation in experiment, the gas-liquid two-phase flow and free surface advection are simulated by COMSOL Multiphysics. The force and flow fields, as well as the movement mode of the disk are analyzed through numerical simulation. Meanwhile, a theoretical model is firstly proposed to investigate the motion equations from the perspectives of energy and force. The stable, metastable and instable motion modes are found out and analyzed in detail by variable-controlled and comparison methods. The critical parameters of stability and instability of the disk are obtained by continuous interpolation method.

A New Look at Diffusion in Vapor Intrusion Assessments; Passive Adsorptive Diffusion Samplers

AbstractVapor intrusion of toxic volatile organic compounds (VOCs) from subsurface soil vapor, through a building slab/floor, and into the indoor air is an important environmental contaminant transport mechanism. It is widely believed that advective flow, driven by the pressure differential between the subslab and indoor air, is the primary mechanism of subslab soil vapor entry into buildings. This paper explores the hypothesis that molecular diffusion through the slab may potentially play a larger role in vapor intrusion than previously believed and may even be the predominant vapor intrusion mechanism when the subslab vapor source strength is sufficiently high or the pressure differential is relatively low. A novel sampling device, referred to as a Passive Adsorptive Diffusion Sampler (PADS), is presented for the purpose of directly measuring the diffusion of VOCs through a building slab. A vacant warehouse was identified as a case study site where historical sampling had determined that vapor intrusion of trichloroethene (TCE) was adversely impacting the indoor air. Calculations using Fick's First Law of Diffusion are presented which demonstrate that diffusion alone can theoretically account for all the TCE observed in the indoor air at this building based on an effective diffusion coefficient for concrete that was calculated from the Johnson and Ettinger Model. Two groups of nine replicate PADS were deployed at two areas on the slab and used to measure the flux and effective diffusion coefficient at each of the 18 total points, which showed an order of magnitude variability within each area and over two orders of magnitude variability overall. These results indicate that diffusion through concrete is inherently variable when measured at a sub‐meter scale. However, when combined over both areas, the overall average approached that calculated from the Johnson and Ettinger Model. An additional 12 PADS were deployed across the building slab (for a total of 30) to quantify the overall building‐wide diffusive flux. This area‐weighted average diffusive flux was consistent with the predicted diffusive flux as calculated from Fick's First Law and the vapor intrusion mass input required to achieve the observed indoor air TCE concentration. The results of this study show that PADS provides a simple way to measure diffusive flux directly without having to drill through the slab. However, significant variability in the measured flux should be expected and will need to be accounted for by the inclusion of a relatively large number of samples including replicates. When using PADs at a new site, the collection of traditional subslab vapors at a select number of locations is recommended for the verification of a building‐specific effective diffusion coefficient, which may not necessarily be the same as for this building.

Positron emission tomography quantifies crystal surface reactivity during sorption reactions

Mechanistic understanding and prediction of solute transport and surface reactions in the pore space of rocks and soils is critical for a variety of processes, including sediment diagenesis, contaminant remediation, and long-term waste storage strategies. Positron emission tomography (PET) using β+-emitting radiotracers is an established and reliable method for investigating advective flow and diffusive flux in porous geomaterials. Here we present a conceptual strategy for spatially resolved quantification of crystal surface reactivity for sorption reactions based on PET techniques. The deconvolution of tracer breakthrough curves quantifies the sorption of F− tracer on calcite crystal surfaces and identifies the varying surface reactivity in the complex porous material. Using an artificial sediment as a model system, we demonstrate the quantifiability of sorption effects down to 10 pmol/mm3. The proposed strategy outlines how spatially resolved surface reactivities and their temporal changes over time can generally be determined using PET.

High mantle helium flux unveils active mantle melting beneath the cathaysia block, south China

Helium (He) in hydrological systems is used to constrain the deep structure and improve interpretations of geophysical techniques beneath the Cathaysia Block (CB), of which the Cenozoic tectonic and geodynamic processes are controversial. The air-corrected 3He/4He ratio of both geothermal and non-geothermal waters in the CB range from 0.10 to 6.41Ra (Ra is the atmospheric 3He/4He ratio), displaying an increasing trend from inland to coastal area, aligning with thinning crust and younger magmatic activities. Mantle-derived He fluxes in the CB vary from 0.11 to 33.41 × 1010 atoms m−2s−1, surpassing those in stable continental areas by up to three orders of magnitude. Due to the absence of active volcanic surface manifestations and identifiable crustal magma chambers, the mantle-derived volatiles are possibly transported from the mantle through the faults with advective flow rates ranging from 1.26 to 154 mm y−1. Two distinct modes of mantle He releases lead to differences in mantle He fluxes between the interior and coastal CB. Seismic activity enhances permeability in the interior CB, leading to the leakage of mantle He. In the coastal CB, high mantle He fluxes with characteristics of volcanic degassing imply degassing from the partial mantle melting. The presence of high 3He/4He ratios (up to 6.41 Ra) and regional thermal anomaly provide evidence for an ongoing process of crustal underplating by mantle melting. Combined with the underplate layer revealed by geophysical results, this implies the continuous compensation mechanism involving mantle influx to counter extension-induced crustal thinning since the Mesozoic–Cenozoic in the coastal CB.

Evaluation of accurate measurement methods for radon exhalation rate with advection effect and application in field

In decommissioning of a uranium tailings pond, radon exhalation rates on a beach surface should meet regulatory standards. Accurate measurements of the radon exhalation rate are demanded. However, current studies fail to consider the impact of advection under temperature variations or pressure gradients caused by gas movement on measurements using an accumulation chamber. Two proposed methods were therefore evaluated to accurately measure radon exhalation rates on the loose medium surface under advective conditions. Repeated experiments were conducted on a laboratory experimental platform filled with uranium tailings sand under advective flow rates of 0.03 and 0.3 L/min to validate the stability and reliability. Deviations between measured and true values were 0.1–6.1 % and 6.3–29.2 % for the two methods, respectively. Subsequently, numerical simulation was used to analyze defects of traditional methods and mechanisms of the new methods. In a field study, all methods were compared, and a predictive map of radon exhalation rates was created using interpolated data from 20 random sites using the new method. Results from the proposed methods, compared with traditional ones, were closer to true values under advective conditions, and accurate assessment of beach surface treatment was expected.