Tracer Front Research Articles

Dynamics of diabatically forced anticyclonic plumes in the stratosphere

AbstractA new class of vortices has been observed in the stratosphere following extreme wildfires (Canada 2017, Australia 2020) and volcanic eruptions (Raikoke 2019). These vortices are long‐lived mesoscale anticyclones (hundreds to 1,000 km in diameter) trapping plumes of aerosols and combustion/volcanic compounds. Owing to their unusual composition, these anticyclonically trapped plumes (ATPs) are associated with a significant radiative heating, which fuels their ascent through the stratosphere. This article investigates the dynamics of ATPs using two complementary approaches: analytically, in a potential vorticity (PV) perspective, and through idealised numerical simulations with the Weather Research and Forecasting (WRF) model. In both cases, we consider the vortical flow forced by a heating Lagrangian tracer. By reformulating the problem in the potential radius–potential temperature coordinate system introduced for tropical cyclones, we first clarify that ATP formation is concomitant with the injection of air into the stratosphere at extratropical latitudes. Then, we derive a set of simplified one‐dimensional equations describing the subsequent evolution of the flow after the injection. The equation obtained for the tracer is a variant of the classical Burgers' equation. In qualitative agreement with the three‐dimensional WRF simulations, this theoretical model predicts that ATPs develop an upper tracer front associated with sustained near‐zero anticyclonic PV, followed by a smooth tracer tail of cyclonic PV. Radiative relaxation of the temperature perturbations induced by the anticyclone and the presence of an initial PV anomaly tend to stabilise ATPs during their ascent. Finally, we note that the theory predicts a similar relationship between the plume and anticyclonic PV for cooled ATPs, which is supported by three‐dimensional simulations and may apply to the 2022 Hunga volcanic plume.

Long‐Distance/Time Surf‐Zone Tracer Evolution Affected by Inner‐Shelf Tracer Retention and Recirculation

AbstractThe evolution of a surf‐zone released tracer ( Liter over 4 hr) was observed for h. Surf‐zone tracer was transported alongshore (y) with relatively steady mean speed vSZ ≈ 0.18 m s−1, consistent with obliquely incident wave forcing. Maximum in situ surf‐zone tracer concentration decayed exponentially with 1.6 km alongshore e‐folding length scale, that is, 2.5 hr advective time scale. Surf‐zone tracer time‐series evolved downstream of the release from a top‐hat structure for y ≤ 1 km to increasingly skewed farther downstream. Within km of the northward propagating tracer front, inner‐shelf tracer was confined to onshore of (surf‐zone width LSZ ≈ 100 m) and was alongshore patchy. A coupled surf‐zone/inner‐shelf tracer advection‐diffusion‐exchange box model reproduces the observed surf‐zone downstream max concentration decay and temporal skewness, with surf‐zone flushing time h. A weaker inner‐shelf unidirectional‐exchange rate kIS ≈ kSZ/2 indicates reduced horizontal mixing outside the surf‐zone. Surf‐zone temporal skewness is linked to inner‐shelf tracer storage, differential surf‐zone/inner‐shelf advection, and recirculation, that is, non‐asymptotic shear dispersion. On the inner‐shelf (), tracer vertical structure differed in the morning versus afternoon suggesting internal tide and solar forced thermal modulation. Model parameters representing surf‐zone processes are well constrained by existing observations and scales. However, the many overlapping inner‐shelf processes make a single process based generalization of inner‐shelf cross‐shore exchange rate (i.e., kIS) and alongshore transport difficult.

Improving well productivity – Ways to reduce the lag between the diffusive and convective time of flight in shale wells

This short communication explains how the key to improve shale well productivity lies in reducing the gap between the diffusive and convective time of flight. Diffusive time of flight in a hydrocarbon reservoir controls which reservoir regions will be affected by the pressure transient at a certain time after the onset of production. The convective time of flight is a measure of tracer front advance as well as for fluid withdrawal rates due to a well system draining the reservoir. The difference between the advance rate of a diffusive pressure front and a convective tracer front has been long recognized. However, only recently has it become clear that a thorough grasp of the two concepts is crucial for better decisions about fracture and well spacing in low permeability reservoirs. Although the diffusive time of flight is highlighted in the traditional pressure depletion models, the convective time of flight has a more direct impact on well productivity because it controls the advance rate, to the well system, of hydrocarbon fluids to be produced. The two concepts, diffusive and convective time of flight, are briefly highlighted below, with emphasis on their relevance for decision-making in shale field development solutions.

Comparison of pressure front with tracer front advance and principal flow regimes in hydraulically fractured wells in unconventional reservoirs

Pressure depletion patterns near hydraulically fractured wells in ultra-low permeability reservoirs occupy a much larger region than outlined by fluid withdrawal contours. When used for reservoirs with moderate or high permeability (say k > 1 mD), industry never needed to caution using pressure depletion plots as a proxy for fluid removal or drained rock volume (DRV). For such conventional reservoirs, pressure depletion plots are good proxies for fluid removal or drained rock volume (DRV). The fluid that moves from the margins of a pressure front, indicative of the so-called diffusive depth of investigation (DOI), will reach the well on the time-scale of economic life of the well. However, a whole new way of thinking is required when studying ultra-low permeability shales (nanoDarcy range), where the distinction between pressure depletion zones and the DRV regions (where fluid is recovered from) becomes crucial to understand which factors control the hydrocarbon recovery process. Our detailed analysis compares the diffusive DOI, of the leading edge of the pressure depletion front, and that of the convective DRV given by a tracer front. Results show that the pressure depletion front between the fractures quickly approaches the adjoining fracture pressure, and most of the production originates from the fracture tips after 12 months. The pressure depletion pattern vastly overestimates the DRV, whereas the tracer front shows the actual produced rock volume. A final section evaluates the specific flow regimes that correspond to certain diffusive DOI and convective DRV growth stages in the life of the well. The discussion section explains how production diagnostic plots can be used to identify the moment in the well life when enhanced oil recovery methods should be applied. This study can facilitate a better understanding of the fluid flow in fractured porous media and can be used to optimize fracture and well spacing to maximize the DRV.

Viscous Flow Approach to “Pushing Out Old Water” from Undisturbed and Repacked Soil Columns

Core Ideas Breakthrough of Br is substantially delayed with respect to the wetting front. Film thickness and specific contact area of infiltrating water are calculated. Viscous flow approach is successful in both undisturbed and repacked soil columns. “Pushing out old water” already occurs at the scale of soil columns. The application of bromide tracer in transient infiltration–drainage experiments on undisturbed and repacked soil columns revealed distinct delays of tracer front arrivals with respect to the wetting front arrivals. A viscous flow approach was matched to the flow data that resulted in the parameters film thickness F and specific contact area L of the mobile water with the stagnant parts of the porous medium. Various combinations of F and L were related to the tracer delays, where the combinations L × F, L × F 3, and L × F 2 increasingly showed the highest correlation. The volume of mobile water was turned over 3.8 to 29.5 times between the arrivals of the wetting fronts and the arrivals of the tracer fronts. The delays and their statistical interpretations in view of the viscous flow approach are considered a significant contribution to the understanding of the phenomenon of “pushing out old water” that is still obscured in the hydrology of hillslopes and catchments. Despite the lack of a satisfying explanation for the high numbers of mobile‐water turnovers, the fact that the phenomenon of “pushing out old water” already occurs in soil columns may stimulate its investigation at the soil profile scale.

English

This paper determines contaminant dispersivity in the coastal sand of River Mersey Outer Estuary using results of field measurement from dye solute injection. Dispersivity is an alternative parameter for describing the characteristic behaviour of porous media. In this case, 3-D plumes in 2-D Photo-images were taken from multiple injection zones across the foreshore. The colour images were calibrated, standardized and the optical intensity converted into concentration maps. Apparent dispersivity of the subsurface foreshore was graphically determined from spatial covariance tensors. The variances were obtained by subtracting the products of the zeroth moments and square of the mean location of mass centers from the second order moments, and results were divided by the zeroth moment. The variance tensors (xʹxʹ2, and yʹyʹ2) were plotted against their respective travel distance intervals. The result predicts linear increase of the variance of spread distance, enhancing estimate of dispersivity using the best fit relations   and  for the x-x and y-y planes. The dispersivity coefficient varied with increase in travel distance. It quantifies the rate of spreading of the mean concentration of the tracer front which shows dependence on the distance traveled by the front. The effect on the mean concentration at the injection point is therefore described by the area of the effective dispersivity coefficient controls. Key words: Sand aquifers, porous media, injectate, vadose zone, ground water, transport of pollutant, contaminant transport.

Cavern Formation in Non-Newtonian Media in a Vessel Agitated by Submerged Recirculating Liquid Jets

Cavern formation is a widely reported phenomenon in agitated vessels processing liquids with a yield stress. This study employs electrical resistance tomography and flow visualization techniques to monitor the spread of a tracer in a vessel containing complex non-Newtonian media agitated using submerged recirculating liquid jets. The experimental media are aqueous solutions of xanthan gum with varying concentrations. It has been demonstrated that when agitated using submerged jet, the tracer is localized within a fixed volume of the vessel and does not spread with time in the more concentrated xanthan gum solution, a phenomenon that is analogous to a cavern structure found in mechanical mixing. A Herschel–Bulkely model with a positive yield stress can be fitted to part of the rheogram of the more concentrated xanthan gum solution. The less-concentrated solution cannot be fitted with a positive yield stress model, and when agitated in a vessel, the tracer front pervades the entire volume of the vessel and ...

Subsurface Transport of in Soils of Wisconsin's Carbonate Aquifer Region.

is a waterborne pathogen known to have a significant reservoir in bovine manure. Land-dependent manure disposal may not result in significant or reliable pathogen attenuation and, therefore, presents a risk for transport of pathogenic spp. to groundwater. One factor missing in the existing literature is the role soil characteristics play in affecting oocyst transport. Of specific concern in regions with carbonate geology are macropores and other soil structures that contribute to preferential flow. Therefore, research is needed to understand soil type effects and important transport pathways for pathogens such as oocysts to drinking water wells. This study investigated transport potential in several soils overlying Wisconsin's vulnerable carbonate aquifer and related the soil transport to soil series and textural class. Experimental work involved monitoring the transport of oocysts through intact soil columns of different soil series under simulated rain conditions. Results demonstrate that a significant portion of oocysts will sorb or be physically entrapped in the soil, especially in soil with high clay content. However, silt loam soils with comparatively lower clay content demonstrated an ability to transport oocysts through the soil profile primarily via the first flush of water infiltrating through soil macropores. The rate of oocyst migration in silt loam soils paralleled the bromide tracer front, thus bypassing the soils' ability to strain or adsorb oocysts out of infiltrating water. Nevertheless, proper manure treatment and management are necessary to minimize public health risks.

Trajectory‐based modeling of fluid transport in a medium with smoothly varying heterogeneity

AbstractUsing an asymptotic methodology, valid in the presence of smoothly varying heterogeneity and prescribed boundaries, we derive a trajectory‐based solution for tracer transport. The analysis produces a Hamilton‐Jacobi partial differential equation for the phase of the propagating tracer front. The trajectories follow from the characteristic equations that are equivalent to the Hamilton‐Jacobi equation. The paths are determined by the fluid velocity field, the total porosity, and the dispersion tensor. Due to their dependence upon the local hydrodynamic dispersion, they differ from conventional streamlines. This difference is borne out in numerical calculations for both uniform and dipole flow fields. In an application to the computational X‐ray imaging of a saline tracer test, we illustrate that the trajectories may serve as the basis for a form of tracer tomography. In particular, we use the onset time of a change in attenuation for each volume element of the X‐ray image as a measure of the arrival time of the saline tracer. The arrival times are used to image the spatial variation of the effective hydraulic conductivity within the laboratory sample.

Estimation of Recharge from Long‐Term Monitoring of Saline Tracer Transport Using Electrical Resistivity Tomography

We conducted a field experiment at the agricultural field site Voulund within the Danish hydrological observatory, HOBE, with the purpose of estimating recharge using geophysical methods. In September 2011, a saline tracer was added across a 142‐m2 area at the surface at an application rate mimicking natural infiltration. The movement of the saline tracer front was monitored using cross‐borehole electrical resistivity tomography (ERT); data were collected on a daily to weekly basis and continued for 1 yr after tracer application. The ERT data were inverted and corrected for temperature changes in the subsurface, and spatial moment analysis was used to calculate the tracer mass, position of the center of mass, and thereby the downwardly recharging flux. The recovered mass was underestimated by the ERT data by up to 50%. Mass balance errors are widely recognized and are a result of variable resolution of the tomographic models and smoothing applied in the inversion routine. The results were nonetheless in very good agreement with pore water samples collected and analyzed from five cores extracted within the tracer application area during the same period. Recharge during the 7.5 mo from September 2011 to the end of April 2012 was estimated to be about 500 mm using the ERT data. This value is in good accord with recharge estimates made based on drainage data from buried lysimeters located only meters away from the cross‐borehole ERT array. This suggests that long‐term automated ERT monitoring of a surface‐applied tracer is a promising technique for estimating groundwater recharge.

Sand box experiments with bioclogging of porous media: Hydraulic conductivity reductions

Tracer experiments during clogging and de-clogging experiments in a 2D sand box were via an image analysis used to establish a data set on the relation between changes in hydraulic conductivity (K) and relative porosity (β). Clogging appears to create a finger-like tracer transport, which could be caused by an initial heterogeneous distribution of biomass in the sand box. De-clogging occurs at a slower rate possibly due to the presence of inert biomass that is not affected by the starvation conditions by sudden removal of the substrate source. The tracer front was observed to get disturbed closer and closer to the substrate source during the experiments suggesting that the zone of clogging moved upstream. Three clogging models, K(β), from the literature were tested for their ability to describe the temporal changes in clogging at the scale of the sand box; the model of Clement et al. (1996) that makes no assumption on biomass distribution, the plug formation model of Thullner et al. (2002a), and the biofilm-plug formation model of Vandevivere (1995). The plug formation and biofilm-plug formation models both match the observed changes between the hydraulic conductivity of the sand box and the relative porosity. Unfortunately our experiments did not reach low relative porosities where the two models predict different behaviors. The model by Clement et al. (1996) underestimates clogging.

Experimental Study of ERT Monitoring Ability to Measure Solute Dispersion

This paper reports experimental measurements performed to test the ability of electrical resistivity tomography (ERT) imaging to provide quantitative information about transport parameters in porous media such as the dispersivity α, the mixing front velocity u, and the retardation factor R(f) associated with the sorption or trapping of the tracers in the pore structure. The flow experiments are performed in a homogeneous porous column placed between two vertical set of electrodes. Ionic and dyed tracers are injected from the bottom of the porous media over its full width. Under such condition, the mixing front is homogeneous in the transverse direction and shows an S-shape variation in the flow direction. The transport parameters are inferred from the variation of the concentration curves and are compared with data obtained from video analysis of the dyed tracer front. The variations of the transport parameters obtained from an inversion performed by the Gauss-Newton method applied on smoothness-constrained least-squares are studied in detail. While u and R(f) show a relatively small dependence on the inversion procedure, α is strongly dependent on the choice of the inversion parameters. Comparison with the video observations allows for the optimization of the parameters; these parameters are found to be robust with respect to changes in the flow condition and conductivity contrast.

Single-hole GPR reflection imaging of solute transport in a granitic aquifer

Identifying transport pathways in fractured rock is extremely challenging as flow is often organized in a few fractures that occupy a very small portion of the rock volume. We demonstrate that saline tracer experiments combined with single-hole ground penetrating radar (GPR) reflection imaging can be used to monitor saline tracer movement within mm-aperture fractures. A dipole tracer test was performed in a granitic aquifer by injecting a saline solution in a known fracture, while repeatedly acquiring single-hole GPR sections in the pumping borehole located 6 m away. The final depth-migrated difference sections make it possible to identify consistent temporal changes over a 30 m depth interval at locations corresponding to fractures previously imaged in GPR sections acquired under natural flow and tracer-free conditions. The experiment allows determining the dominant flow paths of the injected tracer and the velocity (0.4-0.7 m/min) of the tracer front.

Tracer experiments within composite soil column investigated by MRI

The magnetic resonance imaging (MRI) technique was used for the visualisation and interpretation of flow paths. A set of tracer-infiltration experiments was performed on soil columns filled with packed homogeneous sand and with a composite sand-soil system. The flow paths were visualised using a tracer-solution containing Ni(NO<sub>3</sub>)<sub>2</sub> which is characterised by relaxation times different from that of the infiltrating water. The tracer pulse was added under hydraulic steady state conditions. Small disturbances in the tracer front were observed during the breakthrough in the case of a homogeneous sample. More pronounced effects were seen with the composite sample. The vertical components of the velocity fields were evaluated for the experiments presented. The irregularities in the tracer front and in the velocity fields were in this case attributed to the preferential flow phenomena in combination with air bubble entrapment. Beside that, two consecutive tracer pulses were performed with the aim of testing the potential influence of the different solute concentrations on the adsorption power. Both concentrations had negligible impacts on the acquired image. The presented results are constrained by the limits of the described technology; further investigations are being carried out using more advanced equipment. I

Multi tracer test for the implementation of enhanced in-situ bioremediation at a BTEX-contaminated megasite

At the Centre for Environmental Research Leipzig-Halle (UFZ) research site in Zeitz, Germany, benzene contaminates the lower of two aquifers with concentrations of up to 20 mg/l. Since the benzene plume has a minimum length of approximately 1 km, enhanced natural attenuation measures are being considered as a remediation strategy. This study describes the performance and evaluation of a multi-species reactive tracer test using the tracers fluorescein and bromide as conservative tracers and toluene as reactive tracer. Sampling was performed over a period of six months using a detailed network of multilevel sampling wells. Toluene was only slightly retarded in comparison to bromide, whereas fluorescein was retarded considerably stronger. Therefore, it was not possible to use fluorescein as an in situ tracer for the determination of groundwater velocities. The ionic nature of fluorescein is assumed to be the major reason for its retardation. The results show that the infiltration conditions were suitable to produce a wide spreading of the tracer front along the full thickness of the aquifer. Thus, a large aquifer volume can be treated in future enhanced bioremediation measures. The total quantity of infiltrated toluene (24 l) was degraded under sulfate-reducing conditions over a flow path of 50 m. Benzylsuccinate was identified as a metabolite of toluene degradation under sulfate-reducing conditions at this site. The modelling results show that toluene degradation was described more accurately using Monod kinetics than first-order kinetics. Since toluene was only slightly retarded in comparison to bromide, sorption and desorption processes were considered to be negligible.

Unsaturated flow in coarse porous media

Design of effective capillary barrier systems requires a thorough understanding of the soil–water interactions that take place in both coarse- and fine-grained unsaturated soils. Experimental observations of water flow through coarse porous media are presented to gain greater understanding of the processes and mechanisms that contribute to the movement and retention of water in coarse-grained unsaturated soils. The use of pendular ring theory to describe how water is held within a porous material with relatively low volumetric water contents is explored. Experimental measurements of seepage velocity and volumetric water content were obtained for columns of 12 mm glass beads using digital videography to capture the movement of a dye tracer front at several infiltration rates. An estimated curve for hydraulic conductivity versus matric suction is shown and compared to a theoretical curve. The method is shown to provide a reasonable predictive tool.Key words: soil-water characteristic curve, hydraulic conductivity curve, water permeability function, capillary barrier, matric suction.

Magnetic resonance imaging of molecular transport in living morning glory stems

MRI was applied to investigate the transport pathways in Morning Glory plant stems. The study was carried out on living plants without affecting their integrity. The architecture of a dicotyledonous plant was deeply characterized: the root system structure and the vascular bundle location were identified, the presence of central voids caused by cell maturation and loss were observed in the stem. Molecular transport components were recognized, by observing the concentration profile of a tracer, which changed with time after its absorption by the plant roots. MRI analysis revealed the presence of an axial transport as the progress of the tracer front through the vascular bundles and a radial molecular transport from the vascular bundles toward the surface of the stem. As a result, the tracer molecular transport formed the parabolic tracer front (PTF). A model was built up through the analysis of the PTF that consisted of an axial front at the peak position and a radial front at the width of the parabolic tail. PTF analysis revealed differences between the tracer transport velocities in the axial and the radial directions in the plant stem. The model revealed that the width of the parabolic tail reflected the magnitudes of diffusion and permeation of the tracer in the plant stem.

The dipole-flow test with a tracer: a new single-borehole tracer test for aquifer characterization

We propose a new aquifer characterization test, the dipole-flow test with a tracer (DFTT), and develop its interpretation methodology. Combining the dipole-flow test (DFT) and a tracer test, the DFTT is a single-borehole, forced-gradient tracer test. The DFTT device isolates an injection and an extraction chamber in a well with inflatable packers and utilizes a small pump to create a dipole-flow pattern. After a steady-state flow field is reached and the pumping rate and chamber drawdowns are measured, a tracer is released into the injection chamber, and the concentration breakthrough curve is recorded in the extraction chamber. In developing the DFTT model, we assume that the aquifer is homogeneous on the scale of the test and that the well has no skin zone. We use a streamtube approach to semi-analytically simulate the tracer transport in a DFTT and determine the necessary relationships for estimating the longitudinal dispersivity as well as the radial and vertical hydraulic conductivities. The arrival time of the peak concentration is linearly related to the anisotropy ratio, and the arrival time of the tracer front is related to the longitudinal dispersivity. We present data from preliminary DFTTs conducted with Rhodamine WT (RWT) as a tracer at the Lizzie Field Site located between Farmville and Maury, NC. Our results demonstrate that this single-borehole tracer test is feasible and that its estimates of dispersivity are consistent with those reported in literature, whereas its estimates of hydraulic conductivity differ from the flowmeter-test estimates by less than an order of magnitude. This difference is most likely caused by the natural aquifer heterogeneity and different characterization scales of the two tests. The sorption of RWT and its composition of two differently sorbing isomers complicate the nature of the DFTT breakthrough curve and its interpretation. The use of a conservative tracer, such as bromide, should eliminate this complication. The skin effects readily manifest themselves in the DFTT breakthrough curve as extra and/or recirculated peaks. The presented interpretation methodology applies to cases with insignificant skin effects.

High liquid holdup airlift tower loop reactor: II. Downcomer hydrodynamic modeling

Hydrodynamics in the downcomer of a high liquid holdup airlift tower loop reactor, whose riser has been previously studied, is presented. A visual procedure with colorimetric tracers injected into the top part of the riser and monitored by means of a video camera system, for subsequent treatment of the obtained images, was used. The tracer front advance curves have suggested a compartmental model of three parameters to be used for physical transport processes. © 2000 Society of Chemical Industry

High liquid holdup airlift tower loop reactor: II. Downcomer hydrodynamic modeling

Hydrodynamics in the downcomer of a high liquid holdup airlift tower loop reactor, whose riser has been previously studied, is presented. A visual procedure with colorimetric tracers injected into the top part of the riser and monitored by means of a video camera system, for subsequent treatment of the obtained images, was used. The tracer front advance curves have suggested a compartmental model of three parameters to be used for physical transport processes. © 2000 Society of Chemical Industry