Apparent Dispersion Research Articles

Mid-Miocene cosmogenic upper limit for 10Be/21Ne burial age

Abstract Cosmogenic nuclide burial dating provides an alternative method for sediment dating, and the combination of radionuclide 10Be with noble gas isotope 21Ne would theoretically extend the burial dating range up to around 15 Ma ago, which endows 10Be/21Ne pair with huge potential in middle-Miocene sediment dating. Especially in the magnetostratigraphic studies of the Cenozoic sedimentary strata, the 10Be/21Ne pair could be expected to provide absolute age marker when the well-dated volcanic ash layer or bedded mammalian fossils are absent. However, the validity and accuracy of the 10Be/21Ne burial dating when used close to its limiting middle Miocene ages has not been assessed in case study. Here, we show our dating results of the sedimentary stratum samples, which deposited at the same bedding of the Vertebrate fossils of Platybelodon tongxinensis that offer an independent age constraint between 12 and 15 Ma. The 10Be/21Ne analyses yielded a mean burial age of 13.25 ± 0.33 Ma, which agrees well with the paleontological proxy, and the MSWD value of 0.66 indicates the good agreement of the apparent dispersion of six aliquots data with their individual measurement errors. In view of the extended applicable dating range of 10Be/21Ne pair, it is promising in providing absolute age marker or valuable chronological information for a variety of fields in Earth science and beyond.

The anchoring effect-long-term dormancy and genetic population structure.

Understanding the genetic structure of populations is key to revealing past and present demographic and evolutionary processes in a species. In the past decade high genetic differentiation has been observed in many microbial species challenging the previous view of cosmopolitan distribution. Populations have displayed high genetic differentiation, even at small spatial scales, despite apparent high dispersal. Numerous species of microalgae have a life-history strategy that includes a long-term resting stage, which can accumulate in sediments and serve as refuge during adverse conditions. It is presently unclear how these seed banks affect the genetic structure of populations in aquatic environments. Here we provide a conceptual framework, using a simple model, to show that long-term resting stages have an anchoring effect on populations leading to increased genetic diversity and population differentiation in the presence of gene flow. The outcome that species with resting stages have a higher degree of genetic differentiation compared to species without, is supported by empirical data obtained from a systematic literature review. With this work we propose that seed banks in aquatic microalgae play an important role in the contradicting patterns of gene flow, and ultimately the adaptive potential and population dynamics in species with long-term resting stages.

Experimental observation of increased apparent dispersion and mixing in a beach aquifer due to wave forcing

Abstract Solute dispersion and mixing in beach aquifers is strongly influenced by highly transient flow induced from wave forcing. While transport at the groundwater-ocean interface has been modelled, little is known about the quantitative effect of wave forcing on solute dispersion and mixing in beach aquifers. We use a prototype-scale laboratory flume experiment to conduct tracer transport experiments at two locations within a wave-forced beach aquifer. For the first time we demonstrate by systematic laboratory experimentation that transient conditions in the re-circulation zone due to run-up and beach face infiltration as well as head oscillations caused by wave forcing strongly disperse and mix subsurface solute plumes. Wave forcing can increase the apparent dispersion by an order of magnitude depending on conditions, compared to solute transport without waves. Our findings illustrate that beach aquifer transport models need to consider the additional dispersion to correctly quantify mixing and biogeochemical processes in this highly dynamic zone.

Influence of Particle Size Distribution on Lifetime and Thermal Stability of Ostwald Ripening of Supported Particles

AbstractStability of dispersed particles on their supports is one of the central topics in heterogeneous catalysis and quantifying the influence of the particle size distribution on lifetime and thermal stability is highly valuable for rational design of efficient nanocatalysts. We report here a theoretical study of Ostwald ripening of supported particles, in particular, the kinetic evolution of particle number, average size, and dispersion with respect to time and thermal temperature in a wide range of size and monodispersity. Phase diagrams of half‐lifetime and onset temperature of ripening as functions of size and monodispersity were constructed. If decreasing the average particle size, though there is a modest gain in the dispersion, the stability declines dramatically; specifically, the half‐lifetime of ripening decreases exponentially and the corresponding onset temperature decreases by hundreds of Kelvin. Decline in stability owing to the decrease in size could, however, be systematically compensated by increasing the monodispersity of the size distribution. We find that the supported particles with the same half‐life time and onset temperature could originate from different particle size distributions, whereas the particle size distribution with the same apparent dispersion could have very different onset temperature and half‐lifetime. The result highlights the importance of both size and monodispersity in particle size distribution to the ripening resistance of supported particles, and the methodology developed for simulating ripening kinetics could be used to accelerate the aging protocols.

Analysis of temperature dependency of elastic moduli in heavy oil deposits

Seismic monitoring of oil sands during hot or cold production requires a valid rock physics model. The effective elastic properties of heavy oil deposits depend on temperature variations, which consequently alter the P- and S-wave velocities in thermal recovery processes. The authors discuss the relationship between temperature, apparent shear modulus, frequency, and velocity dispersion. We use log data from wells drilled in heated and cold zones of a bitumen reservoir to calculate bulk and shear moduli along the wellbore. We apply various filters to control the unwanted effects of other variables, such as porosity and water saturation. We demonstrate that sonic velocities of steam-saturated sands at reservoir conditions can be lower than the compressional velocity of seismic waves in water. We use modulus-temperature crossplots to verify the existence of the liquidation temperature and apparent shear modulus of the heavy oil. In our study, we observe two rapid-decline events in bulk modulus at around 20°C and 200°C. The first event exhibits the ideas of the viscoelastic model of Maxwell. We attribute the second event to effective replacement of liquid phase with steam. For temperatures between 20°C and 200°C, we use a linear relationship to model bulk modulus decline with temperature. The calculated shear modulus shows a wide range of variations at cold temperatures due to a slight change in the bitumen's API gravity with depth. We attribute the weakening of the shear moduli at temperatures greater than 100°C to thermal expansion of the rock.

Spatial extent of analysis influences observed patterns of population genetic structure in a widespread darter species (Percidae)

Abstract Connectivity among stream fish populations allows for exchange of genetic material and helps maintain genetic diversity, adaptive potential and population stability over time. Changes in species demographics and population connectivity have the potential to permanently alter the genetic patterns of stream fish, although these changes through space and time are variable and understudied in small‐bodied freshwater fish. As a spatially widespread, common species of benthic freshwater fish, the variegate darter (Etheostoma variatum) is a model species for documenting how patterns of genetic structure and diversity respond to increasing isolation due to large dams and how scale of study may shape our understanding of these patterns. We sampled variegate darters from 34 sites across their range in the North American Ohio River basin and examined how patterns of genetic structure and diversity within and between populations responded to historical population changes and dams within and between populations. Spatial scale and configuration of genetic structure varied across the eight identified populations, from tributaries within a watershed, to a single watershed, to multiple watersheds that encompass Ohio River mainstem habitats. This multiwatershed pattern of population structuring suggests genetic dispersal across large distances was and may continue to be common, although some populations remain isolated despite no apparent structural dispersal barriers. Populations with low effective population sizes and evidence of past population bottlenecks showed low allelic richness, but diversity patterns were not related to watershed size, a surrogate for habitat availability. Pairwise genetic differentiation (FST) increased with fluvial distance and was related to both historic and contemporary processes. Genetic diversity changes were influenced by underlying population size and stability, and while instream barriers were not strong determinants of genetic structuring or loss of genetic diversity, they reduce population connectivity and may impact long‐term population persistence. The broad spatial scale of this study demonstrated the large spatial extent of some variegate darter populations and indicated that dispersal is more extensive than expected given the movement patterns typically observed for small‐bodied, benthic fish. Dam impacts depended on underlying population size and stability, with larger populations more resilient to genetic drift and allelic richness loss than smaller populations. Other darters that inhabit large river habitats may show similar patterns in landscape‐scale studies, and large river barriers may impact populations of small‐bodied fish more than previously expected. Estimation of dispersal rates and behaviours is critical to conservation of imperilled riverine species such as darters.

Transport of Reactive Species in Oscillatory Annular Flow

The present article purposes to investigate the solute dispersion through an annular pipe in the presence of heterogeneous chemical reactions among the species and wall of the annulus. The solute is considered to experience a kinetic reversible phase exchange with the inner wall layer and irreversible absorption into the wall. Two kinds of oscillatory flow (Poiseuille and Couette flow) are considered in order to track the complex interactions between the velocity distributions and the reaction parameters. The method of moments as proposed by Aris-Barton is used to determine the apparent dispersion coefficient. The moment equations has been solved by using a standard finite difference implicit scheme, valid for small as well as large times. Dispersion coefficient due to the combined effect of reversible and irreversible reactions has been discussed in a variety of flow situations. Dispersion coefficient may be enhanced owing to the reversible and irreversible heterogeneous reactions in the boundary. On the basis of flow characteristics, radius ratio provides a mixed behaviour of the dispersion coefficient. Dimensionless mass proves to be an increasing function of reversible and irreversible boundary reaction parameters.

CONTRIBUCIÓN DE LAS FRECUENCIAS CARACTERÍSTICAS AL PERIODO DE VIBRACIÓN DOMINANTE EN LA CIUDAD DE QUERÉTARO, MÉXICO

En este trabajo se presenta la metodología desarrollada para evaluar la contribución de la frecuencia característica de cada estrato al período fundamental de vibración a partir de la inversión de las curvas de dispersión aparente obtenidas del levantamiento en campo de ondas superficiales con fuente activa, sin importar la velocidad de cada capa. El cálculo del período fundamental se realizó con el procedimiento simplificado de un medio estratificado lineal desarrollado por Dobry et al. (1976). Como referencia para la clasificación por clases o categorías de velocidades se utilizó el elaborado por el International Building Code (IBC, 2009) y modificado para los materiales del subsuelo de la Ciudad de Querétaro. Las unidades geológicas principales se agruparon en tres secuencias estratigráficas: la Rocosa que abarca las mesetas y elevaciones topográficas que rodean el valle, la de Ladera que es la zona de pendiente variable constituida por depósitos volcánicos y sedimentarios (aluviales), y la secuencia de Planicie que comprende depósitos estratificados del tipo lacustre, fluvial y volcánico. A partir del análisis de los perfiles de velocidades de ondas de corte se observó que existen estratos con velocidades bajas confinados por otros de alta velocidad; lo que indica la presencia de estratos de baja rigidez a profundidades variables. En la zona rocosa el período fundamental de vibración es corto (menor que 0.1 s), mientras que en los depósitos de la planicie es más largos (mayor que 0.3 s). En la zona de ladera el período es relativamente largo (variable entre 0.1 s a 0.3 s) posiblemente debido a un efecto topográfico. Los resultados obtenidos muestran que la metodología propuesta para evaluar el período dominante a partir de la integración de las frecuencias características obtenidas con ondas superficiales permite estimar los espesores y velocidades de corte de cada estrato, el periodo fundamental de vibración de cada secuencia estratigráfica y finalmente, sus espectros de sitio.

Transport of dissolved organic matter in Boom Clay: Size effects

A coupled experimental-modelling approach was developed to evaluate the effects of molecular weight (MW) of dissolved organic matter (DOM) on its transport through intact Boom Clay (BC) samples. Natural DOM was sampled in-situ in the BC layer. Transport was investigated with percolation experiments on 1.5cm BC samples by measuring the outflow MW distribution (MWD) by size exclusion chromatography (SEC). A one-dimensional reactive transport model was developed to account for retardation, diffusion and entrapment (attachment and/or straining) of DOM. These parameters were determined along the MWD by implementing a discretisation of DOM into several MW points and modelling the breakthrough of each point. The pore throat diameter of BC was determined as 6.6–7.6nm. Below this critical size, transport of DOM is MW dependent and two major types of transport were identified. Below MW of 2kDa, DOM was neither strongly trapped nor strongly retarded. This fraction had an averaged capacity factor of 1.19±0.24 and an apparent dispersion coefficient ranging from 7.5×10−11 to 1.7× 10−11m2/s with increasing MW. DOM with MW>2kDa was affected by both retardation and straining that increased significantly with increasing MW while apparent dispersion coefficients decreased. Values ranging from 1.36 to 19.6 were determined for the capacity factor and 3.2×10−11 to 1.0×10−11m2/s for the apparent dispersion coefficient for species with 2.2kDa<MW<9.3kDa. Straining resulted in an immobilisation of in average 49±6% of the injected 9.3kDa species. Our findings show that an accurate description of DOM transport requires the consideration of the size effects.

Alternatives for characterizing macroporous polyacrylamide monolithic ion exchanger columns

The development of new adsorbents to separate biocompounds and polyacrylamide monolithic cryogels shows great potential for purifying macromolecules from non‐clarified media. In this work, alternatives for functionalization and characterization of polyacrylamide monoliths grafted with 2‐acrylamide‐2‐methyl‐1‐propanosulfonic acid were explored. Dynamic grafting did not affect swelling capacity, or graft density. Pore size was evaluated by optical microscopy, which presented the same values obtained by scanning electronic microscopy. The method of moments was used to determine porosity, apparent axial dispersion coefficient, and the height of the theoretical plates of the columns, presenting results consistent with those in the literature. The permeability constant was determined by varying the viscosity of a Newtonian fluid at constant pressure, with the value being the same as that obtained by varying the hydrostatic pressure over the column. Results demonstrated these techniques were adequate for characterizing polyacrylamide cryogels, opening new perspectives in this field. POLYM. ENG. SCI., 58:1717–1725, 2018. © 2017 Society of Plastics Engineers

A Multi-telescope Campaign on FRB 121102: Implications for the FRB Population

Abstract We present results of the coordinated observing campaign that made the first subarcsecond localization of a fast radio burst, FRB 121102. During this campaign, we made the first simultaneous detection of an FRB burst using multiple telescopes: the VLA at 3 GHz and the Arecibo Observatory at 1.4 GHz. Of the nine bursts detected by the Very Large Array at 3 GHz, four had simultaneous observing coverage at other observatories at frequencies from 70 MHz to 15 GHz. The one multi-observatory detection and three non-detections of bursts seen at 3 GHz confirm earlier results showing that burst spectra are not well modeled by a power law. We find that burst spectra are characterized by a ∼500 MHz envelope and apparent radio energy as high as 1040 erg. We measure significant changes in the apparent dispersion between bursts that can be attributed to frequency-dependent profiles or some other intrinsic burst structure that adds a systematic error to the estimate of dispersion measure by up to 1%. We use FRB 121102 as a prototype of the FRB class to estimate a volumetric birth rate of FRB sources Mpc−3 yr−1, where N r is the number of bursts per source over its lifetime. This rate is broadly consistent with models of FRBs from young pulsars or magnetars born in superluminous supernovae or long gamma-ray bursts if the typical FRB repeats on the order of thousands of times during its lifetime.

Pressure and Fluid Effect on Frequency‐Dependent Elastic Moduli in Fully Saturated Tight Sandstone

AbstractWe developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones, especially at seismic frequencies. Calibration of the new system revealed that the system can operate reliably at frequencies of [2–200, 106] Hz. Tight sandstone with a “crack–pore” microstructure was tested under nitrogen gas (dry), brine, and glycerin saturation. A frequency‐dependent effect was not found for the dry case. However, apparent dispersion and attenuation for the undrained/unrelaxed transition was clearly observed for sample under brine or glycerin saturation, the magnitude of which was largely suppressed by increasing effective pressure. The measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range. A simple squirt‐flow model with dual‐porosity scheme was used to compare with the measurement results. Although the estimated values deviated slightly from the data, the trend fitted the saturated data relatively well, especially at low effective pressures. Therefore, considering the crack–pore microstructure of the tight sandstone, dispersion and attenuation are induced predominantly by the squirt‐flow stiffening effect from cracks to pores.

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&nbsp;&nbsp; and&nbsp; 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.

Clinical application value of 3.0T MR diffusion tensor imaging in grade diagnosis of gliomas.

The clinical value of MR diffusion tensor imaging (DTI) in grade diagnosis of gliomas was investigated. A total of 31 patients with glioma were administered 3.0T MR convention and DTI examination, with quantitative measurement of anisotropy coefficient fractional anisotropy (FA) and apparent dispersion coefficient (ADC) value, and the comparison of quantitative parameters of glioma between low- and high-grade, which was detected by Mann-Whitney U test. The receiver operation characteristic (ROC) curve was drawn to take the value of ADC and FA in tumor ROI as a critical point, to calculate the area under the curve and to confirm the diagnosis threshold value and evaluate its diagnostic efficiency. The FA value of 14 low-grade glioma cases was 139.4±81.3, with an ADC value of (1.36±0.21) ×10−3 mm2/sec. The FA value of 17 high-grade glioma cases was 103.1±41.5, with ADC value of (1.09±0.28)−3 mm2/sec; the difference between the two groups was statistically significant (P<0.05). The ADC value was taken as the critical point to judge tumor grade and draw the ROC curve; the area under the curve was 0.79. As the diagnosis threshold value, the ADC value of 1.11×10−3 mm2/sec was used to distinguish between low- and high-grade tumor with a sensitivity of 58.8% and specificity of 92.9%. The FA value was taken as a critical point to judge tumor grade and draw the ROC curve; the area under the curve was 0.62. As the diagnosis threshold value, the FA value of 178.9 was applied to distinguish between low- and high-grade tumor sensitivity of 94.1% and specificity of 35.7%. Therefore, the FA value and ADC value in DTI has an important estimated value for the pathological grade of glioma.

Lensing of Fast Radio Bursts by Plasma Structures in Host Galaxies

Abstract The amplitudes of fast radio bursts (FRBs) can be strongly modulated by plasma lenses in their host galaxies, including that of the repeating FRB 121102 at ∼1 Gpc luminosity distance. Caustics require the lens’ dispersion measure depth ( ), scale size (a), and distance from the source ( ) to satisfy . Caustics produce strong magnifications ( ) on short timescales ( hours to days) that appear as narrow spectral peaks (0.1–1 GHz). They also suppress the flux density in longer-duration (∼months) troughs. Multiply imaged bursts will arrive differentially by to tens of ms with different apparent dispersion measures, pc cm−3. When differing by less than the burst width, interference effects in dynamic spectra will be seen. Larger arrival time perturbations may mask any underlying periodicity with period . Strong lensing requires sources smaller than , which includes compact objects such as neutron star magnetospheres but excludes active galactic nuclei. We discuss constraints on densities, magnetic fields, and locations of plasma lenses related to the conditions needed for lensing to occur. Much of the phenomenology of the repeating FRB source FRB 121102 can be accounted for in this picture, which can be tested by obtaining wideband spectra of bursts (from to 10 GHz and possibly higher) that will also help characterize the plasma environment near FRB sources. A rich variety of phenomena is expected from an ensemble of lenses near an FRB source.

A Measurement of Radius Inflation in the Pleiades and Its Relation to Rotation and Lithium Depletion

Abstract Precise measurements of eclipsing binary parameters and statistical studies of young clusters have suggested that some magnetically active low-mass dwarfs possess radii inflated by ∼5%–15% relative to theoretical expectations. If true, this effect should be pronounced in young open clusters, due to the rapid rotation and strong magnetic activity of their most extreme members. We explore this possibility by determining empirical radii for 83 members of the nearby Pleiades open cluster, using spectral energy distribution fitting to establish with a typical accuracy of ≈3% together with color and spectro-photometric indices to determine . We find several Pleiades members with radii inflated above radius- models from state-of-the-art calculations, and apparent dispersions in radii for the K-dwarfs of the cluster. Moreover, we demonstrate that this putative radius inflation correlates strongly with rotation rate, consistent with inflation of young stars by magnetic activity and/or starspots. We argue that this signal is not a consequence of starspot-induced color anomalies, binarity, or depth effects in the cluster, employing Gaia DR1 distances as a check. Finally, we consider the lithium abundances of these stars, demonstrating a triple correlation between rotation rate, radius inflation, and enhanced lithium abundance. Our result—already significant to ∼99.99% confidence—provides strong support for a magnetic origin of the inflated radii and lithium dispersion observed in young, low-mass stars.

Improved seismic profiling by minimally invasive multimodal surface wave method with standard penetration test source (MMSW-SPT)

Abstract Surface waves propagating in layered media inherently possess multimodal dispersion characteristics. However, traditional surface wave testing methods employing measurements at the free surface usually capture only a single apparent dispersion curve, especially when using short geophone arrays common to near surface and geotechnical-scale investigations. Such single-mode or fragmented multimode apparent dispersion curves contain only a fraction of the possible dispersion information, thus limiting the accuracy of inverted profiles. To enable more robust measurement of higher Rayleigh-wave modes, a recently developed hybrid minimally invasive multimodal surface wave method is combined herein with the widely used geotechnical standard penetration test (SPT), which is employed as a practical and ubiquitous downhole source. Upon superimposing surface wave dispersion data for a range of SPT impact depths within the soil, higher modes can be measured more consistently and reliably relative to traditional non-invasive testing methods. As a result, misidentification of multiple dispersion modes can be practically eliminated, significantly improving the accuracy and certainty of inversion results.

Electrodialytic Capillary Suppressor for Open Tubular Ion Chromatography.

We describe an electrodialytic capillary suppressor for suppressed conductometric open tubular ion chromatography (SC-OTIC). For practical eluent concentrations, suppressor active lengths less than ∼1 mm are adequate. In its preferred embodiment, the suppressor consists of a solid polymer ion exchanger block. Two 0.45-mm-diameter parallel cylindrical passages are provided: these provide for passage of regenerant water and placement of platinum wire electrodes. The suppression channel is made by making a crack in the soft polymer block, using a needle 0.3 mm in diameter. The suppression channel runs parallel to and is flanked by the two aforementioned electrode channels through which water is pumped. The suppressor ends of the separation and the detection capillaries are tapered. The tapered ends of the capillaries are inserted into the suppression channel with the tips 0.4-1.1 mm apart. Using a suppression length of 1 mm, we were able to suppress 100 mM hydroxide @ 100 nL/min (10 neq/min). With such a suppressor coupled to an AS18 latex-coated surface-sulfonated cyclo-olefin polymer (COP) capillary column with an inner diameter (i.d.) of 28 μm and using an on-capillary admittance detector (AD), the feasibility of both isocratic and gradient SC-OTIC was demonstrated. At 170 nL/min (substantially above the Van Deemter optimum), the plate count for fluoride exceeded 70 000 plates/m under isocratic conditions. The dispersion induced by the suppressor could not be determined, because the peak half-widths after suppression were paradoxically less than those without suppression; this apparent negative dispersion is likely an artifact of the detector.

Ultrasound-contrast-agent dispersion and velocity imaging for prostate cancer localization.

Prostate cancer (PCa) is the second-leading cause of cancer death in men; however, reliable tools for detection and localization are still lacking. Dynamic Contrast Enhanced UltraSound (DCE-US) is a diagnostic tool that is suitable for analysis of vascularization, by imaging an intravenously injected microbubble bolus. The localization of angiogenic vascularization associated with the development of tumors is of particular interest. Recently, methods for the analysis of the bolus convective dispersion process have shown promise to localize angiogenesis. However, independent estimation of dispersion was not possible due to the ambiguity between convection and dispersion. Therefore, in this study we propose a new method that considers the vascular network as a dynamic linear system, whose impulse response can be locally identified. To this end, model-based parameter estimation is employed, that permits extraction of the apparent dispersion coefficient (D), velocity (v), and Péclet number (Pe) of the system. Clinical evaluation using data recorded from 25 patients shows that the proposed method can be applied effectively to DCE-US, and is able to locally characterize the hemodynamics, yielding promising results (receiver-operating-characteristic curve area of 0.84) for prostate cancer localization.

Implications of the change in confinement status of a heterogeneous aquifer for scale-dependent dispersion and mass-transfer processes

A series of experimental tracer tests were performed to explore the implications of the change in the pressure status of a heterogeneous bimodal aquifer for scale-dependent dispersion and mass-transfer processes. The sandbox was filled with sands and gravel channels and patches to form an alluvial-like bimodal aquifer. We performed multiple injections of a conservative tracer from 26 different locations of the sandbox and interpreted the resulting depth-integrated breakthrough curves (BTCs) at the central pumping well to obtain a scale-dependent distribution of local and field-integrated apparent longitudinal dispersivity (respectively, αLloc and αLapp). We repeated the experiments under confined (CS) and unconfined (UNS) pressure status, keeping the same heterogeneous configuration. Results showed that αLloc(associated with transport through gravel zones) was poorly influenced by the change in aquifer pressure and the presence of channels. Instead, αLapp(i.e. macrodispersion) strongly increased when changing from CS to UNS. In specific, we found αLapp≈0.03r for the CS and αLapp≈0.15r for the UNS (being r the distance from the well). Second-to-fourth-order temporal moments showed strong spatial dependence in the UNS and no spatial dependence in the CS. These results seem consistent with a “vadose-zone-driven” kinetic mass-transfer process occurring in the UNS but not in the CS. The vadose zone enhances vertical flow due to the presence of free surface and large contrasts in hydraulic conductivity triggered by the desaturation of gravel channels nearby the pumping well. The vadose zone enhances vertical mixing between gravel and sands and generates BTC tailing. In the CS vertical mixing is negligible and anomalous transport is not observed.