Nonreactive Tracer Research Articles

Convection rather than diffusion for fast efficient mRNA vaccine purification

In the rush to produce mRNA vaccines and ensure integrity and purity, capital-intensive purification steps were used, typically involving slow diffusion-driven, inefficient, and costly chromatographic resin processes. We present the first results that microporous affinity synthetic polymer membranes can outperform commercial monolithic and resin approaches for capture of mRNA, thereby promoting fast purification of mRNA vaccines and therapeutics. Our seminal finding is that capture of Fluc-mRNA by synthetic affinity membranes enables efficient intact mRNA recovery (∼100 % of bound mass), faster processing times and lower dispersion by leveraging convective flow with short diffusional path lengths. We also demonstrate higher ligand accessibility and device productivity. Accessible ligand per device volume was 87–94 %, ∼74 % and ∼53 % for the membranes, monoliths, and resin column, respectively. A calibrated mathematical framework predicted mRNA pulse breakthrough with dispersion that was measured in the process tubing and membrane module using a non-reactive tracer and modeled using ideal reactors.

PFAS transport under lower water-saturation conditions characterized with instrumented-column systems

The transport of PFOS and PFOA in well-characterized sand was investigated for relatively low water saturations. An instrumented column was used for some experiments to provide real-time in-situ monitoring of water saturation and matric potential. The results showed that water saturations and matric potentials varied minimally during the experiments. Flow rates were monitored continuously and were essentially constant. These results demonstrate that surfactant-induced flow and other nonideal hydraulic processes did not materially impact PFAS transport for the experiment conditions. Air-water interfacial adsorption was demonstrated to provide the great majority of retention for PFOS and PFOA. Retention was significantly greater at the lower water saturations (0.35–0.45) compared to the higher saturations (∼0.66) for both PFAS, due to the larger extant air-water interfacial areas. Retardation factors were 5 and 3-times greater at the lower water saturations for PFOS and PFOA, respectively. Early breakthrough was observed for the PFAS but not for the non-reactive tracers at the lower water saturations, indicating the possibility that air-water interfacial adsorption was rate-limited to some degree. Independently determined retention parameters were used to predict retardation factors for PFOS and PFOA, which were similar to the measured values in all cases. The consistency between the predicted and measured values indicates that PFAS retention was accurately represented. In addition, air-water interfacial adsorption coefficients measured from the transport experiments were consistent with independently measured equilibrium-based values. Based on these results, it appears that the air-water interfacial adsorption processes mediating the magnitude of PFOS and PFOA retention under lower water-saturation conditions are consistent with those for higher water saturations. This provides some confidence that our understanding of PFAS retention obtained from work conducted at higher water saturations is applicable to lower water saturations.

Study of Hydrodynamics and Residence Time Distribution in a Trickle Bed Reactor

This study focuses on investigating the hydrodynamics using computational fluid dynamics (CFD) simulation and residence time distribution (RTD) within a trickle bed reactor (TBR). CFD simulation was performed on a packed column filled with structured packing of glass beads, while the experiments for RTD studies were conducted in a TBR filled with random packing of Raschig rings. The geometry of TBR and size of packing during CFD simulation was kept close to that of the experimental setup. CFD simulation results were obtained which predicted the flow of fluid through the TBR at different vertical planes along the column. The fluctuations of liquid velocity at various positions inside the reactor were predicted by the CFD model. RTD studies were performed for both pulse input and step input using a non-reactive tracer such as sodium hydroxide. The flowrate of water was varied from 0.6 LPH to 6 LPH for pulse input, while for step input the ratio of water to tracer flowrates was kept at 1:1, 1:2 and 2:1. This study allows us to understand how hydrodynamics and residence time distribution may have an effect on the performance of TBR.

Formulation, Implementation and Validation of a 1D Boundary Layer Inflow Scheme for the QUIC Modeling System

AbstractRecent studies have highlighted the importance of accurate meteorological conditions for urban transport and dispersion calculations. In this work, we present a novel scheme to compute the meteorological input in the Quick Urban & Industrial Complex () diagnostic urban wind solver to improve the characterization of upstream wind veer and shear in the Atmospheric Boundary Layer (ABL). The new formulation is based on a coupled set of Ordinary Differential Equations (ODEs) derived from the Reynolds Averaged Navier–Stokes (RANS) equations, and is fast to compute. Building upon recent progress in modeling the idealized ABL, we include effects from surface roughness, turbulent stress, Coriolis force, buoyancy and baroclinicity. We verify the performance of the new scheme with canonical Large Eddy Simulation (LES) tests with the GPU-accelerated FastEddy"Equation missing" solver in neutral, stable, unstable and baroclinic conditions with different surface roughness. Furthermore, we evaluate QUIC calculations with and without the new inflow scheme with real data from the Urban Threat Dispersion (UTD) field experiment, which includes Lidar-based wind measurements as well as concentration observations from multiple outdoor releases of a non-reactive tracer in downtown New York City. Compared to previous inflow capabilities that were limited to a constant wind direction with height, we show that the new scheme can model wind veer in the ABL and enhance the prediction of the surface cross-isobaric angle, improving evaluation statistics of simulated concentrations paired in time and space with UTD measurements.

Hydraulic containment of TCE contaminated groundwater using pulsed pump-and-treat: Performance evaluation and vapor intrusion risk assessment

Remedial actions for groundwater contamination such as containment, in-situ remediation, and pump-and-treat have been developed. This study investigates the hydraulic containment of Trichloroethylene (TCE) contaminated groundwater by using pulsed pump-and-treat technology. The hypothetical research site assumed the operation of pulsed pump-and-treat to manage groundwater contaminated with 0.1 mg/L of TCE. at the pump-and-treat facility. Numerical models, employing MODFLOW and MT3DMS for groundwater flow and contamination simulations, were used for case studies to evaluate the performance and risks of pump-and-treat operation strategies. Evaluation criteria included capture width, removal efficiency, and contaminant leakage. Health risks from TCE leakage were assessed using a vapor intrusion risk assessment tool in adjacent areas. In the facility-scale case study, the capture width of the pump-and-treat was controlled by pumping/injection well operations, including schedules and rates. Pumping/injection well configurations impacted facility efficiencies. Pulsed operation led to TCE leakage downstream. Site-scale case studies simulated contaminant transport through pump-and-treat considering various operation stages (continuous; pulsed), as well as various reactions of TCE in subsurface environment (non-reactive; sorption; sorption and biodegradation). Assuming non-reactive tracer, TCE in groundwater was effectively blocked during continuous operation stage but released downstream in the following pulsed operation stage. Considering chemical reactions, the influences of the pump-and-treat operation followed similar trends of the non-reactive tracer but occurred at delayed times. Groundwater contamination levels were reduced through biodegradation. Cancer and non-cancer risks could occur at points of exposure (POEs) where the contamination levels approached or fell below TCE groundwater standards.

Comparing the Fate and Transport of MS2 Bacteriophage and Sodium Fluorescein in a Karstic Chalk Aquifer.

Groundwater flow and contaminant migration tracing is a vital method of identifying and characterising pollutant source-pathway-receptor linkages in karst aquifers. Bacteriophages are an attractive alternative tracer to non-reactive fluorescent dye tracers, as high titres (>1012 pfu mL-1) can be safely released into the aquifer, offering improved tracer detectability. However, the interpretation of bacteriophage tracer breakthrough curves is complicated as their fate and transport are impacted by aquifer physicochemical conditions. A comparative tracer migration experiment was conducted in a peri-urban catchment in southeast England to characterise the behaviour of MS2 bacteriophage relative to sodium fluorescein dye in a karstic chalk aquifer. Tracers were released into a stream sink and detected at two abstraction boreholes located 3 km and 10 km away. At both sites, the loss of MS2 phage greatly exceeded that of the solute tracer. In contrast, the qualitative shape of the dye and phage breakthrough curves were visually very similar, suggesting that the bacteriophage arriving at each site was governed by comparable transport parameters to the non-reactive dye tracer. The colloid filtration theory was applied to explain the apparent contradiction of comparable tracer breakthrough patterns despite massive phage losses in the subsurface. One-dimensional transport models were also fitted to each breakthrough curve to facilitate a quantitative comparison of the transport parameter values. The model results suggest that the bacteriophage migrates through the conduit system slightly faster than the fluorescent dye, but that the former is significantly less dispersed. These results suggest that whilst the bacteriophage tracer cannot be used to predict receptor concentrations from transport via karstic flow paths, it can provide estimates for groundwater flow and solute contaminant transit times. This study also provides insight into the attenuation and transport of pathogenic viruses in karstic chalk aquifers.

Cadmium and copper transport in alluvial soils in the Brazilian semiarid region: column percolation and modeling

ABSTRACT Regarding the Brazilian textile industry, part of the northeast region stands out as the second-largest textile manufacturing hub in the country. Despite its importance, this industrial activity has been bringing relevant environmental concerns regarding the disposal of textile effluents, especially from industrial laundries. This waste contains many chemicals and among them are various types of heavy metals. To assess environmental risks associated with heavy metals, pollutant transfer needs to be investigated. This study evaluated the retention and mobility of heavy metals Cd and Cu in alluvial soil, through soil column tests. The up-flow column percolation tests were performed using a nonreactive tracer (KBr) at a concentration of 0.3 mol L -1 and injecting a metallic solution containing Cu and Cd at 100 and 60 mg L -1 , respectively. The injection flow rate was 0.75 mL min -1 . The hydro-dispersive parameters were obtained by modeling the observed breakthrough curves with the convection-dispersion equation (CDE) and the two-region model, also referred to as the MIM (Mobile-IMmobile waters) model. The transport parameters were obtained from the two-site model (TSS). All elution curves were fitted to the models with the CXTFIT 2.0 program. The Two-Site Sorption Model was the best for the case studied, with R 2 of 0.985 and 0.995 for Cu and Cd, respectively. The values of R were considerably higher than the unit, presenting an average of 2.138 for Cu and 1.907 for Cd. This indicates a delay of these contaminants when leaving the column, which is caused by the interaction of these chemical compounds with the soil. The values obtained for parameter D were 3.469 for Cu and 5.205 for Cd. Thus, the metals in this study present a risk of groundwater contamination for the local alluvial aquifers. The main reason for that is the physicochemical features of the soil, such as high sand content (85 %) and low OM content (2.1 %). The results also indicated greater retention and less mobility for Cu than for Cd, pointing to a greater risk for Cd.

Simulation of Helium transport in fractured rocks: Implementation of a dual continuum model in DarcyTools

The main purpose of this work is to present a new analytical solution to study the transient matrix diffusion problem in fractured rocks. The solution is based on a Dual Continuum Model (DCM) and accounts for a zero-order source term in the matrix. The new solution can therefore be applied to describe transport of naturally occurring elements in rock minerals, such as those generated by radioactive decay, i.e., Helium. Also presented in this paper, is a conceptual model to estimate the DCM parameters from properties of an equivalent continuous porous medium (ECPM), thus allowing to incorporate transport calculations within a secondary continuum, i.e., matrix, in ECPM models.For practical applications, the new analytical solution is implemented in a Fortran module that can be readily coupled to a multi-component transport solver, i.e., DarcyTools. This allows DarcyTools to apply the DCM as a subgrid model and thus carry out simultaneous matrix diffusion and transport calculations in a bedrock, with or without including the source term. Since due to the apparent difference in transport time scales in fracture and matrix, simulation results can be sensitive to the grid size and time step taken by the solver, a comparison study is carried out to evaluate the accuracy of the numerical results in a two-dimensional fracture-matrix system.Implementation of the DCM is also verified by comparing the DarcyTools predicted results with exact solutions. For this purpose, a new semi-analytical solution is also derived in the Laplace domain to calculate a solute breakthrough curve in the fracture. Excellent agreement in results is found for the two solutes considered, namely, a non-reactive tracer and Helium generated as alpha particles by radioactive decay in the matrix. Furthermore, the DCM is incorporated in a large-scale three-dimensional transport model to describe concentration distribution of the given solutes in a fractured bedrock. The results suggest that the DCM implementation can effectively capture the impact of mass exchange between fractures and rock matrix in the bedrock.

Anomalous transport in a porous medium with randomly packed ellipse cavities

We investigate the transport of nonreactive tracers in a binary porous medium with randomly packed ellipse fluid-filled cavities. Anomalous transport features, such as early arrival time and long tailing, are observed due to the high contrast in medium properties and highly complex structure of fluid velocity. We use a particle tracking method to quantify transport features of the domain. Then, a continuous time random walk (CTRW) framework builds on tracer transitions in time and space to represent an upscaled model. We study the effect of several key parameters on the anomalous transport process. The parameters include the cavity aspect ratio, porous background permeability, and the Peclet (Pe) number. With the increase in Pe, a longer tailing and a larger residence time are observed, which presents a stronger anomalous feature. A similar situation corresponds to decreased porous medium permeability, which results in wider breakthrough curves. A longer tailing arises in the case of more elongated cavity of larger aspect ratio. The purely advective transport in the medium is investigated at Pe = ∞. This is considered a limit case for the anomalous behavior of the system. One can refer to this case as the most extended tail possible for each cavity arrangement. The widest breakthrough curves for a Pe = ∞ correspond to larger aspect ratios of the cavity and a lower permeable matrix. We show that the upscaled CTRW model can closely predict breakthrough curves in a binary medium with randomly distributed ellipse cavities. These findings give new insight into transport in vesicular and vuggy porous media.

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems.

Perfluorooctane sulfonic acid (PFOS) is one of the most common per- and polyfluoroalkyl substances (PFAS) and is a significant risk driver for these emerging contaminants of concern. A series of two-dimensional flow cell experiments was conducted to investigate the impact of flow field heterogeneity on the transport, attenuation, and mass removal of PFOS. A simplified model heterogeneous system was employed consisting of a lower-permeability fine sand lens placed within a higher-permeability coarse sand matrix. Three nonreactive tracers with different aqueous diffusion coefficients, sodium chloride, pentafluorobenzoic acid, and β-cyclodextrin, were used to characterize the influence of diffusive mass transfer on transport and for comparison to PFOS results. The results confirm that the attenuation and subsequent mass removal of the nonreactive tracers and PFOS were influenced by mass transfer between the hydraulically less accessible zone and the coarser matrix (i.e., back diffusion). A mathematical model was used to simulate flow and transport, with the values for all input parameters determined independently. The model predictions provided good matches to the measured breakthrough curves, as well as to plots of reductions in mass flux as a function of mass removed. These results reveal the importance of molecular diffusion and pore water velocity variability even for systems with relatively minor hydraulic conductivity heterogeneity. The impacts of the diffusive mass transfer limitation were quantified using an empirical function relating reductions in contaminant mass flux (MFR) to mass removal (MR). Multi-step regression was used to quantify the nonlinear, multi-stage MFR/MR behavior observed for the heterogeneous experiments. The MFR/MR function adequately reproduced the measured data, which suggests that the MFR/MR approach can be used to evaluate PFOS removal from heterogeneous media.

Removal of bacterial plant pathogens in columns filled with quartz and natural sediments under anoxic and oxygenated conditions

Irrigation with surface water carrying plant pathogens poses a risk for agriculture. Managed aquifer recharge enhances fresh water availability while simultaneously it may reduce the risk of plant diseases by removal of pathogens during aquifer passage. We compared the transport of three plant pathogenic bacteria with Escherichia coli WR1 as reference strain in saturated laboratory column experiments filled with quartz sand, or sandy aquifer sediments. E. coli showed the highest removal, followed by Pectobacterium carotovorum, Dickeya solani and Ralstonia solanacearum. Bacterial and non-reactive tracer breakthrough curves were fitted with Hydrus-1D and compared with colloid filtration theory (CFT). Bacterial attachment to fine and medium aquifer sand under anoxic conditions was highest with attachment rates of max. katt1 = 765 day-1 and 355 day-1, respectively. Attachment was the least to quartz sand under oxic conditions (katt1 = 61 day-1). In CFT, sticking efficiencies were higher in aquifer than in quartz sand but there was no differentiation between fine and medium aquifer sand. Overall removal ranged between < 6.8 log10 m−1 in quartz and up to 40 log10 m−1 in fine aquifer sand. Oxygenation of the anoxic aquifer sediments for two weeks with oxic influent water decreased the removal. The results highlight the potential of natural sand filtration to sufficiently remove plant pathogenic bacteria during aquifer storage.

Single-well push-pull tests evaluating isobutane as a primary substrate for promoting in situ cometabolic biotransformation reactions.

A series of single-well push-pull tests (SWPPTs) were performed to investigate the efficacy of isobutane (2-methylpropane) as a primary substrate for in situ stimulation of microorganisms able to cometabolically transform common groundwater contaminants, such as chlorinated aliphatic hydrocarbons and 1,4-dioxane (1,4-D). In biostimulation tests, the disappearance of isobutane relative to a nonreactive bromide tracer indicated an isobutane-utilizing microbial community rapidly developed in the aquifer around the test well. SWPPTs were performed as natural drift tests with first-order rates of isobutane consumption ranging from 0.4 to 1.4day-1. Because groundwater contaminants were not present at the demonstration site, isobutene (2-methylpropene) was used as a nontoxic surrogate to demonstrate cometabolic activity in the subsurface after biostimulation. The transformation of isobutene to isobutene epoxide (2-methyl-1,2-epoxypropane) illustrates the epoxidation process previously shown for common groundwater contaminants after cometabolic transformation by alkane-utilizing bacteria. The rate and extent of isobutene consumption and the formation and transformation of isobutene epoxide were greater in the presence of isobutane, with no evidence of primary substrate inhibition. Modeled concentrations of isobutane-utilizing biomass in microcosms constructed with groundwater collected before and after each SWPPT offered additional evidence that the isobutane-utilizing microbial community was stimulated in the aquifer. Experiments in groundwater microcosms also demonstrated that the isobutane-utilizing bacteria stimulated in the subsurface could cometabolically transform a mixture of co-substrates including isobutene, 1,1-dichloroethene, cis-1,2-dichloroethene, and 1,4-D with the same co-substrate preferences as the bacterium Rhodococcus rhodochrous ATCC strain 21198 after growth on isobutane. This study demonstrated the effectiveness of isobutane as primary substrate for stimulating in situ cometabolic activity and the use of isobutene as surrogate to investigate in situ cometabolic reactions catalyzed by isobutane-stimulated bacteria.

Performance analysis of sheep wool fibres as a water filter medium for human enteric virus removal

Sheep wool is increasingly attracting interest for its applications in water and wastewater treatment because of its unique natural properties and superior environmental sustainability compared with non-renewable synthetic materials. While many studies' findings demonstrate wool's substantial capacity for absorbing heavy metals and organic contaminants from water and wastewater, little information is available describing its virus reduction capabilities during water filtration. This preliminary study examined scoured fine- and coarse- wool fibres regarding their abilities to remove human norovirus, adenovirus and rotavirus during water filtration. For each type of wool, 3 virus challenge experiments were conducted using wool-packed glass columns (30 cm long, 3.5 cm diameter) and bore water under a flow rate of 7.3–7.5 mL/s. The results from both wool types were very similar despite their different fibre sizes. The viruses showed greater reductions than the non-reactive water tracer, NaCl. The mass recovery ratios of the viruses to NaCl were 0.62–0.63 for norovirus, 0.64–0.65 for adenovirus and 0.40–0.42 for rotavirus. The log10 reduction values were 0.22–0.23, 0.21–0.22 and 0.45–0.52 for norovirus, adenovirus and rotavirus, respectively. Compared with previous studies' results, the virus reductions in wool appeared greater than those in polyester and polypropylene filters, and river sand. Our findings suggest that wool has some capacity to remove viruses during water filtration. With surface modifications to enhance its virus removal properties, wool has great potential as an alternative filter medium for treating point-of-use drinking water, especially in resourceless situations requiring practical solutions for accessing safe water.

Greywater adsorption into soil during irrigation

The reuse of greywater has significant potential to reduce the demand on potable water. The greywater produced from laundry is free from oil and grease and hence makes it attractive to reuse for irrigation. This study investigates the adsorption of surfactant-rich laundry greywater into the soil surfaces during irrigation. A series of miscible displacement column experiments was conducted under water-saturated condition using non-reactive (NaCl solution of concentration 0.650 g/L) and reactive tracers (greywater solution of concentration 0.26–0.442 g/L with same background electrolyte). Plasterer’s sand was used as the porous medium. Samples collected at the column outlet every two minutes were measured for pH, electrical conductivity and greywater concentrations. Hydraulic conductivity for each experiment was also determined using constant head method. Separate experiments were conducted to determine the surface tension of greywater solution (with same background electrolyte) and modelled using Gibbs adsorption isotherm. Surface tension of greywater reduces with increasing greywater concentration and becomes constant at greywater concentration of 0.440 g/L. The results revealed that pH is improved and electrical conductivity decreased indicating it may increase the soil salinity. The comparison of breakthrough curves of reactive and non-reactive tracers showed that the greywater adsorptions occur into the soil surfaces and it increased with greywater concentrations, which may make the soil water-repellent. This may be a concern if soil becomes water-repellent, increases hydraulic conductivity and enhances the risk of groundwater pollution. The concentration of greywater needs to be checked before irrigation and if needed, it should be diluted to avoid any risk of soil water-repellence.

The importance of freeze–thaw cycles for lateral tracer transport in ice-wedge polygons

Abstract. A significant portion of the Arctic coastal plain is classified as polygonal tundra and plays a vital role in soil carbon cycling. Recent research suggests that lateral transport of dissolved carbon could exceed vertical carbon releases to the atmosphere. However, the details of lateral subsurface flow in polygonal tundra have not been well studied. We incorporated a subsurface transport process into an existing state-of-the-art hydrothermal model. The model captures the physical effects of freeze–thaw cycles on lateral flow in polygonal tundra. The new modeling capability enables non-reactive tracer movement within subsurface. We utilized this new capability to investigate the impact of freeze–thaw cycles on lateral flow in the polygonal tundra. Our study indicates the important role of freeze–thaw cycles and the freeze-up effect in lateral tracer transport, suggesting that dissolved species could be transported from the middle of the polygon to the sides within a couple of thaw seasons. Introducing lateral carbon transport into the climate models could substantially reduce the uncertainty associated with the impact of thawing permafrost.

A Borehole Test for Chlorinated Solvent Diffusion and Degradation Rates in Sedimentary Rock

AbstractWe present a new field measurement and numerical interpretation method (combined termed “test”) to parameterize the diffusion of trichloroethene (TCE) and its biodegradation products (DPs) from the matrix of sedimentary rock. The method uses a dual‐packer system to interrogate a low‐permeability section of the rock matrix adjacent to a previously contaminated borehole and uses the borehole monitoring history to establish the pretest condition. TCE and its DPs are removed from the groundwater between the packers at the onset of the testing. The parameters estimated by fitting a radial diffusion model to the concentration history and borehole concentration data, also termed back diffusion, are the tortuosity factor and sorption coefficients of TCE and DPs in the rock matrix and the TCE and DP biodegradation rate coefficients in the borehole. We demonstrate the equipment design and the interpretive method using a borehole accessing the gray mudstone at a TCE contaminated site in the Newark Basin. In this test, both nonreactive (bromide) and reactive (trichlorofluoroethene) tracers are used to constrain the estimated parameters; however, the bromide tracer was not needed to estimate the parameters in this test. The parameters estimated from the field test are consistent with values measured independently in laboratory experiments using field samples of similar lithology. From the interpretation, we compute the TCE and DP concentration distributions in the rock matrix prior to the test to illustrate how the results can be used to enhance understanding of contaminant distribution in the rock matrix.

Transport of marine tracer phage particles in soil

Marine phages have been applied to trace ground- and surface water flows. Yet, information on their transport in soil and related particle intactness is limited. Here we compared the breakthrough of two lytic marine tracer phages (Pseudoalteromonas phages PSA-HM1 and PSA-HS2) with the commonly used Escherichia virus T4 in soil- and sand-filled laboratory percolation columns. All three phages showed high mass recoveries in the effluents and a higher transport velocity than non-reactive tracer Br−. Comparison of effluent gene copy numbers (CN) to physically-determined phage particle counts or infectious phage counts showed that PSA-HM1 and PSA-HS2 retained high phage particle intactness (Ip > 81%), in contrast to T4 (Ip < 36%). Our data suggest that marine phages may be applied in soil to mimic the transport of (bio-) colloids or anthropogenic nanoparticles of similar traits. Quantitative PCR (qPCR) thereby allows for highly sensitive quantification and thus for the detection of even highly diluted marine tracer phages in environmental samples.

Effect of spilled diluted bitumen on chemical air-water exchange in boreal lake limnocorrals

Following spills into water, petroleum oils can spread widely and produce surface slicks. Resulting slicks may impede volatilization and possibly increase chemical persistence in water. While the influence of oil films on chemical air-water exchange has been examined through theoretical and laboratory studies, field studies have not been conducted to assess the relevance of these effects following actual oil spill events. Here we evaluated the effect of diluted bitumen (dilbit) experimentally spilled in limnocorrals installed in a boreal lake on the volatilization of sulfur hexafluoride (SF6), a non-reactive volatile tracer gas. Dilbit spills were monitored over 70 days and SF6 was introduced twice (after 7 and 48 days) to evaluate the influence of spilled dilbit on the loss of SF6 from water. Volatilization rate constants of SF6 (kVOL) significantly decreased by up to 80% with increasing total dilbit spill cover. Using a theoretical equation, decreases in kVOL were largely explained by a reduction in open water area where chemical exchange across the air-water interface occurs. Apparent effects of the slick on SF6 mass transfer were estimated to be smaller by comparison (20%).To account for this reduction in volatilization, oil spill fate models should include a correction to consider the impact of spill cover on the air-water exchange of organic chemicals.

Numerical dispersion of solute transport in an integrated surface–subsurface hydrological model

Integrated surface–subsurface hydrological models (ISSHMs) are increasingly being used for the assessment of contaminant transport in the environment, in addition to their more common use in water flow applications. However, the subsurface solute transport solvers in these models are prone to numerical dispersion errors. Numerical dispersion is a well-known issue in groundwater modeling, but its impacts on the results of ISSHM simulations are still poorly understood. In this study, the CATchment HYdrology (CATHY) model is used to assess the potential impacts of numerical dispersion on the simulation of coupled surface–subsurface solute transport. We first simulate the subsurface transport of a nonreactive tracer in two soil column test cases (1D and 3D) with known analytical solutions. The subsurface solute transport solver in CATHY adopts a computationally efficient time-splitting technique whereby the advection component of the governing equation is solved on elements and the hydrodynamic dispersion component is solved on nodes. Comparison between simulation results and analytical solutions with different mesh discretizations and different values for the hydrodynamic dispersion coefficients allows for accurate quantification of the numerical dispersion error and yields insights into the parameters and other factors that control it. It is shown that, taken alone, the advection and dispersion solvers are very robust, but their combination can result in significant numerical dispersion, stemming from the exchange of concentration information from elements to nodes and vice versa in the time-splitting procedure. The tests also show that these errors can be kept under control by ensuring that the grid Péclet number is in the range 0.5-1.0 or smaller. We then apply CATHY in a third test case involving two synthetic hillslopes (concave and convex) in fully coupled surface–subsurface mode, in order to examine the impact of this subsurface numerical dispersion on simulated streamflow hydrographs, in particular with reference to pre-event water contributions to runoff. Here as well the results show that the effect of numerical dispersion can be controlled by keeping the grid Péclet number sufficiently small. This work provides a new set of benchmark test cases for integrated surface–subsurface hydrological models, extending to solute transport the flow-only suite of benchmarks recently published in two intercomparison studies.

Mass Flux Analysis of Abiotic Tetrachloroethene Dense Nonaqueous Phase Liquid Pool Dissolution in a Heterogeneous Flow Environment

AbstractPorous aquifer materials are often characterized by layered heterogeneities that influence groundwater flow and present complexities in contaminant transport modeling. Such flow variations also have the potential to impact the dissolution flux from dense nonaqueous phase liquid (DNAPL) pools. This study examined how these heterogeneous flow conditions affected the dissolution of a tetrachloroethene (PCE) pool in a two‐dimensional intermediate‐scale flow cell containing coarse sand. A steady‐state mass‐balance approach was used to calculate the PCE dissolution rate at three different flow rates. As expected, aqueous PCE concentrations increased along the length of the PCE pool and higher flow rates decreased the aqueous PCE concentration in the effluent. Nonreactive tracer studies at two flow rates confirmed the presence of a vertical flow gradient, with the most rapid velocity located at the bottom of the tank. These results suggest that flow focusing occurred near the DNAPL pool. Effluent PCE concentrations and pool dissolution flux rates were compared to model predictions assuming local equilibrium (LE) conditions at the DNAPL pool/aqueous phase interface and a uniform distribution of flow. The LE model did not describe the data well, even over a wide range of PCE solubility and macroscopic transverse dispersivity values. Model predictions assuming nonequilibrium mass‐transfer‐limited conditions and accounting for vertical flow gradients, however, resulted in a better fit to the data. These results have important implications for evaluating DNAPL pool dissolution in the field where subsurface heterogeneities are likely to be present.