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Tracer Transport (tracer + transport)
Selected AbstractsSingle- and dual-porosity modelling of multiple tracer transport through soil columns: effects of initial moisture and mode of applicationEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2001T. Kätterer Summary We investigated the effect of initial moisture contents and mode of application on the displacement of multiple conservative tracers through undisturbed columns of a Humic Gleysol. Bromide was applied at the soil surface and chloride was injected at 5 cm depth. The columns were irrigated with deuterium-enriched water. A dual-porosity model and two single-porosity models were calibrated separately to Br, and Cl, elution curves in the two columns. Elution curves were almost identical for Br, and Cl, under initially wet conditions, whereas the displacement of Br, was faster than that of Cl, in the initially dry column, indicating rapid transport with preferential flow. Only the dual-porosity model described the long-tailing breakthrough of Cl, in the initially dry column adequately. The parameter values giving acceptable fits for ,Br dry' were not compatible with the description of the three other elution curves, which could be adequately modelled with a single set of parameter values. The estimated set of common parameters was validated by comparing with the elution curves of deuterium water, nitrate and sulphate, as well as with resident tracer concentrations at four depths. The results showed that solutes can be displaced much faster when applied at the surface of initially dry soil than when applied to wet soil or when resident in the soil matrix. The simulation results suggest that solute transport under initially dry conditions was governed by preferential flow of infiltration water through macropores by-passing the matrix due to shrinkage cracks and water repellence of matrix surfaces. [source] Analysis of a Vertical Dipole Tracer Test in Highly Fractured RockGROUND WATER, Issue 5 2002William E. Sanford The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 ,m) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (,50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters. [source] Steady- and unsteady-state lumped parameter modelling of tritium and chlorofluorocarbons transport: hypothetical analyses and application to an alpine karst aquiferHYDROLOGICAL PROCESSES, Issue 17 2005N. Nur Ozyurt Abstract Determination of a groundwater's mean residence time with the aid of environmental tracers is common in hydrogeology. Many of the lumped parameter (LP) applications used for this purpose have been based on steady-state models. However, the results may be misleading if a steady LP model is used to simulate the environmental tracer transport in an unsteady aquifer. To test this hypothesis, the results of steady and unsteady versions of several LP models were evaluated theoretically and in an alpine karst aquifer case by using tritium, oxygen-18 and chlorofluorocarbons (CFCs). The results reveal that the mean residence times obtained may be significantly different between the steady and unsteady versions of the same model. For the karst aquifer investigated, a serially connected exponential and a plug flow model were run under unsteady conditions. It is shown that outflux calibration with an unsteady model provides a firm basis in evaluating the results of models. An outflux-calibrated unsteady model predicted reasonably the observed series of water isotopes. The calibrated model's CFCs output overpredicts the observed concentrations, probably because of the time lag in the unsaturated zone of the alpine karst aquifer. Copyright © 2005 John Wiley & Sons, Ltd. [source] Mesoscale simulations of atmospheric flow and tracer transport in Phoenix, ArizonaMETEOROLOGICAL APPLICATIONS, Issue 3 2006Ge Wang Abstract Large urban centres located within confining rugged or complex terrain can frequently experience episodes of high concentrations of lower atmospheric pollution. Metropolitan Phoenix, Arizona (United States), is a good example, as the general population is occasionally subjected to high levels of lower atmospheric ozone, carbon monoxide and suspended particulate matter. As a result of dramatic but continuous increase in population, the accompanying environmental stresses and the local atmospheric circulation that dominates the background flow, an accurate simulation of the mesoscale pollutant transport across Phoenix and similar urban areas is becoming increasingly important. This is particularly the case in an airshed, such as that of Phoenix, where the local atmospheric circulation is complicated by the complex terrain of the area. Within the study presented here, a three-dimensional time-dependent mesoscale meteorological model (HOTMAC) is employed for simulation of lower-atmospheric flow in Phoenix, for both winter and summer case-study periods in 1998. The specific purpose of the work is to test the model's ability to replicate the atmospheric flow based on the actual observations of the lower-atmospheric wind profile and known physical principles. While a reasonable general agreement is found between the model-produced flow and the observed one, the simulation of near-surface wind direction produces a much less accurate representation of actual conditions, as does the simulation of wind speed over 1,000 metres above the surface. Using the wind and turbulence output from the mesoscale model, likely particle plume trajectories are simulated for the case-study periods using a puff dispersion model (RAPTAD). Overall, the results provide encouragement for the efforts towards accurately simulating the mesoscale transport of lower-atmospheric pollutants in environments of complex terrain. Copyright © 2006 John Wiley & Sons, Ltd. [source] Coupling a mass-conserving semi-Lagrangian scheme (SLICE) to a semi-implicit discretization of the shallow-water equations: Minimizing the dependence on a reference atmosphereTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 646 2010J. Thuburn Abstract In a recent paper, a conservative semi-Lagrangian mass transport scheme SLICE has been coupled to a semi-implicit semi-Lagrangian scheme for the shallow-water equations. The algorithm involves the solution at each timestep of a nonlinear Helmholtz problem, which is achieved by iterative solution of a linear ,inner' Helmholtz problem; this framework, as well as the linear Helmholtz operator itself, are the same as would be used with a non-conservative interpolating semi-Lagrangian scheme for mass transport. However, in order to do this, a reference value of geopotential was introduced into the discretization. It is shown here that this results in a weak dependence of the results on that reference value. An alternative coupling is therefore proposed that preserves the same solution framework and linear Helmholtz operator but, at convergence of the nonlinear solver, has no dependence on the reference value. However, in order to maintain accuracy at large timesteps, this approach requires a modification to how SLICE performs its remapping. An advantage of removing the dependence on the reference value is that the scheme then gives consistent tracer transport. Copyright © 2010 Royal Meteorological Society and Crown Copyright. Published by John Wiley & Sons, Ltd. [source] | |||||||||||||