bluuebunny/crossref_metadata_2025 huggingface dataset:
Water and solute residence times in a catchment: Stochastic‐mechanistic model interpretation of 18O transport
Authors: Simic Eva , Destouni Georgia
Published: September 2002

A stochastic‐mechanistic modeling approach to reactive solute transport through an integrated soil‐groundwater system is used to interpret observed results of subsurface 18O transport through a covered experimental catchment at the Lake Gârdsjön site, Sweden. The model incorporates the effect of variable pathway lengths to the catchment outlet due to the distributed source of 18O over the whole catchment area, the nonuniform mean flow velocity that results from groundwater recharge through the unsaturated zone, the existence of preferential flow and transport pathways and the associated diffusional mass transfer between mobile and relatively immobile water zones, and the random flow heterogeneity that results from spatially variable hydraulic transmissivity. Using available site observations and topographical data from the catchment, most of the required model parameters are quantified by independent estimation. On the basis of this independent estimation and calibration of the remaining few parameter values, a good representation of the 18O transport through the catchment was obtained. The spreading effect of the longitudinal extent of the solute source (due to differences in pathway length to the catchment outlet for different entrance points at the soil surface) does to a large degree explain the observed temporal spreading of 18O at the catchment outlet. Additional solute spreading is obtained by diffusional solute mass transfer between mobile and relatively immobile water regions, associated with the existence of preferential flow paths observed at the site. In comparison with these two spreading mechanisms, flow heterogeneity within the mobile water plays a much less important role. This indicates that natural tracer transport through a whole catchment area does not provide a sufficiently fine tool for distinguishing between and quantifying various flow heterogeneity effects that may be important for smaller solute sources.
Stochastic analysis of virus transport in aquifers
Authors: Rehmann Linda L. Campbell , Welty Claire , Harvey Ronald W.
Published: September 2002

A large‐scale model of virus transport in aquifers is derived using spectral perturbation analysis. The effects of spatial variability in aquifer hydraulic conductivity and virus transport (attachment, detachment, and inactivation) parameters on large‐scale virus transport are evaluated. A stochastic mean model of virus transport is developed by linking a simple system of local‐scale free‐virus transport and attached‐virus conservation equations from the current literature with a random‐field representation of aquifer and virus transport properties. The resultant mean equations for free and attached viruses are found to differ considerably from the local‐scale equations on which they are based and include effects such as a free‐virus effective velocity that is a function of aquifer heterogeneity as well as virus transport parameters. Stochastic mean free‐virus breakthrough curves are compared with local model output in order to observe the effects of spatial variability on mean one‐dimensional virus transport in three‐dimensionally heterogeneous porous media. Significant findings from this theoretical analysis include the following: (1) Stochastic model breakthrough occurs earlier than local model breakthrough, and this effect is most pronounced for the least conductive aquifers studied. (2) A high degree of aquifer heterogeneity can lead to virus breakthrough actually preceding that of a conservative tracer. (3) As the mean hydraulic conductivity is increased, the mean model shows less sensitivity to the variance of the natural‐logarithm hydraulic conductivity and mean virus diameter. (4) Incorporation of a heterogeneous colloid filtration term results in higher predicted concentrations than a simple first‐order adsorption term for a given mean attachment rate. (5) Incorporation of aquifer heterogeneity leads to a greater range of virus diameters for which significant breakthrough occurs. (6) The mean model is more sensitive to the inactivation rate of viruses associated with solid surfaces than to the inactivation rate of viruses in solution.
Semianalytical solutions for parameter estimation in diffusion cell experiments
Authors: Moridis George J.
Published: September 2002

In this paper, semianalytical solutions to the diffusion problem are developed under the conditions of diffusion cell experiments, which involve finite liquid volumes and temporally variable concentrations in the upstream and downstream reservoirs. These solutions account for diffusion in the pores, surface diffusion, mass transfer between the mobile and immobile water fractions, linear sorption (equilibrium or kinetic), and radioactive decay. Fully analytical solutions for both through‐diffusion and reservoir‐depletion studies are obtained in the Laplace space, which are subsequently numerically inverted to provide the solution in time. The effects of the various diffusion, sorption, and geometric parameters on the solutions under both equilibrium and kinetic linear sorption are investigated. The semianalytical solutions are coupled with an optimization algorithm for parameter estimation, and diffusion and sorption parameters are determined using experimental data. The semianalytical solutions are shown to have significant advantages over the conventional graphical approach because (1) they are not based on the often invalid assumption of constant upstream and negligible downstream concentrations, (2) they double the amount of data from which to extract the pertinent diffusion and sorption parameters, and (3) they allow differentiation between equilibrium and kinetic sorption.
Dual‐porosity groundwater contaminant transport in the presence of colloids
Authors: Corapcioglu M. Yavuz , Wang Sookyun
Published: September 2002

Prior studies have shown that colloids can facilitate contaminant migration in unimodal porous media. To investigate the effect of no‐flow regions on flow and contaminant transport in dual‐porosity soils, we model a porous medium composed of two different homogeneous, superposed, and interacting regions: the mobile region and the immobile region. We assume that the advective‐dispersive processes govern the transport of contaminant and colloids in the mobile region, while the diffusion process dominates in the immobile region. The contaminant and colloid mass transfer mechanisms between these two regions are represented by a first‐order mass transfer. Colloid deposition on the solid matrix is expressed by a kinetic sorption relationship. The contaminant sorption with the solid matrix and colloidal surfaces is also incorporated into the model. Coupled with mass transfer terms, two sets of governing equations representing the fate and transport of contaminant and colloids in both the mobile and immobile regions are developed and applied to experimental data available in the literature. Numerical solutions are obtained by employing a fully implicit, finite difference scheme. The numerical results indicate that the colloidal facilitation is increased in a dual‐porosity porous medium compared to a unimodal medium. The model is validated by comparing the numerical results with the experimental data available in the literature for colloid‐facilitated contaminant transport in a single‐porosity medium and contaminant transport in a colloid‐free, dual‐porosity medium. A sensitivity analysis is conducted to deduce the effect of major model parameters on contaminant transport. The analysis demonstrates that, although both the volumetric fraction of the mobile region and the mass transfer rate coefficients between the two regions have effects on the dual‐porosity transport, the early breakthrough is affected mainly by the volumetric fraction of the mobile region, while the tailing is affected largely by the mass transfer rate coefficients.
Effects of hydraulic roughness on surface textures of gravel‐bed rivers
Authors: Buffington John M. , Montgomery David R.
Published: September 2002

Field studies of forest gravel‐bed rivers in northwestern Washington and southeastern Alaska demonstrate that bed‐surface grain size is responsive to hydraulic roughness caused by bank irregularities, bars, and wood debris. We evaluate textural response by comparing reach‐average median grain size (D50) to that predicted from the total bank‐full boundary shear stress (т0bf), representing a hypothetical reference condition of low hydraulic roughness. For a given т0bf, channels with progressively greater hydraulic roughness have systematically finer bed surfaces, presumably due to reduced bed shear stress, resulting in lower channel competence and diminished bed load transport capacity, both of which promote textural fining. In channels with significant hydraulic roughness, observed values D50 can be up to 90% smaller than those predicted from т0bf. We find that wood debris plays an important role at our study sites, not only providing hydraulic roughness but also influencing pool spacing, frequency of textural patches, and the amplitude and wavelength of bank and bar topography and their consequent roughness. Our observations also have biological implications. We find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse.
Hydrologic flow paths control dissolved organic carbon fluxes and metabolism in an Alpine stream hyporheic zone
Authors: Battin Tom J.
Published: September 2002

The objective of the present paper was to link reach‐scale streambed reactive uptake of dissolved organic carbon (DOC) and dissolved oxygen (DO) to subsurface flow paths in an alpine stream (Oberer Seebach (OSB)). The topography adjacent to the stream channel largely determined flow paths, with shallow hillslope groundwater flowing beneath the stream and entering the alluvial groundwater at the opposite bank. As computed from hydrometric data, OSB consistently lost stream water to groundwater with fluxes out of the stream averaging 943 ± 47 and 664 ± 45 L m−2 h−1 at low (Q < 600 L s−1) and high (Q > 600 L s−1) flow, respectively. Hydrometric segregation of streambed fluxes and physicochemical mixing analysis indicated that stream water was the major input component to the streambed with average contributions of 70–80% to the hyporheic zone (i.e., the subsurface zone where shallow groundwater and stream water mix). Surface water was also the major source of DOC with 0.512 ± 0.043 mg C m−2 h−1 to the streambed. The DOC flux from shallow riparian groundwater was lower (0.309 ± 0.071 mg C m−2 h−1) and peaked in autumn with 1.011 mg C m−2 h−1. I computed the relative proportion of downstream discharge through the streambed as the ratio of the downstream length (Ssw) a stream water parcel travels before entering the streambed to the downstream length (Shyp) a streambed water parcel travels before returning to the stream water. The relative streambed DOC retention efficiency, calculated as (input‐output)/input of interstitial DOC, correlated with the proportion (Ssw/Shyp) of downstream discharge (r2 = 0.76, p = 0.006). Also, did the streambed metabolism (calculated as DO uptake from mass balance) decrease with low subsurface downstream routing, whereas elevated downstream discharge through the streambed stimulated DO uptake (r2 = 0.69, p = 0.019)? Despite the very short DOC turnover times (∼0.05 days, calculated as mean standing stock/annual input) within the streambed, the latter constitutes a net sink of DOC (∼14 mg C m−2 h−1). Along with high standing stocks of sediment associated particulate organic carbon, these results suggest microbial biofilms as the major retention and storage site of DOC in an alpine stream where large hydrologic exchange controls DOC fluxes.
Slope curvature influence on soil erosion and deposition processes
Authors: Stefano C. Di , Ferro V. , Porto P. , Tusa G.
Published: September 2002

Sediment water quality problems are not necessarily the direct result of erosion from the most intense soil erosion areas. A study of within‐basin variability of the sediment delivery processes is needed to identify areas where the most effective soil conservation strategies should focus. In this paper the sediment delivery processes is examined along the hydraulic path from sediment source areas to the nearest stream reach. The effects of shape (convex, concave, and uniform) of the hydraulic path are modeled using a power equation for slope profile and revised universal soil loss equation (RUSLE) with two different expressions of the topographic factors. A criterion is proposed for the erosion active slope length, i.e., the slope length in which no deposition processes occur. Then, using the continuity equation for sediment transport, a Ψ factor is deduced to correct the topographic factors of RUSLE and those proposed by Moore and Burch [1986], depending on slope curvature. Finally, two relationships for correcting the topographic factors for an irregular slope are experimentally tested comparing the calculated sediment yields with net soil erosion values estimated by the proportional method of Martz and de Jong [1987] and cesium 137 data available from a Sicilian basin.
An automated method for constructing contour‐based digital elevation models
Authors: Maunder Chris J.
Published: September 2002

A major problem in developing digital elevation models (DEMs) is the realistic representation of flow though complex terrain. A contour‐based DEM, which uses natural flow lines and contours to define the element network in the model, is an elegant solution in that the governing equations of flow can be reduced to one dimension. Previous methods of constructing contour‐based element networks require a high level of user involvement to produce the flow net. A new method for constructing contour‐based element networks is presented that relies solely on contour data and does not rely on user‐defined high points, saddle points, streamlines, or boundary files. Critical points such as high points and saddle points are automatically detected and a robust scheme is used to create flow lines. An improved method for calculating element attributes for the final DEM is also presented that takes into account the nonplanar nature of typical elements in contour‐based networks. Furthermore, the inherent deficiencies of traditional flow line construction methods are discussed, and to an extent overcome, resulting in a more realistic model. Preparation time for DEM construction has been reduced, and the user is given greater control over the rules for calculating flow lines. The algorithm has been implemented on a typical desktop computer, and comparisons have been made with existing contour‐based element network construction methods. Results show improved speed and robustness over traditional methods and emphasize the convenience of minimal data requirements.
The entrapment and long‐term dissolution of tetrachloroethylene in fractional wettability porous media
Authors: Bradford Scott A. , Vendlinski Richard A. , Abriola Linda M.
Published: September 2002

Theoretical and modeling studies for the prediction of nonaqueous phase liquid (NAPL) entrapment and dissolution have largely assumed that the soil is preferentially water‐wet. Many natural systems, however, have both water‐ and NAPL‐wet solids as a result of spatial and temporal variations in fluid and soil properties. This condition is referred to as fractional wettability. This work presents long‐term dissolution data for tetrachloroethylene (PCE) entrapped in porous media representing a range of grain sizes, gradations, and fractional wettabilities. Entrapment data suggest that for given wettability conditions, initial residual NAPL saturations tend to increase with decreasing soil mean grain size. In addition, residual NAPL saturations in finer‐textured sands were observed to reach a minimum at intermediate wetting conditions, whereas residual saturations in coarser‐textured media decreased asymptotically with increasing fraction of NAPL‐wet sand. In dissolution studies, an increase in the NAPL‐wet fraction tended to result in decreased PCE dissolution time, characterized by a longer period of high effluent concentrations, followed by a more rapid reduction in concentration. Increases in NAPL‐wet fraction, however, were also associated with higher sorptive capacities, leading to enhanced PCE concentration tailing. The influence of fractional wettability on PCE dissolution behavior was also found to depend on media grain size distribution, particularly for more water‐wet soils. Experimental observations are discussed in the context of pore‐scale conceptual models of entrapment.
Silk‐producing stream insects and gravel erosion: Significant biological effects on critical shear stress
Authors: Statzner Bernhard , Arens Marie‐Françoise , Champagne Jean‐Yves , Morel Robert , Herouin Eric
Published: September 2002

There is growing evidence for impacts of small stream organisms on the transport of fine sediments. However, such impacts on the transport of coarser sediments remained unconsidered. Therefore we studied whether a silk‐producing stream insect (Hydropsyche siltalai) consolidated gravel through silk fixed among gravel pieces. We exposed gravel for 2 months in a stream along a gradient of Hydropsyche abundance and measured the critical shear stress for this gravel in a laboratory flume. The critical gravel shear stress was very significantly (p = 7.7 × 10−11) related to a variable (overall biomass) describing gravel consolidation by Hydropsyche silk. We showed that Hydropsyche in a range of abundance typical of many streams increases critical shear stress for gravel by as much as a factor of 2. Surface gravel arrangement (i.e., the spatial distribution and orientation of different gravel sizes and forms) had a less significant impact on critical shear stress, while other sediment variables so far considered in erosion had no significant impact. Future studies on the transport of solids and on habitat conditions should consider the high potential of stream invertebrates to consolidate gravel.
Effects of pore scale dead‐end air fingers on relative permeabilities for air sparging in soils
Authors: Clayton Wilson S.
Published: September 2002

Measurements of relative permeability‐saturation‐capillary head relationships (kr−S−h c) during unsteady state displacement of water by air in soil cores reveal several unique behaviors that have not been previously described. Extremely low air relative permeability (kra) was observed, which increased independent of saturation, by up to an order of magnitude to approach Brooks‐Corey kra(S) estimates. These effects were more extreme at faster displacement rates and are inferred to reflect the development and subsequent breakthrough of pore scale dead‐end air fingers, which initially do not conduct air flow. Furthermore, kra(hc) estimates tended to be accurate, and where low kra(S) was observed, hc(S) was significantly below moisture retention data. Where kra(S) approximated Brooks‐Corey estimates, hc(S) approximated moisture retention data. A calculated backbone (conductive) air saturation, which is the total air saturation minus the dead‐end air saturation, was found to control both kra and hc. Generally, water relative permeability data followed common functional relationships.
A simple model for simulation of water content, soil frost, and soil temperatures in boreal mixed mires
Authors: Granberg G. , Grip H. , Löfvenius M. Ottosson , Sundh I. , Svensson B. H. , Nilsson M.
Published: September 2002

In this paper we present a model that can reconstruct water table position and soil temperature profiles to 3 m depth in boreal mixed mire systems using the readily available climate data on air temperature and precipitation as driving variables. The model simulates complete, multiple annual cycles including winter conditions and freeze‐thaw processes. The major requisite for an accurate description of the soil heat flux in a mire is an accurate description of the water content of the profile because of the high porosity of the soil and the thermal properties of water. The soil moisture profile in this model is described as a function of water table position derived from empirical published data of soil moisture profile measurements at different water table positions. The parameters of the model were set and tested for a lawn community, one of the most dominant plant communities occurring in boreal mixed mires. The model is optimized for the period 1995–1997 at the mixed mire Degerö Stormyr and was validated for the period 1997–1998 at the same location and for 1991–1993 at Storåmyran, 60 km away. The mean deviation of simulated to measured soil temperature from 10 cm to 3 m depth in the validation data sets was <1.6°C with a maximal standard deviation of 1.8°C. A response analysis of the amount of winter precipitation showed the influence of snow cover depth on soil frost fluctuations. A simulated precipitation corresponding to 25% of the measured precipitation during winter 1995–1996, with all other factors unchanged, prolonged the period of frozen ground in the model from June 8 to August 1. With a doubling of the winter precipitation the modeled period of frozen ground lasted only until May 6.
Diffusion processes in composite porous media and their numerical integration by random walks: Generalized stochastic differential equations with discontinuous coefficients
Authors: LaBolle Eric M. , Quastel Jeremy , Fogg Graham E. , Gravner Janko
Published: September 2002

Discontinuities in effective subsurface transport properties commonly arise (l) at abrupt contacts between geologic materials (i.e., in composite porous media) and (2) in discrete velocity fields of numerical groundwater‐flow solutions. However, standard random‐walk methods for simulating transport and the theory on which they are based (diffusion theory and the theory of stochastic differential equations (SDEs)) only apply when effective transport properties are sufficiently smooth. Limitations of standard theory have precluded development of random‐walk methods (diffusion processes) that obey advection dispersion equations in composite porous media. In this paper we (1) generalize SDEs to the case of discontinuous coefficients (i.e., step functions) and (2) develop random‐walk methods to numerically integrate these equations. The new random‐walk methods obey advection‐dispersion equations, even in composite media. The techniques retain many of the computational advantages of standard random‐walk methods, including the ability to efficiently simulate solute‐mass distributions and arrival times while suppressing errors such as numerical dispersion. Examples relevant to the simulation of subsurface transport demonstrate the new theory and methods. The results apply to problems found in many scientific disciplines and offer a unique contribution to diffusion theory and the theory of SDEs.
Channel network simulation models compared with data from the Ashley River, New Zealand
Authors: Ibbitt Richard P. , Willgoose Garry R. , Duncan Maurice J.
Published: September 2002

This paper compares the ability of two channel network simulation models to simulate the channel network properties of the Ashley River in the foothills of the Southern Alps of the South Island of New Zealand. The basin was chosen because it contains a large‐scale topographic feature, a central ridge, the simulation of which would provide a measure of each model's ability to handle spatial nonuniformities. The two models assessed were the optimal channel network (OCN) model and a catchment evolution model (SIBERIA) catchment evolution model. The ability of these models to replicate observed geomorphic statistics and relationships was assessed. The models were also compared against each other to assess their relative suitability for simulating the observed geomorphology. Model performance was gauged using the single‐valued measures of catchment convergence, hypsometric integral, and energy expenditure by the network; and plots of the width function, the slope of the cumulative area diagram, and the hypsometric curve of basin topography. The effects of different forcings were examined. The basic forcing was one in which the climate and the geological and tectonic properties of the basin were assumed to be unchanging in both space and time. The first variant on the basic forcing looked at what happens when there is a permanent spatial gradient in the rainfall over a basin with spatially uniform geological and tectonic properties. The second variant considered the effects of constant, spatially uniform rainfall on a basin in which spatially variable tectonic uplift is occurring. Neither model adequately simulated the observed geomorphology when the spatially nonuniform tectonic forcing was ignored. When spatially nonuniform tectonic effects at length scales of tens of kilometers were simulated, SIBERIA performed more satisfactorily. The effect of nonuniform rainfall was found to be small for both models. The performance of the OCN as gauged by single‐valued measures improved markedly when energy expenditure calculations were consistent with the geometric length of flow paths.
Tidal inflow to aquifers
Authors: Ross Benjamin
Published: September 2002

Where an aquifer discharges into a tidal estuary, flow can reverse at high tide. When the aquifer and estuary waters differ chemically, it is useful to know whether flow reverses and, if so, how much. For the case of a confined aquifer discharging into a tidal estuary, a formula for the ratio of inflow to outflow during one tidal cycle is derived.
Stream chemistry modeling of two watersheds in the Front Range, Colorado
Authors: Meixner Thomas , Bales Roger C. , Williams Mark W. , Campbell Don H. , Baron Jill S.
Published: September 2002

We investigated the hydrologic, geochemical, and biogeochemical controls on stream chemical composition on the Green Lakes Valley and Andrews Creek watersheds using the alpine hydrochemical model (AHM). Both sites had comparable data sets from 1994 and 1996, including high‐resolution spatial data and high‐frequency time series of hydrology, geochemistry, and meteorology. The model of each watershed consisted of three terrestrial subunits (soil, talus, and rock), with the routing between the subunits determined by spatial land cover data. Using 1994 data for model calibration and 1996 data for evaluation, AHM captured the dominant processes and successfully simulated daily stream chemical composition on both watersheds. These results confirm our procedure of using spatial and site‐specific field and laboratory data to generate an initial catchment model and then calibrating the model to calculate effective parameters for unmeasured processes. A net source of nitrogen was identified in the Andrews Creek watershed during the spring snowmelt period, whereas nitrogen was immobilized in the Green Lakes Valley. This difference was most likely due to the larger and more dominant area of talus in the Andrews Creek watershed. Our results also indicate that routing of snowmelt through either soil or talus material is sufficient for retention of H+ and release of base cations but that N retention is more important on areas mapped as soil. Owing to the larger ionic pulse and larger fraction of surface runoff the Green Lakes Valley was more sensitive to a doubling of wet deposition chemistry than the Andrews Creek watershed.
Estimating the theoretical semivariogram from finite numbers of measurements
Authors: Zheng Li , Silliman Stephen E.
Published: September 2002

We investigate from a theoretical basis the impacts of the number, location, and correlation among measurement points on the quality of an estimate of the semivariogram. The unbiased nature of the semivariogram estimator
is first established for a general random process Z(x). The variance of
is then derived as a function of the sampling parameters (the number of measurements and their locations). In applying this function to the case of estimating the semivariograms of the transmissivity and the hydraulic head field, it is shown that the estimation error depends on the number of the data pairs, the correlation among the data pairs (which, in turn, are determined by the form of the underlying semivariogram γ(r)), the relative locations of the data pairs, and the separation distance at which the semivariogram is to be estimated. Thus design of an optimal sampling program for semivariogram estimation should include consideration of each of these factors. Further, the function derived for the variance of
is useful in determining the reliability of a semivariogram developed from a previously established sampling design.
Role of boundary conditions in convergence and nonlocality of solutions to stochastic flow problems in bounded domains
Authors: Bonilla F. Alejandro , Cushman John H.
Published: September 2002

Steady flow through a heterogeneous porous medium in a bounded domain is investigated using a recursive perturbation scheme. The effect of boundary conditions on a two‐dimensional flow with arbitrary variation of the mean flow (allowing large gradients) is investigated using analytical expressions for the head and velocity covariance functions. Boundary conditions were decomposed into deterministic and stochastic components. Two flow cases with the same zero‐order and different first‐order boundary conditions were analyzed. Boundary conditions are deterministic for the first case and random for the second. Significant differences between the two cases indicate random (or absence of randomness) processes must be modeled at boundaries. First‐order solutions for the head and velocity covariance functions for a bounded rectangular domain are derived. The resulting integral kernels involve Greens functions, and they are evaluated by numerical integration. Boundary conditions for higher‐order problems influence the absolute value and shape of the kernels (and therefore of the head and velocity variance and covariance functions). It is found that the validity of the perturbation scheme is dependent on the magnitude of the kernels and not only on the condition σf2 ≪ 1, where σf is the variance of the log hydraulic conductivity, assumed Gaussian and weakly homogeneous in space. The type of boundary conditions affects the values of the kernels and therefore determine the convergence limits for the problem. Milder head gradients also allow larger values of σf2. Nonlocality is also contingent on boundary type. Weakly homogeneous log‐fluctuating conductivity fields give rise to head and velocity covariances which are not weakly homogeneous. The inhomogeneous fields are obtained from a linear filter of the solution to the problem without stochasticity. Higher‐gradient regions induce higher head and velocity variances. In the presence of space‐varying gradients, nonlocal effects are most important away from the boundaries lpar;“center of the domain”). Small local head gradients result in small head and velocity gradients, and therefore where observations are made in stagnation regions, data should be analyzed taking into account this effect. The results may be used to interpret experimental data for columns or data taken where conditions do not fit the average uniform flow assumption and when processes at the boundaries influence the flow and therefore the mixing of contaminants.
Stochastic modeling of heterogeneous phreatic aquifers
Authors: Tartakovsky Daniel M.
Published: September 2002

Phreatic flow in heterogeneous aquifers is analyzed by treating hydraulic conductivity as a random field with known statistics. A set of equations for the first and second ensemble moments of hydraulic head and phreatic surface is derived. These equations allow one to predict the behavior of phreatic aquifers, as well as to assess the uncertainty associated with such predictions. Perturbation analysis in variance of log hydraulic conductivity is employed to close the moments equations. This leads to a recursive initial boundary value problem.
Spectral analysis of base flow separation with digital filters
Authors: Spongberg M. E.
Published: September 2002

Base flow separation has often been portrayed as the process of removing a high‐frequency event (runoff) from a streamflow time series to determine the low‐frequency component (base flow). Fourier decomposition of several models of streamflow components suggests that this view is inaccurate. Base flow is a predominantly low‐frequency phenomenon, but runoff has a broad bandwidth with a significant low‐frequency “signal.” Base flow separation with digital filters is the attempt to isolate these two signals. Perfect separation is not possible because of the overlapping frequency content. Removal of low‐frequency runoff signal will also partially attenuate base flow. Some principles of filter theory are used to suggest optimal filtering procedures for one commonly used recursive filter. To maximize runoff removal while minimizing base flow attenuation and phase distortion, it is usually best to use two high‐attenuation filter passes as opposed to several low‐attenuation passes.
Effect of groundwater flow dimensionality on mass transfer from entrapped nonaqueous phase liquid contaminants
Authors: Saba Tarek , Illangasekare Tissa H.
Published: September 2002

Mass transfer from entrapped nonaqueous phase liquids (NAPLs) at subsurface locations of environmental contamination typically takes place in three‐dimensional groundwater flow fields. Yet most laboratory studies of NAPLs dissolution have been one‐dimensional, eliminating more realistic conditions such as the heterogeneous distribution of entrapped NAPL ganglia and the potential for flow bypassing due to reduced water permeability in contaminated zones. In this study, experiments in two‐dimensional flow fields were used to evaluate the effects of flow dimensionality on NAPL dissolution. Modifications of the transport code MT3D provided the capability to simulate NAPL dissolution. Regression analysis, matching experimental observations to simulated predictions, provided parameter values for a proposed phenomenological model of dissolution in two‐dimensional flow fields. The proposed model predicted lower NAPL dissolution rates relative to models developed on the basis of published one‐dimensional experimental measurements. The results indicate potential for significant errors using the one‐dimensionally based models for NAPL dissolution in field applications.
Determination of particle transfer in random walk particle methods for fractured porous media
Authors: Liu H. H. , Bodvarsson G. S. , Pan L.
Published: September 2002

A commonly used numerical scheme to simulate solute transport in the subsurface is the random walk particle method (RWPM). While significant progress has been made in developing random walk particle algorithms for porous media, an RWPM for fractured porous media, represented by dual continua, has not yet been developed in the literature. A key issue in developing an RWPM for fractured porous media is the determination of particle transfer between the two continua. In this study a relatively simple, yet physically based scheme to determine the particle transfer is developed. The usefulness of the scheme is demonstrated by comparing solute transport results determined from the proposed scheme with (1) an analytical solution for solute transport in a fractured porous medium with parallel fractures and (2) a numerical simulation obtained using the TOUGH2 code, a simulator for multiphase and multicomponent flow and transport in fractured porous media.
Role of mixing corrosion in calcite‐aggressive H2O‐CO2‐CaCO3 solutions in the early evolution of Karst Aquifers in limestone
Authors: Gabrovšek Franci , Dreybrodt Wolfgang
Published: September 2002

Two cave‐forming mechanisms in limestone are discussed currently. First, when two H2O‐CO2‐CaCO3 solutions, saturated with respect to calcite but with different chemical compositions mix, renewed aggressiveness to limestone dissolution occurs. This process called mixing corrosion [Bögli, 1964, 1980], in combination with linear dissolution kinetics, has been suggested as cave forming. Second, it has been shown that solely the action of nonlinear dissolution kinetics can generate extended karst conduits. This paper combines both mechanisms. By digital modeling of the evolution of the aperture widths of a confluence of two fractures into a third one it is shown that the first mechanism does not create large cave conduits. The combination of mixing corrosion and nonlinear kinetics, however, considerably intensifies karstification, compared to that of nonlinear kinetics solely. The times to terminate early evolution of karst are significantly reduced when the CO2 concentrations of the inflowing solutions differ by no more than 30%. We discuss the underlying mechanisms by inspection of the time dependence of the evolution of aperture widths, flow rates through them, and of the renewed undersaturation of the mixed solution at the confluence of two fractures. Finally, the evolution of a karst aquifer on a two‐dimensional percolation network is modeled when mixing corrosion is present, and compared to that on an identical net with identical nonlinear dissolution kinetics, but mixing corrosion excluded. Large differences in the morphology of the net of cave conduits are found and also a reduction of the time of their evolution. From these findings we conclude that climatic changes, which influence the pCO2 in the soil, can divert the evolving cave patterns.
Prediction of solute spreading during vertical infiltration in unsaturated, bounded heterogeneous porous media
Authors: Sun Alexander Y. , Zhang Dongxiao
Published: September 2002

In this study we investigate the effect of a water table boundary on solute spreading during infiltration in the vadose zone of heterogeneous soils. It has been found recently that the presence of the water table significantly affects unsaturated flow in a heterogeneous vadose zone and causes the flow to be spatially nonstationary. Because a vadose zone is by definition bounded by the water table at the bottom and the contaminants present in the vadose zone migrate into the groundwater through the water table, it is of great interest to study the behaviors of a solute plume near the water table and the effects of flow nonstationarity caused by the water table on solute spreading. To do so, we develop a Lagrangian stochastic approach for predicting field‐scale solute spreading in a spatially nonstationary velocity field. Through first‐order approximations the statistical moments of particle displacement are related to the moments of the Eulerian velocity field. Closed form expressions are obtained for the special case of unidirectional but nonuniform mean flow such as vertical infiltration. We illustrate our theoretical developments with some one‐dimensional examples of solute transport during steady state infiltration in bounded vadose zones. It is found that the inclusion of the water table significantly impacts the behaviors of the mean particle displacement and the displacement covariance, which quantify expected solute displacement and spreading about the expected value, respectively. At late times the particle displacement covariance is generally much smaller when the effect of the water table is included than when it is neglected. In particular, for the case of one‐dimensional infiltration in a bounded vadose zone the particle displacement covariance decreases with time at late times. This finding is in contrast to the result that the particle displacement covariance increases linearly at late times as found by previous stochastic studies of solute transport in unbounded vadose zones. Hence macrodispersion may be greatly overestimated in previous stochastic studies that disregarded the presence of the water table. The validity of the theoretical approach developed in this paper is investigated and confirmed by Monte Carlo simulations.
Simulation of fluid flow and permeability in cohesionless soils
Authors: Masad E. , Muhunthan B. , Martys N.
Published: September 2002

This paper reports on a new laboratory methodology for the determination of the structure of pores, a method for the analysis of the data to obtain the statistics of the pore structure distribution, simulation of porous media with statistics consistent with those in the specimens, numerical simulation of fluid flow in images of porous media, and the determination of permeabilities from the numerical experiments and comparison with the results of laboratory experiments. The computed flow fields show that flow in porous media is restricted to preferential paths depending on the size and connectivity of pores. Whole areas of the pore structure are relatively isolated from the flow because of bottlenecks. The distribution of the isolated regions depends on the angularity of the grains and the porosity of the medium. The more angular the grains are the greater is the possibility for the presence of isolated regions. For grains with the same angularity a decrease in the porosity leads to concentration of flow along preferential flow paths. The permeability tensor coefficients are derived from the flow fields of the two‐dimensional images as well as the three‐dimensional computer‐simulated images of soil microstructure. The numerical values of permeability and permeability anisotropy ratio compare well with laboratory experimental data.
Karst Aquifer evolution in fractured, porous rocks
Authors: Kaufmann George , Braun Jean
Published: September 2002

The evolution of flow in a fractured, porous karst aquifer is studied by means of the finite element method on a two‐dimensional mesh of irregularly spaced nodal points. Flow within the karst aquifer is driven by surface recharge from the entire region, simulating a precipitation pattern, and is directed toward an entrenched river as a base level. During the early phase of karstification both the permeable rock matrix, modeled as triangular elements, and fractures within the rock matrix, modeled as linear elements, carry the flow. As the fractures are enlarged with time by chemical dissolution within the system calcite–carbon dioxide–water, flow becomes more confined to the fractures. This selective enlargement of fractures increases the fracture conductivity by several orders of magnitude during the early phase of karstification. Thus flow characteristics change from more homogeneous, pore‐controlled flow to strongly heterogeneous, fracture‐controlled flow. We study several scenarios for pure limestone aquifers, mixed sandstone‐limestone aquifers, and various surface recharge conditions as well as the effect of faulting on the aquifer evolution. Our results are sensitive to initial fracture width, faulting of the region, and recharge rate.
Terdiurnal oscillations in OH Meinel rotational temperatures for fall conditions at northern mid‐latitude sites
Authors: Pendleton W. R. , Taylor M. J. , Gardner L. C.
Published: September 2002

High‐precision (∼0.5 K) measurements of OH Meinel (M) (6,2) rotational temperatures above the Bear Lake Observatory, UT (42°N, 112°W) during October 1996 have revealed an interesting and unexpected mean nocturnal pattern. Ten quality nights (>100 h) of data have been used to form a mean night for autumnal, near‐equinoctial conditions. The mean temperature and RMS variability associated with this mean night were 203 ± 5 K and 2.4 K, respectively, and compare very favorably with expectations based on Na‐lidar measurements of mean tidal temperature perturbations over Urbana, IL (40°N, 88°W) during the fall 1996. Furthermore, this comparison shows that the 8‐h tide was the dominant source of the mean nocturnal temperature variability in the OH M region during this period. Additional data, obtained at Fort Collins, CO (41°N, 105°W) in November 1997, illustrate the occurrence of an 8‐h component of OH temperature variability about two months after the equinox and show that daily amplitudes as high as ≅15 K are possible.
Anticipated Voyager crossing of the termination shock
Authors: Whang Y. C. , Burlaga L. F.
Published: September 2002

This paper studies the varying location and conditions of the termination shock over 34‐year period (1966 – 2000). The shock location is anti‐correlated with the sunspot number, moving inward or outward during the rising or declining phase of the solar cycle. For the shock in the upwind direction, plasma and magnetic field conditions in its immediately upstream are extrapolated from 1 AU data using MHD equations including pickup protons, and the shock location estimated by the ACR spectrum is used as a guide. Voyager 1 will cross the termination shock in the region of high‐speed solar wind at latitude of ∼34°. Taking into consideration the high‐latitude effects, the calculation indicates that the shock is now moving inward to within 85 AU. Voyager 1 could cross the shock before Year 2002, and the crossing may occur 3 or more times over a period of about 4 years.
Evidence of long term global decline in the Earth's thermospheric densities apparently related to anthropogenic effects
Authors: Keating G. M. , Tolson R. H. , Bradford M. S.
Published: September 2002

A study was performed of the long‐term orbital decay of five Earth satellites with perigee altitudes averaging near 350km. To decouple long‐term trend measurements from the effects of solar variability, measurements were evaluated during the years of solar minimum (1976, 1986 and 1996). Atmospheric densities derived from these essentially global measurements showed substantial evidence of a decline averaging 9.8 ± 2.5% in thermospheric density over 20 years pointing toward a long‐term cooling of the upper atmosphere. Increases in greenhouse gases induced by human activity are hypothesized to warm the Earth's surface and lower atmosphere, but strongly cool the upper atmosphere. Assuming that the 10% increase in CO2 over these 20 years caused cooling resulting in the 10% decline in density, a doubling of CO2 could cause the thermospheric densities measured near 350km to decrease by a factor of 3. This decrease may shrink the altitude of a constant density surface by 40km before the end of the 21st century.
Application of the bounded anisotropy model for the dayside magnetosheath
Authors: Samsonov A. A. , Pudovkin M. I.
Published: September 2002

Experimental data show existence of the proton temperature anisotropy in the dayside magnetosheath (T⊥p > T‖p where ⊥ and ‖ denote directions perpendicular and parallel, respectively, to the background magnetic field). The adiabatic MHD model for an anisotropic plasma is known as CGL equations. However, an application of the CGL equations for magnetosheath modeling overestimates the temperature anisotropy T⊥p/T‖p. To improve this situation, the CGL model was modified into the bounded anisotropy model [Denton et al., 1994; Gary et al., 1994]. This new class of model takes into account proton pitch angle scattering resulting from a growth of plasma anisotropy instabilities. The present paper contains the calculations of three‐dimensional MHD flow around a spherical body with the use of the bounded anisotropy model as well as a comparison between the obtained results and satellite observations of the dayside magnetosheath. The calculated proton temperature anisotropy obtained by the bounded anisotropy model corresponds reasonably to the observed data. The results of the isotropic and anisotropic MHD models are compared. The calculated magnetosheath width is showrn to be greater for the anisotropic double adiabatic model.