ISSN:0043-1397    

DOI:10.1029/94WR01220

年代:1994

数据来源: WILEY

摘要:

A method of modeling freeze‐thaw cycles of naturally deposited snowpacks is presented. The model involves the Stefan condition as an independent governing equation on the exterior moving boundary to calculate snowpack thinning, flow of water through a variably saturated layered porous medium as described by the Richards equation, and heat conduction with a phase change. The heat conduction problem was treated in two ways. Local heat conduction between a snow grain and its surrounding water film was treated by using a simple energy balance. Global heat conduction with a phase change (the Stefan problem) was introduced to calculate the space‐time temperature distribution. In order to handle multiple interior moving boundaries, a specific form of the enthalpy formulation was used for heat conduction with a phase change. Changing material properties were considered according to the calculated meltwater refreez

ISSN:0043-1397    

DOI:10.1029/94WR00764

年代:1994

数据来源: WILEY

摘要:

A flux‐corrected transport (FCT) based numerical algorithm is developed to model the solute transport equation in heterogeneous variably saturated soils. The fully Eulerian algorithm is designed to model very steep gradients without the characteristic numerical diffusion/dispersion found in standard finite difference schemes. This algorithm can handle both small and infinite grid Peclet numbers but is limited to grid Courant numbers less than or equal to 0.5. Since useful results can be obtained for infinite Peclet numbers, the algorithm can be used to trace water movement when coupled with a numerical model for Richards' equation. We used the FCT based algorithm in conjunction with Richards' equation to model a hypothetical experiment at the Las Cruces Trench site. In this numerical experiment, water with tracer is applied across the top of a 24‐m‐wide domain. The two‐dimensional distribution for the hydraulic properties estimated during site characterization is used to define the soil model. The predicted flow and tracer transport is quite complex. The movement of the wetting front appears to be heavily influenced by the old water, whereas the new water tends to bypass much of the old water indicating preferenti

ISSN:0043-1397    

DOI:10.1029/94WR01216

年代:1994

数据来源: WILEY

摘要:

Simulation of water flow and transport processes in soils rely on field representative soil hydraulic functions. The linear variability concept in combination with the inverse technique was used to estimate in situ soil hydraulic properties in a 32‐ha field. Measured cumulative drainage curves were scaled yielding scaling factors. Subsequently, the drainage and moisture content distribution of the scaled reference profile were input to a numerical model to optimize the soil water retention and hydraulic conductivity curves for the reference soil profile by inverse solution of the scaled Richards equation. Field hydraulic functions for each location were computed from the reference curves and scaling factors. In addition, undisturbed soil cores taken from 0.3‐m and 0.6‐m depths at 44 locations were used to determine soil texture, and soil water retention and hydraulic conductivity curves in the laboratory using the multistep outflow technique. These hydraulic functions were scaled using the simultaneous scaling technique. The reference field hydraulic functions compared well with those determined from the soil cores taken from the 0.6‐m depth. In situ saturated hydraulic conductivity variability was one order of magnitude less than that of the soi

ISSN:0043-1397    

DOI:10.1029/94WR00756

年代:1994

数据来源: WILEY

摘要:

Dynamical systems describe the time evolution of moving spatial points under the influence of a smooth, bounded vector field. The theory of these systems is focused on the global properties of their flow paths, not on their integration, and so gives general, qualitative information that does not depend on the details of the spatial variability of the vector field. This approach was applied to describe the global consequences of the Darcy law for steady groundwater flows in isotropic, heterogeneous aquifers. No particular model of the spatial variability of the hydraulic conductivity (K) was assumed. Vorticity in the flow paths was shown to exist wherever isoconductivity and equipotential surfaces intersect transversely, and the importance of the Lamb vector (the vector product of vorticity and specific discharge) for the geometry of flow paths was established. Because steady groundwater flows governed by the Darcy law have zero helicity, they cannot exhibit tangled vorticity lines or become chaotic. The absence of chaos is related closely to the impossibility of closed flow paths, the asymptotic stability of isolated minima of the hydraulic head (H), and the existence of a function ℋ(K,H) on whose level surfaces all flow paths are confined. This last function also permits groundwater flows to be represented by moving points in theK,Hplane, with motion there generated by a form of Hamilton's equations. The results obtained are not related to any stochastic approach to aquifer spatial variability, but instead may be applied to constrain stochastic models on purely dynamical ground

ISSN:0043-1397    

DOI:10.1029/94WR01328

年代:1994

数据来源: WILEY

摘要:

The available approaches for analyzing pulse tests assume that the initiating pressure pulse is instantaneous. However, in practice this is difficult to achieve due to equipment response lags. A new model is derived which includes pulses of finite duration. This model is an extension of the instantaneous model of Bredehoeft and Papadopulos (1980), who presented an integral expression to explain behavior during a pulse test. In this paper, equations are derived for both instantaneous and finite duration cases in Laplace space. These equations are numerically inverted to real space using the algorithm of Stehfest (1970). The finite duration pulse test model differs from the instantaneous model in an integral term which acts as a correction to the flow response. In order to numerically invert the finite duration equation the integral term is first numerically evaluated. An approximation is introduced to enable the explicit numerical integration of this term. The use of the instantaneous pulse model to analyze pulse tests which involve a finite duration results in error in the estimation of formation hydraulic properties. In one example considered, where the pulse was 30 s, the error was near zero for a specific storage capacity less than 1 × 10−9m−1, and the formation conductivity less than 1 × 10−9m s−1. However, for the same test conditions, a conductivity of 1 × 10−8m s−1and a specific storage capacity of similar magnitude would result in an error of approximately 50% i

ISSN:0043-1397    

DOI:10.1029/94WR00755

年代:1994

数据来源: WILEY

摘要:

Remediation of contaminated groundwater aquifers can be extremely difficult when desorption of the contaminant from the solid phase to the aqueous phase is slow. The effects of desorption rates on the feasibility of in situ bioremediation are investigated using an analytical solution to the solute transport equation with rate‐limited desorption and first‐order decay. Sensitivity analyses are used to identify when desorption or degradation is the rate‐limiting process for bioremediation for the range of desorption rates, degradation rates, and equilibrium distribution coefficients reported in the literature. The results show that when the desorption coefficient is large relative to the degradation coefficient (degradation is rate limiting), the performance of in situ bioremediation compared to groundwater extraction is improved as the degradation rate increases. Degradation in the aqueous phase increases the concentration gradient between solid and aqueous phases, increasing the rate of desorption and thus the rate of remediation. However, when the desorption coefficient is small relative to the degradation coefficient (desorption is rate limiting), increasing the degradation rate any further will not improve the performance of in situ bioremediation. For sites that conform to the assumptions of the model and for which model parameters can be measured or estimated, the results can be used to perform a preliminary assessment of the feasibility of in situ bioremedi

ISSN:0043-1397    

DOI:10.1029/94WR01272

年代:1994

数据来源: WILEY

摘要:

Organic and inorganic compounds infiltrating into groundwater are subject to redox reactions catalyzed by bacteria. These bacteria tend to form conglomerates and to adsorb to the solid matrix. Assuming a biofilm concept, the mass transport in and near such a biofilm is a controlling factor for different processes affecting the microbial activity, such as the supply of the substrates to the bacteria or the detachment of biofilm parts by shear forces. We present here a macroscopic model for the transport of dissolved substances in groundwater‐biofilrn systems. This model conceptualizes the diffusion dominated microscopic transport processes within the biofilm by using a logistic approach based on a dififusional limitation. As biological processes, the growth, the maintenance, and the decay of the bacterial population are explicitly accounted for. The model is formulated for a one‐dimensional flow and transport field and is numerically solved with an operator splitting technique as described by Zysset et al. (1994). It is able to reproduce the transport of nitrate and sulfate in two laboratory column experiments in which these substances served as dissolved electron acceptors in bacterially mediated redox reacti

ISSN:0043-1397    

DOI:10.1029/94WR01045

年代:1994

数据来源: WILEY

摘要:

An approach to probabilistic modeling of contaminant transport based on the first‐ and second‐order reliability methods (FORM and SORM) is presented. FORM and SORM were initially developed for structural reliability applications to estimate the occurrence of low‐probability events. They can be readily used with both analytical and numerical models and do not require restrictive assumptions about the problem geometry or about the properties of the media. Sensitivity information is obtained as an integral part of these analyses and is used to identify the variables or parameters which have a major influence on the estimate of probability. Example reliability analyses of one‐ and two‐dimensional transport are used to illustrate the approach, and the accuracy of the reliability methods is evaluated in comparison with Monte Carlo simulations. The results show that FORM increasingly overestimates the probability of exceedance as the spatial variability of the domain increases. SORM, on the other hand, accounts for the nonlinearity of the limit state surface and gives results consistent with Monte Carlo simulation over a range of coefficient of variation ofKfrom 0.1 to 0.7. In addition, the FORM/SORM analyses are shown to provide a computational advantage over the Monte Carlo simulation for low‐probability events, because the computational effort is independent of the probability and the results also include sensitivity information. Finally, an example application of system reliability using FORM and SORM shows that problems with multiple limit state surfaces can be readily analyzed and the computational effort is proportional to the number and complexity of the limit stat

ISSN:0043-1397    

DOI:10.1029/93WR03554

年代:1994

数据来源: WILEY

摘要:

This paper presents a new method of semianalytical particle tracking analysis under transient conditions. To permit accurate particle tracking in transient flows, the particle velocity is interpolated linearly in both space and time coordinates within each finite difference cell and time step. By using the total derivative and this interpolation scheme, the particle velocity is derived and integrated analytically to obtain the particle's trajectory within each cell and time step. Errors are much smaller than in numerical integration schemes. Numerical experiments show that the transient semianalytical solutions are efficient and accurate.

ISSN:0043-1397    

DOI:10.1029/94WR01219

年代:1994

数据来源: WILEY