摘要:

Buoyancy‐induced flow (BIF) of both18O‐enriched (δ18O, +160 to +5195‰) and18O‐depleted (δ18O, −500‰) water has been studied in a 10‐cm‐diameter tube that simulates a section of a well. BIF was detected in all the experiments for a density contrast (Δρ) between 0.03 and 1.0 σ units (equivalent to a total dissolved solids contrast of 30−1000 mg/L). The magnitude of the Rayleigh number suggests that the natural convection flow is turbulent. A mathematical model is presented to predict vertical tracer propagation inside the cavity. The model assumes a local mixing length scale proportional to the tube diameter. A possible transition to the laminar natural convection regime, with a significant reduction in mixing rate compared to that predicted by the model, is detected when the density contrast is reduced to Δρ = 0.03 σ units. This smaller mixing rate is still orders of magnitude larger than diffusion alone. BIF may be an important groundwater transport and mixing mechanism in open conduits in aquifers. Its effects should also be considered when performing tracer

ISSN:0043-1397    

DOI:10.1029/95WR00201

年代:1995

数据来源: WILEY

摘要:

A laboratory flow apparatus was used to visualize and measure two‐phase gas‐liquid flows in natural rough‐walled rock fractures. Experiments at carefully controlled flow rate and pressure conditions have been performed using a natural fracture and three transparent fracture replicas. Two‐phase flow exhibited persistent instabilities with cyclic pressure and flow rate variations even under conditions of constant applied boundary conditions. Visual observations of changes in pore occupancy showed that the instabilities could be explained as resulting from an interplay between capillary effects and pressure drop due to viscous flow. Measurements of relative permeabilities indicated strong phase interference, with relative permeabilities reduced to very small values at intermediate saturations for both wetting and nonwetting phases. These results run counter to a conventional view of fracture relative permeabilities that assumes that the relative permeability of each phase is equal to its saturation, but the results are consistent with recent models that view fractures as two‐dimensional heterogeneous por

ISSN:0043-1397    

DOI:10.1029/95WR00171

年代:1995

数据来源: WILEY

摘要:

Water and solute input rates on the soil surface may vary considerably from place to place. Distribution flow, i.e., the process of water and solute flow in a lateral direction over and through the very first millimeter or centimeter of the soil profile, is an extremely important process in distributing the rainfall toward places where vertical transport occurs. This study was carried out to quantify the process of distribution flow and its underlying process mechanism. A KBr tracer was applied on two water repellent sandy soils to follow the actual flow paths of water and solutes in the upper part of the profile. On both experimental fields, distribution flow actually displaced the applied bromide laterally through a very thin layer on the top of the soil profile, referred to here as the “distribution layer.” Distribution flow was directed to locations within the 0‐ to 2.5‐cm layer, where the soil was the least water repellent. On these relatively wet areas, the highest concentrations of bromide could be found. There was an acceleration in the vertical transport of water and bromide on these sites. Effects of distribution flow on the local concentration of water and solutes can be expected to be even more pronounced in uneven terrains, where lateral displacements may increase from the millimeter‐centimeter scale to tens of meters. Implications for modeling field‐scale water and solute flow a

ISSN:0043-1397    

DOI:10.1029/94WR02979

年代:1995

数据来源: WILEY

摘要:

In this paper we present a partitioning interwell tracer test (PITT) technique for the detection, estimation, and remediation performance assessment of the subsurface contaminated by nonaqueous phase liquids (NAPLs). We demonstrate the effectiveness of this technique by examples of experimental and simulation results. The experimental results are from partitioning tracer experiments in columns packed with Ottawa sand. Both the method of moments and inverse modeling techniques for estimating NAPL saturation in the sand packs are demonstrated. In the simulation examples we use UTCHEM, a comprehensive three‐dimensional, chemical flood compositional simulator developed at the University of Texas, to simulate a hypothetical two‐dimensional aquifer with properties similar to the Borden site contaminated by tetrachloroethylene (PCE), and we show how partitioning interwell tracer tests can be used to estimate the amount of PCE contaminant before remedial action and as the remediation process proceeds. Tracer tests results from different stages of remediation are compared to determine the quantity of PCE removed and the amount remaining. Both the experimental (small‐scale) and simulation (large‐scale) results demonstrate that PITT can be used as an innovative and effective technique to detect and estimate the amount of residual NAPL and for remediation performance assessment in subsurface for

ISSN:0043-1397    

DOI:10.1029/95WR00174

年代:1995

数据来源: WILEY

摘要:

The propagation of dense nonaqueous phase liquids (DNAPLs) in water‐saturated, homogeneous porous media was investigated. The static distribution of DNAPL after gravity‐driven displacement was studied using a number of three‐dimensional spill experiments. Fingering intrinsic to the displacement systems was observed in all experiments. The effects of physical and chemical parameters on flow instability were examined for a range of sand grain sizes (fine, medium, and very coarse) and for different DNAPLs (trichloroethylene, trichloroethane, and dibutyl phthalate). Gravitational, viscous, and capillary forces were observed to have a varied influence on the flow behavior in these experiments. Our observations show that the development of finger patterns is sensitive to the porous media grain size, properties of the DNAPL, and spill conditions. By controlling experimental parameters, results are reproducible and yield insight into finger formation and preferential DNAPL flow in homogeneous aquifer materials. This paper discusses the experimental results qualitatively; a companion paper discusses their quantification with fractal con

ISSN:0043-1397    

DOI:10.1029/95WR00428

年代:1995

数据来源: WILEY

摘要:

Fingering of dense nonaqueous phase liquids (DNAPLs) as seen in three‐dimensional experirments with saturated, homogeneous porous media was analyzed. A consistent geometrical quantification of finger configurations was obtained using concepts of fractal and multifractal scaling. Fractal patterns that determine the probabilistic distribution of the DNAPL were found to be representative for every experimental combination of sand and DNAPL. These patterns could be attributed to either capillary or viscous fingering regimes. With multifractal formalisms we were able to give a description of the underlying process kinetics. The generalized dimensionDqrelates results to diffusion‐limited aggregation (DLA) or invasion percolation type models. The spectrum of singularities ƒ(α) is invariable for cross sections of an experiment and in turn can be used for a classification of the displacement system. The width of the ƒ(α) curve in the range of positive moments quantifies displacement instability. Phase transitions are indicated for the more stable displacement

ISSN:0043-1397    

DOI:10.1029/95WR00429

年代:1995

数据来源: WILEY

摘要:

The Brooks‐Corey and van Genuchten functions were modified to adequately represent retention at all matric suctions. The modification consisted of replacing the residual water content with an adsorption equation (Campbell and Shiozawa, 1992). The modified functions retain the form of the original functions in the wet range and the form of an adsorption equation in the dry range. The modified functions provided excellent fits to data from six soils with textures ranging from sand to silty clay. The modified van Genuchten function can use previously determined parameters to obtain a reasonable representation of the high matric suction range, thus allowing for the use of existing parameter sets. The modified functions, as well as the function proposed by Rossi and Nimmo (1994), were combined with the Mualem conductivity model to generate closed‐form analytical expressions for the calculation of hydraulic conductiv

ISSN:0043-1397    

DOI:10.1029/95WR00173

年代:1995

数据来源: WILEY

摘要:

The method of Bouwer and Rice (1976) for analyzing slug test data is widely used to estimate hydraulic conductivity (K). Based on steady state flow assumptions, this method is specifically intended to be applicable to unconfined aquifers. Therefore it is of practical value to investigate the limits of accuracy of theKestimates obtained with this method. Accordingly, using a numerical model for transient flow, we evaluate the method from two perspectives. First, we apply the method to synthetic slug test data and study the error in estimated values ofK. Second, we analyze the logical basis of the method. Parametric studies helped assess the role of the effective radius parameter, specific storage, screen length, and well radius on the estimated values ofK. The difference between unconfined and confined systems was studied via conditions on the upper boundary of the flow domain. For the cases studied, the Bouwer and Rice analysis was found to give good estimates ofK, with errors ranging from 10% to 100%. We found that the estimates ofKwere consistently superior to those obtained with Hvorslev's (1951) basic time lag method. In general, the Bouwer and Rice method tends to underestimateK, the greatest errors occurring in the presence of a damaged zone around the well or when the top of the screen is close to the water table. When the top of the screen is far removed from the upper boundary of the system, no difference is manifest between confined and unconfined conditions. It is reasonable to infer from the simulated results that when the screen is close to the upper boundary, the results of the Bouwer and Rice method agree more closely with a “confined” idealization than an “unconfined” idealization. In effect, this method treats the aquifer system as an equivalent radial flow permeameter with an effective radius,Re, which is a function of the flow geometry. Our transient simulations suggest thatRevaries with time and specific storage. Thus the effective radius may be reasonably viewed as a time‐averaged mean value. The fact that the method provides reasonable estimates of hydraulic conductivity suggests that the empirical, electric analog experiments of Bouwer and Rice have yielded shape factors that are better than the shape factors implicit in the Hvorsl

ISSN:0043-1397    

DOI:10.1029/94WR03292

年代:1995

数据来源: WILEY

摘要:

Transport of bacteria in a soil was delayed relative to the mobile pore water by rate‐limited rather than instantaneous sorption to soil particles. Desorption rates were found to be faster than adsorption rates, and sorption kinetics were dependent on the density of bacteria and on the flow rate of water. Sorption kinetics were revealed by a combination of model descriptions of the mass balance and sorption kinetics of a groundwater bacterium entering a porous system and laboratory experiments using a stirred flow chamber and a soil column. The flow chamber was designed and operated to (1) distinguish instantaneous equilibrium reactions from time‐dependent reactions, (2) estimate sorption rate coefficients at different cell densities and water velocities, and (3) obtain rate coefficients that could be used to predict the breakthrough of bacteria in a soil column under saturated, continuous flow conditions. Sorption rates in the stirred flow chamber were expressed by three rate laws, first order, Langmuir, and second order, and the transport of bacteria through the soil column was described and predicted by a model assuming a mixture of instantaneous and time‐dependent sorption reactions and, alternatively, by a kinetic model. The sorption rate was primafly dependent on the density of sorbed bacteria, but the desorption rate was, in addition, influenced by the density of cells in the aqueous phase. Rate coefficients obtained in the flow chamber could be used to accurately predict transport of bacteria in the soil column at the corresponding density and flow rate, but extrapolations are complicated by the nonlinear relationship between density and sorption rate. Timescales for reaching equilibrium estimated from the rate coefficients were in the range from 10–15 hours to several days, longer for low densities and vel

ISSN:0043-1397    

DOI:10.1029/95WR00023

年代:1995

数据来源: WILEY

摘要:

The purpose of this study is to derive a general solution for the problem of flow in nonstationary geological formations where the nonstationarity manifests itself in the form of a spatial trend in the mean log conductivity. A stochastic frame of reference is adopted to account for the spatial variability of the hydraulic conductivity. For a complete stochastic description we derive the expected values and spatial covariances of the hydraulic head and the fluid flux vector, as well as a relation between the expected values of the head and the fluxes. These expressions are obtained using a perturbation expansion of the log conductivity about its nonstationary mean, and they are correct to the first order in the variance of the log conductivity. The expressions we derive are applicable for any space dimensionality and for arbitrary orientation of the trend in space. A general methodology is outlined for derivation of these expressions for any type of spatial covariance; and for demonstration, explicit results are obtained for a Gaussian isotropic covariance.

ISSN:0043-1397    

DOI:10.1029/94WR03002

年代:1995

数据来源: WILEY