摘要:

The dissolution of nonaqueous phase liquids (NAPLs) trapped at residual saturation is an important problem at many contaminated groundwater sites. It is well known that NAPL ganglia trapped within the pore space reduce the permeability of the medium to aqueous phase flow. When fluid flow is imposed on such a system, the aqueous phase may interact with the dissolution‐induced permeability changes, leading to fingered patterns. This mechanism is very similar to that of mineral dissolution instabilities, which are a particular example of reactive infiltration instabilities. Extending that literature, we present a nonlinear model describing the dissolution of NAPL ganglia and perform a linear stability analysis of the resultant moving free boundary problem, demonstrating that instabilities may develop from a planar dissolution front. Predicted finger wavelengths are a function of both residual NAPL saturation and the imposed aqueous phase flow rate; they range from centimeters to meters. Experimental observations of dissolution fingering are presented in a companion paper [Imhoff et al., this issue] and are compared with predictions from this model. Dissolution fingering may affect the solubilization of NAPL ganglia in natural environments and in experimental studies of NAPL dissolution intended to quantify mass transfer rate

ISSN:0043-1397    

DOI:10.1029/96WR00602

年代:1996

数据来源: WILEY

摘要:

Nonaqueous phase liquids (NAPLs) are a common source of contamination at polluted groundwater sites, where they frequently remain trapped within the pore space at residual saturation and reduce the permeability of the medium to aqueous phase flow. The model presented in a companion paper [Imhoff and Miller, this issue] suggested that when fluid flow is imposed on such a system, the aqueous phase may interact with dissolution‐induced permeability changes, and lead to fingered patterns. In this investigation, a two‐dimensional flow cell was used to study the effects of porous medium structure, Darcy flux, initial residual NAPL saturation, median particle diameter, gravity, and NAPL composition on dissolution fingering. Fingering occurred when two conditions were met: (1) 11 to 80e‐fold times had elapsed, wheree‐fold time is the time required for the instability to grow by a factoreand was predicted from the linear stability analysis in the companion paper; and (2) the length of the dissolution front before finger development was smaller than the zone of NAPL residual. Where fingers formed, finger structure was similar and showed no systematic variation within the parameters investigated. Observed finger wavelengths compared well with model predictions. A single experiment in a three‐dimensional cell, 1 m long, demonstrated that fingers can grow to at least 30 cm in length. When experimental observations in this cell were compared with predictions of NAPL dissolution based on models that did not include fingering, the measurements of changing NAPL saturation differed significantly from model pr

ISSN:0043-1397    

DOI:10.1029/96WR00601

年代:1996

数据来源: WILEY

摘要:

Experimental evidence is presented on the influence of specific surface area on the transport of the sorbing tracer strontium in fractures with uniform, but differing aperture. Specific surface area is defined as the ratio of the fracture surface area to the volume of mobile water in the fracture. Static sorption experiments on granite coupons suggest hysteresis in the sorption process, showing higher surface distribution coefficients for desorption than for sorption. Strontium was subject to significantly greater dispersion than the nonreactive tracer tritium. This enhanced dispersion is believed to be the result of chemical heterogeneity at the mineral grain scale, hysteresis in sorption, and limited transverse mixing across the fracture aperture. The influence of fracture aperture on retardation is much greater than predicted by the commonly used definition of the surface retardation factor. Strontium retardation was approximately an order of magnitude greater in a smaller‐aperture fracture (450 µm,Ra∼ 45) than in a large‐aperture fracture (780 µm,Ra∼ 3.5). We hypothesize that hysteresis in sorption, in conjunction with limited transverse mixing across the aperture, caused the apparent increase in sorption strength (Ka) with a decrease in fracture

ISSN:0043-1397    

DOI:10.1029/96WR00895

年代:1996

数据来源: WILEY

摘要:

Microscale experiments can provide mechanistic insights into larger‐scale flow and transport phenomena. Studies of the microscale mechanics involved in preferential flow in general, and unsaturated fast flow paths in particular, require the development of new experimental techniques. A new method for constructing glass micromodels has been developed which permits direct visualization and quantification of flow and transport phenomena in fractured porous media. In the fracture‐matrix micromodels a sequential etching procedure was developed in order to provide the necessary contrast of depths between matrix pores and fracture apertures. This high contrast in etching depths ensures that very different capillary properties are associated with micromodel “fractures” and “matrix” blocks. Improved techniques were also developed for reducing the pore sizes of the matrix to a natural fine‐grained sandstone pore scale. The improved micromodel pattern designs allow for previously unachievable control of boundary conditions. Various saturated and unsaturated fracture flow and transport processes can be visually and quantitatively studied with these micromodels. A method for directly measuring pore‐scale flow velocity distribution through tracing trajectories of suspended fluorescent microspheres was also developed. Examples of applications include measurements of velocity profiles in fractures, imbibition, fracture‐matrix transient flow, and matrix diffusion. In general, the improved micromodel method provides a unique tool for exploring some of the previously unrecognized flow and transport processes in fractured porous media. This research is directed at providing microscale explanations to some currently unresolved flow and transport issues important in predicting the larger‐s

ISSN:0043-1397    

DOI:10.1029/96WR00755

年代:1996

数据来源: WILEY

摘要:

A cone penetrometer method for measuring hydraulic conductivity of unsaturated soils at depth is under development. Successful advancement of this method hinges on using parameter estimation to obtain hydraulic parameter values from pore water pressure and flow rate data. A finite element model is employed to predict flow responses, and objective functions describe differences between “true” and simulated responses. Contour plots in parameter space show the relative sensitivity of objective functions to field‐saturated hydraulic conductivity,Kfs, field‐saturated moisture content,θfs, and the van Genuchten hydraulic parameters, α andn. Principal curvatures and directions in parameter space describe the nature of objective functions near “true” parameter values. An objective function based on flow rate and pore water pressures does not provide better parameter sensitivity than one based on pore water pressures alone. It appears possible to obtain estimates ofKfsand α but unlikely that the other parameters will b

ISSN:0043-1397    

DOI:10.1029/96WR00894

年代:1996

数据来源: WILEY

摘要:

We consider steady flow of water in a confined aquifer toward a fully penetrating well of radiusrw(Figure 1). The hydraulic conductivityKis modeled as a three‐dimensional stationary random space function. The two‐point covariance ofY= In (K/KG) is of axisymmetric anisotropy, withIandIυ, the horizontal and vertical integral scales, respectively, andKG, the geometric mean ofK. Unlike previous studies which assumed constant flux, the well boundary condition is of given constant head (Figure 1). The aim of the study is to derive the mean head 〈H〉 and the mean specific discharge 〈q〉 as functions of the radial coordinaterand of the parameters σy2,e=I/Iυandrw/I. An approximate solution is obtained at first‐order in σy2, by replacing the well by a line source of strength proportional toKand by assuming ergodicity, i.e., equivalence between,, space averages over the vertical, and 〈H〉 〈q〉, ensemble means. An equivalent conductivityKeqis defined as the fictitious one of a homogeneous aquifer which conveys the same dischargeQas the actual one, for the given headHwin the well and a given headin a piezometer at distancerfrom the well. This definition corresponds to the transmissivity determined in a pumping test by an observer that measuresHw,, andQ. The main result of the study is the relationship (19)Keq=KA(1 − λ) +Kefuλ, whereKAis the conductivity arithmetic mean andKefuis the effective conductivity for mean uniform flow in the horizontal direction in the same aquifer. The weight coefficient λ<1 is derived explicitly in terms of two quadratures and is a function ofe,rw/Iandr/I. HenceKequnlikeKefu, is not a property of the medium solely. Forrw/IKefu, our result indicates that the transmissivity is overestimated in a pumping test in a steady state and it decreases with the distance from the well. However, the difference betweenKAandKefuis small for highly anisotropic formations for whiche≪ 1 . A nonlocal effective conductivity, which depends only on the heterogeneous structure, is derived in Appendix A a

ISSN:0043-1397    

DOI:10.1029/96WR00990

年代:1996

数据来源: WILEY

摘要:

A small‐perturbation semianalytical solution is derived for solute transport in porous media with multiple spatially variable reaction processes. Specific reactions of interest include reversible sorption, reversible mass transfer, and irreversible transformation (such as radioactive decay, hydrolysis reactions with fixedpH, and biodegradation). Laplace transform is employed to eliminate the time derivatives in the linear transport equations, and the transformed equations are solved analytically. The transient solution is ultimately obtained by use of an efficient quotient‐difference inversion algorithm. Results indicate that spatial variation of transformation constants for the solution phase and the sorbed phases decreases the global rate of mass loss and enhances solute transport. If the sorbed‐phase transformation constant is spatially uniform but not zero, a similar effect is observed when there is spatial variation of the equilibrium sorption coefficient. The global rate of mass loss and apparent retardation are decreased when the spatial variability of the sorbed‐phase transformation constant is positively correlated with the spatial variability of the equilibrium sorption coefficient and increased for a negative corr

ISSN:0043-1397    

DOI:10.1029/96WR01046

年代:1996

数据来源: WILEY

摘要:

One‐dimensional vertical flows through spatially heterogeneous areas are studied with numerical solutions for a variety of soil types, to ascertain the behavior of the ensemble. Soil heterogeneity is treated using the distributions of parameters describing the characteristic relations of conductivity, water content, and pressure head. Values of the critical parameters, taken from a modified Brooks‐Corey or van Genuchten model, have previously 7lsqb;Smith and Diekkrüger, 1992] been shown in two extensive data sets to exhibit little interdependence. The static ensemble characteristics, except for saturated and residual water content parameters, are shown to have regular bias compared to a soil with the mean characteristic parameters. The measured data also are shown not to support the popular similar media heterogeneity model. Using both analytic approximate flow models and an ensemble solution of Richards' equation, the effective dynamic ensemble characteristics in three example soils are studied. As suggested in previous studies, such effective characteristics are conditional to flow conditions and are here shown to be quite different from ensemble static characteristics. This indicates that irrespective of the sample size, effective soil dynamic characteristics are not measurable for samples possessing significant heterogen

ISSN:0043-1397    

DOI:10.1029/96WR01048

年代:1996

数据来源: WILEY

摘要:

The response of a conceptual soil water balance model to storm events is compared to a detailed finite element solution of the one‐dimensional Richards equation in order to test the capabilities of the former in calculating the local contributions to infiltration excess runoff in a distributed catchment scale model. Local infiltration excess runoff is computed from ground level precipitation using the time compression approximation and a Philip infiltration capacity curve with Brooks‐Corey constitutive equations. The validity of applying the conceptual model for local runoff and soil water balance calculations is investigated by performing numerical experiments over a range of soil types, control volume depths, and initial soil moisture conditions. We find that a good agreement between the conceptual and detailed models is obtained when the gravitational infiltration rate in Philip's formula is set to the saturated hydraulic conductivity, and when percolation from the control volume is updated as a function of the soil moisture content in a stepwise fashion. The comparison between these two models suggests that the simpler (and much less computer‐intensive) conceptual water balance technique could be incorporated into distributed models for large scale complex terrains as an efficient means of retaining consideration of spatial variability effects in catchment scale hydrologie simulations. This is illustrated in an application to the Rio Missiaga catchment in the eastern Italian Alps, where the local contributions to surface and subsurface runoff are routed onto a digital elevation model‐based conceptual transport network via a simple numerical scheme based on the Muskingum‐Cun

ISSN:0043-1397    

DOI:10.1029/96WR00897

年代:1996

数据来源: WILEY

摘要:

Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment‐1987 showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with at−½dependency at the daily timescale and (2) that this day‐to‐day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion‐type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylor's α, the evaporative fraction, and a few others), with an assumption of self‐preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2‐week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation int

ISSN:0043-1397    

DOI:10.1029/96WR00995

年代:1996

数据来源: WILEY