ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00554.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractComputer software with embedded knowledge has the potential to improve the utility and usability of computer models. We explore these opportunities with the software package Expert ROKEY that includes EXPAR, a knowledge‐based system to assist in the preparation of input data for a simple mass transport model. EXPAR is partitioned into two levels. At the top level is a set of computer forms that contain groups of related parameters and corresponding entry fields. Users can either volunteer parameter values or request help from an assistance program. Elaboration programs provide supporting information for each parameter and brief tutorials for the mass transport processes included in the model. The sets of assistance and elaboration programs represent the bottom level of EXPAR. This model preprocessor contains useful features like tutorial and data‐base information, systematic procedures for deriving parameter values, capabilities for checking the completeness and consistency of data, and a virtually transparent user interface with the computer, which are applicable to codes of all ty

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00555.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractOne method for the disposal of hazardous liquid wastes is by injection into deep aquifers. Although these aquifers may be separated from underground sources of drinking water by thick formations of low permeability, their mobility due to different migration mechanisms has to be studied carefully, since the injected wastes remain toxic over periods of thousands of years. One possible mechanism for waste movement is density‐driven flow and transport, due to density differences between the waste and the surrounding water in the injection zone. In the present paper the importance of this phenomenon is studied mathematically by means of analytical and numerical calculations for typical deep injection conditions. The analytical estimates reveal that density‐driven movement of liquid wastes in sloping aquifers can be much stronger than plume migration due to natural hydraulic gradients. This finding is emphasized by the results of a two‐dimensional vertical finite element model, which is applied for detailed numerical simulations. Results show that during the initial stage, waste can be expected to spread into all directions due to density‐induced stratification effects. Later on, it mainly moves laterally along the slope of either aquifer top or aquifer bottom, depending on the waste density. If regional ground‐water flow is directed the same way, transport is accelerated. If regional ground‐water flow is in the opposite direction, on the other hand, transport to both sides must be expected to occur. Thus, the aquifer slope and regional hydraulic gradient may be equally significant factors in estimating potential migration of disposed li

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00556.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractWaste water injected into aquifers bounded by aquitards can potentially leak into the aquitards and eventually reach other layers. Based on the mathematical framework of an earlier model, the current study deals with steady‐state and transient solutions for solute transport from an injection well into a three‐layer leaky aquifer system consisting of a pumped and an unpumped aquifer separated by an aquitard. Solutes are assumed to be transported by horizontal advection within the pumped aquifer, by advection and dispersion in a vertical flow field in the aquitard, and by molecular diffusion under hydrostatic conditions in the unpumped aquifers. Proper leaky aquifer well hydraulics are used to describe the ground‐water flow fields in the model, and radioactive (biological) decay and linear isotherm adsorption are considered. This model study shows that adsorption in the pumped aquifer has an overall retardation effect on solute movement in the whole system. Adsorption in the aquitard does not significantly affect solute movement in the pumped aquifer. Type curves are prepared to indicate whether the unpumped aquifer would be contaminated by the injection into the pumped aquifer. Pumping rate is an important hydraulic factor determining the leakage conditions of an aquifer (in addition to some hydrogeological conditions of the systems). Analytical solutions are obtained for steady‐state conditions. For transient conditions, however, only semianalytical solutions can be determined by numerically inverting the solutions of the model in the Laplace transform

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00557.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTwo liquid hazardous wastes (an alkaline brine‐like solution and a dilute acidic waste) were mixed with finely ground rock samples of three injection‐related lithologies (sandstone, dolomite, and siltstone) for 155 to 230 days at 325°K‐10.8 MPa. The pH and inorganic chemical composition of the alkaline waste were not significantly altered by any of the rock samples after 230 days of mixing. The acidic waste was neutralized as a consequence of carbonate dissolution, ion exchange, or clay‐mineral dissolution, and hence was transformed into a nonhazardous waste.Mixing the alkaline waste with the solid phases yielded several reaction products: brucite, Mg(OH)2; calcite, CaCO3; and possibly a type of sodium metasilicate. Clay‐like minerals formed in the sandstone, and hydrotalcite, Mg6Al2‐CO3(OH)16· 4H2O, may have formed in the siltstone at trace levels. Mixing the alkaline waste with a synthetic brine yielded brucite, calcite, and whewellite (CaC2O4· H2O). The thermodynamic model PHRQPITZ predicted that brucite and calcite would precipitate from solution in the dolomite and siltstone mixtures and in the alkaline waste‐brine system.The dilute acidic waste did not significantly alter the mineralogical composition of the three rock types after 155 days of contact. The model PHREEQE indicated that the calcite was thermodynamically stable in the dolomite and siltstone mixtures, and was detected in small quantities in aged‐solid samples. The sandstone‐waste system appeared to equilibrate with amorphous silica, whereas silica equilibria may not have been attained in the dolomite and siltstone systems.Computer models like PHRQPITZ and PHREEQE may be useful tools for estimating mineral equilibria in deep‐well scenarios, but there is a need to expand the database used in these kinds of calculations. Caution must be applied in interpreting the predicted equilibria. Fate modeling based on thermodynamic principles can predict simple geochemical interactions, but empirical, laboratory‐based investigations may be needed in addition to modeling for a reliable assessment of the

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00558.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractEquations describing the three‐phase equilibrium of contaminants between the pore‐gas, pore‐water, and solid‐sorbed phases in the unsaturated zone are presented. These relationships are then used to derive general expressions for the temperature‐dependence of the equilibrium in a three‐phase system, and those relationships are applied to derive expressions for the temperature‐dependence of the concentrations in the three phases when the total contaminant mass present and the gas‐filled and water‐filled porosities are constant. The temperature‐dependence of the three‐phase equilibrium is dictated by the enthalpies of phase transition and the appropriate equilibrium constants. As the temperature increases, contaminants move from the more‐dense to the less‐dense phases, so that the pore‐gas‐phase concentration increases to an upper limit; the pore‐water‐phase concentration increases, reaches a maximum, and then decreases; and the solid‐sorbed concentration decreases. Equations describing the effects of changes in the water‐filled porosity, such as those that would be caused by infiltration of either clean or contaminated pore water, on the gas‐phase and pore‐water‐phase concentrations of contaminants are presented. For compounds with a Henry's Law constant (H, dimensionless) greater than one, an increase in water‐filled porosity results in an increase in the pore‐gas concentration, while for compounds with H less than one, such a change results in a decrease in the pore‐gas concentration. The applicability of the thermodynamic relationships developed is limited by the assumption of ideal‐gas behavior, isothermal phase transition, and isobaric conditions. The stoichiometric relationships developed are limited by the assumption of complete mixing. They are used to develop equations that allow calculation of the equilibrium fraction of the total contaminant mass present in each phase on the basis of pore‐gas monitoring data, site properties, and equilibrium constants. Based on observed natural seasonal changes in subsurface temperatures and moisture contents, the effects predicted in the derived equations could result in significant changes in gas‐phase concentration

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00559.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTo show the effect of sorption model selection, the one‐dimensional transport of naphthalene in two saturated porous media was simulated using numerical models, which included the processes of dispersion, advection, and sorption. Three vdifferent models describing the sorption process: (1) a local equilibrium model, (2) a first‐order kinetic sorption model, and (3) a two‐site model (a combination of equilibrium and kinetic sorption model), were examined. In addition, this study considered two types of boundary conditions: constant concentration and constant flux boundary conditions. Simulations were made for data of naphthalene transport through a laboratory soil column packed with Lincoln or Eustis soil series. Numerical simulation results show that the selection of the sorption model had significant effects on the numerical solutions. The numerical solution obtained using either the local equilibrium model or the first‐order kinetic sorption model described the experimental data very well from the Lincoln soil column. In contrast, the numerical solution obtained using the two‐site model described the experimental data for the Eustis soil better than either the equilibrium or kinetic models. Different boundary conditions had little influence on the numerical solutions. Different amounts of organic carbon in the Eustis (0.66%) and Lincoln (0.036%) soils are thought to cause the differences in breakthrough curves and the response to the sorption

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00560.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTwo‐dimensional analysis of ground‐water flow using graphical flow net analysis or numerical modeling is applied to many field settings. However, the assumption of hydrostatic head conditions, implicit in two‐dimensional depth‐averaged planimetric analyses, is not correct for situations where there is vertical flow such as near a partially penetrating stream. Vertical flow is particularly important when predicting flow paths for contaminant transport. An analytical solution is presented, which calculates the flow paths in the vicinity of a partially penetrating stream. The derivation assumes radial flow towards the stream in the vicinity of the stream, and predominantly horizontal Dupuit flow in the remainder of the region. The analytical solution may be used as a correction to apply to flow paths generated by two‐dimensional analysis, or as a screening tool to determine the error when vertical flow is neglected by two‐dimensional analysis. To verify the analytical solution, it is compared to numerical simulations of three‐dimensional flow in an idealized aquifer for a number of cases with varying degrees of hydraulic conductivity anisotropy. An example illustrates the use of the analytical solution. For the example presented, two‐dimensional analysis can underestimate the distance which a contaminant travels before discharging into the stream b

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00561.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe occurrence of selenium in agricultural drain water from the western San Joaquin Valley, California, has focused concern on the semiconfined ground‐water flow system, which is underlain by the Corcoran Clay Member of the Tulare Formation. A two‐step procedure is used to calibrate a preliminary model of the system for the purpose of determining the steady‐state hydraulic properties. Horizontal and vertical hydraulic conductivities are modeled as functions of the percentage of coarse sediment, hydraulic conductivities of coarse‐textured (Kcoarse) and fine‐textured (Kfine) end members, and averaging methods used to calculate equivalent hydraulic conductivities. The vertical conductivity of the Corcoran (Kcorc) is an additional parameter to be evaluated.In the first step of the calibration procedure, the model is run by systematically varying the following variables: (1) Kcoarse/Kfine, (2) Kcoarse/Kcorc, and (3) choice of averaging methods in the horizontal and vertical directions. Root mean square error and bias values calculated from the model results are functions of these variables. These measures of error provide a means for evaluating model sensitivity and for selecting values of Kcoarse, Kfine, and Kcorcfor use in the second step of the calibration procedure. In the second step, recharge rates are evaluated as functions of Kcoarse, Kcorc, and a combination of averaging methods. The associated Kfinevalues are selected so that the root mean square error is minimized on the basis of the results from the first step.The results of the two‐step procedure indicate that the spatial distribution of hydraulic conductivity that best produces the measured hydraulic head distribution is created through the use of arithmetic averaging in the horizontal direction and either geometric or harmonic averaging in the vertical direction. The equivalent hydraulic conductivities resulting from either combination of averaging methods compare favorably to field‐ and laboratory

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00562.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThis paper examines the behavior of dense, nonaqueous phase liquids (DNAPLs) in fractured clay and rock. The conditions under which a DNAPL will enter an initially water‐saturated, rough‐walled fracture are outlined and expressed in a number of ways, including the height to which a DNAPL pool can accumulate above a fracture prior to initial entry. To study the behavior of DNAPL in a rough‐walled fracture following initial entry, numerical simulations are carried out both in the plane of a fracture using a discrete representation of fracture roughness, and at a larger scale of averaging using an equivalent homogeneous porous media approach. The simulations illustrate that DNAPL will migrate through the larger aperture regions of a fracture plane, and that the DNAPL has the potential to enter progressively smaller aperture fractures with depth as it migrates. Additional numerical simulations indicate that the time taken for a nonaqueous phase liquid to traverse a fractured aquitard is inversely proportional to the fracture aperture, the fracture dip from the horizontal, and the height of the pool collected above the aquitard. It is also demonstrated that upward hydraulic gradients across a fractured aquitard can significantly slow the downward rate of DNAPL migration while downward water gradients enhance the rate of DNAPL migration across the aqu

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00563.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY