摘要:

A quasi three‐dimensional finite element model based on the conjugate gradient technique is developed and applied to the analysis of subsurface flow occurring in complex heterogeneous multiaquifer systems. Transient leakage across the aquitards is fully taken into consideration using the convolution approach originally derived by Herrera (1970). The convolution integrals enhance the favorable features of the modified conjugate gradients (MCG), which prove to be a very efficient tool for the solution of the integrodifferential model. The high performance of MCG results from two combined factors: (1) the diagonal dominance of the assembled finite element matrix, which is emphasized by the integral contributions; (2) the essential reducibility of the matrix for a wide range of time step values, which is due to the virtual decoupling of the aquifer equations. The preconditioning of the conjugate gradient scheme is therefore improved, and a good convergence occurs after a number of iterations smaller than (N/n)½,Nandnbeing the overall number of nodes and aquifers, respectively. The computational cost of the model is generally low, and its relative efficiency tends to increase for small to moderate time steps and for complex multiaquifer systems where several interconnected aquifers are simulat

ISSN:0043-1397    

DOI:10.1029/WR022i007p01003

年代:1986

数据来源: WILEY

摘要:

A simulation study of two‐well injection‐withdrawal tracer tests in stratified granular aquifers at two widely separated sites is presented. The first site is located near the Chalk River Nuclear Laboratories in Canada, and the second site is located in Mobile, Alabama. Field data and test conditions at these sites are substantially different in terms of vertical distributions of hydraulic conductivity, well spacings, flow rates, test durations, and tracer travel distances. Furthermore, the test at the Chalk River site was conducted in a recirculating mode, whereas the test at the Mobile site was conducted in a nonrecirculating mode. Simulations of these tests were performed in three dimensions using the curvilinear finite element model developed in the previous paper of this series. The simulations incorporated measured vertical variations in relative hydraulic conductivity and local dispersivity values that are small fractions (between 1/1000 and 1/100) of the spacing between the injection and the withdrawal wells. The local dispersivities are used to account for local hydrodynamic dispersion and are chosen independently so that they are not affected by the scales of the tests. Simulation results obtained from the model are presented. Interpretation of these results is made in conjunction with measured breakthrough curves at the withdrawal well and multilevel observation wells. For the Chalk River site, predicted and measured breakthrough curves at the withdrawal well are in good agreement over the earlier part of the test duration. Deviation of the field data from the model prediction occurs over the second part, where the predicted breakthrough curves show a declining trend but the field data plot does not. For the Mobile site, predicted and measured breakthrough curves at the withdrawal well show similar trends throughout the entire test duration and are in good agreement overall. Model predictions of the effect of hydraulic conductivity stratification on temporal distributions of concentration are generally consistent with data from multilevel observation wells at both sites. The model results also show interesting effects of stratification on spatial distributions of concentration, and on the evolution and spreading pattern of tracer plumes. These effects have not been fully assessed in the field measurement and should be investigated furt

ISSN:0043-1397    

DOI:10.1029/WR022i007p01016

年代:1986

数据来源: WILEY

摘要:

During the late summer of 1984 a two‐well tracer test was performed which involved travel distances of 38.3 m to 90 m. A bromide tracer slug was injected at one well while groundwater was simultaneously pumped from the other well. The two‐well test, which required 32 days to complete, was then simulated by GeoTrans, Inc., using a three‐dimensional advection‐dispersion model with a longitudinal dispersivity of 0.15 m, a transverse dispersivity of 0.05 m, and the permeability distribution being measured independently in a single‐well test. (Single‐well test results, which involved a travel distance of 5.5 m, indicated that the hydraulic conductivity varied by about a factor of 4 throughout the aquifer thickness.) The two‐well test simulation, made without knowledge of the test results, and without any calibration, was remarkably good. Local hydrodynamic dispersion was negligible, so that the concentration versus time breakthrough at the withdrawal well could be simulated just as well with a quasi‐three‐dimensional advection model having zero hydrodynamic dispersion. This implies that over the travel distances applicable to the two‐well test the spreading of the slug in the aquifer depended mainly on macroscopic velocity variations that were quantifiable in terms of the inferred permeability distribution. This result, which pertains to an aquifer in a fluvial terrace deposit of Quaternary age, is similar to that observed by Pickens and Grisak in a sandy glaciofluvial aquifer near the Chalk River Nuclear Laboratories in Canada. Such flow systems can result in fully three‐dimensional plumes with high concentrations of contaminant moving over large horizontal distances in the high

ISSN:0043-1397    

DOI:10.1029/WR022i007p01031

年代:1986

数据来源: WILEY

摘要:

The relationship between the aperture and gas conductivity of a single natural fracture was investigated in the laboratory. Fracture conductivity was evaluated as a function of both the applied fluid pressure gradient and average fracture aperture, the latter ranging from 600 to 200μm. Fracture apertures were determined independently on the basis of both fracture deformation and fracture volume measurements. Flow generally occurred in the linear and transitional flow regime between linear and fully nonlinear flow. The transition was found to be smooth and well described by an equation of the form: −(dp/dx) =av+bv2, wheredp/dxis the pressure gradient andvis the fluid velocity. The linear and nonlinear fracture conductivities were found to be functions of the aperture and surface roughness of the fracture in agreement with existing empirical equations. A new physical model for fracture flow is also formulated based on an analogy to pipe fl

ISSN:0043-1397    

DOI:10.1029/WR022i007p01038

年代:1986

数据来源: WILEY

摘要:

Condie silt from near a polychlorinated biphenyls (PCBs) and chlorinated benzenes spill site was studied with respect to the sorption and predicted retardation of these compounds. The soil organic carbon fraction (foc) of the silt was measured to be 0.0036 ± 0.00088. The sorption partition coefficient (Kp′ milliliters/gram) values were: 2,2′,5,5′‐tefrachlorobiphenyl (4‐PCB), 5000; 2,2′,4,5,5′‐pentachlorobiphenyl (5‐PCB), 9400; and 2,2′,4,4′,5,5′‐hexachlorobiphenyl (6‐PCB), 26,000. The direct measurement of theKpvalues allowed an improved estimation of sorption over that possible with the measuredfocand the available literatureKocdata. Mean transport retardation factors (R) for 4‐PCB, 5‐PCB, and 6‐PCB in saturated silt were calculated to be 2.7E+ 04, 5.0E+ 04, and 1.4E+ 05, respectively. This implies that dissolved PCBs will move only very slowly through unfractured Condie silt. Since theKpvalues of the PCBs were not affected when 20 mg/L of 1,2,4‐trichlorobenzene was added to the aqueous phase, PCB transport at the site will not be accelerated by the presence of the chlorinated benzenes. Therefore if fractures are not present in the silt in its middle and lower levels, then there is little chance that the PCBs pose a threat to the underlying Regina aquifer system.Rvalues predicted for 1,2,4‐trichlorobenzene and 1,2,4,5‐tetrachlorobenzene were 120 and 600, respectively; they may also not pose an immediate threat to the un

ISSN:0043-1397    

DOI:10.1029/WR022i007p01051

年代:1986

数据来源: WILEY

摘要:

Scattering analog techniques are used to analyze steady quasilinear infiltration from buried discs and to develop related results. The exact solution involves spheroidal wave functions; but, more usefully, we express the far‐field wetting function and the discharge function as expansions appropriate for small and large values ofs, the dimensionless disc radius. Other simple techniques for estimating the moisture field include boundary layer and Green's function methods. Further aspects of the study include: useful comparisons with Wooding's classical results for the surface disc; and smallsexpansions, not hitherto available, for steady infiltration from cylindrical and spherical cavities. Buried discs (with aspect ratioh= 0), oblate spheroids (0

ISSN:0043-1397    

DOI:10.1029/WR022i007p01058

年代:1986

数据来源: WILEY

摘要:

To simulate water desalination in solar earth stills, we developed a numerical one‐dimensional model, which considers four layers: cover, soil, enclosed, and external air. The soil layer is divided into thin homogeneous sublayers, whose thermal and hydraulic properties are computed, during the simulation, by taking into account their temperature and humidity. Given adequate photometric properties, any covering material can be simulated. The model was experimentally verified and applied to two different climatological regions in Israel, examining the influence of water table depth on the amount of collected fresh water in two different types of solar earth stills. The model is able to predict accurately the amount of drinking water collected in a solar earth still. This method of water desalination is applicable in regions where the water table is near the soil surfac

ISSN:0043-1397    

DOI:10.1029/WR022i007p01067

年代:1986

数据来源: WILEY

摘要:

The construction of two bridges on Rock Creek, a high elevation stream in the Sierra Nevada, resulted in the elevation of stream water pH, conductivity, alkalinity, and calcium concentrations. Relative to an upstream unaffected site, the following maximum increases were observed: pH from 6.83 to 11.22, conductivity from 9 to 341μS cm−1, alkalinity from 70 to 2001μeq L−1, and calcium from 54 to 2227μeq L−1. Major effects were likely the result of the leaching of calcium oxide hydration products from cement in the bridge structures. Chemical effects proved to be transitory, lasting less than 1 day past the time of concrete pouring. Observed chemical changes may have had a detrimental effect on the stream biota and suggest that great care should be taken to isolate concrete structures from contact with poorly buffered surface waters during the period of cement h

ISSN:0043-1397    

DOI:10.1029/WR022i007p01077

年代:1986

数据来源: WILEY

摘要:

A generalized network flow model has been developed to simulate the allocation of additional water supplies in a river basin with observance of the prior appropriation doctrine of water rights and other legal requirements such as interstate compact agreements. The computer model, called MODSIMR, is capable of simulating complex river basin morphology while incorporating a relational data base management system for efficiently accessing prioritized water rights. Program MODSIMR is a generalized model designed to be applicable to a wide variety of river basins operating under an appropriative water rights system. As a demonstration of its usage, MODSIMR was applied to the Rio Grande Basin of Colorado, New Mexico, and Texas for predicting allocation and use of increased runoff from simulated silvicultural activities on the Rio Grande National Forest. Results indicate that under the current institutional framework, increased runoff would primarily be allocated to agricultural users in Colorado. Computer results also showed the potential value to Colorado of the Closed Basin Project in the San Luis Valley and the possibility of determining optimal pumping schemes for the Project using MODSIMR. Program MODSIMR will be useful in future economic studies to determine the benefits of the augmented water supply under various water use scenarios.

ISSN:0043-1397    

DOI:10.1029/WR022i007p01083

年代:1986

数据来源: WILEY

摘要:

Results of updating and reanalyzing streamflow data from studies in two experimental watersheds in western Oregon suggest that clearcut logging has altered snow accumulation and melt enough to have increased the size of peak flows caused by snowmelt during rainfall. In a 96‐ha clearcut watershed in the transient snow zone, peak flows with return periods of roughly 3–8 years were higher than predicted by prelogging data. In a similarly clearcut 10‐ha watershed, sizes of peak flows caused by melting of relatively deep snowpacks during rainfall were also higher after logging. Higher peak flows indicate a higher rate of water delivery to soils, which, in turn, suggests increased potential for both hillslope and channel er

ISSN:0043-1397    

DOI:10.1029/WR022i007p01095

年代:1986

数据来源: WILEY