摘要:

An exact Laplace transform solution to the problem of dispersion, advection, and adsorption of a tracer due to its injection in a steady, horizontal, radially convergent flow field in a densely fractured, porous formation (double‐porosity aquifer) is presented. The porous blocks were assumed to be covered with a layer of material (fracture skin) of negligible volume and storage capacity that provides a resistance to diffusion in the rock matrix. Longitudinal dispersion, advection, and adsorption dominate transport of the tracer in the fractures, and diffusion and adsorption dominate movement of the tracer in the blocks. Dimensionless breakthrough curves are used to illustrate the influence of various aquifer and tracer properties. In support of the model a detailed analysis is performed of a published multitracer field test, conducted in a layer of densely fractured chalk in Béthune, France. Of the three tracers analyzed, two are nonsorptive but have widely different free water diffusion coefficients, and one is slightly sorptive. Analysis of measured breakthrough curves, matched by trial and error to theoretical responses, reveals that by allowing for fracture skin on block surfaces, one can obtain (1) pure‐advection arrival times that are independent of the tracer used, (2) values of mass recovery consistent with measured values, and (3) relative values of effective diffusion coefficients that are consistent with known free water diffusion coefficients for the separate tracers. Reasonable estimates of longitudinal dispersivity and fracture porosity are also obta

ISSN:0043-1397    

DOI:10.1029/95WR01275

年代:1995

数据来源: WILEY

摘要:

A water desaturation zone develops around a tunnel in water‐saturated rock when the evaporative water loss at the rock surface is larger than the water flow from the surrounding saturated region of restricted permeability. We describe the methods with which such water desaturation processes in rock materials can be quantified. The water retention characteristic θ (ψ) of crystalline rock samples was determined with a pressure membrane apparatus. The negative water potential, identical to the capillary pressure, ψ, below the tensiometric range (ψ<−0.1 MPa) can be measured with thermocouple psychrometers (TP), and the volumetric water contents, θ, by means of time domain reflectometry (TDR). These standard methods were adapted for measuring the water status in a macroscopically unfissured granodiorite with a total porosity of approximately 0.01. The measured water retention curve of granodiorite samples from the Grimsel test site (central Switzerland) exhibits a shape which is typical for bimodal pore size distributions. The measured bimodality is probably an artifact of a large surface ratio of solid/voids. The thermocouples were installed without a metallic screen using the cavity drilled into the granodiorite as a measuring chamber. The water potentials observed in a cylindrical granodiorite monolith ranged between −0.1 and −3.0 MPa; those near the wall in a ventilated tunnel between −0.1 and −2.2 MPa. Two types of three‐rod TDR probes were used, one as a depth probe inserted into the rock, the other as a surface probe using three copper stripes attached to the surface for detecting water content changes in the rock‐to‐air boundary. The TDR signal was smoothed with a low‐pass filter, and the signal length determined based on the first derivative of the trace. Despite the low porosity of crystalline rock these standard methods are applicable to describe the unsaturated zone in solid rock and may also be used in other consolidated ma

ISSN:0043-1397    

DOI:10.1029/95WR01487

年代:1995

数据来源: WILEY

摘要:

The occurrence of selenium in agricultural drainage water derived from the central part of the western San Joaquin Valley has focused concern on alternatives to agricultural drains for managing shallow, poor‐quality groundwater. A transient, three‐dimensional simulation model was developed to evaluate the response of the water table to alternatives that affect recharge to or discharge from the groundwater flow system. The modeled area is 551 mi2(1 mi2= 2.59 km2) and includes both the semiconfined and confined zones above and below the Corcoran Clay Member of the Tulare Formation of Pleistocene age. The simulation model was calibrated using hydrologic data from 1972 to 1988, and was extended to the year 2040 to forecast for various management alternatives, including maintenance of present practices, land retirement, reduced recharge, increased groundwater pumping, and combinations of these alternatives. Maintenance of present practices results in a worsening of the situation: the total area subject to bare‐soil evaporation increases from 224 mi2in 1990 to 344 mi2in 2040, and drain flow increases from 25,000 ac ft/yr (1 ac ft = 1234 m3) to 28,000 ac ft/yr. Although land retirement results in elimination of bare‐soil evaporation and drain flow in the areas retired, it has little to no effect in adjacent areas. In contrast, regional‐scale changes in recharge and pumping are effective for regional management. The area subject to bare‐soil evaporation can be reduced to 78 mi2, and drain flow to 8000 ac ft/yr if (1) recharge is reduced by 15% (26,000 ac ft/yr) in areas that currently use surface and groundwater (362 mi2); (2) recharge is reduced by 40% (28,000 ac ft/yr) in areas that currently use only surface water (137 mi2); and (3) pumping rates are uniformly incremented by 0.5 ft/yr (160,000 ac ft/yr) in both areas. If these water budget changes were to be implemented in the study area, and in adjacent areas with similiar Hydrogeologic characteristics, then approximately 400,000 ac ft/yr of surface water would be made available. Thus a shift in the hydrologic budget in the central part of the western San Joaquin Valley improves the prospects for sustaining agriculture in the area, and could provide substantial water resources for

ISSN:0043-1397    

DOI:10.1029/95WR01328

年代:1995

数据来源: WILEY

摘要:

A method for the estimation of subsurface unsaturated hydraulic conductivity in situ was developed and field tested. The new method is based on Philip's (1992) approximation of water flow from a small subsurface cavity. Measurements of water pressure within the cavity (P) and associated discharge (Q) are used to infer the soil's saturated hydraulic conductivity (Ks) and capillary length (α). A probe for conducting the measurements was designed and field tested, and a measurement protocol was established. Comparisons were made with existing surface‐based methods by exposing the soil surface at the depth of the subsurface measurement. The method provided estimates of the soil parameters that were in close agreement with those obtained using surface‐based in situ met

ISSN:0043-1397    

DOI:10.1029/95WR01140

年代:1995

数据来源: WILEY

摘要:

Abandoned and improperly sealed boreholes, monitoring wells, and water supply wells are common features at many contaminated sites. These features can act as conduits that transmit contaminants between aquifers separated by otherwise continuous aquitards. In this work the leaky boreholes are represented as highly conductive one‐dimensional line elements superimposed onto a mesh of three‐dimensional finite elements representing the porous medium. Simulation results are presented for a series of scenarios involving a simple hydrogeologic setting composed of an upper unconfined aquifer, a middle aquitard, and a lower confined aquifer. The simulations examine the effect of varying the borehole properties, the vertical hydraulic gradient across the aquitard, and the borehole location. The results show that a contaminant can rapidly migrate downward along a leaky borehole and create an extensive plume in the lower aquifer, even if the borehole is filled with aquifer sediments. If the borehole is an open feature across the aquitard, the entire plume, or a significant portion of it, that is migrating into the upper aquifer can be diverted into the lower one if the vertical hydraulic gradient across the aquitard is sufficiently strong. The peak concentration arriving at a pumping well located in the lower aquifer and the time of arrival are functions of the proximity of the leaky borehole to the pumping well and its angular offset from the central flow line passing through the surficial source. Overpressurization of the lower aquifer due to injection can overcome downward preexisting hydraulic gradients across the aquitard such that contaminants can rapidly migrate upward along the leaky borehole and cause contamination of the otherwise protected upper aqui

ISSN:0043-1397    

DOI:10.1029/95WR00661

年代:1995

数据来源: WILEY

摘要:

The prediction of infiltration of water into field soils requires measurements of the field‐saturated hydraulic conductivity (Kfs) and the matric flux potential corresponding to field saturation (ϕm). We develop analytical solutions for (1) early‐time, one‐dimensional infiltration into slowly permeable soils such as clay liners under the conditions of an initial constant head followed by a falling head phase and (2) steady state, three‐dimensional infiltration into permeable soils under the condition of steady state flow at a constant head followed by falling head conditions.Kfsand ϕmgovern the shape of both solutions. The parameters are simultaneously identifiable, unique, and stable within the early‐time solution; however, there is a limited range in which the parameters of the steady state solution are unique. Laboratory and field experiments based on both solutions gave physically realistic best fit parameter values using the solutions in inversion procedures. Field measurements gave an excellent correlation betweenKfsfrom steady state constant head and falling head techniques. The correlation between ϕmvalues was so

ISSN:0043-1397    

DOI:10.1029/95WR01139

年代:1995

数据来源: WILEY

摘要:

We present an efficient method for determining temporal moments of concentration for a solute subject to first‐order and diffusive mass transfer in steady velocity fields. The differential equations for the moments of all orders have the same form as the steady state nonreactive transport equation. Thus temporal moments can be calculated by a solute transport code that was written to simulate nonreactive steady state transport, even though the actual transport system is reactive and transient. Higher‐order moments are found recursively from lower‐order moments. For many cases a small number of moments sufficiently describe the movement of a solute plume. The first four moments describe the accumulated mass, mean, spread, and skewness of the concentration histories at all locations. Actual concentration histories at any location can be approximated from the moments by applying the principle of maximum entropy, a constraint consistent with the physical process of dispersion. The forms of the moment‐generating equations for different mass transfer models provide insight into reactive transport through heterogeneous aquifers. For the mass transfer models we considered, the zeroth moment in a heterogeneous aquifer is independent of the mass transfer coefficients. Thus, if the velocity field is known, the mass transported past any point, or out any boundary, can be calculated without knowledge of the spatial pattern of mass transfer coefficients and, in fact, without knowledge of whether mass transfer is occurring. Also, for both first‐order and diffusive mass transfer models, the mean arrival time depends on the distribution coefficient but is independent of the values of the rate coefficients, regardless of the spatial variability of groundwater velocity and mass transfer coe

ISSN:0043-1397    

DOI:10.1029/95WR01231

年代:1995

数据来源: WILEY

摘要:

The numerical model CompFlow is used to study the mechanisms controlling vacuum extraction, coupled with air sparging, as a means for remediation of heterogeneous formations contaminated with dense nonaqueous phase liquids (DNAPLs). Two dominant mechanisms are demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing relatively rapid transfer of contaminant from the nonaqueous phase to the gas phase and subsequent removal by the vacuum extractor. Second, at later times, remediation is controlled by the transfer of contaminant from the nonaqueous phase to the aqueous phase below the water table. During this time the vacuum extractor pumps both liquid and vaporized water in the aqueous and gas phases. This causes the contaminant that is dissolved in the aqueous phase to migrate vertically upward across the permeability layers toward the vacuum extractor where it is removed. This intermediate to late time removal mechanism is shown to be controlled by contaminant dissolution, which is a slower transfer process than the direct DNAPL vaporization that occurs at early time. Our analysis indicates that as long as both air and water are actively flushed through the DNAPL zone, both early‐time vaporization and intermediate‐ to late‐time dissolution are effective mechanisms leading to the removal of the DNAPL. We show that it may be possible to design the remedial system so as to reduce its performance sensitivity to geologic heterogeneity. A lack of sensitivity of a remedial design to heterogeneity is highly desirable because a robust design implies that the degree of site characterization required for reasonable success will be less than that needed for a less robust s

ISSN:0043-1397    

DOI:10.1029/95WR00172

年代:1995

数据来源: WILEY

摘要:

We propose three nontraditional dielectric geometries and present an experimental and theoretical analysis and comparison of time domain reflectometry (TDR) performances for them. The traditional geometry (the probes inserted in material of essentially infinite extent) is compared to three nontraditional geometries where the probes are affixed outside of a core sample, inside of a bore, or flat on the surface of a semi‐infinite solid. Our derivation relates the velocity of electromagnetic wave propagation to the complex permittivities and permeabilities of the media and the geometry for the three nontraditional configurations. Experimental results for air, styrofoam, dry sand, wet sand of varying water content, nylon, dry wood, and ferromagnetic steel are obtained for the three proposed configurations and are in fair agreement with the literature within the experimental uncertainties. Through experiments and theoretical analysis, the TDR performance is found to be the same within the experimental uncertainties for the three nontraditional geometries. The proposed geometries yield slightly lower sensitivities compared to the traditional geometry. Advantages and disadvantages of the geometries compared to the traditional geometry are also discusse

ISSN:0043-1397    

DOI:10.1029/95WR00788

年代:1995

数据来源: WILEY

摘要:

The coupled transport process through an integrated soil‐groundwater system is quantified for kinetically sorbing solute that originates from a time dependent source at the soil surface and is transported by steady random velocity. The derived expressions of ensemble mean solute breakthrough at some arbitrary control plane normal to the mean flow direction involve probability density functions (pdfs) of advective solute travel time through the unsaturated and the saturated zone of the transport domain. A nonstationary travel time pdf is derived for the saturated zone, to account for possible effects of flow nonuniformity due to recharge of water from the unsaturated zone. Nonuniform mean flow in the saturated zone decreases the relative influence of spatial variability within that zone on the ensemble mean solute breakthrough curve. Factors such as the longitudinal extent of the solute source and the unsaturated zone variability become more important for the spreading of the expected solute breakthrough as the degree of flow nonuniformity in the saturated zone increases. This implies that possible far‐field simplifications based on the assumption that the transport process in an integrated soil‐groundwater system is dominated by the transport conditions in the saturated zone may not be valid in cases with significant groundwater recharge from the unsaturated

ISSN:0043-1397    

DOI:10.1029/95WR01330

年代:1995

数据来源: WILEY