摘要:

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil‐water flux and recharge. Soil‐water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil‐water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial m

ISSN:0043-1397    

DOI:10.1029/96WR00564

年代:1996

数据来源: WILEY

摘要:

The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured rocks of various lithologies that have low primary permeability and porosity, and (3) a shallow residuum of medium permeability and high total porosity. Conceptual models of shallow groundwater flow and solute transport in this system have been developed but are difficult to evaluate using physical characterization or short‐term tracer methods due to extreme spatial variability in hydraulic properties. In this paper we describe how chlorofluorocarbon 12,3H, and3He were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are<0.05 m, suggesting that concentrations of these tracers in fractures and in the matrix have time to diffusionally equilibrate. The relatively smooth nature of tracer concentrations with depth in the residuum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr−1. In contrast, groundwater flow within the unweathered rock appears to be controlled by fractures with spacings of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes in the unweathered rock cannot be estimated from the tracer d

ISSN:0043-1397    

DOI:10.1029/96WR00354

年代:1996

数据来源: WILEY

摘要:

In situ neutron activation of35Cl within the rock and groundwater of geologic deposits that have elevated concentrations of uranium provides a hydrogeological tracer. We determine the production rate and mobility of36Cl in the 1.3‐billion‐year‐old Cigar Lake uranium ore deposit. Accelerator mass spectrometry was used to map the concentrations of36Cl in the ore and in the groundwater that were up to 100 times greater than those encountered in unmineralized portions of the host sandstone aquifer. The residence time of this mobile anion in groundwater within the mineralized zone ranged from 14 to 280 kyr. These residence times are consistent with the hydraulic and geochemical data, suggesting significant control of Cl−and groundwater movement by the clay‐rich matrix of the mineral

ISSN:0043-1397    

DOI:10.1029/95WR03821

年代:1996

数据来源: WILEY

摘要:

A fracture simulation model which incorporates the physics of fracture growth is used to investigate how the mechanics of fracture formation affect the flow characteristics of fractured systems. Fractures are assumed to grow subcritically with the growth rate given by a power law function of the energy available for fracture growth. Flow characteristics are quantified in terms of the percent of networks percolating and the average effective conductivity as a function of the fracture density. For all flaw densities considered and for values of the growth rate exponent α ≤ 1, the flow characteristics primarily depend on the fracture spatial density and are similar to the flow characteristics of networks generated stochastically by assuming the fractures are randomly located. For α ≫ 1, the mechanical interaction of the flaws and fractures imparts an organized structure to the network resulting in isolated fractures, or zones of fractures, which form extensive, connected pathways at significantly lower fracture densities. Experimentally measured values of α for subcritical fracture growth are typically greater than one, suggesting that the flow characteristics of randomly located fractures may not be representative of natural fracture networks thought to have grown subcritically. An analysis of published fracture trace maps suggests that many natural fracture networks have fracture spatial densities near the percolation threshold. It is suggested that this may be due to the existence of a self‐limiting mechanism in fracture network f

ISSN:0043-1397    

DOI:10.1029/96WR00711

年代:1996

数据来源: WILEY

摘要:

Transient moisture flow in a variably saturated quasi‐three‐dimensional fracture‐rock matrix system is investigated. The fracture is assumed to possess a spatially variable aperture in its two‐dimensional plane, whereas the rock matrix is treated as a two‐dimensional homogeneous and tight porous medium. The aperture fluctuations in the fracture plane are described stochastically. Moisture exchange between the fracture and the rock matrix is accounted for via an advective coupling term that governs the transfer of moisture at the fracture‐matrix interface and takes into account the effect of a fracture‐surface coating material. Although the variable aperture fracture is two‐dimensional, the coupling term between the fracture and the rock matrix accounts for the three‐dimensional nature of the physical system. The stochastic nonlinear set of partial differential equations is solved numerically by the Galerkin finite element method in conjunction with the Picard iterative scheme and an automatic time step marching. Simulations are performed to investigate phenomena which have been ignored in previous studies. It is demonstrated that, for the case of no moisture exchange with the rock matrix, the moisture follows preferential flow paths within the fracture plane and exhibits pronounced fingering effects. Furthermore, it is shown that the larger the fracture aperture fluctuations the more extended the moisture flow in the fracture. In addition, for the case where there exists moisture exchange with the rock matrix, the movement of the moisture front is considerably reduced, whereas fracture‐surface coatings tend to slow down moisture absorption

ISSN:0043-1397    

DOI:10.1029/96WR00656

年代:1996

数据来源: WILEY

摘要:

A stochastic analysis of the transient characteristics of sorption at field scales shows that sorption macrokinetics arise as a result of aquifer physical and chemical heterogeneities. Analytical expressions for the time evolution of the field scale retardation factor and longitudinal macrodispersivity for a reactive solute are obtained and compared with previous theoretical findings. Sorption macrokinetics are characterized by a timescale related to the average solute velocity, mean retardation factor, and variability correlation scale. A comparison of this timescale with the intragrain diffusion characteristic timescale provides a quantitative criterion to determine the relative importance of intragrain processes and aquifer physical and chemical heterogeneities on the characteristic field scale features of reactive solute plumes: retardation and dispersion. Calculations performed for the Borden field site in Ontario, Canada, show that the macrokinetics timescale is potentially much larger than the intragrain diffusion characteristic time, rendering natural heterogeneities as the controlling mechanism in the transient development of reactive plumes in the field. Analytically derived expressions for spatial moments of a reactive plume are evaluated to obtain results for the mass in solution, center of mass displacement, and longitudinal second moment. These results compare favorably with numerical simulations, but a comparison with field measurements is limited at this time owing to a lack of reliable data.

ISSN:0043-1397    

DOI:10.1029/96WR00791

年代:1996

数据来源: WILEY

摘要:

A two‐dimensional multiphase flow and species transport model was developed and applied to the case of nonaqueous phase liquid (NAPL) emplacement and dissolution in both homogeneous and heterogeneous porous media systems. Simulations were performed to observe dissolution rate variations and the degree of NAPL‐aqueous phase nonequilibrium as a function of two aqueous phase velocities and five forms of the NAPL‐aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL‐aqueous phase nonequilibrium. Mass removal rates for homogeneous media were insensitive to the form of the NAPL‐aqueous phase mass transfer formulation, yielding results similar to a local equilibrium approach for all but one mass transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, which was manifested as a decrease in NAPL dissolution rates as the NAPL volume fraction decreased. Variations in mass elution rates between homogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for horizontal correlation lengths. In heterogeneous media, decreases in dissolution rates were attributed to the existence of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so‐called NAPL pools. These results illustrate the importance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolutio

ISSN:0043-1397    

DOI:10.1029/96WR00291

年代:1996

数据来源: WILEY

摘要:

Transport of nonlinearly adsorbing solutes in homogeneous and heterogeneous porous formations is considered. Initially, a fixed amount of solute is assumed to be present in the domain. Transport is characterized in terms of the first and second spatial moments. Nonlinear equilibrium adsorption is described by the Freundlich isotherm, with the Freundlich exponentn, 0

ISSN:0043-1397    

DOI:10.1029/96WR00636

年代:1996

数据来源: WILEY

摘要:

Statistical moments of solute plumes from small sources in variably saturated, heterogeneous porous media are analyzed by using a newly developed, efficient high‐resolution Monte Carlo technique. In agreement with previous theoretical work, it is illustrated that the prediction of such solute plumes is associated with large uncertainties for dimensionless travel times,t′, exceeding 40, particularly predictions of plumes in highly heterogeneous soils (σy2>2). Uncertainty about the travel path of the plume center contributes significantly to overall concentration uncertainty as flux fields become more variable. It is shown that the concentration coefficient of variation at the center of the plume initially increases but stagnates or decreases at later times. For highly heterogeneous soil flux conditions or for the common case of soils with strongly anisotropic conditions, analytical models underestimate transverse spreading of the mean concentration plume at any given time, while overestimating longitudinal spreading. At identical mean plume displacement distances, analytical models underestimate both transverse and longitudinal spreading and overestimate the variance of solute flux (breakthrough curve) by up to a factor 4. As an alternative to the statistical analysis of solute flux, we propose to analyze statistical properties of time associated with peak solute flux and with first exceedance of a given solute flux l

ISSN:0043-1397    

DOI:10.1029/96WR00502

年代:1996

数据来源: WILEY

摘要:

A method is developed for the conditional (Monte Carlo) simulation of steady state flow and transient transport from point sources in variably saturated porous media. It combines the geostatistical method, a linearized approximation of the soil water tension perturbation solution, and a finite element numerical model. The method is used to investigate the usefulness of conditional simulation for predicting solute transport under a variety of sampling network designs applied to a number of hypothetical soils. Saturated hydraulic conductivity data yield the largest reduction of conditional uncertainty in relatively wet soils with mild heterogeneities. In highly heterogeneous soils or under dry conditions, soil water tension data by themselves, taken at a sampling density of one to two correlation scales along the expected mean travel path, can greatly reduce prediction uncertainty about solute concentration. Parameter uncertainty about statistical properties of the independent random variables becomes less important as the number of conditioning data increases. However, even with a very high number of sampling data, uncertainty of predicted concentration levels remains significant.

ISSN:0043-1397    

DOI:10.1029/96WR00503

年代:1996

数据来源: WILEY