摘要:

Our current understanding of wetlands is insufficient to assess the effects of past and future wetland loss. While knowledge of wetland hydrology is crucial, groundwater flows are often neglected or uncertain. In this paper, groundwater inflows were estimated in wetlands in southwestern Wisconsin using traditional Darcy's law calculations and three independent methods that included (1) stable isotope mass balances, (2) temperature profile modeling, and (3) numerical water balance modeling techniques. Inflows calculated using Darcy's law were lower than inflows estimated using the other approaches and ranged from 0.02 to 0.3 cm/d. Estimates obtained using the other methods generally were higher (0.1 to 1.1 cm/d) and showed similar spatial trends. An areal map of groundwater flux generated by the water balance model demonstrated that areas of both recharge and discharge exist in what is considered a regional discharge area. While each method has strengths and weaknesses, the use of more than one method can reduce uncertainty in the estimates.

ISSN:0043-1397    

DOI:10.1029/95WR03724

年代:1996

数据来源: WILEY

摘要:

The response of water in the unsaturated zone to seasonal changes of temperature (T) is determined analytically using the theory of nonisothermal water transport in porous media, and the solutions are tested against field observations of moisture potential and bomb fallout isotopic (36Cl and3H) concentrations. Seasonally varying land surface temperatures and the resulting subsurface temperature gradients induce thermal vapor diffusion. The annual mean vertical temperature gradient is close to zero; however, the annual mean thermal vapor flux is downward, because the temperature‐dependent vapor diffusion coefficient is larger, on average, during downward diffusion (occurring at highT) than during upward diffusion (lowT). The annual mean thermal vapor flux is shown to decay exponentially with depth; the depth (about 1 m) at which it decays toe−1of its surface value is one half of the corresponding decay depth for the amplitude of seasonal temperature changes. This depth‐dependent annual mean flux is effectively a source of water, which must be balanced by a flux divergence associated with other transport processes. In a relatively humid environment the liquid fluxes greatly exceed the thermal vapor fluxes, so such a balance is readily achieved without measurable effect on the dynamics of water in the unsaturated zone. However, if the mean vertical water flux through the unsaturated zone is very small (<1 mm y−1), as it may be at many locations in a desert landscape, the thermal vapor flux must be balanced mostly by a matric‐potential‐induced upward flux of water. This return flux may include both vapor and liquid components. Below any near‐surface zone of weather‐related fluctuations of matric potential, maintenance of this upward flux requires an increase with depth in the annual mean matric potential; this theoretical prediction is supported by long‐term field measurements in the Chihuahuan Desert. The analysis also makes predictions, confirmed by the field observations, regarding the seasonal variations of matric potential at a given depth. The conceptual model of unsaturated zone water transport developed here implies the possibility of near‐surface trapping of any aqueous constituent introd

ISSN:0043-1397    

DOI:10.1029/95WR03489

年代:1996

数据来源: WILEY

摘要:

A series of infiltration and tracer experiments was conducted in unsaturated sand and gravel deposits on Cape Cod, Massachusetts. A network of 112 porous cup lysimeters and 168 time domain reflectometry (TDR) probes was deployed at depths from 0.25 to 2.0 m below ground surface along the centerline of a 2‐m by 10‐m test plot. The test plot was irrigated at rates ranging from 7.9 to 37.0 cm h−1through a sprinkler system. Transient and steady state water content distributions were monitored with the TDR probes and spatial properties of water content distributions were determined from the TDR data. The spatial variance of the water content tended to increase as the average water content increased. In addition, estimated horizontal correlation length scales for water content were significantly smaller than those estimated by previous investigators for saturated hydraulic conductivity. Under steady state flow conditions at each irrigation rate, a sodium chloride solution was released as a tracer at ground surface and tracked with both the lysimeter and TDR networks. Transect‐averaged breakthrough curves at each monitoring depth were constructed both from solute concentrations measured in the water samples and flux concentrations inferred from the TDR measurements. Transport properties, including apparent solute velocities, dispersion coefficients, and total mass balances, were determined independently from both sets of breakthrough curves. The dispersion coefficients tended to increase with depth, reaching a constant value with the lysimeter data and appearing to increase continually with the TDR data. The variations with depth of the solute transport parameters, along with observations of water and solute mass balance and spatial distributions of water content, provide evidence of significant three‐dimensional flow during the irrigation experiments. The TDR methods are shown to efficiently provide dense spatial and temporal data sets for both flow and solute transport in unsaturated sediments with minimal sediment and flow field disturbance. Combined implementation of lysimeters and TDR probes can enhance data interpretation particularly when three‐dimensional flow conditions are

ISSN:0043-1397    

DOI:10.1029/95WR02972

年代:1996

数据来源: WILEY

摘要:

Subsurface flow is primarily controlled by the distribution of hydrogeological properties. Spatially correlated random fields, currently the primary means of describing heterogeneity in these properties, underutilizes the existing knowledge of geological systems. A new computer code, the Braided Channel Simulator (BCS‐3D), is used to geometrically simulate the spatial heterogeneity of hydrogeological properties using representations of surface topography to estimate the three‐dimensional arrangement of subsurface units. Four methods of assigning conductivity were used to generate conductivity fields as a basis for saturated flow simulation. Variograms measured for each of the conductivity fields give no indication of underlying discrete structure. However, particle‐tracking simulations showed that flow occurred along paths of preferentially high hydraulic conductivity. Results indicate that in systems where the contrast in conductivity is sufficiently great, the location and magnitude of flow is constrained by discrete internal structure. Additionally, geometrical simulation techniques such as BCS‐3D can be used to produce property fields that embody these discrete geological str

ISSN:0043-1397    

DOI:10.1029/95WR03399

年代:1996

数据来源: WILEY

摘要:

This work investigates the dependence of soil permeability to air on sampling scale in near‐surface unsaturated soils. A new dual‐probe dynamic pressure technique was developed to measure permeability in situ over different length scales and different spatial orientations in the soil. Soils at three sites were studied using the new technique. Each soil was found to have higher horizontal than vertical permeability. Significant scale dependence of permeability was also observed at each site. Permeability increased by a factor of 20 as sampling scale increased from 0.1 to 2 m in a sand soil vegetated with dry grass, and by a factor of 15 as sampling scale increased from 0.1 to 3.5 m in a sandy loam with mature Coast Live Oak trees (Quercus agrifolia). The results indicate that standard methods of permeability assessment can grossly underestimate advective transport of gas phase contaminants through so

ISSN:0043-1397    

DOI:10.1029/95WR03637

年代:1996

数据来源: WILEY

摘要:

Waste management problems for shallow land burial facilities in the humid eastern United States are usually complicated by slow but continuous movement of wastes through the soil matrix and discrete but rapid pulses of wastes through macropores and fractures. Multiple‐pore‐region models employed to describe flow and solute transport in the soils usually consist of multiple mass transfer coefficients that cannot be measured experimentally, and their effects on subsurface mass transport are poorly understood. The objective of this research was to study the individual and concurrent effects of interaggregate advection and diffusion on mass transport in a structured soil. The interactions of these two mass transfer processes and local solute concentration equilibrium are examined for a heterogeneous soil. Pore region water retention, hydraulic conductivity, and dispersivities, obtained from independent measurements and published calibration results, were used to test a novel three‐pore‐region, one‐dimensional numerical model. Advective and diffusive mass transfer coefficients were estimated using mass transfer equations and fracture spacings published in the literature. The mass transfer coefficients were then varied systematically, and the sensitivity of local fluid pressure and solute concentration nonequilibrium to interregion mass transfer were analyzed. Our results indicated that time‐dependent interaggregate advection and diffusion were important processes controlling solute mobility in heterogeneous media. Under transient flow conditions, interaggregate advection may reduce the significance of interaggregate diffusion that otherwise dominates interaggregate mass transfer under steady state conditions. Nonetheless, the equilibrium of local solute concentrations was 20 times more sensitive to diffusive mass transfer than to advective mass transfer, which suggests that site characterization efforts should be directed more toward the former process. Unfortunately, characterization efforts of this type are not commonplace and if available are frequently ignored because they add a difficult reality to complex waste management problems. Since advective and diffusive mass transfer may be important processes limiting the efficiency of cleanup activities such as pump and treat, it is perhaps time to include the characterization of these processes and quantification of the timescale of physical nonequilibrium in site remediat

ISSN:0043-1397    

DOI:10.1029/95WR03397

年代:1996

数据来源: WILEY

摘要:

A higher‐order theory is presented for steady state, mean uniform saturated flow and nonreactive solute transport in a random, statistically homogeneous natural log hydraulic conductivity fieldY. General integral expressions are derived for the spatial covariance of fluid velocity to second order in the variance σ2ofYin two and three dimensions. Integrals involving first‐order (in σ) fluctuations in hydraulic head are evaluated analytically for a statistically isotropic two‐dimensionalYfield with an exponential autocovariance. Integrals involving higher‐order head fluctuations are evaluated numerically for this same field. Complete second‐order results are presented graphically for σ2=1 and σ2=2. They show that terms involving higher‐order head fluctuations are as important as those involving lower‐order ones. The velocity variance is larger when approximated to second than to first order in σ2. Discrepancies between second‐ and first‐order approximations of the velocity autocovariance diminish rapidly with separation distance and are very small beyond two integral scales. Transport requires approximation at two levels: the flow level at which velocity statistics are related to those ofY, and the advection level at which macrodispersivities are related to velocity fluctuations. Our results show that a second‐order flow correction affects transport to a greater extent than does a second‐order correction to advection. Asymptotically, the second‐order transverse macrodispersivity tends to zero as does its first‐order counterpart. An approximation of advection alone based on Corrsin's conjecture, coupled with either a first‐ or a second‐order flow approximation, leads to a transverse macrodispersivity which is significantly larger than that obtained by standard perturbation and tends to a nonzero asymptote. Published Monte Carlo results yield macrodispersivities that lie significantly below those predicted by first‐ and second‐order theories. Considering that Monte Carlo simulations often suffer from sampling and computational errors, that standard perturbation approximations are theoretically valid only for σ2<1, and that Corrsin's conjecture represents the leading term in a renormalization group perturbation which contains contributions from an infinite number of high‐order terms, we find it difficult to tell which of these approximations is closest to representing transport i

ISSN:0043-1397    

DOI:10.1029/95WR03492

年代:1996

数据来源: WILEY

摘要:

The random‐walk method for simulating solute transport in porous media is typically based on the assumption that the velocity and velocity‐dependent dispersion tensor vary smoothly in space. However, in cases where sharp interfaces separate materials with contrasting hydraulic properties, these quantities may be discontinuous. Normally, velocities are interpolated to arbitrary particle locations when finite difference or finite element methods are used to solve the flow equation. The use of interpolation schemes that preserve discontinuities in velocity at material contacts can result in a random‐walk model that does not locally conserve mass unless a correction is applied at these contacts. Test simulations of random‐walk particle tracking with and without special treatment of material contacts demonstrate the problem. Techniques for resolving the problem, including interpolation schemes and a reflection principle, are reviewed and tested. Results from simulations of transport in porous media with discontinuities in the dispersion tensor show which methods satisfy continuity. Simulations of transport in two‐dimensional heterogeneous porous media demonstrate the potentially significant effect of using a nonconservative model to compute spatial moments and breakthrough of a sol

ISSN:0043-1397    

DOI:10.1029/95WR03528

年代:1996

数据来源: WILEY

摘要:

Similar media scaling and geostatistical analyses are used to characterize the spatial variability of soil hydraulic properties at the Las Cruces Trench Site in New Mexico. A simple method is described for conditioning the hydraulic properties used for unsaturated water flow and solute transport modeling, based on the spatial distributions of initial field‐measured water contents and a set of scale‐mean hydraulic parameters determined from the scaling analysis. This method is used to estimate hydraulic properties for numerical simulations of the latest field‐scale flow and transport experiment conducted at the Las Cruces Trench Site. Relatively good matches between the observed and simulated flow and transport behavior are obtained without model calibration. The results of this study suggest that using similar media scaling in conjunction with the described conditioning procedure can significantly reduce the uncertainty in predictions of water flow and solute transport in spatially variable

ISSN:0043-1397    

DOI:10.1029/95WR03398

年代:1996

数据来源: WILEY

摘要:

The general formulation of the environmental risk problem captures the entire process of identifying the source term of the risk agent, its fate and transport through porous media, estimation of human exposure, and conversion of such exposure into the risk level. The contaminant fate and transport is modeled using the solute flux formulation evaluated with its first two moments, which explicitly account for the spatial variability of the velocity field, sorption properties, and parametric uncertainty through the first‐order analysis. The risk level is quantified on the basis of carcinogenicity using the risk factor (which describes the risk per unit dose or unit intake) employed to the total doses for individuals potentially consuming radionuclide‐contaminated groundwater. As a result of the probabilistic formulation in the solute flux and uncertainty in the water intake and dose‐response functions, the total risk level is expressed as a distribution rather than a single estimate. The results indicate that the geologic heterogeneity and uncertainty in the sorption estimate are the two most important factors for the risk evaluation from the physical and chemical processes, while the mean risk factor is a crucial parameter in the risk formul

ISSN:0043-1397    

DOI:10.1029/95WR03530

年代:1996

数据来源: WILEY