摘要:

Detailed depth profiles of Chlorofluorocarbons CFC‐11(CFCl3(, CFC‐12 (CF2Cl2) and CFC‐113 (C2F3Cl3) have been obtained from a well‐characterized field site in central Ontario. Aquifer materials comprise predominantly silty sands, with a mean organic carbon content of 0.03%. Nearly one‐dimensional flow exists at this site, and the vertical migration of a well‐defined3H peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with3H profiles enables estimation of chlorofluorocarbon transport parameters. CFC‐12 appears to be the most conservative of the CFCs measured. Sorption at this site is low (Kd<0.03), and degradation does not appear to be important. CFC‐ 113 is retarded both with respect to CFC‐12 and with respect to3H (Kd= 0.09−0.14). CFC‐11 appears to be degraded both in the highly organic unsaturated zone and below 3.5 m depth in the aquifer, where dissolved oxygen concentrations decrease to below 0.5 mg L−1. The half‐life for CFC‐11 degradation below 3.5 m depth is less than 2 years. While apparent CFC‐12 ages match hydraulic ages to within 20% (up to 30 years), apparent CFC‐11 and CFC‐113 ages significantly overestimate

ISSN:0043-1397    

DOI:10.1029/94WR02528

年代:1995

数据来源: WILEY

摘要:

Mobile colloids in soils and their underlying strata may play an important role in the translocation of some contaminants from surface sources to groundwater. This study was conducted to evaluate the role of adsorbed natural organic matter (NOM) in the transport of submicron soil colloids through a commonly occurring type of saprolite in North Carolina. Intact saprolite columns from 4 m below the soil surface were used to study the movement of a conservative tracer (3H2O) and of soil colloids with and without adsorbed NOM. For natural (i.e., untreated) soil colloids having high colloidal stability due to adsorbed NOM, the filier efficiency of the saprolite decreased rapidly to zero as increasing amounts of colloids were deposited on the pore walls in the saprolite (blocking effect). Colloid breakthrough curves exhibited little tailing, indicating that colloid deposition was largely irreversible. The colloids were excluded from about 33% of the water‐filled pore space, resulting in faster transport of colloids as compared to3H2O. When the colloids were treated with NaOCl to remove adsorbed NOM, colloidal stability and mobility were strongly decreased. For these suspensions the filter efficiency of the columns increased as increasing amounts of colloids were deposited in the saprolite (filter ripening). After addition of small amounts of humic acid (1 mg L−1) to the NaOCl‐treated colloids, they exhibited very similar transport behavior as the untreated soil colloids. Stabilization of colloids by NOM and the possible occurrence of the blocking effect or filter ripening must be considered in future models of subsurface colloid tran

ISSN:0043-1397    

DOI:10.1029/94WR02676

年代:1995

数据来源: WILEY

摘要:

We used time domain reflectometry (TDR) probes installed vertically at the soil surface beneath a constant‐rate rainfall simulator to measure cumulative water storage and the soil's unsaturated hydraulic conductivity. The slope from linear regression of water storage on time before any applied water infiltrates to the bottom of the TDR probe gives an estimate of the local infiltration rate. Local infiltration rates measured by TDR in the field were plotted against the corresponding local steady state water contents to give an estimate of the soil's unsaturated hydraulic conductivity over a range in water content of 20% using only two applied rainfall rates. The spatial variability in local infiltration rates may be the result of infrequent high‐intensity pulses of rainfall leading to temporary ponding and redistribution of water at the soil surface. Nonlinear optimization was used to estimate the saturated hydraulic conductivity and inverse capillary length scale from TDR d

ISSN:0043-1397    

DOI:10.1029/94WR02535

年代:1995

数据来源: WILEY

摘要:

The complexity of the formation and removal of nonwetting‐phase residual in porous media provides a significant challenge to the development of theories needed to advance more effective aquifer remediation schemes. Constitutive theories are required to overcome theoretical deficiencies in describing behaviors such as the formation, dissolution, and mobilization of residual. Current macroscopic approaches are often unable to capture adequately the complexity of such processes; in these cases, pore‐scale research should provide the necessary conceptual and quantitative basis for constitutive theory development. This work contributes to these efforts by investigating factors affecting residual in strongly wet, capillary‐dominated systems. Porous media are modeled as three‐dimensional networks of pore bodies and throats. A random network approach is developed, which provides stochastic geometry and variable connectivity. Immiscible displacement occurs via piston‐displacement and film‐flow mechanisms. Residual ganglia become trapped during imbibition. Wetting‐phase continuity is maintained during drainage, disallowing the entrapment of wetting‐phase ganglia. Results show that pore‐ scale geometric parameters significantly affect residual saturation, the ganglia‐volume distribution, and nonwetting‐wetting‐phase interfacial area. Thus constitutive theory of residual should incorporate pore‐scale knowledge to be valid for a wide range of conditions and porous media types. Comparison with quantitative experimental ganglia‐ volume distributions yields good agreement with the data trends but poor quantitative agreement. These results indicate the need for further experimental comparisons with theoretical results, linking macroscale and micro

ISSN:0043-1397    

DOI:10.1029/94WR02849

年代:1995

数据来源: WILEY

摘要:

A new methodology for solution of the inverse problem in groundwater hydrology is proposed and applied to a site in southeastern New Mexico with extensive hydrogeologic data. The methodology addresses the issue of nonuniqueness of the inverse solutions by generating an ensemble of transmissivity fields considered to be equally likely, each of which is in agreement with the measured transmissivity and pressure data. It consists of generating a selected number of conditionally simulated transmissivity fields and then calibrating each of the fields to match the measured steady state or transient pressures, in a least squares sense. The calibration phase involves an iterative implementation of an automated pilot point approach coupled with conditional simulations. Pilot points are the parameters of calibration. They are synthetic transmissivity data which are added to the transmissivity database to produce a revised conditional simulation during calibration. Coupled kriging and adjoint sensitivity analysis is employed for the optimal location of pilot points, and gradient search methods are used to derive their optimal transmissivities. The pilot point methodology is well suited for characterizing the spatial variability of the transmissivity field in contrast to methods using zonation. Pilot points are located where their potential for minimizing the objective function is the highest. This minimizes the perturbations in the transmissivities which are optimally assigned to the pilot point and results in minimal changes to the covariance structure of the transmissivity field. The calibrated fields honor the transmissivity measurements at their locations, preserve the variogram, and match the measured pressures in a least squares sense. Since there are numerous options in the execution of this methodology, computational experiments have been conducted to identify the most efficient among them. The method has been applied to the Waste Isolation Pilot Plant (WIPP) site, in southeastern New Mexico, where the U.S. Department of Energy is conducting probabilistic system assessment for the permanent disposal of transuranic nuclear waste. The resulting calibrated transmissivity fields are input to a Monte Carlo analysis of the total system performance. The present paper, paper 1 of a two‐paper presentation, describes the methodology. Paper 2, a companion paper, presents the methodology's application to the WIPP sit

ISSN:0043-1397    

DOI:10.1029/94WR02258

年代:1995

数据来源: WILEY

摘要:

This paper, the second in a two‐part series, presents the application of a methodology to assess spatial variability of the transmissivities within a regional aquifer in the vicinity of the Waste Isolation Pilot Plant (WIPP), the Culebra dolomite. An innovative aspect of this methodology is the generation of an ensemble of conditional simulations of the transmissivity field which preserve the statistical moments and spatial correlation structure of the measured transmissivity field and honors the measured transmissivity values at their locations. Each simulation is then calibrated, using an iterative procedure, to match an exhaustive set of steady state and transient pressure data. A fully automated inverse algorithm using pilot points as parameters of calibration was employed. The application of this new methodology to the Culebra dolomite flow system produced 70 conditional simulations which were consistent with all the measured transmissivity and head data at the site. Based on an analysis of the calibrated transmissivity fields, the spatial variability of the transmissivity fields was increased as a result of the calibration process. This increase is in part due to the addition of a high‐transmissivity feature to each of the transmissivity fields which is needed to match both steady state and transient state head d

ISSN:0043-1397    

DOI:10.1029/94WR02259

年代:1995

数据来源: WILEY

摘要:

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field‐average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregate

ISSN:0043-1397    

DOI:10.1029/94WR02534

年代:1995

数据来源: WILEY

摘要:

Typical models for the representation of naturally fractured systems generally rely on the double‐porosity Warren‐Root model or on random arrays of fractures. However, field observations have demonstrated the existence of multiple length scales in a variety of naturally fractured media. Present models fail to capture this important property of self‐similarity. We first use concepts from the theory of fragmentation and from fractal geometry to construct numerically a network of fractures that exhibits self‐similar behavior over a range of scales. The method is a combination of fragmentation concepts and the iterated function system approach and allows for great flexibility in the development of patterns. Next, numerical simulation of unsteady single‐phase flow in such networks is described. It is found that the pressure transient response of finite fractals behaves according to the analytical predictions of Chang and Yortsos (1990) provided that there exists a power law in the mass‐radius relationship around the test well location. Finite size effects can become significant and interfere with the identification of the fractal structure. The paper concludes by providing examples from actual well tests in fractured systems which are analyzed using fractal pressure trans

ISSN:0043-1397    

DOI:10.1029/94WR02260

年代:1995

数据来源: WILEY

摘要:

This paper describes a nonstationary spectral theory for analyzing flow in a heterogeneous porous medium with a systematic trend in log hydraulic conductivity. This theory relies on a linearization of the groundwater flow equation but does not require the stationarity assumptions used in classical spectral theories. The nonstationary theory is illustrated with a two‐dimensional analysis of a linear trend aligned with the mean flow direction. In this case, closed‐form solutions can be obtained for the effective hydraulic conductivity, head covariance, and log conductivity‐head cross covariance. The effective hydraulic conductivity decreases from the geometrical mean as the mean slope of the log conductivity increases. Trending leads to a reduction of head variance and a structural change in the head covariance and the log conductivity‐head cross covariance. Such changes have important implications for measurement conditioning (or cokriging) methods which rely on the head covariance and log conductivity‐head covariance. The nonstationary spectral analysis is also compared with classical spectral analysis. This comparison indicates that the classical spectral method correctly predicts the normalized head covariance in a linear trending media. The stationary spectral method fails to capture the qualitative influence of trends on the effective hydraulic conductivity and the log conductivity‐head cross covariance, although the magnitude of the error is relatively small for realistic values of the mean log conductivity slope. The stationary and nonstationary results are the same when there is no trend in log conductivity. The trending conductivity example illustrates that the nonstationary spectral method has all the capabilities of the classical spectral approach while not requiring as many restrictive

ISSN:0043-1397    

DOI:10.1029/94WR02531

年代:1995

数据来源: WILEY

摘要:

A model is introduced for microbial kinetics in porous media that includes effects of transients in the metabolic activity of subsurface microorganisms. The model represents the microbial metabolic activity as a functional of the history of aqueous phase substrates; this dependence is represented as a temporally nonlocal convolution integral. Conceptually, this convolution represents the activity of a microbial component as a fraction of its maximum activity, and it is conventionally known as the metabolic potential. The metabolic potential is used to scale the kinetic expressions to account for the metabolic state of the organisms and allows the representation of delayed response in the microbial kinetic equations. Calculation of the convolution requires the definition of a memory (or kernel) function that upon integration over the substrate history represents the microbial metabolic response. A simple piecewise‐linear metabolic potential functional is developed here; however, the approach can be generalized to fit the observed behavior of specific systems of interest. The convolution that results from the general form of this model is nonlinear; these nonlinearities are handled by using two separate memory functions and by scaling the domains of the convolution integrals. The model is applied to describe the aerobic degradation of benzene in saturated porous media. Comparative simulations show that metabolic lag can be used to consistently describe observations and that a convolution form can effectively represent microbial lag for this system. Simulations also show that disregarding metabolic lag when it exists can lead to overestimation of the amount of substrate degrade

ISSN:0043-1397    

DOI:10.1029/94WR02533

年代:1995

数据来源: WILEY