bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation of accretionary wedges in response to seamount subduction: Insights from sandbox experiments
Authors: Dominguez S. , Malavieille J. , Lallemand S. E.
Published: September 2002

Sandbox experiments, using a two‐dimensional and a three‐dimensional approach, are used to study the deformation of margins in response to seamount subduction. Successive mechanisms of deformation are activated during the subduction of conical seamounts. First, reactivation of the frontal thrusts and compaction of the accretionary wedge is observed. Then, back thrusting and, conjugate strike‐slip faulting develops above the leading slope of the subducted seamount. The basal decollement is deflected upward in the wake of the subducting high, and a large shadow zone develops behind the seamount trailing slope. Consequently, frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone. When the main decollement returns to its basal level in the wake of the seamount, the margin records a rapid subsidence and a new accretionary wedge develops, closing the margin reentrant. The sediments underthrusted in the wake of the seamount into the shadow zone, are underplated beneath the rear part of the accretionary wedge. Substantial shortening and thickening of the deformable seaward termination of the upper plate basement, associated with basal erosion is observed. Seamount subduction induces significant material transfer within the accretionary wedge, favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin. The subduction and underplating of relatively undeformed, water‐ladden sediments, associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal decollement. Consequently, significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount.
High mountains in a zone of extended crust: Insights into the Neogene‐Quaternary topographic development of northeastern Iberia
Authors: Lewis Claudia J. , Vergés Jaume , Marzo Mariano
Published: September 2002

Topographic analysis combined with geological and geophysical observations indicate that much of the present configuration of the northeastern Iberian Peninsula derives from Neogene‐Quaternary opening of the western Mediterranean basins following the Pyrenean orogeny. Mountainous topography up to 2000 m lines the Mediterranean coastal margin of Cataluña (Spain) despite a crustal thickness of 22 km at the coast. Geometric analysis of topographic profiles indicates a strong link between topography and extensional tectonics. Trends in maximum, mean and minimum elevations show marked regional increases in a SW‐NE direction along the axis of the Ebro basin towards the active Amer‐Brugent normal fault. From patterns of local relief, characteristics of the fluvial drainage network, location of drainage divides, and degree of escarpment retreat, we distinguish two overlapping areas of mountainous topography, a southern area related to late Oligocene to middle Miocene rifting and a northern area related to late Miocene to Quaternary rifting. The youthful topography in the northern area likely results from mechanical unloading of the footwall of the Amer‐Brugent normal fault. In the southern area (encompassing the Catalan Coastal Ranges), erosion continuously renews topography produced by rifting while causing headward erosion of coastal rivers into the Ebro basin. Elevated topography along the Catalan margin derives principally from flexural isostatic rebound due to tectonic and erosional denudation, causing exhumation of the rifted margin. Nonetheless, the height of the mountains and the thinness of the crust suggest an additional component of dynamic or thermal support of mountainous topography, likely buoyancy forces from density differences in the mantle.
Disturbances of temperature‐depth profiles due to surface climate change and subsurface water flow: 1. An effect of linear increase in surface temperature caused by global warming and urbanization in the Tokyo Metropolitan Area, Japan
Authors: Taniguchi Makato , Shimada Jun , Tanaka Tadashi , Kayane Isamu , Sakura Yasuo , Shimano Yasuo , Dapaah‐Siakwan S. , Kawashima Shinichi
Published: September 2002

A series of type curves is presented for evaluating vertical groundwater fluxes under the condition of a linear increase in surface temperature. The depths of minimum groundwater temperature in the temperature‐depth profiles indicate the magnitude of the downward groundwater flux. The type curve method has been applied to the subsurface thermal regime observed in Tokyo metropolitan area, Japan, to estimate the vertical groundwater fluxes under the condition of surface warming caused by global warming and urbanization. The groundwater fluxes obtained from the type curves and the depths of minimum groundwater temperature agree well with the values obtained from the other studies in the Tokyo metropolitan area. The inversion due to the surface warming could be a good tracer to detect the groundwater flow system.
A comparison of relative nonwetting‐phase permeability models
Authors: Dury O. , Fischer U. , Schulin R.
Published: September 2002

We compared three groups of models commonly used to describe the dependence of relative nonwetting‐phase permeability on phase saturation in porous media and tested the capability of these models to predict relative air permeabilities using two experimental data sets. The first group of models is purely empirical, involving no fitting parameters. In models of the second type, denoted here as geometry‐based models, relative nonwetting‐phase permeability is derived from the wetting‐phase retention curve on the basis of an idealized concept of pore space geometry. According to these models, nonwetting‐phase permeability should vary over the full range of saturation and should be a function of total nonwetting‐phase saturation. In models of the third type this variation is limited to a narrower range based on the concept that only part of the nonwetting phase contributes to permeability and assuming that the relationship between the “effective” saturation of this continuous nonwetting phase can be described by the same mathematical functions as between total nonwetting‐phase saturation and relative permeability in the geometry‐based models. Since this implies some kind of analogy, we refer to this group of models as “analogy‐based.” We found that matching the saturation at which nonwetting‐phase permeability emerges, i.e., the emergence point, is crucial for the description or prediction of relative nonwetting‐phase permeability in the range of high wetting‐phase saturations. Using analogy‐based models which were derived from the geometry‐based models by simply rescaling the effective saturation range to fit this endpoint, predictions were found to depend sensitively on whether the Burdine or the Mualem model was chosen to relate the permeability function to the retention curve. Differences between these two approaches were found to be primarily due to the different exponents in the tortuosity term. When we made this parameter flexible and determined it by curve‐fitting from measured data, we obtained very good fits of experimental air permeabilities.
Solute transport in heterogeneous porous media with long‐range correlations
Authors: Federico Vittorio Di , Zhang You‐Kuan
Published: September 2002

Transport of finite plumes in three‐dimensional, heterogeneous, and statistically isotropic aquifers is investigated where the log hydraulic conductivity is characterized by a fractional Gaussian noise (fGn) covariance structure of Hurst coefficient H. Leading‐order analytical expressions for velocity autocovariance functions uij, one‐particle displacement covariance Xij, and macrodispersivity tensor αij are derived under ergodic conditions and mean‐uniform steady state flow. Nonergodic transport is then discussed for a line source of finite length, either normal or parallel to the mean flow, by evaluating time‐dependent ensemble averages of the second spatial moments,Zij ≡ 〈Aij〉 − Aij(0) = Xij − Rij and the effective dispersivity tensor, γij defined as (0.5/μ)(d〈Aij〉/dt), where Aij(0) is the initial value of the second spatial moments of a plume Aij, and Rij is the plume centroid covariance. The main finding is that in a fGn log K field the spreading of a solute plume is never ergodic; as H increases, effective dispersivity results differ more from their ergodic counterparts, since a larger H implies the medium is more correlated. The most interesting results are as follows: for a source parallel to flow, γ22 does not decrease below zero at large time but remains strictly positive, in variance with exponential or Gaussian covariance. For a source normal to flow, γ11 reaches a large‐time asymptote, whose value depends on H as follows: it decreases with H for a small source, it increases, reaches a peak, and then decreases as H goes from 1/2 to 1 for intermediate and large sources; for H = 1, γ11 is zero irrespective of the source size.
Simultaneous measurement of water retention and electrical conductivity in soils: Testing the Mualem‐Friedman Tortuosity Model
Authors: Weerts A. H. , Bouten W. , Verstraten J. M.
Published: September 2002

A conceptual model that accounts for the influence of pore geometry is needed to obtain soil water conductivity from bulk soil electrical conductivity measurements in unsaturated soils. Mualem and Friedman [1991] proposed such a model, based on the hypothesis that the tortuosity factor affecting bulk soil electrical conductivity is identical to that defined for prediction of (odel output) of two soils were measured simultaneously over a wide range of soil water conductivity using a multistep outflow (MSO) experiment in combination with time domain reflectometry. The model with only one free parameter can describe the measured data. Predictions indicate a large dependence of the model outcome on the value of the fitting parameter. The fitted tortuosity parameters were used for inverse modeling of the water flow during the MSO experiments, obtaining unique estimates of the hydraulic conductivity.
Multifrequency radar observations of bare surface soil moisture content: A laboratory experiment
Authors: Mancini Marco , Hoeben Rudi , Troch Peter A.
Published: September 2002

This paper reports on a laboratory experiment that investigates the use of active microwave observations to estimate volumetric soil moisture content. The experiment, held in 1995, was set up at the European Microwave Signature Laboratory, Joint Research Centre of the European Communities, Ispra (Italy). Full polarimetric radar observations of a 2 m diameter cylindric container filled with a sandy loam soil were performed. During successive wetting and drying cycles, different soil moisture profiles were generated and observed in situ by means of time domain reflectometry probes. The radar data are analyzed based on the Integral Equation Model that simulates radar backscattering given known surface characteristics, such as moisture content and roughness. This allows the evaluation of the predictive power of the simulation model. We find general good agreement between measurements and simulations, but problems occur at high incidence angles. The model is then used to invert soil moisture information from radar measurements. It is shown that, in spite of the complexity of the model involved, it is possible to retrieve under certain circumstances reliable soil moisture estimates with similar accuracies as the in situ measurements.
Characterization of leaky faults: Study of air flow in faulted vadose zones
Authors: Shan Chao , Javandel Iraj , Witherspoon Paul A.
Published: September 2002

Characterization of leaky faults in vadose zones of large thickness is very important for engineering problems such as high‐level nuclear waste disposal. This paper presents analytical solutions for air flow in faulted vadose zones of large or small thicknesses. The focus is on those sites where the fault zone is more permeable than its adjacent rock matrix. On the basis of the assumptions of a two‐dimensional air flow in the matrix and a one‐dimensional air flow in the fault zone, analytical solutions are presented for a sinusoidal atmospheric pressure variation. A procedure is then provided for extending the application of the solutions to an arbitrary atmospheric pressure variation. The solutions can help determine air permeability of leaky faults in the vadose zone.
One‐dimensional nonlinear steady infiltration
Authors: Basha H. A.
Published: September 2002

An approximate solution of the one‐dimensional nonlinear steady infiltration with arbitrary plant withdrawal functions is presented. The solution is derived using perturbation expansion methods and is applicable for homogeneous soils for which the hydraulic conductivity function is of the rational power form. The soil domain is either a semi‐infinite flow medium or a finite‐depth medium overlying a shallow water table. The boundary condition at the surface of the soil is taken as the steady constant flux, and the bottom boundary condition is a prescribed head at a depth. The water extraction functions used herein include the uniform, the piecewise constant, and the exponential uptake. The perturbation solution showed very good agreement with the exact implicit solutions for various soil types and surface flux conditions. The approximate solution provided a basis on which explicit algebraic equations for parameter estimation can be derived. Numerical results show the effects of the soil type, surface flux, and rooting depths on the pressure distribution in the soil.
Quantification of transient CO2 production in a sandy unsaturated zone
Authors: Hendry M. J. , Mendoza C. A. , Kirkland R. A. , Lawrence J. R.
Published: September 2002

Temporal and spatial respiration rates were determined in a 5.7‐m thick, sandy, unsaturated zone over a 550‐day period using measured CO2 concentrations, CO2 fluxes to the atmosphere, moisture contents, and temperatures. Cyclical patterns in CO2 concentrations were measured in duplicate nests of nine gas samplers. Maximum CO2 gas concentrations occurred during the summer (0.85–1.22%), and minimum concentrations occurred during the winter (0.04–0.24%). CO2 gas concentrations decreased with increasing depth during the summer and increased with depth during the winter. A one‐dimensional finite element model was developed to quantify transient respiration rates through the unsaturated zone. The model was calibrated to the measured CO2 concentrations. Temperature and moisture content variations were represented with an analytical expression and linear interpolation of field‐measured values, respectively, in the model. Simulation results provided very good approximations to the field‐measured CO2 concentrations, but predicted CO2 fluxes to the atmosphere were higher than measured. Respiration rates ranged from 5 µg C g−1 d−1 in the soil horizon during the summer to about <10−4 µg C g−1 d−1 in unsaturated sections of the C horizon. A sensitivity analysis showed that the respiration rates in the C horizon must be <10−3 µg C g−1 d−1 and that the majority of the elevated CO2 concentrations in this thick unsaturated zone are the result of respiration in the soil horizon. Overall, roots contribute about 75% of the CO2 in the summer months. O2 gas, microbial analyses, and the distribution of root biomass supported this conclusion. These observations also imply that although microorganisms are present in subsurface environments their in situ activity in this sandy unsaturated zone may be very low.
Calibration of transfer function–noise models to sparsely or irregularly observed time series
Authors: Bierkens Marc F. P. , Knotters Martin , van Geer Frans C.
Published: September 2002

A method is presented to calibrate transfer function–noise (TFN) models, operating at the same frequency as the input (auxiliary) variables, to sparsely or irregularly observed time series of the output (target) variable. Once calibrated, the TFN models can be used to predict or simulate the output variable at the same frequency as the input variable. Consequently, the method provides a useful tool for filling in gaps of irregularly or sparsely observed hydrological time series. Although generic and suitable for any type of time series, the method is described through the modeling of a time series of groundwater head data with precipitation surplus (precipitation minus potential evapotranspiration) as input variable. First, the TFN model is written in vector notation, yielding the state equation of a linear discrete stochastic system. Subsequently, the state equation is embedded in a Kalman filter algorithm. The Kalman filter is then combined with a maximum likelihood criterion to obtain estimates of the parameters of the TFN model for small time steps (e.g., 1 day) while using sparsely (e.g., two times a month) or even irregularly observed time series of groundwater head data. The method is illustrated using (subsets of) time series of groundwater head data with varying regular and irregular observation intervals.
Adsorption and capillary condensation in porous media: Liquid retention and interfacial configurations in angular pores
Authors: Tuller Markus , Or Dani , Dudley Lynn M.
Published: September 2002

Conventional models of liquid distribution, flow, and solute transport in partially saturated porous media are limited by the representation of media pore space as a bundle of cylindrical capillaries (BCC). Moreover, the capillary model ignores the dominant contribution of adsorptive surface forces and liquid films at low potentials. We propose two new complementary elements for improving our understanding of liquid configuration in porous media: (1) an approach for considering the individual contributions of adsorptive and capillary forces to the matric potential and (2) a more realistic model for pore space geometry. Modern interface science formalism is applied to determine the thickness of adsorbed liquid films as a function of thermodynamic conditions and specific surface area of the medium. The augmented Young‐Laplace (AYL) equation provided the necessary framework for combining adsorptive and capillary processes. A new pore space geometry composed of an angular pore cross section (for capillary processes) connected to slit‐shaped spaces with internal surface area (for adsorption processes) offers a more realistic representation of natural porous media with explicit consideration of surface area (absent in the standard BCC model). Liquid‐vapor configuration, saturation, and liquid‐vapor interfacial area were calculated for different potentials and pore (unit cell) dimensions. Pore dimensions may be easily related to measurable soil properties such as specific surface area and porosity. Rigorous calculations based on the AYL equation were simplified and led to the development of algebraic expressions relating saturation and interfacial area of liquid in the proposed pore space geometry to chemical potential. These simple expressions are amenable to upscaling procedures similar to those presently used with the BCC model.
Low‐flow frequency exacerbation by irrigation withdrawals in the agricultural midwest under various climate change scenarios
Authors: Eheart J. Wayland , Tornil Daniel W.
Published: September 2002

As a possible outcome of global climate change, reductions in precipitation, while directly decreasing surface water streamflows and reserves, can also initiate or exacerbate the surface water demand by regional agriculture. Driven by the desire to maintain consistent yields and hedge the risk of drought, farmers may implement or increase supplemental irrigation applications, obtaining water from either surface sources or aquifers. This paper presents results of an investigation of the impact of these effects on streamflows in the midwestern United States. The investigation examines the direct effect of precipitation reduction and the indirect effects of riparian and nonriparian irrigation. The crop‐growth and farm‐scale hydrologie model erosion‐productivity impact calculator is used in conjunction with the routing features of the basin‐scale hydrologic model soil water analysis tool to simulate direct and indirect effects of global change on streamflows. The analysis shows that the coupled effects of decreases in runoff and increases in irrigation may be devastating to the streams of Illinois and other midwestern states. For the example basin a 25% decrease in mean precipitation under optimal irrigation practices by both riparian and nonriparian farmers yields an increase in the annual (mean) occurrence of the historical 7Q10 from 3 days/year to 13 days/year, despite the streamflow accretion from groundwater‐supplied irrigators.
Nonideal transport of reactive solutes in heterogeneous porous media: 5. Simulating regional‐scale behavior of a trichloroethene plume during pump‐and‐treat remediation
Authors: Zhang Zhihui , Brusseau Mark L.
Published: September 2002

“Pump and treat” is widely used for containment and remediation of groundwater contaminant plumes. However, it is commonly observed that pump‐and‐treat systems begin to exhibit reduced efficiency at some point. A number of factors and processes may contribute to the reduced efficiency, among them being those associated with nonideal transport behavior, such as subsurface heterogeneity, nonlinear, rate‐limited sorption/desorption, and rate‐limited dissolution of immiscible liquid. We use numerical modeling to analyze the regional‐scale (∼49 km2) nonideal transport behavior of trichloroethene in a contaminated aquifer undergoing pump and treat remediation. The pump‐and‐treat system has been in operation for ∼12 years, during which time the composite concentrations of trichloroethene in the treatment plant influent have decreased from >300 to ∼100 μg L−1. However, as is typically observed elsewhere, the system is exhibiting extensive concentration tailing, wherein the concentrations have remained relatively constant at ∼100 μg L−1 for the past 8 years. Various factors that may be contributing to this tailing phenomenon are evaluated using a three‐dimensional solute transport model specifically developed for the site, which is located in Tucson, Arizona. The values for almost all of the input parameters of the model were obtained independently of the historic concentration data being simulated. The hydraulic conductivity field was generated using information obtained from borehole logs and pumping tests, the sorption and local‐scale mass transfer parameters were obtained from laboratory experiments conducted with aquifer material collected from the site, and the initial immiscible‐liquid saturations were based on the results of partitioning tracer tests conducted in a representative source zone at the site. On the basis of our analyses we conclude that while rate‐limited desorption and large‐scale spatial variability of hydraulic conductivity have significant impacts on trichloroethene transport, the dissolution of immiscible‐liquid saturation associated with the source zones is most likely the primary cause of the extensive concentration tailing observed at the site. The impact of nonlinear sorption and local‐scale mass transfer on trichloroethene removal appears to be insignificant.
Assessing rooting depths of an austrian pine stand by inverse modeling soil water content maps
Authors: Musters P. A. D. , Bouten W.
Published: September 2002

Rooting depths in a forested stand (0.11 ha) were estimated indirectly by inverse modeling maps of soil water contents from time domain reflectometry (TDR) measurements at 150 points. These maps were described with a calibrated one‐dimensional soil water flow model, with specific values for the rooting depth, van Genuchten's [1980] α parameter, and throughfall fraction at each point. At about one third of the 150 points, modeled rooting depths did not fall within the a priori likely range of rooting depths; these values were discarded when kriging the point estimates to a map of rooting depths. The resulting rooting depths range from 0.8 m to 3.5 m within the stand in good agreement with root observations (R2 = 0.85). Several factors contributed positively to this good match, such as the fairly uniform soil hydraulic properties and the high sensitivity of soil water dynamics to root water uptake. Overall, the results demonstrate the suitability of soil water content maps based on TDR measurements to quantify spatial variability in soil water dynamics.
Analysis of groundwater migration from artificial recharge in a large urban aquifer: A simulation perspective
Authors: Tompson Andrew F. B. , Carle Steven F. , Rosenberg Nina D. , Maxwell Reed M.
Published: September 2002

The increased use of reclaimed water for artificial groundwater recharge purposes has led to concerns about future groundwater quality, particularly as it relates to the introduction of new organic and inorganic contaminants into the subsurface. Here we review the development and initial application of a detailed numerical model of groundwater flow and migration in a region encompassing a large groundwater recharge operation in Orange County, California. The model is based upon a novel representation of geologic heterogeneity, which has long been known to influence local flow and transport behavior in the subsurface. The model and complementary series of isotopic analyses provide an improved scientific basis to understand flow paths, migration rates, and residence times of recharged groundwater, as well as to identify the source composition of water produced in wells near the recharge operation. From a management perspective these issues need to be confronted in order to respond to proposed regulatory constraints that would govern the operation of recharge facilities and nearby production wells. While model calibration is greatly aided by isotopic source and residence time analyses, the model also provides unique insights on the interpretation of isotopic data themselves. Isotopic estimates of groundwater age help discriminate between several equally acceptable simulations calibrated to head data only. However, the results also suggest that groundwater reaching a well spans a wide‐ranging distribution of age, demonstrating the importance of geologic heterogeneity in affecting flow paths, mixing, and residence times in the vicinity of recharge basins and wells.
A finite cell method for simulating the mass transport process in porous media
Authors: Sun Ne‐Zheng
Published: September 2002

This paper presents a new numerical method, the finite cell method (FCM), for simulating complex biological and chemical transport phenomena in porous media. It has the same advantages as the random walk method (RW): there is no advection‐dispersion equation to be solved, it is unnecessary to calculate the concentration distribution in each time step, and there is no numerical dispersion for the case of high Peclet number. The concept of FCM, however, is different from the RWM in several aspects. The minimum unit in the FCM is a cell that has a certain volume and carries variable mass, while in the RWM the minimum unit is a particle that has no volume but carries a certain mass. In the FCM we use multiple sets of cells to represent different phases in a porous medium. A mobile or an immobile cell may carry multiple biological and/or chemical components. Mass exchanges may occur not only between cells but also within cells. These mass exchanges can be described by different rules: equilibrium or nonequilibrium, linear or nonlinear. In this paper, a one‐dimensional FCM code is developed and used to simulate the solute transport with nonequilibrium adsorption, colloid transport with kinetic deposition and release, and colloid facilitated transport with kinetic mass exchanges. The FCM solutions have been compared with available analytical solutions and the solutions obtained by the finite difference method (FDM). Results show that both FCM and FDM solutions match analytical solutions quite well when the concentration front is flat. When the concentration front is sharp, however, the FDM solutions become inaccurate because of numerical dispersion, but the FCM can still produce very accurate peak and tailing concentrations. From the example of colloid‐facilitated transport, we can see how a complex chemical and biological transport procedure in porous media can be simulated directly by the FCM without solving any advection‐dispersion equation.
A stochastic model for deriving the basic statistics of J‐day averaged streamflow
Authors: Yue Sheng , Hashino Michio
Published: September 2002

This study provides the reader with a methodology for directly deriving basic streamflow statistics (mean, variance, and correlation coefficient) from long‐term recorded daily rainfall data. A daily streamflow sequence is considered as a filtered point process where the input is a storm time sequence that is assumed to be a marked point process. The mark is the storm magnitude that is constructed from a daily rainfall time series, and the correlation of the daily rainfall during the storm is considered. The number of storms is a counting process represented by either the binomial, the Poisson, or the negative binomial probability distribution, depending on its ratio of mean versus variance. As a pulse‐response function for a filtered point process, the model of three serial tanks with a parallel tank is adopted to describe the physical process of rainfall‐runoff. Thus the basic statistics (mean, variance, and covariance function) of J‐day averaged streamflows can be estimated in terms of the constants expressing stochastic properties of a rainfall time series and the tank model's parameters representing the causal relationship between rainfall and runoff. The method is used to derive the streamflow statistics of an actual dam basin, the Sameura Dam basin, located in Shikoku island, Japan. The resulting computed means and variances of 5‐day averaged streamflows show a good correspondence with observed ones.
Arsenic mobilization in the hyporheic zone of a contaminated stream
Authors: Nagorski Sonia A. , Moore Johnnie N.
Published: September 2002

Arsenic behavior was examined in a contaminated stream by sampling the dissolved (<0.45 μm) arsenic and metals in surface water, shallow hyporheic zone water, and adjacent ground water. Surface water was oxic and slightly basic, and ground water was anoxic and acidic. Hyporheic zone water had pH values of 6–7, dissolved oxygen concentrations mostly between 0 and 3 mg L−1, and mean concentrations of most metals inbetween surface and ground water sample concentrations. However, arsenic and iron were enriched in the hyporheic zone. Most of the hyporheic zone dissolved arsenic was in the form of As(III), which is considered to be more toxic to some organisms than As(V). In the oxic surface water, 20% of the total dissolved As was found to occur in its reduced form. We hypothesize that upon burial and reduction of Fe‐oxyhydroxides in the streambed, sediment‐bound arsenic is transferred into the dissolved phase as As(III), and it is subsequently released into the surface water, where it does not immediately reoxidize. A continual flux of reduced As to the surface water maintains As(III) concentrations above that expected in oxygenated surface waters.
Mobilization of trichloroethene (TCE) during ethanol flooding in uniform and layered sand packs under confined conditions
Authors: Grubb Dennis G. , Sitar Nicholas
Published: September 2002

Five horizontal flow experiments were conducted in homogeneous and layered sand packs to evaluate the effectiveness of mobilization of trichloroethene (TCE) by ethanol flooding. A small quantity (5–7 mL) of pure TCE was injected into the center of each sand pack, and then horizontal ethanol floods were conducted at darcy velocities between 4 and 12 m d−1. Once ethanol contacted the region initially affected by TCE in uniform sand packs, rapid downward and upgradient flow of TCE occurred along sloping ethanol‐water interfaces. In layered sand packs the bulk mobilization of TCE appeared to be associated more with phenomena resulting from interfacial tension reduction rather than with enhanced dissolution processes. Heterogeneities promoted a variety of bypassing and mixing phenomena that resulted in, but were not limited to, physical displacement of TCE at the edges of discrete fine‐grained lenses due to localized confined flow conditions; upward migration of ethanol through finer‐grained soils due to buoyancy; rapid downward mobilization of TCE; upgradient mobilization of TCE around heterogeneities; and migration of TCE into finer‐grained soils at locations both upgradient and downgradient of the TCE source zone. Enhanced TCE dissolution was also observed.
A new soil metric potential sensor based on time domain reflectometry
Authors: Or Dani , Wraith Jon M.
Published: September 2002

We developed and tested a new sensor based on time domain reflectometry (TDR) to measure soil matric potential (h). The TDR‐matric (TM) sensor is constructed of porous disks having different known maximum pore sizes and stacked within a coaxial cage. The constant relationship between water content (θ) and h of the TM porous matrix is initially calibrated and subsequently used to infer matric potential of the surrounding soil, similar to existing porous heat dissipation and electrical resistance sensors. The θ of the sensor's porous matrix in hydraulic equilibrium with the surrounding soil is measured by TDR travel time analysis. Prototype sensors were constructed using porous ceramic and plastic disks having maximum pore diameters between 120 μm(2.5 kPa) and about 0.6 μm (0.5 MPa). Calibration tests to evaluate sensor θ‐h relationships were completed in a pressure chamber apparatus using four soils. These results and those from sensors installed in soil lysimeters in the presence of growing plants showed consistent θ‐h relationships and synchronized responses of soils and TM sensors to changing water status. Pairing standard TDR probes with the new TM sensors facilitates in situ determination of soil θ(h) relationships, using conventional TDR instrumentation. The sensor design accommodates construction of media‐ or application‐specific sensors using combinations of disks having different pore sizes. There is a trade‐off between the TM sensor's matric potential range and its sensitivity to changes in the surrounding soil. Additionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse‐textured soils) can lead to hydraulic decoupling of these and other porous sensors.
Pumping tests in networks of multilevel sampling wells: Motivation and methodology
Authors: Butler J. J. , McElwee C. D. , Bohling G. C.
Published: September 2002

The identification of spatial variations in hydraulic conductivity (K) on a scale of relevance for transport investigations has proven to be a considerable challenge. Recently, a new field method for the estimation of interwell variations in K has been proposed. This method, hydraulic tomography, essentially consists of a series of short‐term pumping tests performed in a tomographic‐like arrangement. In order to fully realize the potential of this approach, information about lateral and vertical variations in pumping‐induced head changes (drawdown) is required with detail that has previously been unobtainable in the field. Pumping tests performed in networks of multilevel sampling (MLS) wells can provide data of the needed density if drawdown can accurately and rapidly be measured in the small‐diameter tubing used in such wells. Field and laboratory experiments show that accurate transient drawdown data can be obtained in the small‐diameter MLS tubing either directly with miniature fiber‐optic pressure sensors or indirectly using air‐pressure transducers. As with data from many types of hydraulic tests, the quality of drawdown measurements from MLS tubing is quite dependent on the effectiveness of well development activities. Since MLS ports of the standard design are prone to clogging and are difficult to develop, alternate designs are necessary to ensure accurate drawdown measurements. Initial field experiments indicate that drawdown measurements obtained from pumping tests performed in MLS networks have considerable potential for providing valuable information about spatial variations in hydraulic conductivity.
Rate‐limited mass transfer or macrodispersion: Which dominates plume evolution at the macrodispersion experiment (MADE) site?
Authors: Harvey Charles , Gorelick Steven M.
Published: September 2002

We present a model of solute transport that explains the large‐scale behavior of the solute tracer‐test plumes at the Macrodispersion Experiment (MADE) site as the result of advection and rate‐limited mass transfer between mobile and small‐scale immobile domains. This model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles. Compared to the macrodispersion model, the mass transfer model better represents the change in mobile dissolved mass with time, the peak of the concentration profile, and the profile asymmetry. Specifically, unlike the macrodispersion model, the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times. We suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain. The immobile domain consists of a combination of low‐permeability zones on the scale of centimeters to decimeters (the Darcy‐scale immobile domain), and intragranular porosity, dead‐end pores, and surface sorption (the pore‐scale immobile domain). We suggest that the mobile domain was sampled preferentially when water was extracted. Therefore, at early times, relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations. Similarly, the mass at late times was underestimated because solute trapped in the pore‐scale immobile domain was not extracted during sampling and therefore ignored. The combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only ∼5 m by the termination of the experiment, as well as the different behavior of bromide and tritium tracers.
Self‐consistency of a heterogeneous continuum porous medium representation of a fractured medium
Authors: Jackson C. Peter , Hoch Andrew R. , Todman Steve
Published: September 2002

A possible approach to representing fractured rocks is to use heterogeneous continuum porous medium (CPM) models in which each grid block has an effective permeability appropriate to represent the network of fractures within the block. It needs to be demonstrated that the approach is self‐consistent; that is, the effective permeability on a length scale larger than the grid blocks derived using the heterogeneous CPM model is close to that derived directly from the underlying network model. For models based on the Borrowdale Volcanic Group at Sellafield in the United Kingdom and the granite at Äspö in Sweden, it is shown that the use of heterogeneous CPM models is self‐consistent, both for models using effective permeabilities determined directly from the underlying network of discrete features model and for models based only on the correlation structure of the effective permeability on the length scale of the grid blocks.
Cokriging estimation of the conductivity field under variably saturated flow conditions
Authors: Li Bailing , Yeh T.‐C. Jim
Published: September 2002

A linear estimator, cokriging, was applied to estimate hydraulic conductivity, using pressure head, solute concentration, and solute arrival time measurements in a hypothetical, heterogeneous vadose zone under steady state infiltrations at different degrees of saturation. Covariances and cross‐covariances required by the estimator were determined by a first‐order approximation in which sensitivity matrices were calculated using an adjoint state method. The effectiveness of the pressure, concentration, and arrival time measurements for the estimator were then evaluated using two statistical criteria, L1 and L2 norms, i.e., the average absolute error and the mean square error of the estimated conductivity field. Results of our analysis showed that pressure head measurements at steady state flow provided the best estimation of hydraulic conductivity among the three types of measurements. In addition, head measurements of flow near saturation were found more useful for estimating conductivity than those at low saturations. The arrival time measurements do not have any significant advantage over concentration. Factors such as variability, linearity, and ergodicity were discussed to explain advantage and limitation of each type of data set. Finally, to take advantage of different types of data set (e.g., head and concentration), a computationally efficient estimation approach was developed to combine them sequentially to estimate the hydraulic conductivity field. The conductivity field estimated by using this sequential approach proves to be better than all the previous estimates, using one type of data set alone.
When do logs move in rivers?
Authors: Braudrick Christian A. , Grant Gordon E.
Published: September 2002

Large woody debris is an integral component of forested, fluvial systems throughout the world, yet we know little about hydraulic thresholds for movement and transport of logs. We developed theoretical models of entrainment and performed flume experiments to examine thresholds for wood movement in streams. Both the model and the experiments indicate that log entrainment is primarily a function of the piece angle relative to flow direction, whether or not the log had a rootwad, the density of the log, and the piece diameter. Stability increased if the pieces had rootwads or were rotated parallel to flow. Although previously reported as the most important factor in piece stability, piece length did not significantly affect the threshold of movement in our experiments or our physically based model, for logs shorter than channel width. These physically based models offer a first‐order approach to evaluating the stability of either naturally derived woody debris or material deliberately introduced to streams for various management objectives.
Preface [to special section on Recent Loch Vale Watershed Research]
Authors: Baron Jill S. , Williams Mark W.
Published: September 2002

Catchment‐scale intensive and extensive research conducted over the last decade shows that our understanding of the biogeochemical and hydrologic processes in subalpine and alpine basins is not yet sufficiently mature to model and predict how biogeochemical transformations and surface water quality will change in response to climatic or human‐driven changes in energy, water, and chemicals. A better understanding of these processes is needed for input to decision‐making regulatory agencies and federal land managers. In recognition of this problem the National Research Council [1998] has identified as a critical research need an improved understanding of how global change will affect biogeochemical interactions with the hydrologic cycle and biogeochemical controls over the transport of water, nutrients, and materials from land to freshwater ecosystems. Improved knowledge of alpine and subalpine ecosystems is particularly important since high‐elevation catchments are very sensitive to small changes in the flux of energy, chemicals, and water. Furthermore, alpine ecosystems may act as early warning indicators for ecosystem changes at lower elevations.
Surface water‐groundwater interaction near shallow circular lakes: Flow geometry in three dimensions
Authors: Townley Lloyd R. , Trefry Michael G.
Published: September 2002

Steady flow regimes for three‐dimensional lake‐aquifer systems are studied via idealized mathematical models that are extensions of earlier simplified vertical section models of interaction between shallow lakes and underlying aquifers. The present models apply to a shallow circular lake at the surface of a rectangular aquifer of finite depth, yielding a truly three‐dimensional representation of the resulting flow system. Flux boundary conditions are applied at the ends of the aquifer, with net vertical recharge or evapotranspiration at the water table. The lake is defined by a region with constant head. By determining and visualizing solutions to the discretized saturated flow equations, a range of possible flow regimes is identified, and their topological properties are studied. Tools for analyzing flow regimes are described, including a method for locating and mapping three‐dimensional dividing surfaces within steady flow fields. Results show strong similarities between two‐ and three‐dimensional systems, including a large number of flow‐through, recharge, and discharge regimes and reverse flow cells. Flow lines calculated on a vertical plane through the middle of a lake resemble but are not identical to two‐dimensional streamlines for a range of aquifer flow and recharge conditions. Estimates of the widths and depths of capture and release zones for various lake‐aquifer geometries are asymptotic to earlier results for two‐dimensional systems. Numerical predictions are compared with analytical results for certain limiting flow regimes.
Groundwater waves in a coastal aquifer: A new governing equation including vertical effects and capillarity
Authors: Li L. , Barry D. A. , Stagnitti F. , Parlange J.‐Y.
Published: September 2002

Groundwater waves, that is, water table fluctuations, are a natural phenomenon in coastal aquifers. They represent an important part of the interaction between the ocean and aquifer and affect the mass exchange between them. This paper presents a new groundwater wave equation. Because it includes the effects of vertical flows and capillarity, the new equation is applicable to both intermediate‐depth aquifers and high‐frequency waves. Compared with the wave equation derived by Nielsen et al. [1997], the present equation provides a closer representation of groundwater waves. In particular, it predicts high‐frequency water table fluctuations as observed in the field. A validation of the new equation has been carried out by comparing the analytical solutions to it with predictions from direct simulations using the numerical model SUTRA. The effects of various physical parameters and their relative importance are also discussed.
Local reaction and diffusion in porous media transport models
Authors: Mo Zhimin , Friedly John C.
Published: September 2002

The problem of when advection‐dispersion models apply for reactive transport in porous media is addressed. Assuming local mass balances, including arbitrary homogeneous and interfacial chemical reactions, are known, volume averaging is applied to obtain a set of equations for the average concentrations. It is shown that timescale constraints must be satisfied in addition to the well‐known length‐scale constraint needed for volume averaging. The timescale for simulation must be longer than a diffusion timescale in the representative elementary volume, t/TD ≫ 1. In addition, interfacial reaction timescales must be larger than meaningful diffusion timescales, Tr/TD ≫ 1. When these constraints are satisfied, the usual dispersion coefficient exists and is time‐invariant and independent of reactions. Reaction rate expressions and all mass transfer fluxes can be expressed in terms of the average concentrations of the macroscopic model. Even when surface reactions are fast, it is shown that the fluid volume can be subdivided into small enough regions such that the appropriate time constraint Tr/TD ≫ 1 is satisfied. An average model can be obtained that includes mass transfer resistance expressed in terms of a mass transfer coefficient. The mass transfer coefficient is defined and is shown to depend only on the geometry of the porous medium and the flow field. This work provides a theoretical basis for the commonly used advection‐dispersion models for reactive transport at the Darcy scale and provides both length‐scale and timescale constraints for when they apply.