bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic setting and U/Pb zircon dating of the plutonic Socorro Complex in the Transpressive Rio Paraíba do Sul Shear Belt, SE Brazil
Authors: Ebert H. D. , Chemale F. , Babinski M. , Artur A. C. , Van Schmus W. R.
Published: February 2004

The Precambrian Rio Paraíba do Sul Shear Belt comprises a 200‐km‐wide anastomosing network of NE‐SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic‐migmatitic, and granitoid terrains as well as low‐ to medium‐grade metavolcano‐sedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west‐northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen‐parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike‐slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right‐lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono‐metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calc‐alkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones.
Active folding of Pleistocene unconformities on the edge of the Australian‐Pacific plate boundary zone, offshore North Canterbury, New Zealand
Authors: Barnes Philip M.
Published: February 2004

In North Canterbury, New Zealand, the southeastern edge of an active fold and thrust belt lies 20 km offshore beneath the continental shelf. High‐resolution marine seismic reflection profiles and a detailed Quaternary sequence stratigraphy are integrated to map and characterise the seismic potential of actively growing folds. Eleven large‐scale folds occur within the upper few hundred metres of a Pliocene to Recent succession of silty mudstone, representing the upper part of a sedimentary cover sequence up to 2 km thick. The folds are gentle, NE‐SW trending, overlapping, asymmetric structures approximately 10–32 km in length, which verge consistently to the northwest in accordance with major thrust faults and folds exposed in nearby coastal hills. The folds are inferred to overlie a system of blind, southeast dipping thrust faults that are accommodating a small component of regional NW‐SE crustal shortening in North Canterbury and to develop by coseismic uplift during thrust fault earthquakes. The critical interactions between the rates of folding, regional tilting, coastal uplift, outer shelf subsidence, and the local sedimentation response to high‐amplitude Quaternary sea‐level cycles have led to the development and preservation of a stack of folded sedimentary sequences, an essentially smooth, gently sloping seafloor throughout most of the off‐shore deformation zone, and net uplift close to zero at the deformation front. Eleven unconformity‐bounded sedimentary units of middle Pleistocene to Recent age (< 0.75 Ma) constrain the rates and timing of folding and enable comparisons with onshore deformation rates, examination of the coastal tectonic gradient, and speculation about the rates of upper crustal deformation beneath the shelf. Deposition, folding, and coastal uplift have occurred contemporaneously throughout the last 0.75 m.y., and all measurable deformation can be accommodated within approximately 0.8 m.y. Fold amplitude growth rates of 0.02 m/kyr to 0.14 m/kyr (typically 0.05–0.09 m/kyr) near the deformation front are low and up to 25 times lower than some actively growing folds exposing basement rocks onshore. There is a significant decline in strain rate across the coastal zone and inner shelf, toward the off‐shore deformation front. It is inferred that blind thrust faults beneath the offshore folds have slip rates typically of the order of 0.1–0.9 mm/yr and that the probable recurrence interval of moderately large magnitude (M6.2–6.8 ± 0.3) thrust earthquakes beneath individual folds is of the order of several tens of thousands of years.
Listric normal faulting during postorogenic extension revealed by 40Ar/39Ar thermochronology near the Robertson Lake shear zone, Grenville orogen, Canada
Authors: Busch Jay P. , van der Pluijm Ben A. , Hall Chris M. , Essene Eric J.
Published: February 2004

The Robertson Lake shear zone is a major plastic to brittle extensional shear zone in the Grenville orogen that bounds the Mazinaw and Sharbot Lake domains and provides information on the style of late extension and the unroofing history of the orogen. Argon isotope data were collected from hornblende and micas to determine 40Ar/39Ar ages, constrain the temperature‐time histories of these two domains, and infer the unroofing history of the region. Hornblende cooling ages across the Mazinaw domain (footwall) show little variation, indicating uniform unroofing of the footwall since 950 Ma. Phlogopite, muscovite, and biotite cooling ages of the footwall are 924 to 890 Ma. The cooling history of the Mazinaw domain is characterized by slow cooling after peak metamorphism (circa 1000 Ma), accelerated cooling (4°–5°C/m.y.) from 950 Ma to 890 Ma, and an average cooling rate of ∼1°C/m.y. to circa 590 Ma, when these rocks were at or near the surface. The cooling history of Sharbot Lake (hanging wall) domain is drastically different than that of the Mazinaw domain. Hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 Ma, respectively, indicating a cooling rate of 5°C/m.y. after slow cooling from metamorphic temperatures. Biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve, indicating that the cooling history varied across the domain. Cooling rates in the hanging wall adjacent to the shear zone are low (2°C/m.y.). A biotite cooling age (1029 Ma) and preservation of an amphibole growth age (1205 Ma) in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 Ma. These data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone. The varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement.
Analysis of Land Surface Heat Fluxes Using the Orthonormal Wavelet Approach
Authors: Katul Gabriel G. , Parlange Marc B.
Published: February 2004

Heat fluxes under unstable atmospheric conditions are measured and analyzed using orthonormal wavelet expansions. Both wavelet and Fourier power spectra display a −1 power law that can be derived from dimensional arguments for latent and sensible heat flux in the turbulent production subrange. The wavelet expansion is used to investigate the spatial structure of the heat fluxes for those scales that exhibit a −1 power law. Dimensionless statistical measures which provide spatial information at different scales are developed and applied to the sensible and latent heat flux measurements. Deviations from Gaussian statistics were observed at the turbulent production subrange. The large flux events (both positive and negative) in the heat flux signals contribute directly to the energy and spatial structure of the −1 power law. The wavelet transform is used to identify the scale of turbulent action directly responsible for the tails observed in the horizontal gradient probability density function of both heat fluxes.
Product Periodic Autoregressive Processes for Modeling Intermittent Monthly Streamflows
Authors: Chebaane Mohamed , Salas Jose D. , Boes Duane C.
Published: February 2004

Several attempts have been made in the past to model hydrological processes such as monthly streamflows in dry regions. One of the crucial problems in modeling this type of process is the handling of zero flows. A stochastic model is presented herein which enables the reproduction of the percentage of zero flows in each month, the monthly mean and variance, and the month‐to‐month correlation of the intermittent flows. The model considers the intermittent monthly flow process as a product of a periodic binary discrete process and a periodic continuous process. Both the discrete and the continuous processes are periodic first‐order autoregressive. Parameter estimation has been developed based on the method of moments, method of transition probability, and method of maximum likelihood.
Estimation of Discharge From Braided Glacial Rivers Using ERS 1 Synthetic Aperture Radar: First Results
Authors: Smith L. C. , Isacks B. L. , Forster R. R. , Bloom A. L. , Preuss I.
Published: February 2004

Using multitemporal ERS 1 synthetic aperture radar (SAR) satellite imagery and simultaneous ground measurements of streamflow, a strong correlation (R2 = 0.89) was found between water surface area and discharge for a braided glacial river in British Columbia, Canada. Satellite‐derived effective width (We) was found to vary with discharge (Q) as We = 27.5Q0.42, where We is defined as the total water surface area within a 10 km × 3 km control section, divided by the section length. This “area/discharge rating curve” yields instantaneous discharge estimates with a mean error of ±275 m3/s for ground‐measured flows that ranged from 242 to 6350 m3/s.
State Space Identification of Linear Deterministic Rainfall‐Runoff Models
Authors: Ramos José , Mallants Dirk , Feyen Jan
Published: February 2004

Rainfall‐runoff models of the black box type abound in the water resources literature (i.e., transfer function, autoregressive moving average (ARMA), ARMAX, state space, etc.). The corresponding system identification algorithms for such models are known to be numerically efficient and accurate, leading in most cases to good parsimonious representations of the rainfall‐runoff process. Alternatively, every model in transfer function, ARMA, and ARMAX form has an equivalent state space representation. However, state space models do not necessarily have simple system identification algorithms, unless the system matrices are restricted to some canonical form. Furthermore, state space system identification algorithms that work with the rainfall/runoff data directly (i.e., covariance free), require initial conditions and are inherently iterative and nonlinear. In this paper we present a state space system identification theory which overcomes these limitations. One advantage of such a theory is that the corresponding algorithms are highly robust to additive noise in the data. They are referred to as “subspace algorithms” due to their ability to separate the signal subspace from the noise subspace. The main advantages of the subspace algorithms are the automatic structure identification (system order), geometrical insights (notions of angle between subspaces), and the fact that they rely on robust numerical procedures (singular value decomposition). In this paper, two algorithms are presented. The first one is a two‐step procedure, where the impulse response (unit hydrograph ordinates for the single‐input, single‐output case) are computed from the input/output data by solving a constrained deconvolution problem. These impulse response ordinates are then used as inputs for identifying the system matrices by means of a Hankel‐based realization algorithm. The second approach uses the data directly to identify the system matrices, bypassing the deconvolution step. The algorithms are tested with real data from the Voer catchment in Belgium.
Desorption and the two Stages of Drying of Natural Tallgrass Prairie
Authors: Brutsaert Wilfried , Chen Daoyi
Published: February 2004

During the First ISLSCP (International Satellite Land Surface Climatology Project) Field experiment (FIFE) the first stage of drying (with the atmosphere controlling) in these natural grasslands occurred when the soil was moist with a volumetric soil moisture content SM >27% after rainfall. Both at the local scale and at the regional scale, once SM in the top 10 cm of the soil dropped below about 17% and its vertical gradient started to exceed about 1.1%/cm at 5 cm, the cumulative daily evaporation rate could be taken to be proportional to the square root of time, t½, which is not unlike a second stage of drying (with the soil moisture content controlling). Between these two drying stages, there was a transition period which lasted from several days to 2 weeks depending on the soil moisture conditions and on the season. The longer transition periods were observed under conditions of lower net radiation and of higher soil moisture content at depths in excess of, say, 50 cm. The present findings of gradual transitions from the first stage to a desorptive second stage of drying for a surface covered with grass (which can extract water from greater depths) are in contrast to earlier findings of relatively abrupt transitions for bare soil.
On Advective Transport in Fractal Permeability and velocity Fields
Authors: Neuman Shlomo P.
Published: February 2004

We consider advective transport in a steady state random velocity field with homogeneous increments. Such a field is self‐affine with a power law dyadic semivariogram γ(s) proportional to d2ω, where d is distance and ω is a Hurst coefficient. It is characterized by a fractal dimension D = E + 1 − ω, where E is topological dimension. As the mean and variance of such a field are undefined, we condition them on measurement at some point x0. We then introduce a tracer at another point y0 and invoke elementary theoretical considerations to demonstrate that its conditional mean dispersion is local at all times. Its conditional mean concentration and variance are given explicitly by well‐established expressions which, however, have not been previously recognized as being valid in fractal fields. Once the conditional mean travel distance s of the tracer becomes large compared to the distance between y0 and x0, the corresponding dispersion and dispersivity tensors grow in proportion to s1+2ω, where 0 < ω < 1. This supralinear rate of growth is consistent with that exhibited by apparent longitudinal dispersivities obtained by standard methods of interpretation from tracer behavior observed in a variety of geologic media under varied flow and transport regimes. Filtering out modes from the fractal velocity field with correlation scales larger than some s0 allows an asymptotic transport regime to develop when s ≫ s0. The corresponding asymptotic dispersivities grow in proportion to s02ω, when 0 < ω ≤ ½. This linear to sublinear rate of growth is consistent with that exhibited by apparent longitudinal dispersivities obtained from calibrated numerical models in a variety of media. A self‐affine natural log permeability field gives rise to a self‐affine velocity field, while s is sufficiently small to insure that the variance of the log permeabilities, which grows as a power of s, remains nominally less than one. An analysis of published apparent longitudinal dispersivity data in light of the above theoretical results supports my earlier conclusion that when one juxtaposes data from a large number of generally dissimilar geologic media from a variety of locales, one observes a tendency toward self‐affine behavior with a Hurst coefficient ω ≃ 0.25. At any given locale such media may or may not exhibit fractal behavior; if they do, ω may or may not be close to 0.25.
Discrete Time‐ and Length‐Averaged Solutions of the Advection‐Dispersion Equation
Authors: Leij Feike J. , Toride Nobuo
Published: February 2004

Solute concentrations obtained from displacement experiments in porous media frequently represent discrete values as a result of averaging over a finite sampling interval. For example, effluent curves are made up of time‐averaged concentrations while volume‐averaged concentrations are obtained from core samples. The discrete concentrations are often described by continuous solutions of macroscopic solute transport equations such as the advection‐dispersion equation (ADE). The continuous solution is often shifted to describe the average concentration. This paper compares continuous and time‐ or length‐averaged solutions of the one‐dimensional ADE cast in terms of flux‐averaged and resident concentrations. Expressions for the time‐ and length‐averaged concentrations are presented for solute applications described by Dirac delta or Heaviside functions (instantaneous and continuous releases of the solute) using four different combinations of solute application and detection modes. A temporal and spatial moment analysis was conducted to compare the traditional continuous description with the discrete time‐ or length‐averaged approach. Graphical and tabular data are presented to evaluate the accuracy of continuous solutions of the ADE for determining transport parameters. Although significant errors may occur for extreme cases with low dispersion coefficients and large sampling intervals, shifting the continuous solution by half the sampling interval generally yields results similar to those obtained with the time‐ or length‐averaged analysis. An advantage of averaged concentrations is that they permit greater flexibility to conduct experiments, since averaged concentrations provide an exact description of the data regardless of the sampling interval.
Influence of Choice Set Considerations in Modeling the Benefits From Improved Water Quality
Authors: Peters T. , Adamowicz W. L. , Boxall P. C.
Published: February 2004

Random utility models have become popular tools for the measurement of economic benefits from environmental quality changes. These models, however, require assumptions about the choice sets held by the individuals in the sample. Typically, researchers specify the choice set and assume that individuals are aware of all elements of this set. This paper compares the standard approach with an alternative that uses an estimate of each sampled individual's specific choice set. The latter approach produces model parameters and welfare estimates that are quite diflferent from those using standard assumptions.
Role of Near‐Bed Turbulence Structure in Bed Load Transport and Bed Form Mechanics
Authors: Nelson Jonathan M. , Shreve Ronald L. , McLean Stephen R. , Drake Thomas G.
Published: February 2004

The interactions between turbulence events and sediment motions during bed load transport were studied by means of laser‐Doppler velocimetry and high‐speed cinematography. Sweeps (u′ > 0, w′ < 0) which contribute positively to the mean bed shear stress, collectively move the majority of the sediment, primarily because they are extremely common. Outward interactions (u′ > 0 w′ > 0) which contribute negatively to the bed shear stress and are relatively rare, individually move as much sediment as sweeps of comparable magnitude and duration, however, and much more than bursts (u′ < 0, w′ > 0) and inward interactions (u′ < 0, w′ < 0). When the magnitude of the outward interactions increases relative to the other events, therefore, the sediment flux increases even though the bed shear stress decreases. Thus, although bed shear stress can be used to estimate bed load transport by flows with well‐developed boundary layers, in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity, it is not accurate for flows with developing boundary layers, such as those over sufficiently nonuniform topography or roughness, in which significant spatial variations in the magnitudes and durations of the sweeps, bursts, outward interactions, and inward interactions occur. These variations produce significant peaks in bed load transport downstream of separation points, thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms.
Ground Subsidence of Finite Amplitude Due to Pumping and Surface Loading
Authors: Ng Chiu‐On , Mei Chiang C.
Published: February 2004

Extending a finite‐strain theory of ground subsidence due to water pumping in a soil stratum, we study the joint effects of pumping and surface loads on ground subsidence. The soil system consists of a hard aquifer sandwiched by two soft and semipervious layers. Pumping is highly localized and is represented by a single well. On the uncovered part of the ground surface, the pore pressure is assumed to be maintained at a constant value. Beneath a building foundation, two types of boundary conditions are considered: one for a monolithic large foundation and one for numerous small and unconnected foundations. The total area covered by the building foundation is assumed to be large in horizontal dimensions compared with the soil thickness. In terms of the small permeability ratio, a perturbation analysis is performed to yield the leading order asymptotic equations. A numerical scheme based on finite differences is developed to solve the quasi‐three‐dimensional initial boundary value problem which is nonlinear in the soil behavior and the finite settlement. Numerical results are obtained for circular foundations. Space and time variations of the ground settlement are examined for unsealed or sealed foundations. It is found that a rigid and well‐sealed foundation can settle much faster than its surrounding soil. With pumping, the contact stress between the soil and the rigid foundation will be redistributed with time, resulting in increased stress near the outer edge of the foundation and diminished stress at the center. The physical mechanism is explained by examining the pore pressure and void ratio directly beneath the foundation.
Effect of Solute Size on Transport in Structured Porous Media
Authors: Hu Qinhong , Brusseau Mark L.
Published: February 2004

The purpose of this work was to investigate the effect of solute size on transport in structured porous media. Miscible displacement experiments were performed with tracers of different sizes (i.e., tritiated water (3H2O), pentafluorobenzoate (PFBA), 2,4‐dichlorophenoxyacetic acid (2,4‐D), and hydroxypropyl‐β‐cyclodextrin (HPCD)) in aggregated, stratified, and macroporous media. The breakthrough curves exhibited both early breakthrough and tailing, indicative of nonideal transport in these structured media. Comparison of breakthrough curves revealed that the extent of nonideality (e.g., tailing) was HPCD > PFBA, 2,4‐D > 3H2O. This behavior is consistent with the impact of solute size on the relative degree of “nonequilibrium” experienced by solutes whose transport is constrained by diffusive mass transfer. The capability of the first‐order, dual‐porosity mobile‐immobile model to represent solute transport in these structured systems was evaluated by comparing independently determined values of the input parameters to values obtained by curve fitting of the experimental measurements. The calculated and optimized values compared quite well for the aggregated and stratified media, but not for the macroporous media. xperiments performed with tracers of different size are useful for characterizing the nature of the porous medium through which transport is occurring.
Steady State Groundwater Flow Across Idealized Faults
Authors: Haneberg William C.
Published: February 2004

The effects of vertical faults on steady state groundwater flow are simulated using simple analytical solutions for horizontal flow across three domains that are linked by requiring continuity of head and flux. Hydraulic properties are vertically averaged, so that changes in transmissivity can be used to account for changes in aquifer thickness, juxtaposition of different aquifers across the fault, or some combination of the two. Vertical flow along the fault can be included through the use of a source‐sink term. Different combinations of transmissivity contrast and recharge/discharge produce distinct head profiles, which can be used to qualitatively infer the nature of real hydrogeologic systems. Observed hydraulic gradient ratios can also be used to calculate transmissivity ratios, and if information about fault zone thickness and an independent estimate for one of the three transmissivities is known, the magnitudes of the remaining two transmissivities can be calculated. Use of the model to interpret real flow systems is demonstrated using an example from the Albuquerque Basin, New Mexico.
Effects of Subbasin Size on Topographic Characteristics and Simulated Flow Paths in Sleepers River Watershed, Vermont
Authors: Wolock David M.
Published: February 2004

The effects of subbasin size on topographic characteristics and simulated flow paths were determined for the 111.5‐km2 Sleepers River Research Watershed in Vermont using the watershed model TOPMODEL. Topography is parameterized in TOPMODEL as the spatial and statistical distribution of the index ln (a/tan B), where In is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the slope gradient. The mean, variance, and skew of the ln (a/tan B) distribution were computed for several sets of nested subbasins (0.05 to 111.5 km2)) along streams in the watershed and used as input to TOPMODEL. In general, the statistics of the ln (a/tan B) distribution and the simulated percentage of overland flow in total streamflow increased rapidly for some nested subbasins and decreased rapidly for others as subbasin size increased from 0.05 to 1 km2, generally increased up to a subbasin size of 5 km2, and remained relatively constant at a subbasin size greater than 5 km2. Differences in simulated flow paths among subbasins of all sizes (0.05 to 111.5 km2) were caused by differences in the statistics of the ln (a/tan B) distribution, not by differences in the explicit spatial arrangement of ln (a/tan B) values within the subbasins. Analysis of streamflow chemistry data from the Neversink River watershed in southeastern New York supports the hypothesis that subbasin size affects flow‐path characteristics.
Convergent Radial Dispersion in a Double‐Porosity Aquifer with Fracture Skin: Analytical Solution and Application to a Field Experiment in Fractured Chalk
Authors: Moench Allen F.
Published: February 2004

An exact Laplace transform solution to the problem of dispersion, advection, and adsorption of a tracer due to its injection in a steady, horizontal, radially convergent flow field in a densely fractured, porous formation (double‐porosity aquifer) is presented. The porous blocks were assumed to be covered with a layer of material (fracture skin) of negligible volume and storage capacity that provides a resistance to diffusion in the rock matrix. Longitudinal dispersion, advection, and adsorption dominate transport of the tracer in the fractures, and diffusion and adsorption dominate movement of the tracer in the blocks. Dimensionless breakthrough curves are used to illustrate the influence of various aquifer and tracer properties. In support of the model a detailed analysis is performed of a published multitracer field test, conducted in a layer of densely fractured chalk in Béthune, France. Of the three tracers analyzed, two are nonsorptive but have widely different free water diffusion coefficients, and one is slightly sorptive. Analysis of measured breakthrough curves, matched by trial and error to theoretical responses, reveals that by allowing for fracture skin on block surfaces, one can obtain (1) pure‐advection arrival times that are independent of the tracer used, (2) values of mass recovery consistent with measured values, and (3) relative values of effective diffusion coefficients that are consistent with known free water diffusion coefficients for the separate tracers. Reasonable estimates of longitudinal dispersivity and fracture porosity are also obtained.
Steady Infiltration Into a Two‐Layered Soil from a Circular Source
Authors: Shan Chao , Stephens Daniel B.
Published: February 2004

Infiltration of water from a surface source is highly relevant to a wide range of problems in soil science, hydrology, and engineering. This article presents analytical solutions to steady infiltration in layered soils caused by a constant influent through a circular area (such as a pond or a ring infiltrometer) at the land surface. An exponential relationship between the unsaturated hydraulic conductivity and the pressure head, K = K0 exp (αψ), is assumed for both layers, and α is assumed to be the same for both layers. The solutions for two kinds of lower boundary conditions are discussed. The first condition assumes a semiinfinite system with very dry soil at an infinite depth; the second condition considers an impermeable boundary. Solutions are given for single layer and two‐layer systems. As a special case of the two‐layer solution, the single layer solution agrees very well with previous solutions when the boundary conditions are approximately the same. The effects of soil properties and layer thickness are also discussed.
Microbial Clogging of Saturated Soils and Aquifer Materials: Evaluation of Mathematical Models
Authors: Vandevivere Philippe , Baveye Philippe , de Lozada Diego Sanchez , DeLeo Paul
Published: February 2004

Bacterial reductions of the saturated hydraulic conductivity of natural porous media appear to be caused by a wide range of mechanisms, few of which have been carefully studied. Nevertheless, a number of mathematical models have been developed in recent years to describe the microbial clogging process, based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. In the present study, two independent sets of experimental data available in the literature are used to test the existing bioclogging models. To broaden the scope of the assessment, an additional model, initially developed to describe the deep filtration of suspended colloids, is also included in the comparisons. Analysis of the experimental data reveals a clear relationship between the texture of a porous medium and the ability of a given level of biomass to reduce its saturated hydraulic conductivity; at equal biomass, clogging is much more pronounced in fine‐textured materials than in coarse‐textured ones. In addition, the results of the model comparisons suggest that none of the existing models can predict satisfactorily the saturated hydraulic conductivity reductions observed in fine sands, whereas they fare somewhat better in coarser materials. It is argued that this inadequacy of existing models is due to the continuous biofilm assumption on which they are founded. Indeed, a simplistic model that assumes the biomass to be distributed as plugs instead of as continuous biofilms produces quantitatively much improved predictions of the saturated hydraulic conductivity reductions. Reference is made to the consequences of this observation in terms of future research.
Combined Use of Groundwater Dating, Chemical, and Isotopic Analyses to Resolve the History and Fate of Nitrate Contamination in Two Agricultural Watersheds, Atlantic Coastal Plain, Maryland
Authors: Böhlke J. K. , Denver J. M.
Published: February 2004

The history and fate of groundwater nitrate (NO3−) contamination were compared in 2 small adjacent agricultural watersheds in the Atlantic coastal plain by combined use of chronologic (CCl2F2, 3H), chemical (dissolved solids, gases), and isotopic (δ15N,δ13C, δ34S) analyses of recharging groundwaters, discharging groundwaters, and surface waters. The results demonstrate the interactive effects of changing agricultural practices, groundwater residence times, and local geologic features on the transfer of NO3− through local flow systems. Recharge dates of groundwaters taken in 1990–1992 from the surficial aquifer in the Chesterville Branch and Morgan Creek watersheds near Locust Grove, Maryland, ranged from pre‐1940 to the late 1980’s. When corrected for localized denitrification by use of dissolved gas concentrations, the dated waters provide a 40‐year record of the recharge rate of NO3−, which increased in both watersheds by a factor of 3–6, most rapidly in the 1970's. The increase in groundwater NO3− over time was approximately proportional to the documented increase in regional N fertilizer use, and could be accounted for by oxidation and leaching of about 20–35% of the fertilizer N. Groundwaters discharging upward beneath streams in both watersheds had measured recharge dates from pre‐1940 to 1975, while chemical data for second‐order reaches of the streams indicated average groundwater residence times in the order of 20+ years. At the time of the study, NO3− discharge rates were less than NO3− recharge rates for at least two reasons: (1) discharge of relatively old waters with low initial NO3− concentrations, and (2) local denitrification. In the Chesterville Branch watershed, groundwaters remained oxic throughout much of the surficial aquifer and discharged relatively unaltered to the stream, which had a relatively high NO3− concentration (9–10 mg/L as N). In the Morgan Creek watershed, groundwaters were largely reduced and denitrified before discharging to the stream, which had a relatively low NO3− concentration (2–3 mg/L as N). Chemical and isotopic data indicate that quantitative denitrification occurred within buried calcareous glauconitic marine sediments that are present at relatively shallow depths beneath the Morgan Creek watershed. NO3− removal by forests, wetlands, and shallow organic‐rich soils in near‐stream environments was largely avoided by groundwaters that followed relatively deep flow paths before converging and discharging rapidly upward to the streams.
Prediction of Ground Surface Temperature and Soil Moisture Content by the Force‐Restore Method
Authors: Hu Zhenglin , Islam Shafiqul
Published: February 2004

The parsimony and computational efficiency offered by the force‐restore approximation of the diffusion equation have motivated its widespread application in modeling ground surface temperature. Different assumptions regarding the definition of ground surface temperature have resulted in different versions of the force‐restore method. Here, four existing versions of the force‐restore method for ground surface temperature are compared and contrasted. An improved version of the force‐restore method is developed by minimizing the error produced by the force‐restore approximation of the heat diffusion equation. The proposed model performs well for the physically realistic ranges of scaled soil thickness and reproduces amplitude and phase that are quite close to the exact solution of the diffusion equation under a single periodic forcing. It is shown that neglect of higher harmonics can produce appreciable errors in the force‐restore method if the upper soil thickness is less than the damping depth of the diurnal forcing. The success of the force‐restore approximation in modeling ground surface temperature has prompted its application in the prediction of soil moisture content. However, extension of the force‐restore method for the prediction of soil moisture content is not straightforward. There are two major difficulties in modeling soil moisture content by the force‐restore method. One is the situation‐dependent relative importance of the suction term and gravity term in the Richards equation and the other is the choice of state variable, moisture content versus suction head, in the solution of the force‐restore method for soil moisture prediction. Both of these could produce appreciable errors in the force‐restore treatment of soil moisture evolution.
Analytical Solutions for Determining Vertical Air Permeability in Unsaturated Soils
Authors: Shan Chao
Published: February 2004

The study of flow and transport in unsaturated soils requires a good knowledge of the soil properties. When advective gas phase transport is of interest, air permeability is the key parameter of soil characterization. Although an air permeability test can easily be done in a laboratory, the reported values are frequently found to be not very representative of the air permeability in situ. Air permeability determined by a field test is useful both in the study of gas flow through unsaturated soils and in the prediction of the saturated hydraulic conductivity. This paper presents analytical solutions for determining the vertical air permeability of unsaturated soils using observed air pressure data at the land surface and at depth in the vadose zone. One solution is developed for a single‐layer soil, and the other is developed for layered soils. Example calculations using field data have demonstrated the significant potential of the solution applications in field tests.
A Scenario‐Based Framework for Multicriteria Decision Analysis in Water Resources Planning
Authors: Stewart Theodor J. , Scott Leanne
Published: February 2004

This paper describes the evolution of a general group decision‐making or negotiation support procedure for water resources planning at a strategic level (such as regional development planning), based on principles of multicriteria decision analysis. A number of difficulties have been encountered in applying many of the standard multiple‐criteria decision‐making (MCDM) tools and techniques directly in this context. The paper introduces a broader framework in which these difficulties may be overcome, so that MCDM tools can be used effectively for evaluating strategic planning options. This framework is based on direct evaluation of sequences of “policy scenarios,” which are neither a full set of possible planning “alternatives,” nor fully specified down to the finest level of detail. The overall procedure and some experiences in applying it within the context of regional water resource planning in South Africa are discussed.
Observation and Three‐Dimensional Simulation of Chloride Plumes in a Sandy Aquifer Under Forced‐Gradient Conditions
Authors: Jim Yeh T.‐C. , Mas‐Pla J. , Williams T. M. , McCarthy J. F.
Published: February 2004

Two‐well, forced‐gradient tracer experiments over a distance of 5 m were carried out in a coastal sandy aquifer at Georgetown, South Carolina. The evolution of three‐dimensional chloride plumes during two tracer experiments was observed. A three‐dimensional finite element model for flow and transport was used with extensive hydraulic conductivity data obtained from slug tests to assess our ability to predict solute transport in the aquifer. Results showed that our predictive ability is limited to the bulk behavior of the plumes, which is mainly controlled by some “significant” heterogeneities. In addition, hydraulic conductivity values estimated by the Hvorslev method for analysis of slug tests best represented the hydraulic conductivity distribution of the sandy aquifer. Finally, the results of a long‐term experiment illustrate the importance of temporal variability in boundary conditions in the prediction of solute transport in aquifers.
Multiscaling and Skew Separation in Regional Floods
Authors: Dawdy David R. , Gupta Vijay K.
Published: February 2004

Matalas et al. (1975) (MSW) found that the simulated values of flood peaks in a region using various common forms of flood frequency distributions did not reproduce the empirical skew statistics for 14 different regions covering the conterminous United States. Specifically, the field data always demonstrated a higher value of standard deviation of skew for a given sample value of mean skew than did the simulations. MSW termed this difference “separation” and further showed that it could not be explained either by autocorrelation of flood peaks or as a small sample property. In this paper, we discuss an explanation of this property using the recently developing scaling theories of regional floods. It is shown that in a homogeneous region, recently defined by us, separation would result from the multiscaling structure of flood peaks. Separation would not occur if floods obey simple scaling, nor would separation necessarily occur with heterogeneity or mixing among different homogeneous regions. Mixing must be of a particular kind in order to cause separation. The use of normalized flood frequencies having mean of zero and variance of 1 in the simulations carried out by MSW is shown to be consistent with the assumption of index flood or simple scaling but not multiscaling. In the 14 “megaregions” analyzed by MSW, mixing among subregions within each megaregion may add to the magnitude of separation. The separation in 14 regions is physically interpreted based on different physical mechanisms that have been recently hypothesized by us to be responsible for the presence of simple scaling or multiscaling in floods.
Modeling Hexavalent Chromium Reduction in Groundwater in Field‐Scale Transport and Laboratory Batch Experiments
Authors: Friedly J. C. , Davis J. A. , Kent D. B.
Published: February 2004

A plausible and consistent model is developed to obtain a quantitative description of the gradual disappearance of hexavalent chromium (Cr(VI)) from groundwater in a small‐scale field tracer test and in batch kinetic experiments using aquifer sediments under similar chemical conditions. The data exhibit three distinct timescales. Fast reduction occurs in well‐stirred batch reactors in times much less than 1 hour and is followed by slow reduction over a timescale of the order of 2 days. In the field, reduction occurs on a timescale of the order of 8 days. The model is based on the following hypotheses. The chemical reduction reaction occurs very fast, and the longer timescales are caused by diffusion resistance. Diffusion into the secondary porosity of grains causes the apparent slow reduction rate in batch experiments. In the model of the field experiments, the reducing agent, heavy Fe(II)‐bearing minerals, is heterogeneously distributed in thin strata located between larger nonreducing sand lenses that comprise the bulk of the aquifer solids. It is found that reducing strata of the order of centimeters thick are sufficient to contribute enough diffusion resistance to cause the observed longest timescale in the field. A one‐dimensional advection/dispersion model is formulated that describes the major experimental trends. Diffusion rates are estimated in terms of an elementary physical picture of flow through a stratified medium containing identically sized spherical grains. Both reduction and sorption reactions are included. Batch simulation results are sensitive to the fraction of reductant located at or near the surface of grains, which controls the amount of rapid reduction, and the secondary porosity, which controls the rate of slow reduction observed in batch experiments. Results of Cr(VI) transport simulations are sensitive to the thickness and relative size of the reducing stratum. Transport simulation results suggest that nearly all of the reductant must be located in the reducing stratum. Within this context and as long as there is adequate reductive capacity present, the transport simulation results are insensitive to the parameters important for the batch simulations. The results illustrate how a combination of field measurements and batch laboratory studies can be used to improve predictive modeling of contaminant transport.
Sampling Design Methods For Groundwater Modeling Under Uncertainty
Authors: Wagner Brian J.
Published: February 2004

A sampling network design model is presented that evaluates the trade‐off between the varying costs of different types of data and the contribution of those data to improving model reliability. The methodology couples parameter‐estimate and model‐prediction uncertainty analyses with optimization to identify the mix of hydrogeologic information (e.g., head, concentration, and/or hydraulic conductivity measurement locations) that will minimize model prediction uncertainty for a given data collection budget. Two alternative optimization algorithms are presented and compared: a branch‐and‐bound algorithm and a genetic algorithm. A series of synthetic examples are presented to demonstrate the adaptability of the methodology to different sampling scenarios. The examples reveal two important properties of this network design problem. First, model‐parameter and model‐prediction uncertainty analyses are important components of the network design methodology because they provide a natural framework for evaluating the cost/information trade‐off for different types of data and different sampling network designs. Second, the genetic algorithm can identify near‐optimal solutions for a small fraction of the computational effort needed to determine the globally optimal solutions of the branch‐and‐bound algorithm.
A finite element method for simulating fault block motion and hydrothermal fluid flow within rifting basins
Authors: Wieck J. , Person M. , Strayer L.
Published: February 2004

We study the influence of vertical fault block motion on the hydrodynamics of continental rift systems in this paper using finite element analysis. Numerical solutions to governing groundwater flow and heat transport equations within independently moving fault blocks are obtained using triangular elements. Hybrid, four‐node “slip elements” developed for this study are employed along fault surfaces; standard three‐node elements are used everywhere else. The model is used in a sensitivity study to assess the effects of permeability variations, subsidence/erosion rates, and water table configuration on basin hydrodynamics. Idealized simulations of rifting over a 1.5 m.y. period were constructed using two (horst and graben) fault blocks that are vertically offset by 4.5 km during the simulation period. Numerical results indicate that significant interaction between different fluid flow–impelling mechanisms (compaction‐, topography‐, and density‐driven flow) occurs over a permeability range of 10−16.5 to 10−14.5 m2. Many of the flow fields, especially those in high‐permeability (10−14.5 m2) horst and graben blocks, produce areas of high and low heat flow relative to conductive conditions. Models which incorporate alternating layers of permeable and less permeable layers indicate that the truncation/reconnection of aquifers can induce transient thermal and hydrologie behavior. The sensitivity results suggest a plausible mechanism for transient fluid flow on geologic timescales (105 to 107 years) within faulted basins and may help to explain the occurrence of some banded ore deposits or multiple episodes of sediment diagenesis preserved within the sedimentary rock record.