bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic setting of the Sandia pluton: An orogenic 1.4 Ga granite in New Mexico
Authors: Kirby Eric , Karlstrom Karl E. , Andronicos Chris L. , Dallmeyer R. David
Published: February 2004

Structural studies of the circa 1.42 Ga Sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the “anorogenic” label associated with this and other 1.4 Ga granites in the southwestern United States. The SE margin of the pluton is a 1‐ to 2‐km‐wide NW dipping ductile shear zone. Field and microstructural observations (melt‐filled shear bands, high‐temperature dynamic recrystallization of K‐feldspar megacrysts, and crosscutting pegmatite dikes) indicate that top‐to‐the‐NW (normal) movement in the shear zone took place in the presence of melt. Subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top‐to‐the‐NW shear sense, suggesting that over large regions of the pluton, magmatic flow mimicked solid‐state strain. In the northern aureole, contact metamorphic aluminosilicate porphyroblasts grew during the formation of a NE striking crenulation cleavage (S3) and related folds of late‐stage pegmatite dikes. These features document the synchroneity of magma emplacement, shortening, and metamorphism and indicate that the Sandia pluton is syntectonic, not anorogenic. We interpret the kinematic consistency of structural elements from the base of the pluton, the interior of the pluton, and the northern aureole to reflect a regional (larger than the pluton) strain field and suggest that the “orogeny” recorded in and around the Sandia pluton involved a three dimensional strain field with subhorizontal extension (N–S) and contraction (E–W) directions. N–S extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high‐strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole. East to SE shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated S3 crenulation cleavage, and east to SE shortening (or least extension) directions in the pluton proper are documented by the intersections of orthogonal dikes. Thus emplacement of the Sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin, rather than local strains generated by emplacement.
Fluid inclusion constraints on the kinematics of footwall uplift beneath the Brenner Line normal fault, eastern Alps
Authors: Selverstone Jane , Axen Gary J. , Bartley John M.
Published: February 2004

Dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear. Each class of model predicts a different incremental strain history that should be recorded in the footwall. In the Tauern Window (eastern Alps), postmylonitic structures in the footwall of the Brenner Line normal shear zone predominantly consist of closely spaced, steep, west down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear. In contrast, formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed. However, both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion‐lined microfaults with the macroscopic west down and east down faults, obtaining density data for the inclusions, and correlating these data with previously obtained geochronologic data. The results indicate that the west down structures formed at depths of 10–20 km and temperatures >450°C in the mid to late Oligocene and that the east down structures formed at 2‐ to 10‐km depth and temperatures of 300 ± 50°C in the mid‐Miocene. These data support the hypothesis that a "rolling hinge" was present in the footwall of the Brenner Line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear. The depths at which west down and east down faulting occurred, coupled with the angle of dip of the Brenner Line, yield a minimum lateral displacement on the fault of 15–26 km. Approximately coeval ductile shearing and brittle faulting at depths of 15–20 km and temperatures in excess of 400°C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation.
Paleomagnetism and 40Ar/39Ar geochronology of gabbro sills at Mariscal Mountain anticline, southern Big Bend National Park, Texas: Implications for the timing of Laramide tectonism and vertical axis rotations in the southern Cordilleran orogenic belt
Authors: Harlan S. S. , Geissman J. W. , Henry C. D. , Onstott T. C.
Published: February 2004

Mariscal Mountain anticline is a major Laramide asymmetric fold located along the frontal margin of the Cordilleran orogenic belt in the Trans‐Pecos region of southwest Texas. Exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement, based largely on their arcuate outcrop geometry, has been interpreted to predate folding. Paleomagnetic fold tests on samples from large (≥ 30 m thick) and thin (≤ l m thick) gabbro sills are negative at the 99% confidence level. Baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro, whereas sample directions from a site well removed from thermal effects of the gabbro are scattered. Results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization. The negative fold test indicates that sill intrusion at Mariscal Mountain anticline postdated Laramide folding. Two 40Ar/39Ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37.0 ± 1.3 Ma (2σ). This date is similar to those from rocks of similar composition elsewhere in Trans‐Pecos Texas, but significantly younger than whole rock and pyroxene K‐Ar dates of Late Cretaceous and early Eocene age commonly cited for the Mariscal Mountain gabbro. The paleomagnetic and geochronologic data indicate that Laramide deformation had clearly ceased by late Eocene/early Oligocene time, consistent with absence of contractional deformation in middle Eocene to lower Oligocene volcanic and sedimentary rocks elsewhere in the region. Although paleomagnetic data from the gabbro sills may not adequately average secular variation, the in situ group mean direction (D = 348.6°, I = 50.9°, k = 65.1, α95 = 4.3°, n = 18 sites) is indistinguishable from expected mid‐Tertiary reference directions. This indicates no evidence of significant vertical axis rotation in the Mariscal Mountain area. Analysis of paleomagnetic data from studies elsewhere in the Big Bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations. Thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with Basin and Range extension is equivocal.
An 40Ar/39Ar thermochronology of the Ofoten‐Troms region: Implications for terrane amalgamation and extensional collapse of the northern Scandinavian Caledonides
Authors: Coker Janet E. , Steltenpohl Mark G. , Andresen Arild , Kunk Michael J.
Published: February 2004

Fifteen 40Ar/39Ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the Ofoten nappe stack of northern Norway. Eight cooling ages on hornblende range from 425 to 394 Ma and seven muscovite ages, from the same or nearby outcrops as the hornblendes, range from 400 to 373 Ma. These data are compared with 40Ar/39Ar ages from over a large part of the northern Caledonides to evaluate regional mineral cooling patterns. Results indicate that (1) Scandian (Silurian‐Devonian) metamorphism was predominant; (2) most of the nappes investigated contain some vestige of pre‐Scandian tectonism and/or metamorphism; (3) hornblende and muscovite cooling ages are progressively younger to the west and south, which suggests a hinged‐to‐the‐east mineral cooling pattern; and (4) a late, out‐of‐sequence thrust is the only disruption of this cooling pattern. Synmetamorphic amalgamation of the nappes resulted from Scandian A type subduction. The hinged‐to‐the‐east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping, crustal‐scale extensional fault, located somewhere west of the present Norwegian coast, during late synorogenic gravitational collapse. The late out‐of‐sequence fault formed contemporaneously with uplift in the hinterland, implying a kinematic and temporal connection with east directed contractional faulting in the foreland.
Flow and solute transport through a levee separating fluids with different densities
Authors: Brooker Andrew M. H. , Townley Lloyd R.
Published: February 2004

Steady seepage through a levee or a dam separating reservoirs with different levels and densities is shown to result in five possible flow configurations. If densities are equal, the problem reduces to the classical problem of flow through a dam. If the lower reservoir is less dense, a stationary wedge may or may not form inside the levee near the downgradient reservoir level. If the lower reservoir is more dense, a wedge forms near the toe of the levee, as in the classical problem of saltwater intrusion. Another possibility is that the dense wedge can connect the two reservoirs, thus allowing bidirectional flow. Conditions are derived which define when these different flow configurations occur. For the case of a rectangular dam with vertical walls a closed form solution is obtained for the net mass flux between the reservoirs. Net volumetric flux is determined analytically for all flow configurations except the case of bidirectional flow. The expression for volumetric flux is a generalization of a result based on the Dupuit assumption for the case of constant density. A range of parameters is identified in which net volumetric flux is in one direction and net mass flux in the other. A numerical model based on the boundary integral equation method allows calculation of net volumetric flux for the case of bidirectional flow and also allows simulation of more complex levee geometries. The solution has direct application to levees separating ponds in solar salt production, to the causeway across the Great Salt Lake in Utah, and to the role of exchange flows in the genesis of evaporites in nearshore lagoons.
Flood frequency analysis with systematic and historical or paleoflood data based on the two‐parameter general extreme value models
Authors: Frances Felix , Salas Jose D. , Boes Duane C.
Published: February 2004

Historical and paleoflood data have become an important source of information for flood frequency analysis. A number of studies have been proposed in the literature regarding the value of historical and paleoflood information for estimating flood quantiles. These studies have been generally based on computer simulation experiments. In this paper the value of using systematic and historical/paleoflood data relative to using systematic records alone is examined analytically by comparing the asymptotic variances of flood quantiles assuming a two‐parameter general extreme value marginal distribution, type 1 and type 2 censored data, and maximum likelihood estimation method. The results of this study indicate that the value of historical and paleoflood data for estimating flood quantiles can be small or large depending on only three factors: the relative magnitudes of the length of the systematic record (N) and the length of the historical period (M); the return period (T) of the flood quantile of interest; and the return period (H) of the threshold level of perception. For instance, for N = 50, M = 50 and T = 500, the statistical gain for type 2 censoring becomes significantly larger than for type 1 censoring as H becomes greater than 100 years. In addition, computer experiments have shown that the results regarding the statistical gain based on asymptotic considerations are valid for the usual sample sizes.
Dating of shallow groundwater: Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr
Authors: Ekwurzel Brenda , Schlosser Peter , Smethie William M. , Plummer L. Niel , Busenberg Eurybiades , Michel Robert L. , Weppernig Ralf , Stute Martin
Published: February 2004

This paper describes a direct comparison of apparent ages derived from 3H/3He, chlorofluorocarbons (CCl3F and CCl2F2), and 85Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved‐gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption‐desorption processes at this location.
Interpretation of isotopic data in groundwater‐rock systems: Model development and application to Sr isotope data from Yucca Mountain
Authors: Johnson Thomas M. , DePaolo Donald J.
Published: February 2004

A model enabling extraction of hydrologic information from spatial and temporal patterns in measurements of isotope ratios in water‐rock systems is presented. The model describes the evolution of isotope ratios in response to solute transport and water‐rock interaction. In advective systems, a single dimensionless parameter (a Damköhler number, ND) dominates in determining the distance over which isotopic equilibrium between the water and rock is approached. Some isotope ratios act as conservative tracers (ND ≪ 1), while others reflect only interaction with the local host rock (ND ≫ 1). If ND is close to one (i.e., the distance for equilibration is close to the length scale of observation), isotope ratio measurements can be used to determine ND, which in turn may yield information concerning reaction rates, or spatial variations in water velocity. Zones of high velocity (e.g., as a result of greater fracture density), or less reactive zones, may be identified through observation of their lower ND values. The model is applied to paleohydrologic interpretations of Sr isotope data from calcite fracture fillings in drill cores from Yucca Mountain, Nevada (Marshall et al., 1992). The results agree with other studies suggesting “fast path” transport in the unsaturated zone. Also, we find that the data do not give a conclusive indication of paleowater table elevation because of the effects of water‐rock interaction.
Unsaturated zone tritium and chlorine 36 profiles from southern Australia: Their use as tracers of soil water movement
Authors: Cook P. G. , Jolly I. D. , Leaney F. W. , Walker G. R. , Allan G. L. , Fifield L. K. , Allison G. B.
Published: February 2004

In this paper we present seven unsaturated zone profiles of 36Cl and three 3H profiles from southern Australia. All profiles show single peaks corresponding to high radionuclide fallout from nuclear testing in the 1950s and 1960s. The profiles are used to estimate rates of water movement leading to recharge of the groundwater. Among these profiles is the first profile on which high concentrations of 36Cl have been found below a 2‐m depth. In this profile, 3H and 36Cl peaks coincide. In six of the seven profiles, total 36Cl fallout was found to be between 1.2 and 2.4 × 1012 atoms m−2, and is of similar magnitude to that found at similar latitudes in the northern hemisphere. Comparisons of soil water fluxes estimated with 3H, 36Cl, and chloride are briefly discussed.
Modeling of chemically reactive groundwater transport
Authors: Zysset Andreas , Stauffer Fritz , Dracos Themistocles
Published: February 2004

A complete reactive groundwater transport model must account for both chemical and transport processes. For the chemical processes one has to decide whether to formulate them kinetically or to assume a local chemical equilibrium state. This decision in the chemical model part determines the mathematical structure of the overall model. In a kinetic formulation, the linear partial differential equations of the transport have to be coupled with a nonlinear system of ordinary differential equations describing the kinetic development, whereas in an equilibrium formulation, the equations of the transport are coupled to a nonlinear system of algebraic equations describing the equilibrium state. Basically, two kinds of methods for solving reactive transport systems may be distinguished, namely, one‐step methods which simultaneously solve the transport and the chemical model parts and two‐step methods which solve these model parts separately. We here present a sequential two‐step method for kinetic transport models and an iterative two‐step method for equilibrium transport models. We conduct a timescale analysis to check whether the error of the sequential two‐step method is tolerable and whether a given kinetic transport system can be reduced to an equilibrium one. The numerical methods and the timescale analysis are applied to two test cases. Zysset et al. (1994) present a further application of the kinetic transport model to laboratory column experiments governed by biodegradation.
Gravel saltation: 1. Experiments
Authors: Niño Yarko , García Marcelo , Ayala Luis
Published: February 2004

Laboratory observations of the saltation of natural gravel particles in a steep, movable‐bed channel are reported. Standard video‐imaging techniques were used to measure and analyze particle motion. The saltation of gravel particles is described in terms of statistical properties of particle trajectories, such as mean values and standard deviations of saltation length, height, and streamwise particle velocity. The results obtained are compared with available empirical data, and a general good agreement is obtained. Particle collision with the bed is also analyzed, and friction and restitution coefficients are estimated from the experimental observations. Nonvanishing values of the restitution coefficient and values of the friction coefficient lower than unity are obtained, which contradicts previous discussions on the subject. The dynamic friction coefficient associated with particle motion is also estimated from the experimental data, and a mean value of 0.3 is obtained, which is about half of that proposed by Bagnold (1973) but similar to those found in previous experiments.
Road surface drainage, channel initiation, and slope instability
Authors: Montgomery David R.
Published: February 2004

Field surveys of road drainage concentration at three sites in the western United States are used to test simple models relating channel initiation and shallow landsliding to ground slope and contributing area thresholds. The form of boundaries between data for locations where road drainage concentration is associated with either shallow landsliding, channel initiation by overland flow, or no observable geomorphic effect is consistent with theoretically derived drainage area‐slope relations. Comparison of survey data with results of previous studies in these areas indicates that the drainage area required to support a channel head is smaller for road‐related runoff than for undisturbed slopes. Contrary to current land management paradigms in the Pacific Northwest, drainage concentration from ridgetop roads may cause both landsliding and integration of the channel and road networks. Road drainage concentration increases the effective length of the channel network and strongly influences the distribution of erosional processes in each of the study areas. The approach of using field reconnaissance to establish thresholds for erosion associated with road drainage provides a useful method to define regional criteria for road design that should reduce impacts on downstream channel systems.
Estimation of seasonal flood risk using a two‐stage transformation
Authors: Atan Ismail Bin , Metcalfe A. V.
Published: February 2004

Hydrological time series are often asymmetric in time, insomuch as rises are more rapid than recessions, as well as having highly skewed marginal distributions. A two‐stage transformation is proposed for deseasonalized series. Rises are stretched, and recessions are squashed until the series is symmetric over time. An autoregressive moving average (ARMA) model is then fitted to the natural logarithms of this new series. The residuals from the ARMA model are represented by a double mixture of Weibull and exponential distributions. The method is demonstrated with 24 years of daily flows from the River Cherwell in the south of England, and a 40‐year record from the upper reaches of the Thames. Seasonal estimates of flood risk are given, and these can be conditioned on catchment wetness at the time of prediction.
A three‐dimensional boundary element method for steady unsaturated quasi‐linear flow in porous media
Authors: Subia S. R. , Ingber M. S. , Martinez M. J.
Published: February 2004

A three‐dimension boundary element method is developed to study unsaturated groundwater flows in heterogeneous porous media. The method is based on the Kirchhoff transformation of variables which transforms the nonlinear Richards equation into the Fokker‐Planck equation. Several benchmark problems are considered involving cavity infiltration and exclusion to demonstrate the reliability of the boundary element formulation. As an example application problem, the effective conductivity of a porous medium containing randomly positioned impermeable spherical inclusions is considered.
Cosmogenic 36Cl accumulation in unstable landforms: 1. Effects of the thermal neutron distribution
Authors: Liu Beiling , Phillips Fred M. , Fabryka‐Martin June T. , Fowler Malcolm M. , Stone William D.
Published: February 2004

Cosmogenic nuclides produced in situ within minerals at the surface of the Earth are proving to be an effective means of assessing geomorphic histories. The use of multiple cosmogenic nuclides permits both exposure times and erosion rates to be determined. However, if two nuclides are produced only by spallation reactions, the systematic differences in their accumulation rates depend only on the differences in their production rates and half‐lives. The relatively small differences that result require a high degree of analytical precision to yield useful results. In contrast to other spallogenic nuclides, 36Cl is also produced by low‐energy neutron absorption, which creates a different pattern of production as a function of depth. We have measured the thermal flux with depth in a concrete block using 3He‐filled neutron detectors. The measured thermal neutron profile agrees well with predictions from a simple diffusion‐ based thermal neutron distribution model. Calculations of 36Cl production using the model suggest that the use of 36Cl along with a purely spallogenic nuclide to determine erosion rates and exposure times should be less sensitive to analytical error than are determinations from two purely spallogenic nuclides.
Monitoring network design to provide initial detection of groundwater contamination
Authors: Meyer Philip D. , Valocchi Albert J. , Eheart J. Wayland
Published: February 2004

The design of a monitoring network to provide initial detection of groundwater contamination at a waste disposal facility is complicated by uncertainty in both the characterization of the subsurface and the nature of the contaminant source. In addition, monitoring network design requires the resolution of multiple conflicting objectives. A method is presented that incorporates system uncertainty in monitoring network design and provides network alternatives that are noninferior with respect to several objectives. Monte Carlo simulation of groundwater contaminant transport is the method of uncertainty analysis. The random inputs to the simulation are the hydraulic conductivity field and the contaminant source location. The design objectives considered are (1) minimize the number of monitoring wells, (2) maximize the probability of detecting a contaminant leak, and (3) minimize the expected area of contamination at the time of detection. The network design problem is formulated as a multiobjective, integer programming problem and is solved using simulated annealing. An application of the method illustrates the configurations of noninferior network solutions and the trade‐offs between objectives. The probability of detection can be increased either by using more monitoring wells or by locating the wells farther from the source. The latter case results in an increase in the average area of the detected contaminant plumes at the time of initial detection. If monitoring is carried out very close to the contaminant source to reduce the expected area of a detected plume, a large number of wells are required to provide a high probability of detection. A sensitivity analysis showed that the predicted performance of a given number of wells decreases significantly as the heterogeneity of the porous medium increases. In addition, a poor estimate of hydraulic conductivity was shown to result in optimistic estimates of network performance. In general, the trade‐offs between monitoring objectives are an important factor in network design unless the cost (as expressed by the number of monitoring wells) is of limited concern.
X ray and visible light transmission for laboratory measurement of two‐dimensional saturation fields in thin‐slab systems
Authors: Tidwell Vincent C. , Glass Robert J.
Published: February 2004

Two independent techniques, X ray absorption and light transmission, are developed and demonstrated for imaging transient saturation fields in thin‐slab porous systems. The techniques yield full two‐dimensional saturation fields with high spatial and temporal resolution. In the time required to make a single measurement by one of the traditional methods (e.g., gravimetric or gamma densitometry) an entire image consisting of hundreds of thousands of points is acquired by either the X ray or light technique. These methods are also very sensitive, capable of resolving a hundred or more levels of saturation at each of these points. Evaluation of these techniques is accomplished by direct comparison of X ray and light data as well as comparison with gravimetric and gamma densitometry data. Results of the comparison show very close agreement between the four techniques (on average within 5% saturation). These techniques represent useful tools for investigating processes governing unsaturated flow and transport through porous media.
Kernel quantite function estimator for flood frequency analysis
Authors: Moon Young‐Il , Lall Upmanu
Published: February 2004

A kernel estimator (KQ) of the quantile function is presented here. Boundary kernels are used for extrapolation of tail quantiles. The bandwidth of the estimator is chosen using an automatic, “plug‐in” method. Confidence intervals for the estimated quantile are estimated by bootstrapping. Comparisons of the estimator with selected tail probability estimators are offered. The KQ estimator presented here is shown to be competitive with other estimators.
Early development of karst systems: 1. Preferential flow path enlargement under laminar flow
Authors: Groves Christopher G. , Howard Alan D.
Published: February 2004

Modeling of flow and solutional processes within networks of interconnected conduits in limestone aquifers indicates that enlargement occurs very selectively during the early stages of karst aquifer development under laminar flow. If initial flow paths are uniform in size, almost all enlargement occurs along a single set of connected conduits that lie along a direct path between recharge and discharge locations and are aligned along the hydraulic gradient. With a sufficiently large variation in initial aperture widths, enlargement occurs along the flow path offering the least resistance to flow, but since flow rates in laminar flow are proportional to the fourth power of diameter but only linearly proportional to hydraulic gradient, the preferentially enlarged set of fractures may follow an indirect path. Results disfavor earlier suggestions that nonselective cave patterns result from artesian flows (at least under laminar flow conditions) and that all passages should be competitive until the onset of turbulent flow.
Steady groundwater flow as a dynamical system
Authors: Sposito Garrison
Published: February 2004

Dynamical systems describe the time evolution of moving spatial points under the influence of a smooth, bounded vector field. The theory of these systems is focused on the global properties of their flow paths, not on their integration, and so gives general, qualitative information that does not depend on the details of the spatial variability of the vector field. This approach was applied to describe the global consequences of the Darcy law for steady groundwater flows in isotropic, heterogeneous aquifers. No particular model of the spatial variability of the hydraulic conductivity (K) was assumed. Vorticity in the flow paths was shown to exist wherever isoconductivity and equipotential surfaces intersect transversely, and the importance of the Lamb vector (the vector product of vorticity and specific discharge) for the geometry of flow paths was established. Because steady groundwater flows governed by the Darcy law have zero helicity, they cannot exhibit tangled vorticity lines or become chaotic. The absence of chaos is related closely to the impossibility of closed flow paths, the asymptotic stability of isolated minima of the hydraulic head (H), and the existence of a function ℋ(K,H) on whose level surfaces all flow paths are confined. This last function also permits groundwater flows to be represented by moving points in theK,Hplane, with motion there generated by a form of Hamilton's equations. The results obtained are not related to any stochastic approach to aquifer spatial variability, but instead may be applied to constrain stochastic models on purely dynamical grounds.
Modeling phosphorus trapping in wetlands using generalized additive models
Authors: Reckhow Kenneth H. , Qian Song S.
Published: February 2004

In the past several years, wetlands have become increasingly used for wastewater treatment purposes. As a consequence, nutrient trapping and accumulation, particularly for phosphorus, have emerged as important environmental management issues. To predict the effectiveness of wetlands for phosphorus removal, data from a large cross‐sectional study and from a single wetland over time were obtained with the objective of developing a predictive model. From the analysis of these data, it was found that phosphorus trapping is predictable using a simple function of phosphorus input and water loading. Several statistical models of phosphorus trapping were developed and evaluated using the two data sets. Particular emphasis was placed on generalized additive modeling, a graphics‐oriented data‐driven approach, for model diagnostic and model‐building purposes. A nonlinear model was recommended, and suggestions were made for future work.
Effect of sorption isotherm type on predictions of solute mobility in soil
Authors: Hinz C. , Gaston L. A. , Selim H. M.
Published: February 2004

Partitioning of a species between aqueous solution and sorbed phases is commonly described as an equilibrium process and modeled by a sorption isotherm. These sorption models may be derived from the cumulative effect of many underlying specific reactions. However, in heterogeneous systems, such as soils, the full array of these reactions and the relative contribution of each are unknown. This problem may be circumvented, without sacrificing consistency with the underlying reactions, by use of a sorption isotherm which faithfully describes measured sorption behavior. Since transport predictions based on one or another isotherm model may be quite different, careful choice of an isotherm is critical. The objective of this study was to examine the effect of isotherm type on predicted solute mobility in soil. The derivation of several isotherms from either specific reactions or a combination of “local” isotherms with weighting functions is first presented. The use of Freundlich and logarithmic Scatchard plots for selecting the proper sorption isotherm is discussed. Isotherm models, fit to experimental data from the literature, are then used in a transport simulation model. Differences between results are discussed. Also, a brief sensitivity analysis which shows the effects of isotherm parameters on transport behavior is presented.
On the Velocity Covariance and Transport Modeling in Heterogeneous Anisotropic Porous Formations: 1. Saturated Flow
Authors: Russo David
Published: February 2004

Velocity covariances for saturated heterogeneous formations with anisotropic structures were derived for mean flow parallel and perpendicular to the formation bedding and were employed for modeling transport in these formations by the use of a Lagrangian formulation. Relatively simple analytical expressions, each requiring only a single numerical integration, were obtained for the longitudinal and transverse components of the velocity covariance, the displacement covariance, and the macrodispersion coefficient tensors, all as functions of the anisotropy of the formation. Closed‐form analytical expressions were derived for the longitudinal and transverse components of the velocity variance and for the asymptotic values of the transverse components of the displacement covariance. The applicability of this approach for modeling flow and transport in the vadose zone is tested in the companion to this paper by the use of the stochastic theory of Yeh et al. (1985a, b) for unsaturated flow.
Three‐dimensional analysis of infiltration from the disc infiltrometer: 1. A capillary‐based theory
Authors: Smettem K. R. J. , Parlange J. Y. , Ross P. J. , Haverkamp R.
Published: February 2004

The hydraulic properties of an unsaturated homogenous and isotropic soil can be obtained from the unconfined flux out of a disc infiltrometer into the soil over the depth of wetting. The disc infiltrometer is becoming increasingly popular, but methods of analysis have generally relied on the restrictive assumptions of one‐dimensional flow at early times or quasi‐steady state flow at large times. We provide an approximate analytical expression for three‐dimensional unsteady, unconfined flow out of a disc infiltrometer, and this includes the geometric effect of the circular source but ignores gravity. This physically based solution is tested against data obtained from laboratory experiments on repacked material. The results illustrate that the difference between three‐dimensional and one‐dimensional flow is linear with time.
Three‐dimensional analysis of infiltration from the disc infiltrometer: 2. Physically based infiltration equation
Authors: Haverkamp R. , Ross P. J. , Smettem K. R. J. , Parlange J. Y.
Published: February 2004

In situ measurement of soil hydraulic properties may be achieved by analyzing the unconfined efflux from disc tension infiltrometers, once consistent infiltration equations can be derived. In this paper an analytical, three‐dimensional infiltration equation is developed, based on the use of parameters with sound physical meaning and adjustable for varying initial and boundary conditions. The equation is valid over the entire time range. For practical purposes, a simplified solution is also derived. The full and simplified equations give excellent agreement with published experimental results and are particularly useful for determining soil hydraulic properties through application of inverse procedures.
Stochastic weather type simulation for regional climate change impact assessment
Authors: Wilby Robert L.
Published: February 2004

A stochastic model is developed for the synthesis of daily precipitation by weather type analysis. Daily rainfall at two sites in southern England is related to the Lamb weather types by using conditional probabilities. Time series of circulation patterns and hence rainfall are then generated using a Markov representation of matrices of transition probabilities between weather types. The model reproduces key aspects of the historic rainfall regime at daily, monthly, and annual temporal resolutions. The general validity of the methodology as a means of disaggregating general circulation model predictions into finer spatial scales for water resource studies is discussed. Several limitations are identified of which the subjectivity introduced by the weather type classification system and the nonstationarity of the precipitation characteristics are the most serious.
Early Development of Karst Systems: 2. Turbulent Flow
Authors: Howard Alan D. , Groves Christopher G.
Published: February 2004

A simulation model developed to explore patterns of fracture enlargement within incipient limestone karst aquifers has been extended to turbulent flow. In contrast to the highly selective passage enlargement that occurs early in cave network development under laminar flow, the transition to turbulent flow results in more general passage enlargement, leading to maze networks when initial fractures are large and hydraulic gradients are high. These results support previously published hypotheses for the development of maze patterns, including formation within structural settings that have created initially large fractures or within flow systems periodically inundated by flooding. Maze development is also favored under turbulent flow when passages are entirely water filled, and where the groundwater flow system is long‐lived. By contrast, branched patterns are most common when passages become free‐surface subterranean streams, because depression of the piezometric surface along main passages, downcutting along main passages, and possible infilling with sediment of side passages limit the sharing of discharge among interconnected fractures or bedding planes that promote maze development.
A Model for Deuterium and Oxygen 18 Isotope Changes During Evergreen Interception of Snowfall
Authors: Claassen H. C. , Downey J. S.
Published: February 2004

A one‐dimensional, physically based numerical model was constructed to describe the isotopic enrichment observed in throughfall of snow intercepted on evergreens. The process of enrichment is similar to that which results in formation of depth hoar in snowpack. On‐site data were obtained at a high‐altitude (3500 m) watershed in the Colorado Rocky Mountains. The model includes the ambient atmospheric variables of temperature, relative humidity, and water vapor isotopic composition and the intercepted snow variables of temperature profile, permeability for viscous flux, and isotopic composition. Model simulations yield results similar to those observed on site and suggest that the process is dominated by diffusive flux despite the very high permeability of freshly fallen snow. Median enrichments were observed to be 2.1‰ in oxygen 18 and 13‰ in deuterium.
Eulerian‐Lagrangian Analysis of Transport Conditioned on Hydraulic Data: 2. Effects of Log Transmissivity and Hydraulic Head Measurements
Authors: Zhang Dongxiao , Neuman Shlomo P.
Published: February 2004

In paper 1 of this series we described an analytical‐numerical method to predict deterministically solute transport under uncertainty. The method is based on a unified Eulerian‐Lagrangian theory which allows conditioning of the predictions on hydraulic measurements. Conditioning on measured concentrations is also possible, as demonstrated by Neuman et al. (1993). In this paper we condition velocity on log transmissivity and/or hydraulic head data via cokriging. We then combine an early time analytical solution with a pseudo‐Fickian Galerkin finite element scheme for later time to obtain conditional predictions of concentration and lower bounds on its variance and coefficient of variation. The pseudo‐Fickian scheme involves a conditional dispersion tensor which depends on information and varies in space‐time. Hence the predicted plumes travel along curved trajectories and attain irregular, non‐Gaussian shapes. We also compute a measure of uncertainty for the original source location of a solute “particle” whose position at some later time is known from sampling. Spatial maps of this “particle origin covariance” provide vivid images of preferential flow paths and exclusion zones identified by the available data. We illustrate these concepts and results on instantaneous point and area sources.
Application of Fractal Geometry to the Study of Networks of Fractures and Their Pressure Transient
Authors: Acuna Jorge A. , Yortsos Yanis C.
Published: February 2004

Typical models for the representation of naturally fractured systems generally rely on the double‐porosity Warren‐Root model or on random arrays of fractures. However, field observations have demonstrated the existence of multiple length scales in a variety of naturally fractured media. Present models fail to capture this important property of self‐similarity. We first use concepts from the theory of fragmentation and from fractal geometry to construct numerically a network of fractures that exhibits self‐similar behavior over a range of scales. The method is a combination of fragmentation concepts and the iterated function system approach and allows for great flexibility in the development of patterns. Next, numerical simulation of unsteady single‐phase flow in such networks is described. It is found that the pressure transient response of finite fractals behaves according to the analytical predictions of Chang and Yortsos (1990) provided that there exists a power law in the mass‐radius relationship around the test well location. Finite size effects can become significant and interfere with the identification of the fractal structure. The paper concludes by providing examples from actual well tests in fractured systems which are analyzed using fractal pressure transient theory.
Pore Scale Variation in Retardation Factor as a Cause of Nonideal Reactive Breakthrough Curves: 1. Conceptual Model and its Evaluation
Authors: Sugita Fumi , Gillham Robert W.
Published: February 2004

Breakthrough curves (BTCs) for reactive solutes commonly show earlier appearance, greater spreading, and more tailing than can be represented by the classical advection‐dispersion equation. Various types of kinetic models have been developed as a means of representing this nonideal behavior; however, several inconsistencies between the predictions of these models and experimental results have been noted. This paper presents an alternative mechanism that can contribute to the nonideality of BTCs for reactive solutes. The proposed mechanism is based on pore scale variation in the retardation factor (R) resulting from the pore size distribution. The effect of the mechanism on reactive solute transport was investigated using a stochastically derived macrodispersivity equation. In the example presented, the calculated stochastic dispersivity, which accounts for the pore scale variation in R, was almost 1 order of magnitude greater than that determined for a nonreactive solute. The BTC predicted using the stochastically derived dispersivity showed much better agreement with a measured BTC for strontium than that predicted using the nonreactive dispersivity. The simulated curve with stochastically derived dispersivity exhibited the common characteristics of nonideality, including enhanced spreading, early breakthrough, and tailing. Although the analysis was based on a highly idealized model of a porous medium, it is concluded that pore size variation in R can contribute to nonideality in reactive BTCs, even in a homogeneously packed column under conditions of equilibrium solute partitioning.
Hydrologic Processes in a Low‐Gradient Source Area
Authors: Montgomery David R. , Dietrich William E.
Published: February 2004

Discharge, piezometric, and other field observations from a 3‐year monitoring program indicate that runoff from a low‐gradient source area in Marin County, California, is controlled by the interaction of throughflow, macropore flow, and saturation overland flow. Throughflow response integrates multiple storm events and exhibits both seasonal trends and rapid response to midwinter storms upon saturation of highly conductive near‐surface soils. After saturation of the deeper colluvium along the hollow axis, macropore flow responds rapidly to individual storm events and locally provides a ceiling to piezometric response. Saturation overland flow occurs along the axis of the unchanneled valley only during large storms when both soil matrix and macropore transmissivity are exceeded. During large, runoff‐producing storms, saturation overland flow extends continuously over most of the unchanneled valley axis. During smaller runoff‐producing events, however, ground surface saturation may be discontinuous, reflecting either variations in the conductivity of the underlying soil/bedrock or a variable soil thickness along the hollow axis. Results of this study document a sequence of flow path activation in which the temporal distribution of rainfall events within a season determines both the mechanism and magnitude of runoff generation.
Dispersive Transport Dynamics in a Strongly Coupled Groundwater‐Brine Flow System
Authors: Oldenburg Curtis M. , Pruess Karsten
Published: February 2004

Many problems in subsurface hydrology involve the flow and transport of solutes that affect liquid density. When density variations are large (>5%), the flow and transport are strongly coupled. Density variations in excess of 20% occur in salt dome and bedded‐salt formations which are currently being considered for radioactive waste repositories. The widely varying results of prior numerical simulation efforts of salt dome groundwater‐brine flow problems have underscored the difficulty of solving strongly coupled flow and transport equations. We have implemented a standard model for hydrodynamic dispersion in our general purpose integral finite difference simulator, TOUGH2. The residual formulation used in TOUGH2 is efficient for the strongly coupled flow problem and allows the simulation to reach a verifiable steady state. We use the model to solve two classic coupled flow problems as verification. We then apply the model to a salt dome flow problem patterned after the conditions present at the Gorleben salt dome, Germany, a potential site for high‐level nuclear waste disposal. Our transient simulations reveal the presence of two flow regimes: (1) recirculating and (2) swept forward. The flow dynamics are highly sensitive to the strength of molecular diffusion, with recirculating flows arising for large values of molecular diffusivity. For pure hydrodynamic dispersion with parameters approximating those at Gorleben, we find a swept‐forward flow field at steady state rather than the recirculating flows found in previous investigations. The time to steady state is very sensitive to the initial conditions, with long time periods required to sweep out an initial brine pool in the lower region of the domain. Dimensional analysis is used to demonstrate the tendency toward brine recirculation. An analysis based on a dispersion timescale explains the observed long time to steady state when the initial condition has a brine pool in the lower part of the system. The nonlinearity of the equations and the competing effects of dispersion and gravity make this variable‐density problem a challenge for any numerical simulation method.