bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐pressure, high‐temperature rocks from the base of thick continental crust: Geology and age constraints from the Manicouagan Imbricate Zone, eastern Grenville Province
Authors: Indares Aphrodite , Dunning Greg , Cox Richard , Gale David , Connelly Jim
Published: February 2004

The Grenville Province of the Canadian Shield is composed of crustal material with Laurentian affinities, involved in continental collision during the Grenvillian Orogeny between 1.19 and 0.98 Ga. In the eastern Grenville Province, the Manicouagan Imbricate Zone (MIZ) is composed of Paleoproterozoic (Labradorian, ∼1.6 Ga) and Mesoproterozoic (Pinwarian, ∼1.4 Ga) crustal segments that were metamorphosed under high‐pressure, high‐temperature (high‐P‐T) conditions (P∼1800 MPa and 800°<T<900°C) and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite, during a ∼1.0 Ga crustal shortening event in the Grenvillian Orogeny. The gabbroic dykes display high‐P mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high‐P metamorphism. The structural configuration of MIZ is that of a thrust stack (Lelukuau terrane) overlain by an extensional assembly of slices (Tshenukutish terrane). To the north, Lelukuau terrane overlies Archean basement and Paleoproterozoic supracrustal rocks of the Gagnon terrane along a thrust contact. Tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre‐Grenvillian and early Grenvillian lithologic units that are correlative with those in MIZ, but which were at midcrustal levels during the Grenvillian Orogeny (low‐P segment of the eastern Grenville Province), and were intruded by anorthosite and granitoids farther south. A likely tectonothermal evolution of MIZ at ∼1.0 Ga involves crustal shortening by imbrication in a postsubduction phase of continental collision, followed by (convective?) removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust. Exhumation of MIZ likely occurred from the basal levels of the crust and was achieved by (1) NW directed thrusting over a crustal‐scale ramp (Archean basement of the Gagnon terrane) with coeval extension at the higher levels of the pile (stage 1 extension) and (2) a second extension event (stage 2 extension) coeval with emplacement of postmetamorphic granite at the boundary between MIZ and the low‐P segment.
Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez‐Mischabel Nappe, western Pennine Alps, Switzerland
Authors: Markley Michelle J. , Teyssier Christian , Cosca Michael A. , Caby Renaud , Hunziker Johannes C. , Sartori Mario
Published: February 2004

We explore the timing of deformation and exhumation of the Siviez‐Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 40Ar/39Ar ages obtained on both synkinematic white mica from Permo‐Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez‐Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez‐Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 40Ar/39Ar white mica ages from cover units of the central and southern Siviez‐Mischabel Nappe. In cover units from the central and southern Siviez‐Mischabel regions of the study area, 40Ar/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez‐Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez‐Mischabel, however, 40Ar/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez‐Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high‐pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years.
Magma emplacement during arc‐perpendicular shortening: An example from the Cascades crystalline core, Washington
Authors: Paterson Scott R. , Miller Robert B.
Published: February 2004

It is increasingly inferred by others that active arcs at all crustal levels are undergoing arc‐perpendicular extension and that magma utilizes this extension or extension along arc‐parallel, strike‐slip faults during emplacement. However, a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc‐perpendicular shortening without the aid of strike‐slip faulting. This conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens. Evaluation of host rock material transfer processes that operated during emplacement of the 93 Ma, 10‐ to 14‐km‐deep Mount Stuart batholith, Washington, provides one example and includes the following: (1) ductile downward flow in a narrow (< 0.2 body radii) structural aureole, (2) stoping, and, of lesser importance, (3) regional folding, thrusting, and subhorizontal arc‐parallel extension. We see no evidence that arc‐perpendicular extension or local extension along strike‐slip faults made space for this pluton nor for most other plutons in contractional arcs. Instead we argue that both localized and rapid (around plutons) and regional and slower downward flow during growth of crustal roots is a common process in such arcs.
Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean
Authors: Platt J. P. , Soto J.‐I. , Whitehouse M. J. , Hurford A. J. , Kelley S. P.
Published: February 2004

High‐grade metamorphic rocks drilled at Ocean Drilling Program Site 976 in the Alboran Sea show a PT path characterized by decompression from about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550°±50°C to 675°±25°C. The Ar/Ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5–18 Ma, which coincides with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late Cretaceous? to Paleogene contractional orogen, and extension coincided with thrusting in the peripheral parts of the Betic‐Rif arc, which surrounds the basin on three sides. Thermal modeling of the PT path was carried out with the aim of constraining geodynamic models for the formation of the basin. Variables considered in the modeling included (1) the thickness and thermal gradient of the postorogenic lithosphere; (2) the radiogenic heat production in the thickened crust; (3) the time gap (pause) between the end of contractional tectonics and the start of extension; (4) removal of lithospheric mantle below 125, 75, or 62.5 km; and (5) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause (to produce high temperatures in rocks initially at 40 km depth), removal of lithosphere below 62.5 km (to produce further heating during decompression), extension by a factor of 3 in 6 m.y. (to delay the attainment of the maximum temperature until the rocks reached shallow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximum of about 9 m.y. for exhumation from 40 km depth to the surface. Lithospheric stretching in response to plate‐boundary forces such as trench rollback, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature. Only a combination of significant postcollisional radiogenic heating, then wholesale removal of lithospheric mantle below the orogenic crust, followed by rapid stretching can explain the observed PT path. These results appear to support some form of lithospheric delamination as the primary cause for the formation of the Alboran Sea basin.
Tectonics of the Arabian margin associated with the formation and exhumation of high‐pressure rocks, Sultanate of Oman
Authors: Gregory Robert T. , Gray David R. , Miller John McL.
Published: February 2004

Continental crustal rocks, now structurally beneath the allochthonous Samail ophiolite, underwent blueschist to eclogite facies metamorphism prior to the emplacement of the Oman ophiolite onto the Arabian margin. The recognition of a major low angle fault within this polydeformed and polymeta‐morphosed sequence of metacarbonates, metabasites, quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the Saih Hatat window, NE Oman. Carpholite‐bearing upper plate rocks consist of pre‐Permian to Jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate. The lower plate rocks have higher peak pressure and temperature assemblages; they are exposed in two windows separated by Jabal Abu Daud. Although more intensely deformed, the lower plate rocks are recognizable as metamorphosed continental platform sediments. Fold axes are parallel to the regional NNE‐SSW lineation. Sense of shear indicators yield a transport direction of south over north in the lower plate, opposite to the sense of motion inferred for the emplacement of the ophiolite. Lower plate eclogite‐facies metabasalts are only preserved in kilometer‐scale megaboudins found in the easternmost window at As Sifah. These metamorphic assemblages along with their remnant east‐west fabrics define the existence of a short‐lived, Arabian platform‐directed, nascent subduction zone. The stretching lineation elsewhere within the upper and lower plates, the Hatat Schist (the basement), and the metamorphic sole of the ophiolite is consistently NNE‐SSW, suggesting that the exhumation of high‐pressure metamorphic rocks of Saih Hatat is related to the ophiolite obduction. Exhumation of the high pressure rocks was accompanied by intense deformation involving regional‐scale fold nappes in a convergent margin setting. The geotherm remained suppressed for a period (>30 Myr) greater than the thermal relaxation time of the crust. The geometric and thermal constraints from Oman may be applicable to the general problem of the formation and preservation of high‐pressure, low‐temperature rocks.
A comparison of seven geostatistically based inverse approaches to estimate transmissivities for modeling advective transport by groundwater flow
Authors: Zimmerman D. A. , de Marsily G. , Gotway C. A. , Marietta M. G. , Axness C. L. , Beauheim R. L. , Bras R. L. , Carrera J. , Dagan G. , Davies P. B. , Gallegos D. P. , Galli A. , Gómez‐Hernández J. , Grindrod P. , Gutjahr A. L. , Kitanidis P. K. , Lavenue A. M. , McLaughlin D. , Neuman S. P. , RamaRao B. S. , Ravenne C. , Rubin Y.
Published: February 2004

This paper describes the first major attempt to compare seven different inverse approaches for identifying aquifer transmissivity. The ultimate objective was to determine which of several geostatistical inverse techniques is better suited for making probabilistic forecasts of the potential transport of solutes in an aquifer where spatial variability and uncertainty in hydrogeologic properties are significant. Seven geostatistical methods (fast Fourier transform (FF), fractal simulation (FS), linearized cokriging (LC), linearized semianalytical )LS), maximum likelihood (ML), pilot point (PP), and sequential self‐calibration (SS)) were compared on four synthetic data sets. Each data set had specific features meeting (or not) classical assumptions about stationarity, amenability to a geostatistical description, etc. The comparison of the outcome of the methods is based on the prediction of travel times and travel paths taken by conservative solutes migrating in the aquifer for a distance of 5 km. Four of the methods, LS, ML, PP, and SS, were identified as being approximately equivalent for the specific problems considered. The magnitude of the variance of the transmissivity fields, which went as high as 10 times the generally accepted range for linearized approaches, was not a problem for the linearized methods when applied to stationary fields; that is, their inverse solutions and travel time predictions were as accurate as those of the nonlinear methods. Nonstationarity of the “true” transmissivity field, or the presence of “anomalies” such as high‐permeability fracture zones was, however, more of a problem for the linearized methods. The importance of the proper selection of the semivariogram of the log10 (T) field (or the ability of the method to optimize this variogram iteratively) was found to have a significant impact on the accuracy and precision of the travel time predictions. Use of additional transient information from pumping tests did not result in major changes in the outcome. While the methods differ in their underlying theory, and the codes developed to implement the theories were limited to varying degrees, the most important factor for achieving a successful solution was the time and experience devoted by the user of the method.
Approximation of asymptotic dispersivity of conservative solute in unsaturated heterogeneous media with steady state flow
Authors: Hammel Klaus , Roth Kurt
Published: February 2004

The relation between the heterogeneity of hydraulic properties and effective asymptotic transport is studied for microscopically heterogeneous but macroscopically homogeneous unsaturated media with steady flow. Heterogeneity is described by the scaling of the hydraulic functions θ(Ψm) and K(θ). The study is based on an analytical approximation of asymptotic dispersivity under the assumption that matric potential is spatially constant. For the special case of a water‐saturated medium the result of the stochastic continuum theory is recovered. When applied to several published numerical simulations, the approximated asymptotic dispersivities are found to agree well with the numerical values. By exploring the heterogeneous media for various hydraulic states, the approximation resolves some apparent inconsistencies found in the simulations. The cases considered span the entire range from perfect to zero correlation between scaling factors of matric potential and hydraulic conductivity. It is demonstrated that this correlation is crucial for the behavior of asymptotic dispersivity with changing flow rate. Since weak to moderate correlations are often found in soils, this result has significant implications for solute transport through heterogeneous soils.
Costs of water treatment due to diminished water quality: A case study in Texas
Authors: Dearmont David , McCarl Bruce A. , Tolman Deborah A.
Published: February 2004

The cost of municipal water treatment due to diminished water quality represents an important component of the societal costs of water pollution. Here the chemical costs of municipal water treatment are expressed as a function of raw surface water quality. Data are used for a 3‐year period for 12 water treatment plants in Texas. Results show that when regional raw water contamination is present, the chemical cost of water treatment is increased by $95 per million gallons (per 3785 m3) from a base of $75. A 1% increase in turbidity is shown to increase chemical costs by 0.25%.
Transport in multiscale log conductivity fields with truncated power variograms
Authors: Di Federico Vittorio , Neuman Shlomo P.
Published: February 2004

Di Federico and Neuman [this issue] investigated mean uniform steady state groundwater flow in an unbounded domain with log hydraulic conductivity that forms a truncated multiscale hierarchy of statistically homogeneous and isotropic Gaussian fields, each associated with an exponential autocovariance. Here we present leading‐order expressions for the displacement covariance, and dispersion coefficient, of an ensemble of solute particles advected through such a flow field in two or three dimensions. Both quantities are functions of the mean travel distance s, the Hurst coefficient H, and the low‐ and high‐frequency cutoff integral scales λl and λu. The latter two are related to the length scales of the sampling window (region under investigation) and sample volume (data support), respectively. If one considers transport to be affected by a finite domain much larger than the mean travel distance, so that s ≪ λl < ∞, then an early preasymptotic regime develops during which longitudinal and transverse dispersivities grow linearly with s. If one considers transport to be affected by a domain which increases in proportion to s, then λl and s are of similar order and a preasymptotic regime never develops. Instead, transport occurs under a regime that is perpetually close to asymptotic under the control of an evolving scale λl ∼ s. We show that if, additionally, λu ≪ λl, then the corresponding longitudinal dispersivity grows in proportion to λl1+2H or, equivalently, s1+2H. Both these preasymptotic and asymptotic theoretical growth rates are shown to be consistent with the observed variation of apparent longitudinal Fickian dispersivities with scale. We conclude by investigating the effect of variable separations between cutoff scales on dispersion.
A physical explanation of the cumulative area distribution curve
Authors: Perera Hemantha , Willgoose Garry
Published: February 2004

A physical explanation for the behavior of the cumulative area distribution (CAD) based on the Tokunaga channel network model is given. The CAD is divided into three regions. The first region, for small areas, is dependent on hillslope flow accumulation patterns and represents the catchment average of the hillslope flow accumulation in the diffusive erosion‐dominated areas, upstream reaches, of the catchment. The second region represents that portion of the catchment dominated by fluvial erosion. This region is well described by a log‐log linear power law, which results from the scaling properties of the channel network. The scale exponent, ϕ, is highly sensitive to a parameter of the Tokunaga stream numbering scheme. The exponent ϕ converges to −0.5 for higher order Tokunaga networks for parameters consistent with topological random networks. Small networks have lower values of ϕ, which asymptotic converges to ϕ=−0.5 as the catchment scale increase. The third region reflects the lowest reaches of the channel network, the scale of the catchment, and is a boundary effect. An explicit analytical solution to the scaling properties in the second region is derived on the basis of the Tokunaga network model.
Stabilizing self‐organized structures in gravel‐bed stream channels: Field and experimental observations
Authors: Church Michael , Hassan Marwan A. , Wolcott John F.
Published: February 2004

Stable reticulate structures, which we call “stone cells,” have been observed in cobble‐gravel channel beds with low bed material transport rates. Experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the Shields threshold by more than ∼2 times, so that bed material transport is low. They are constructed by particles moving from less stable positions into more stable configurations against each other. Intermediate developments include clusters and stone lines. They reduce sediment transport by orders of magnitude and are evidently a major stability‐promoting mechanism in gravel channels. The timescale for their development suggests that the boundaries of many gravel‐bed channels are not in equilibrium with recent competent flows but reflect the history of recent “dominant” flows.
Development and testing of a compartmentalized reaction network model for redox zones in contaminated aquifers
Authors: Abrams Robert H. , Loague Keith , Kent Douglas B.
Published: February 2004

The work reported here is the first part of a larger effort focused on efficient numerical simulation of redox zone development in contaminated aquifers. The sequential use of various electron acceptors, which is governed by the energy yield of each reaction, gives rise to redox zones. The large difference in energy yields between the various redox reactions leads to systems of equations that are extremely ill‐conditioned. These equations are very difficult to solve, especially in the context of coupled fluid flow, solute transport, and geochemical simulations. We have developed a general, rational method to solve such systems where we focus on the dominant reactions, compartmentalizing them in a manner that is analogous to the redox zones that are often observed in the field. The compartmentalized approach allows us to easily solve a complex geochemical system as a function of time and energy yield, laying the foundation for our ongoing work in which we couple the reaction network, for the development of redox zones, to a model of subsurface fluid flow and solute transport. Our method (1) solves the numerical system without evoking a redox parameter, (2) improves the numerical stability of redox systems by choosing which compartment and thus which reaction network to use based upon the concentration ratios of key constituents, (3) simulates the development of redox zones as a function of time without the use of inhibition factors or switching functions, and (4) can reduce the number of transport equations that need to be solved in space and time. We show through the use of various model performance evaluation statistics that the appropriate compartment choice under different geochemical conditions leads to numerical solutions without significant error. The compartmentalized approach described here facilitates the next phase of this effort where we couple the redox zone reaction network to models of fluid flow and solute transport.
Effects of flow bypassing and nonuniform NAPL distribution on the mass transfer characteristics of NAPL dissolution
Authors: Soerens Thomas S. , Sabatini David A. , Harwell Jeffrey H.
Published: February 2004

Ground water concentrations of nonaqueous phase liquid (NAPL) are usually less than their aqueous solubility; this can be attributed to irregular NAPL distribution, nonuniform flow patterns, dilution effects, and rate‐limited mass transfer between the NAPL and aqueous phases. This paper uses two‐domain and parallel column models to demonstrate the effects of nonuniform NAPL distribution and flow bypassing on apparent mass transfer kinetics for NAPL dissolution. The hypothesis of this study is that much of the apparent nonequilibrium dissolution can be explained by the effects of heterogeneity in NAPL distribution and in porous media properties. Models incorporating two‐domain concepts to represent heterogeneity are able to reproduce the timescale dependency of mass transfer coefficients which has often been observed but is inconsistent with mass transfer theory. Parallel column models, with equilibrium partitioning assumed in each column, are able to reproduce the concentration drop‐off and tailing observed in published column studies of NAPL dissolution. The parallel column model is also able to reproduce the experimentally observed phenomenon of mass transfer zones which lengthen with time and distance traveled. The results of this study support the hypothesis that much of the apparent nonequilibrium mass transfer kinetics of NAPL dissolution can be described by heterogeneity in NAPL distribution and in porous media properties.
Streamflow estimation using kriging
Authors: Huang Wen‐Cheng , Yang Fu‐Ti
Published: February 2004

This study presents a centroid‐based method of regional analysis employing the Kriging technique. A case study in streamflow estimation using Kriging was carried out in the Choushui Chi Basin in central Taiwan. A variable (runoff depth) distributed in space was viewed as a regionalized variable, and the representative value of the runoff depth was allocated on the centroid of the area considered. The results show that the spatial structure of the measurements in the experimental variogram is not purely random and the stationarity hypothesis is appropriate in estimating streamflow.
Efficient three‐dimensional data inversion: Soil characterization and moisture Monitoring from cross‐well ground‐penetrating radar at a Vermont Test Site
Authors: Eppstein Margaret J. , Dougherty David E.
Published: February 2004

We extend our methodology for three‐dimensional parameter structure and value estimation and apply it to a Vermont test site. Ground‐penetrating radar (GPR) cross‐well travel times are inverted for estimation of heterogeneous GPR soil velocities before and after a controlled release of salt water in the unsaturated zone. The method, which is based on an approximation of the extended Kaiman filter in conjunction with data‐driven zonation, automatically estimates not only distributed zone values but also the number of zones, zone geometry, and zone covariance. Resultant GPR velocity estimates are shown to reduce travel time estimation errors and to be consistent with independent cone penetrometer measurements at all five walls at the site. Comparison of velocity estimates before and after forced injection of salt water is used to detect and visualize soil moisture patterns in three dimensions. By varying the “cluster tolerance criterion” in the data‐driven zonation process, the user can obtain a desired resolution of heterogeneity (number of zones used) in the resultant model.
An infiltration model to predict suction changes in the soil profile
Authors: Totoev Yuri Z. , Kleeman Peter W.
Published: February 2004

A computer model for the infiltration at the soil surface is described. The model has been developed for computation of seasonal suction changes in the soil profile. It simulates time dependence of the vector of sources and sinks for the finite element solution of Richard's equation. Lumped meteorological and soil parameters are used. Meteorological input to the model consists of the records for rainfall, pan evaporation, air temperature, and air humidity. As an example of an application of the model, the daily magnitudes of infiltration/evapotranspiration at the soil surface at Maryville, New South Wales, Australia, in March and April 1993 were obtained, and the resulting suction profile was calculated numerically. This example shows that the model adequately reflects major meteorological changes. The resulting suction profile correlates well with the experimental data, better than profiles resulting from the models, which assume periodic or sudden changes in surface suction.
Critical path analysis of the relationship between permeability and electrical conductivity of three‐dimensional pore networks
Authors: Friedman Shmulik P. , Seaton Nigel A.
Published: February 2004

The critical path analysis (CPA) of Ambegaokar et al. [1971] suggests that transport in random systems with a broad enough distribution of conductances is dominated by a controlling conductance of a critical magnitude. The present short, yet systematic, study provides a test of the applicability of the CPA to determine the relationship between two different transport properties of three‐dimensional pore networks. Two characteristic transport properties are examined: viscous (hydraulic) permeability and electrical conductivity, and the three‐dimensional pore networks are represented by diluted simple‐cubic lattices of different average coordination numbers and broadness of a positively skewed pore size distribution. The prediction of the critical path analysis is good for small coordination numbers as the flow is largely through resistances in series and it diverges from the correct permeability–electrical conductivity relationship as the coordination number increases, especially for moderate broadness of the pore size distribution. The critical path analysis is also applicable to the relationships between other transport properties, for example, molecular and Knudsen diffusivities, and also to various pore shapes, other then cylindrical and slit‐shaped, referred to in this study.
Recession flow analysis for aquifer parameter determination
Authors: Szilagyi Jozsef , Parlange Marc B. , Albertson John D.
Published: February 2004

The recession flow analysis of Brutsaert and Nieber [1977] extended by Troch et al. [1993] to estimate aquifer parameters (saturated hydraulic conductivity and mean aquifer depth) is examined by means of a numerical model. It is found to be reliable for the estimation of the catchment‐scale saturated hydraulic conductivity and mean aquifer depth. Increasing the complexity of the synthetic watershed had no impact on the accuracy of the estimated parameters.
Effects of transverse mixing on transport of bacteria through heterogeneous porous media
Authors: Morley Linda M. , Hornberger Geogrge M. , Mills Aaron L. , Herman Janet S.
Published: February 2004

In this paper, we examine two questions: (1) can the effects of transverse mixing of bacteria in a system constructed to have a permeability discontinuity in the direction parallel to the flow be measured; and (2) if the effects are measurable, can they be calculated using a transverse dispersion coefficient estimated from experiments using a conservative tracer? Pulses of chloride and bacteria were transported downward through heterogeneous columns constructed with a tubule of coarse, quartz sand surrounding an annulus of fine, quartz sand. Pulses of each were also transported through homogeneous columns of the two sands. Doubly peaked breakthrough curves resulted from the columns containing two distinct sand sizes. Modeling of the breakthrough curves was performed taking into account advection, dispersion, deposition, entrainment, and pore‐size exclusion. The results revealed that transverse mixing does occur during transport of bacteria through heterogeneous material and that this mixing can be estimated using a conservative tracer.
Land‐surface water vapor and sensible heat flux: Spatial variability, homogeneity, and measurement scales
Authors: Brutsaert Wilfried
Published: February 2004

Available methods to determine the sensible and latent heat fluxes from the Earth's land surfaces are still relatively imprecise; this is due primarily to the inherent irregularity of the turbulent transport mechanisms and also to the pronounced variability of most natural land surfaces. While great progress has been made in the study of turbulence, until recently, surface variability has received relatively little attention in this context. Some thoughts are provided on approaching this issue by focusing on the stochastic structure of the land surface within the framework of homogeneous, i.e., stationary, random functions. In addition, suggestions are made regarding the likely applicability of classical concepts by relating the relevant scales of surface variability structure to those characterizing turbulent transport in the atmospheric boundary layer.
Spatially varying hydraulic and solute transport characteristics of a fractured till determined by field tracer tests, Funen, Denmark
Authors: Sidle Roy C. , Nilsson Bertel , Hansen Martin , Fredericia Johnny
Published: February 2004

A natural gradient tracer test that simulated one‐dimensional flow was conducted in a large (4 m × 4.8 m surface area) isolated block of fractured till in Funen, Denmark, to determine both bulk and spatially distributed hydraulic properties. Hydraulic conductivity (based on free‐flow tests) was higher in the upper 2.5 m (5.81 × 10−5 m s−1) than in the upper 4 m (8.00 × 10−6 m s−1) of till, indicating the more highly fractured and bioturbated nature of the upper part of the till. Chloride was introduced at the surface of the block as NaCl solution (490 mg L−1 Cl−), and breakthrough of Cl− was monitored in 13 horizontal screens at depths of 2.5 and 4 m. Arrival times of half of the input Cl− concentrations (C/c0=0.5) ranged about sevenfold among various screens at each sampling depth; 5–35 hours at 2.5 m and 11–68 hours at 4 m. Chloride breakthrough data for the two depths were fitted to both an equivalent porous media (EPM) model, based on the convective‐dispersive equation, and a discrete fracture model. These results indicate differences in transport characteristics of the till with depth. At the 2.5 m depth the EPM model fit the initial part of the breakthrough curves quite well for most samplers but did not predict the long tails adequately. The converse was generally true for the discrete fracture model. Thus it appears that transport in the upper 2.5 m of till is controlled by a complex interconnected system of fractures and macropores. Field measurements of fractures and macropores at this site substantiate these results. At the 4 m depth the discrete fracture model described solute transport in all samplers better than the EPM model, indicating that vertical fracture flow dominates in the lower portion of the block. Fracture apertures and spacings were estimated based on the cubic law and fitting the breakthrough data to the discrete fracture model. Fitted fracture spacings at the 4 m depth (0.025–0.08 m) were slightly smaller than the minimum spacing (0.09 m) mapped in the field. Calculated apertures at the 4 m depth were large compared with other studies and ranged from 65 to 127 μm (average value of 94 μm).
Bottom buoyancy layer in an ice‐covered lake
Authors: Malm Joakim
Published: February 2004

Field data on temperature and salt content structure in the vicinity of the bottom boundary in an ice‐covered lake are analyzed. A bottom boundary layer is identified, characterized by rapid increases in both temperature and salt concentration and having a thickness of ∼1 dm with respect to temperature and 2–4 cm with respect to salt content. A simple analytical model of the temperature distribution in the near‐bottom layer and the associated downslope flow of the heavier water is proposed. The model describes satisfactorily the temperature field in the near bottom layer. The corresponding modeled advective velocities are near 1 mm s−1 in early winter and 1 order of magnitude lower in late winter, the latter values agreeing well with observations in other lakes.
Export of nitrogen from catchments within a temperate forest: Evidence for a unifying mechanism regulated by variable source area dynamics
Authors: Creed I. F. , Band L. E.
Published: February 2004

Considerable variation in the export of dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) was observed among catchments located within an old‐growth sugar maple forest in central Ontario. Although discharge was a strong predictor of N‐export, rates of export were variable for each catchment, ranging from −50% to +50% from the catchment‐average response for DIN and −25% to +25% from the catchment‐average response for DON. Among the catchments, a unifying flushing behavior was apparent for NO3−‐N, the dominant form of DIN in the discharge waters, providing a basis for explaining the variation in the export of DIN. Flushing occurs when a water table rises to the soil surface with subsequent mobilization of nutrients stored near or at the soil surface to surface waters. Catchment‐specific flushing behaviors were captured in “flushing” characteristic time constants, defined as the time interval required for a decline in N concentrations in discharge waters toe−l(37%) of their initial concentration. Variation in flushing behavior was linked to variation in N export; catchments with short flushing times (interpreted as catchments with source areas that are less variable) were observed to export less N than catchments with long flushing times (source areas that are more variable). A hypothesis was formulated in which catchment topography and its influence on variable source area dynamics accounts for variation in flushing behavior, hence variation in the export of NO3−‐N among the catchments. The implication of this hypothesis is that to predict accurately the export of NO3−‐N from catchments within a landscape, we need first to consider the influence of the topographic complexity of the catchments. Our understanding of the mechanisms of processing and export of DON is not sufficient for accurate prediction at this point, highlighting the need for additional research on DON.
Diurnal fluctuations of tensiometric readings due to surface temperature changes
Authors: Warrick A. W. , Wierenga P. J. , Young M. H. , Musil S. A.
Published: February 2004

Pressure fluctuations in tensiometers in response to temperature changes are examined. Mechanisms considered include temperature variation within an air gap at the top of the tensiometer, the air gap size, saturated water vapor pressure, and hydraulic conductivity of the soil. Pressures measured in a tensiometer generally fall between two simplified, limiting cases. The first limiting case assumes that the tensiometer cup is impermeable for water. This leads to very high fluctuations as air and soil temperatures change. For a cyclical temperature of 35 ± 15°C, variations in water pressure inside the cup can be ±70 cm water head. For the second limiting case the water moves freely between the tensiometer and the soil, which leads to more stable readings, within ±1 cm for the above 15°C fluctuation. While cup impedance was found to be a negligible factor for all cases considered, the analysis presented here suggests that conductivity of the soil immediately around the cup is the main factor governing temperature‐induced pressure fluctuations inside the cup.
Molybdate transport in a chemically complex aquifer: Field measurements compared with solute‐transport model predictions
Authors: Stollenwerk Kenneth G.
Published: February 2004

A natural‐gradient tracer test was conducted in an unconfined sand and gravel aquifer on Cape Cod, Massachusetts. Molybdate was included in the injectate to study the effects of variable groundwater chemistry on its aqueous distribution and to evaluate the reliability of laboratory experiments for identifying and quantifying reactions that control the transport of reactive solutes in groundwater. Transport of molybdate in this aquifer was controlled by adsorption. The amount adsorbed varied with aqueous chemistry that changed with depth as freshwater recharge mixed with a plume of sewage‐contaminated groundwater. Molybdate adsorption was strongest near the water table where pH (5.7) and the concentration of the competing solutes phosphate (2.3 micromolar) and sulfate (86 micromolar) were low. Adsorption of molybdate decreased with depth as pH increased to 6.5, phosphate increased to 40 micromolar, and sulfate increased to 340 micromolar. A one‐site diffuse‐layer surface‐complexation model and a two‐site diffuse‐layer surface‐complexation model were used to simulate adsorption. Reactions and equilibrium constants for both models were determined in laboratory experiments and used in the reactive‐transport model PHAST to simulate the two‐dimensional transport of molybdate during the tracer test. No geochemical parameters were adjusted in the simulation to improve the fit between model and field data. Both models simulated the travel distance of the molybdate cloud to within 10% during the 2‐year tracer test; however, the two‐site diffuse‐layer model more accurately simulated the molybdate concentration distribution within the cloud.
Diurnal fluctuations of water and heat flows in a bare soil
Authors: Schelde K. , Thomsen A. , Heidmann T. , Schjønning P. , Jansson P.‐E.
Published: February 2004

The complexity of coupled transport of heat and moisture at the soil surface necessitates a combination of field and numerical experiments to evaluate the interactions between liquid and vapor phase flow. The near‐surface moisture and temperature conditions of a bare soil were investigated experimentally and by using the SOIL model to assess the importance of water vapor flow. During a 1‐month period in early fall, intensive measurements of water content, water tension, and temperature were made in a bare soil plot. Soil thermal conductivity, measured on soil cores extracted for laboratory analysis, was found to agree with estimates based on the Kersten equation. Simulated water content and soil temperature agreed well with observations. Modeled soil vapor flow was significant compared to liquid flow only during the initial dry days when the inclusion of vapor flow improved the predicted diurnal variation in water tension. Model predictions were sensitive to an accurate representation of the soil surface energy balance, including the consideration of steep gradients in tension near the soil surface, and to the enhancement of vapor flow.
Hydrologic analysis of urban catchments with event‐based probabilistic models: 1. Runoff volume
Authors: Guo Yiping , Adams Barry J.
Published: February 2004

The frequency distributions of rainfall volume, rainfall duration, and interevent time are examined using the historical rainfall record of Toronto, Canada. Exponential probability density functions are found to fit well to the histograms of the rainfall event characteristics. The rainfall‐runoff transformation on an event basis is described by an equation which incorporates the hydrologic processes commonly considered in numerical simulation models. On the basis of this equation and the exponential probability density functions of rainfall event characteristics, closed‐form analytical expressions are derived for average annual runoff volume and runoff event volume return period. Deterministic continuous simulation of various urban catchments are conducted using the Toronto historical rainfall record as input. Close agreement between simulation model results and those from analytical expressions is obtained. The event‐based probabilistic models for the determination of average annual runoff volumes and runoff event volumes with specified return periods from urban catchments are proposed as an alternative to continuous simulation models.
Non–aqueous phase liquid dissolution in heterogeneous systems: Mechanisms and a local equilibrium modeling approach
Authors: Powers Susan E. , Nambi Indumathi M. , Curry Garrey W.
Published: February 2004

Experiments quantifying rates of non–aqueous phase liquid (NAPL) dissolution from heterogeneous media are presented and compared with model simulations. This work specifically addresses the overall dissolution of NAPL entrapped in a coarse sand lens at a high saturation. To explore the mechanisms governing dissolution rates, mathematical models describing the hydrodynamics of flow through the heterogeneous system were developed and coupled with a mass balance equation and a local equilibrium assumption (LEA) to quantify interphase mass transfer processes. Variations in the effective permeabilities as a function of NAPL saturation and the intrinsic permeabilities of the sands were employed to characterize the hydrodynamic aspects of flow through the heterogeneous system. Relative to errors generated by the ill‐defined aqueous phase relative permeabilities at high NAPL saturations, the model incorporating the system hydrodynamics as the sole rate‐limiting process provided a reasonable first estimate of effluent concentrations. With the representative elemental volume defined here, rate‐limited dissolution becomes important for low‐NAPL saturations (Sn < ∼0.05–0.15) causing tailing in the observed dissolution data and deviations between these data and the LEA model.
Solute transport in a heterogeneous soil for boundary and initial conditions: Evaluation of first‐order approximations
Authors: Vanderborght Jan , Mallants Dirk , Feyen Jan
Published: February 2004

We compared four different approaches to derive the statistics of the solute travel time т and horizontal displacement η from spatial covariance functions of the pore water velocity u in an unsaturated heterogeneous soil profile using a Lagrangian framework. The effects of four simplifications that are generally used to derive т and η statistics were evaluated: (1) first‐order approximation of the stochastic flow equation, (2) first‐order expansion of the inverse vertical pore water velocity 1/u2, (3) identical distributions of u and of solute particle velocity w, and (4) vertical solute trajectories. Alternatives that comprehend numerical solutions of the stochastic flow equation to derive distributions of u and 1/u2, using a flux‐weighted distribution of u to represent the distribution of w and using two dimensional covariance functions to represent the effect of horizontal deviations of the particle trajectories, were discussed. The statistics of т and η derived in a Lagrangian framework were compared with the statistics derived from two types of transport simulations in generated, two‐dimensional heterogeneous soil profiles: simulations (1) for uniform solute flux at the soil surface (uniform boundary value problem, UBVP) and (2) for a uniform initial concentration profile (uniform initial value problem, UIVP). The considered heterogeneity of the saturated hydraulic conductivity, Ksat, was relatively large, σ2 1n Ksat=2.55, but it was based on conductivity measurements in a loam soil and found to be realistic for predicting solute transport in this soil. For the UBVP simulations the best predictions of the solute travel time and horizontal displacement statistics were obtained using the flux‐weighted distribution of simulated u. For the UIVP simulations the distribution of w was not stationary but changed from the nonweighted distribution of u for small travel depths to the flux‐weighted distribution of u for larger travel depths.
On space‐time scaling of cumulated rainfall fields
Authors: Rodriguez‐Iturbe Ignacio , Marani Marco , D'Odorico Paolo , Rinaldo Andrea
Published: February 2004

In the study of space‐time rainfall it is particularly important to establish characteristic properties to guide both theoretical and modeling research efforts. In the present paper, new observational analyses on the scaling properties of time‐evolving cumulated rainfall fields are presented, and a theoretical framework for their interpretation is introduced. It is found that the time evolution of the spatial organization of a cumulated rainfall field produces scaling relationships of spatial variance versus time and characteristic values for the scaling exponent. The reproduction of these values constitutes a basic requirement for spatial‐temporal field generators in order to model important properties of real rainfall fields. It is then shown, on theoretical grounds, what properties the instantaneous rainfall intensity fields must obey in order to reproduce the experimental observations and how the size of the observation domain affects the scaling relationships. Some current stochastic models of space‐time precipitation are finally discussed and analyzed in the light of the tools introduced, to show under what circumstances the models considered give acceptable results. Furthermore, it is shown that the assumption of an exponential time correlation function, used in many current rainfall models, is not compatible with observations.