bluuebunny/crossref_metadata_2025 huggingface dataset:
On N. Park's Analytical solution for steady state density‐ and mixing regime—dependent solute transport in a vertical soil column
Authors: Thiele Michael
Published: February 2004

Recently, Park [1996] presented an analytical solution for stationary one‐dimensional solute transport in a variable‐density fluid flow through a vertical soil column. He used the widespread Bear‐Scheidegger dispersion model describing solute mixing as a sum of molecular diffusion and velocity‐proportional mechanical dispersion effects. His closed‐form implicit concentration and pressure distributions thus allow for a discussion of the combined impact of molecular diffusion and mechanical dispersion in a variable‐density environment. Whereas Park only considered the example of vanishing molecular diffusion in detail, both phenomena are taken into account simultaneously in the present study in order to elucidate their different influences on concentration distribution characteristics. The boundary value problem dealt with herein is based on an upward inflow of high‐density fluid of constant solute concentration and corresponding outflow of a lower constant concentration fluid at the upper end of the column when dispersivity does not change along the flow path. The thickness of the transition zone between the two fluids appeared to strongly depend on the prevailing share of the molecular diffusion and mechanical dispersion mechanisms. The latter can be characterized by a molecular Peclet number Pe, which here is defined as the ratio of the column outflow velocity multiplied by a characteristic pore size and the molecular diffusion coefficient. For very small values of Pe, when molecular diffusion represents the exclusive mixing process, density differences have no impact on transition zone thicknesses. A relative density‐;dependent thickness increases with flow velocities (increasing Pe values) very rapidly compared to the density‐independent case, and after having passed a maximum decreases asymptotically to a constant value for the large Peclet number limit when mechanical dispersion is the only mixing mechanism. Hence the special transport problem analyzed gives further evidence for the importance of simultaneously considering molecular diffusion and mechanical dispersion in gravity‐affected solute transport in porous media.
Use of 3H/3He Ages to evaluate and improve groundwater flow models in a complex buried‐valley aquifer
Authors: Sheets Rodney A. , Bair E. Scott , Rowe Gary L.
Published: February 2004

Combined use of the tritium/helium 3 (3H/3He) dating technique and particle‐tracking analysis can improve flow‐model calibration. As shown at two sites in the Great Miami buried‐valley aquifer in southwestern Ohio, the combined use of 3H/3He age dating and particle tracking led to a lower mean absolute error between measured heads and simulated heads than in the original calibrated models and/or between simulated travel times and 3H/3He ages. Apparent groundwater ages were obtained for water samples collected from 44 wells at two locations where previously constructed finite difference models of groundwater flow were available (Mound Plant and Wright‐Patterson Air Force Base (WPAFB)). The two‐layer Mound Plant model covers 11 km2 within the buried‐valley aquifer. The WPAFB model has three layers and covers 262 km2 within the buried‐valley aquifer and adjacent bedrock uplands. Sampled wells were chosen along flow paths determined from potentiometric maps or particle‐tracking analyses. Water samples were collected at various depths within the aquifer. In the Mound Plant area, samples used for comparison of 3H/3He ages with simulated travel times were from wells completed in the uppermost model layer. Simulated travel times agreed well with 3H/3He ages. The mean absolute error (MAE) was 3.5 years. Agreement in ages at WPAFB decreased with increasing depth in the system. The MAEs were 1.63, 17.2, and 255 years for model layers 1, 2, and 3, respectively. Discrepancies between the simulated travel times and 3H/3He ages were assumed to be due to improper conceptualization or incorrect parameterization of the flow models. Selected conceptual and parameter modifications to the models resulted in improved agreement between 3H/3He ages and simulated travel times and between measured and simulated heads and flows.
Stochastic analysis of the relationship between topography and the spatial distribution of soil moisture
Authors: Yeh Pat J.‐F. , Eltahir Elfatih A. B.
Published: February 2004

This paper deals with the issue of the spatial horizontal variability of soil moisture in the root zone of a shallow soil at the large scale. The problem of water flow in the unsaturated zone is formulated so that topography appears explicitly as a forcing for the movement and redistribution of soil moisture. This formulation emphasizes the role of the lateral redistribution of water that is induced by topography. A stochastic theory is developed to relate the statistical distribution of soil moisture to that of elevation. This approach will ultimately facilitate the use of the readily available data sets describing topography for the purpose of defining the large‐scale distribution of soil moisture. The steady state horizontal distribution of soil moisture under homogeneous bare soil conditions is regulated by three distinct factors: topography, climate, and soil properties. First, topography, forces a distribution of soil moisture that tends to mimic the elevation field at large scales. The other two factors are the vertical divergence of water in response to the climate forcing (evaporation) and the capillary resistance to water movement. The climate forcing tends to smooth the spatial distribution of soil moisture. However, the capillary forces exerted by the soil matrix tend to resist displacement of water and hence exert adverse effects against the topography and climate forcings. The variance of the soil moisture distribution increases with the variance of the elevation field and decreases with the correlation scale of the elevation field and the magnitude of the climate forcing. The impact of capillary forces on the vertical fluxes of water is more significant than their impact on the topographically induced horizontal fluxes, owing to the larger hydraulic gradient in the vertical direction resulting from the disparity in scale between the vertical and horizontal directions.
Contribution of the TOPEX NASA Radar Altimeter to the global monitoring of large rivers and wetlands
Authors: Birkett Charon M.
Published: February 2004

For certain major rivers and wetlands, hydrological information can often be difficult to obtain due to the inaccessibility of the region, the sparse distribution of gauge stations, or the slow dissemination of data. Satellite radar altimeters have the potential to monitor height variations over inland waters. Here, it is shown that the NASA radar altimeter (NRA), currently operating on board the TOPEX/POSEIDON satellite, can successfully track both large wetlands and rivers of >1 km width. The coverage, performance, and limitations of the NRA altimeter are discussed with relevance to these regions, and the merits of utilizing the geophysical data records (GDRs) and the sensor data records (SDRs) are explored. Time series of relative water level variations, for the first ∼3.7 years of the mission, have been obtained for a selection of the world's largest rivers, and for major wetlands of international importance. Validation shows the results can be accurate to ∼11 cm rms, offering the potential to observe these regions as part of a long‐term hydrological monitoring program.
Scale invariance properties of the peak of the width function in topologically random networks
Authors: Agnese Carmelo , Criminisi Antonio , D'Asaro Francesco
Published: February 2004

Some scaling properties of the topological width function for an infinite population of networks obeying the random model are analyzed. A Monte Carlo procedure is applied to generate width functions according to the hypothesis of topological randomness. The probability distributions of both peak and distance to peak of the topological width functions, conditioned (1) on the network diameter λ and (2) on λ and parameter β=[log (2μ − l)]/log λ, are studied. The parameter β can be considered a shape factor of the network; indeed, low β values describe elongated networks, while high β values refer to fan‐like networks. Scale invariance for both random variables is established in the first case by using λ as a scale parameter. Also in the second case, scale invariance is observed for both the peak and the distance to peak of the topological width function; in particular, the invariance property for the peak involves a scaling function which is directly related to the shape factor β, allowing determination of the statistical similarity between random networks indexed by the same β. Then, a coarse‐graining procedure is applied to a set of 15,000 width functions with λ=512; a scaling behavior of peaks of the original width function and aggregated ones is observed over a wide range of aggregation scales. Consequently, a statistical self‐similarity for the peaks is also observed, which involved the same β‐related scaling function. Finally, possible implication of the present results on the hydrologic response, at the basin scale, is discussed.
Optimal conjunctive‐use operations and plans
Authors: Philbrick C. Russ , Kitanidis Peter K.
Published: February 2004

Heuristic or intuitive rules based on experience may not be efficient when applied to the management of water supply systems that contain both surface and subsurface storage. In particular, rules that assign subsurface storage the role of a backup to surface storage do not recognize the different capabilities of surface and subsurface storage. We demonstrate how to incorporate the different capabilities of surface and subsurface storage in appropriately cautious long‐term control of conjunctive‐use systems. We also demonstrate how these results may be used to evaluate the benefit of adding groundwater supplies to an existing surface water supply system.
Turbulence structure in a river reach with sand bed
Authors: Sukhodolov Alexander , Thiele Michael , Bungartz Heinz
Published: February 2004

Measurements and analysis of the three‐dimensional turbulence structure in a straight lowland river reach are presented. Accurate measurements of velocity profiles were taken with an acoustic Doppier velocimeter (ADV) and a multichannel micropropeller‐based system. It is shown that analytical expressions derived from investigations of laboratory open‐channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three‐dimensional. At the same time, clear differences of empirical parameters between river and laboratory flows are detected. It was established that river turbulence is isotropic for spatial scales smaller than the river depth. The data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents. Special attention is paid to the investigation of coherent structures. Their spatial scales were evaluated applying the conditional‐average procedure based on uw quadrant analysis. On average the scales of ejection and sweep events are as large as 1.5 times the flow depth.
Numerical analysis of flow and transport in a three‐dimensional partially saturated heterogeneous soil
Authors: Russo David , Zaidel Jacob , Laufer Asher
Published: February 2004

Numerical simulation of flow and transport of tracer and reactive (sorptive) solutes was used to investigate solute spreading and breakthrough in a realistic, three‐dimensional, heterogeneous, partially saturated soil, along with realistic weather boundary conditions, considering water uptake by plant roots. Results of the analyses suggest that the combination of spatially variable soil properties and periodic infiltration with substantial redistribution periods may impart a pseudokinetic behavior to the large‐scale sorption and create a three‐dimensional velocity fluctuation field with significant transverse components. The resultant velocity fluctuation field promotes lateral mixing of the solute, smooths out the extremes in solute convection, slows down the longitudinal spreading of the solute plume and increases its transverse spreading, reduces the skewing of the solute breakthrough, and leads to an approximately Fickian behavior of the transport. Water uptake by the plant roots is shown to increase the variability in the response of the flow domain and to produce a drier soil profile, with lower conductivity and steeper head gradient. Consequently, it reduces the solute velocity in the vertical direction, smooths its heterogeneity, and further diminishes the longitudinal spreading of the solute plume and the skewing of the solute breakthrough. Sorption also reduces the solute velocity in the vertical direction. For the physically plausible situation in which log retardation factor is negatively correlated with log saturated conductivity, sorption contributes to the variability in the solute velocity and increases both the longitudinal spreading of the solute plume and the skewing of the solute breakthrough.
Modeling ionic solute transport in melting snow
Authors: Harrington Robert , Bales Roger C.
Published: February 2004

A numerical model that includes the effects of mass transfer between mobile and immobile liquid phases, advection, hydrodynamic dispersion, and melt‐freeze episodes was developed to simulate ionic solute transport in melting snow. Model calibration using a tracer‐infused laboratory snowpack experiment yielded a dispersivity of 0.05 cm and a mobile‐immobile phase mass‐transfer coefficient of 4 × 10−6 s−1, but these parameter values are tentative because of the artificial nature of the experiment. The modeled concentration of meltwater flowing out the bottom of the snowpack was sensitive to residual water saturation, flow rate, dispersivity, mass‐transfer rate, and the initial distribution of solute within the pack, similar to experimental observations. The model was applied to a small watershed, and it was found that the ability of the model to accurately simulate solute movement depends on the validity of the assumption of one‐dimensional flow and on the accuracy of modeling the snowpack energy balance.
Scaling of constitutive relationships in unsaturated heterogeneous media: A numerical investigation
Authors: Desbarats A. J.
Published: February 2004

Numerical models of groundwater flow in the unsaturated zone require relative conductivity and moisture retention curves at a discretization scale much larger than that at which these constitutive relationships are measured. This study seeks to identify simple scaling laws linking parameters of block‐scale constitutive relationships with the parameters of their measurement‐scale counterparts represented here using Gardner‐Russo models. The two parameters of these models, saturated conductivity and pore‐size distribution parameter, are viewed as cross‐correlated regionalized variables with isotropic or anisotropic spatial correlation structures and are simulated using the sequential Gaussian method. Upscaled constitutive curves for steady gravity drainage through three‐dimensional heterogeneous domains are then constructed by repeating numerical flow experiments for a series of applied infiltration rates and plotting the resulting upscaled relative conductivities and average water saturations against average matric potentials. The experiments reveal clear log‐linear relationships between upscaled relative conductivities and average matric potentials, the slopes of which are used to define an upscaled pore‐size distribution parameter. The reciprocal of this quantity, which represents an upscaled characteristic capillary length, is very well approximated by a spatial power average of its measurement‐scale values. Furthermore, the averaging exponent is found to be the same as that associated with the upscaling of saturated conductivities over the same flow domains. When average matric potentials are normalized using this upscaled characteristic capillary length, relative conductivity and moisture saturation values for widely different flow field realizations collapse along well‐defined curves. However, for wet conditions the form of these curves differs from that of the models assumed at the measurement scale.
Capillary thermomechanics in serially porous media, with implications for randomly porous media
Authors: Miller Robert D.
Published: February 2004

Visions of a single mobile substance present as two rival phases more or less cleanly segregated by capillarity between rival strata of a serially porous medium (a “discontinuum”) imply explicit testable equations for externally measured capillary thermo‐osmotic pressures and capillary thermo‐osmosis, with implications for thermomechanical consolidation, dilation, and cracking. Underlying equations assume fluid phases governed by the laws of surface tension and viscous flow, moderated by an appropriate form of the Clapeyron equation. Derived phenomenological coefficients in macroscopic equations for steady coupled transports of mass and heat include only path‐length‐weighted fluid and heat conductances for rival domains and the heat of phase transformation. Expressions emphasize the phase‐specific nature of Onsager's reciprocity principle and apply to serial media held within permeameters set up for measuring either “isothermal” or “adiabatic” mass transport or held within sealed containers intended for measurements of “thermal conductivities.” Results clarify unmet challenges facing modelers of similar processes and attributes in randomly porous media.
Distribution of sulfate and organic carbon in a prairie till setting: Natural versus industrial sources
Authors: Fennell J. , Bentley L. R.
Published: February 2004

Large lateral variations in the concentrations of sulphate and dissolved organic carbon (DOC) have been observed near a sour gas (natural gas containing hydrogen sulfide) processing facility in southern Alberta, Canada. High concentrations of sulfate are correlated with high concentrations of dissolved organic carbon. The spatial distribution of the high concentrations indicates that the pattern is not related to facility activities. Drilling, chemical analyses, and electrical conductivity measurements produced results consistent with the depression‐focused recharge model that has been proposed for the Dalmeny site in Saskatchewan. Unleached soils contain gypsum and soluble sulfur, and the electrical conductivity is relatively high. In the leached settings, groundwaters have lower total dissolved solids, lower sulfate concentrations, and lower concentrations of DOC. Leached soils contain little soluble sulfur, and the electrical conductivity is relatively low. In contrast to the Dalmeny site, the soils at this study site showed little spatial variation in total organic carbon, because the till at the site contains numerous coal fragments. Spatial variability of sulphate concentrations may be due to natural or industrial sources. Stable isotopes are used to distinguish between the two sources. The highest concentrations of sulfate found in groundwater samples are mainly derived from weathering gypsum and pyrite. The highest concentrations of DOC are interpreted to be predominantly fulvic acid. The isotopic signature of industrial sulfur is found in groundwater near suspected contamination sources, in shallow soils, and possibly in plant material. This case study demonstrates the importance of integrated analysis when studying the effects of industrial contamination in complex prairie till environments and the effectiveness of using stable sulfur isotopes to distinguish between industrial impact and natural site conditions.
Detecting influential observations in nonlinear regression modeling of groundwater flow
Authors: Yager Richard M.
Published: February 2004

Nonlinear regression is used to estimate optimal parameter values in models of groundwater flow to ensure that differences between predicted and observed heads and flows do not result from nonoptimal parameter values. Parameter estimates can be affected, however, by observations that disproportionately influence the regression, such as outliers that exert undue leverage on the objective function. Certain statistics developed for linear regression can be used to detect influential observations in nonlinear regression if the models are approximately linear. This paper discusses the application of Cook's D, which measures the effect of omitting a single observation on a set of estimated parameter values, and the statistical parameter DFBETAS, which quantifies the influence of an observation on each parameter. The influence statistics were used to (1) identify the influential observations in the calibration of a three‐dimensional, groundwater flow model of a fractured‐rock aquifer through nonlinear regression, and (2) quantify the effect of omitting influential observations on the set of estimated parameter values. Comparison of the spatial distribution of Cook's D with plots of model sensitivity shows that influential observations correspond to areas where the model heads are most sensitive to certain parameters, and where predicted groundwater flow rates are largest. Five of the six discharge observations were identified as influential, indicating that reliable measurements of groundwater flow rates are valuable data in model calibration. DFBETAS are computed and examined for an alternative model of the aquifer system to identify a parameterization error in the model design that resulted in overestimation of the effect of anisotropy on horizontal hydraulic conductivity.
Selection and scheduling of rehabilitation alternatives for water distribution systems
Authors: Kleiner Yehuda , Adams Barry J. , Rogers J. Scott
Published: February 2004

Deterioration of the water distribution network due to aging and stress causes increased operation and maintenance costs, water losses, reduction in the quality of service, and reduction in the quality of water supplied. Since the most expensive component of a water supply system is the distribution network these increased costs can be substantial. In Kleiner et al. [this issue] an approach is proposed in which the water distribution network economics and hydraulic capacity are analyzed simultaneously over a predefined analysis period while explicitly considering the deterioration over time of both the structural integrity and the hydraulic capacity of every pipe in the system. In this paper a methodology is presented to implement this approach into a decision support system that facilitates the identification of an optimal rehabilitation strategy. The proposed methodology uses dynamic programming combined with partial and (sometimes) implicit enumeration schemes to select, for each pipe in an existing network, the rehabilitation alternative and the time of its implementation so as to minimize the cost of the rehabilitation investment and all maintenance costs over a predefined time horizon and subject to certain constraints. The methodology is validated (1) by comparing some results to those obtained from a restricted exhaustive enumeration and (2) by devising a survey in which utility managers were required to find an optimal rehabilitation strategy for a hypothetical distribution system and then comparing the managers' strategies with the one obtained from the proposed methodology.
Tracer‐pebble movement along a concave river profile: Virtual velocity in relation to grain size and shear stress
Authors: Ferguson R. I. , Wathen S. J.
Published: February 2004

Over 1400 tracer pebbles 16–256 mm in diameter were tracked for 2 years in six reaches of Allt Dubhaig, Scotland, a small gravel‐bed river along which shear stress and bed surface grain size decrease toward a local base level. Pebble movement was size‐selective both within and between reaches. Within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution. Particle shape has a minor secondary effect. A nondimensional grain velocity, averaged over the duration of competent flow, is used to compare different size classes and reaches. Over 90% of its variance is explained by relative grain size and reach Shields stress. The pattern of size selectivity is consistent with single‐event tracer results elsewhere, bedload trap data from our distal reach, and the concept of partial mobility. It provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level. The nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing.
Pore‐scale dilution of conservative solutes: An example
Authors: Cao J. , Kitanidis P. K.
Published: February 2004

We simulate flow and transport of a conservative nonsorbing tracer in an idealized periodic pore channel using finite element techniques. The concentration is computed; then the slowly varying concentration mean, variance, coefficient of variation, and reactor ratio are calculated through averaging over every cell. The coefficient of variation and reactor ratio are related and quantify the degree of dilution. Then a novel methodology is developed for the evaluation of macroscopic parameters (homogenization), including the variance decay coefficient, which measures the rate with which small‐scale concentration fluctuations tend to diminish, and the large‐time coefficient of proportionality between the concentration variance and the square of the mean concentration variance. The methodology is based on the solution of a steady advection‐dispersion problem in a single cell (which acts as a representative elementary volume); the computed result is then integrated in order to compute the macroscopic parameters. These parameters are compared with the parameters computed through direct simulation on a cell‐by‐cell basis, and they are found to be in reasonably good agreement. When the macroscopic parameters are used in the macroscopic equations, they produce estimates of the concentration mean and variance that are in agreement with the results of direct simulation.
Specific discharge covariance in a heterogeneous semiconfined aquifer
Authors: Zhu Jianting
Published: February 2004

In this study, covariance functions of specific discharge for both one‐dimensional and two‐dimensional flows in a semiconfined aquifer were derived with mean flow parallel to the formation bedding. Spectral theory was used to analyze a unidirectional mean flow field of infinite extent which is perturbed by one‐ and two‐dimensional statistically homogeneous hydraulic conductivity field. Relatively simple closed‐form analytical expressions requiring only some numerical integration for two‐dimensional flows were obtained for the longitudinal and transverse components of specific discharge covariance. The spatial, statistical structure of the specific discharge field was investigated in terms of the leakage factor and spatial structure of hydraulic conductivity field.
Gas transport in a confined unsaturated zone during atmospheric pressure cycles
Authors: Elberling Bo , Larsen Flemming , Christensen Steen , Postma Dieke
Published: February 2004

Gas transport induced by temporal pressure fluctuations in the atmosphere can be an important mechanism for transport of atmospheric oxygen within the unsaturated zone. Moreover, the presence of oxygen in the unsaturated zone may be a factor controlling oxidation of sulphide minerals and other redox processes. A field study was carried out in a glacial aquifer with a 10–12 m thick sandy unsaturated zone to explore gas exchange between the atmosphere and the unsaturated zone. The exchange occurs through a “geological window” in a till layer which covers the sandy unsaturated zone. Observed pressure distribution and oxygen concentrations within the unsaturated zone were compared to numerical simulations with SUTRA, a finite element and fluid density dependent groundwater flow model. The simulations were carried out by modeling the gas pressure distribution within the unsaturated zone based on atmospheric pressure time series. The spatial variation in permeability observed from borehole logging was implemented in the model. The analysis demonstrated a good match between the field observations and the numerical simulations. During an atmospheric pressure cycle, atmospheric oxygen migrated more than 10 md−1 horizontally in the capped unsaturated zone. The analysis shows that both the amplitude and the length of the period of pressure variations are important for the transport of oxygen, and it shows that the combined effects determine the extent of a subsurface zone where atmospheric oxygen can reach.
Free‐surface flow toward a well and interface upconing in stratified aquifers of random conductivity
Authors: Dagan G. , Zeitoun D. G.
Published: February 2004

Aquifers are known to display spatial variability of their properties. To account for the seemingly erratic variation and the uncertainty affecting the permeability, the common approach is to regard it as random and to characterize it statistically. The present note studies the steady axisymmetric water table flow toward a well and interface upconing due to freshwater pumping. The hydraulic conductivity is modeled as a stationary random function of the vertical coordinate, and the Dupuit assumption is adopted, It is found that the impact of heterogeneity upon the mean equivalent conductivity and the mean profiles is small. The standard deviation of these variables may be appreciable, depending on the hydraulic conductivity variance, on the ratio between the well head and the conductivity integral scale, and on the drawdown.
The sample areas of conventional and alternative time domain reflectometry probes
Authors: Ferré P. A. , Knight J. H. , Rudolph D. L. , Kachanoski R. G.
Published: February 2004

We define the sample area in the plane perpendicular to the long axis of conventional and alternative time domain reflectometry (TDR) probes based on the finite element numerical analysis of Knight et al. [1997] and the definition of spatial sensitivity ofKnight [1992]. The sample area of conventional two‐ and three‐rod probes is controlled by the rod separation. Two‐rod probes have a much larger sample area than three‐rod designs. Low dielectric permittivity coatings on TDR rods greatly decrease the sample area. The sample area of coated rod probes decreases as the relative dielectric permittivity of the surrounding medium increases. Two alternative profiling probes were analyzed. The separation of the metal rods of Hook et al. [1992] probes controls the size of the sample area. Reducing the height or width of the rods improves the distribution of sensitivity within the sample area. The relative dielectric permittivity of the probe body does not affect the sample size. The sample size of the Redman and DeRyck [1994] probe is also controlled by the rod separation. Two alternative surface probe designs [White and Zegelin, 1992; Selker et al., 1993] are shown to have similar sample areas.
Analytical solution for solute transport with depth‐dependent transformation or sorption coefficients
Authors: Flury Markus , Wu Q. Joan , Wu Laosheng , Xu Linlin
Published: February 2004

Reaction processes in soils, such as degradation and sorption, are often strongly depth dependent. Microbial activity and organic matter, which play an important role in reaction processes, are mostly confined to the top few decimeters of the soil. The purpose of this paper is to analyze the effects of space‐dependent reaction coefficients on transport of solutes through soil. We derive an analytical solution of the advection‐dispersion equation with depth‐dependent transformation or sorption coefficients. The solution is obtained in Laplace space and is inverted numerically. Breakthrough curves and concentration profiles of degrading and sorbing chemicals are presented to illustrate the influence of spatially variable reactions. It is demonstrated that the concentration profiles for depth‐dependent reactions vary between the two extreme cases of no and constant reactions, depending on how fast the reactions cease with increasing depth. The results emphasize the relevance of depth‐dependent transformation and sorption to transport of chemicals in soils. It is further shown that depth‐dependent sorption may cause a bimodal depth profile, even under steady state water flow regimes.
Base cation concentrations in subsurface flow from a forested hillslope: The role of flushing frequency
Authors: Burns Douglas A. , Hooper Richard P. , McDonnell Jeffrey J. , Freer James E. , Kendall Carol , Beven Keith
Published: February 2004

A 20‐m‐wide trench was excavated to bedrock on a hillslope at the Panola Mountain Research Watershed in the Piedmont region of Georgia to determine the effect of upslope drainage area from the soil and bedrock surfaces on the geochemical evolution of base cation concentrations in subsurface flow. Samples were collected from ten 2‐m sections and five natural soil pipes during three winter rainstorms in 1996. Base cation concentrations in hillslope subsurface flow were generally highest early and late in the storm response when flow rates were low, but during peak flow, concentrations varied little. Base cation concentrations in matrix flow from the 10 trench sections were unrelated to the soil surface drainage area and weakly inversely related to the bedrock surface drainage area. Base cation concentrations in pipe flow were lower than those in matrix flow and were also consistent with the inverse relation to bedrock surface drainage area found in matrix flow. The left side of the trench, which has the highest bedrock surface drainage area, had consistently lower mean base cation concentrations than the right side of the trench, which has the lowest bedrock surface drainage area. During moderate size rain events of about 20–40 mm, subsurface flow occurred only on the left side of the trench. The greater volume of water that has flowed through the left side of the trench appears to have resulted in greater leaching of base cations from soils and therefore lower base cation concentrations in subsurface flow than in flow from the right side of the trench. Alternatively, a greater proportion of flow that bypasses the soil matrix may have occurred through the hillslope on the left side of the trench than on the right side. Flushing frequency links spatial hillslope water flux with the evolution of groundwater and soil chemistry.
Evaluating the source and residence times of groundwater seepage to streams, New Jersey Coastal Plain
Authors: Modica E. , Buxton H. T. , Plummer L. N.
Published: February 2004

A conceptual model of the patterns and residence times of groundwater seepage to gaining streams indicates that groundwater seepage originates from sources that are both near and far from the stream. Consequently, the age of groundwater seepage across a stream‐channel transect increases from its banks to its center and becomes progressively older with distance downstream. A groundwater flow model and particle‐tracking analysis of the Cohansey River Basin in the New Jersey Coastal Plain supports this conceptual model and demonstrates that the orientation of the stream channels with respect to the regional groundwater flow direction, and the heterogeneities of the aquifer and stream‐channel patterns, can shift source area locations and distributions of groundwater residence time from those expected. Groundwater samples collected from stream transects were analyzed for nitrogen, representative of widespread agricultural land use in the basin in recent decades, and for chlorofluorocarbons, used to estimate groundwater ages. The patterns of nitrogen concentration and the age of groundwater entering the stream channel corroborate model inferences. The conceptual model of groundwater seepage to streams presented herein is relevant to unconfined aquifer systems with gaining streams and demonstrates how nonpoint‐source contaminants are transported to streams by groundwater. Results are useful for the design of programs needed to monitor stream‐water quality.
Stability and damping in a dynamical model of hillslope hydrology
Authors: Brandes David , Duffy Christopher J. , Cusumano Joseph P.
Published: February 2004

A stability analysis is performed on the two‐state variable, integral‐balance model of hillslope hydrology developed by Duffy [1996]. The motivation for the research is to develop a physically based, low‐dimensional representation of inflow/outflow behavior within a hillslope‐stream setting. Stability criteria are developed for the model equilibria and are evaluated using the results of numerical solutions of Richards' equation for a convex‐concave hillslope geometry. We show that for homogeneous hillslopes of three widely varying soil types the single moisture equilibrium is classified as a stable node for low precipitation rates and a stable spiral for wet conditions. The spiral equilibrium indicates that the hillslope system is lightly damped, and transient oscillations of the state variables are expected for high precipitation rates. The timescale of these oscillations is of the order of several days to weeks for the model hillslopes examined. Furthermore, we demonstrate that the model contains a Hopf bifurcation from a stable static equilibrium to a stable limit cycle. The amplitude, phase, and frequency of the limit cycle are determined analytically using second‐order averaging. However, this behavior is shown to be nonphysical for the particular homogeneous soils and hillslope geometry investigated. Implications of lightly damped behavior in the hillslope system include moisture oscillations in the field under wet conditions and difficulty in numerical solution of Richards' equation.
Lateral subsurface flow pathways in a semiarid Ponderosa pine hillslope
Authors: Newman Brent D. , Campbell Andrew R. , Wilcox Bradford P.
Published: February 2004

The mechanisms controlling lateral subsurface flow in semiarid environments have received relatively little attention despite the fact that lateral subsurface flow can be an important runoff process in these environments. The objective of the current study is to better understand lateral subsurface flow process in semiarid environments. Natural chloride, dissolved organic carbon, and stable isotope (δD and δ18O) tracers were used to investigate the lateral subsurface flow process and the chemical changes that occur as a result of lateral subsurface flow. Observed differences in chemistry between soil matrix water and lateral subsurface flow were large (for example, chloride concentrations in matrix soil water samples were >200 mg/L, compared with only 2 mg/L in lateral subsurface flow samples obtained at the same time). This difference in chemistry is indicative of a two‐domain flow system in which macropores conduct lateral subsurface flow that is not in chemical or hydrological equilibrium with the soil matrix. The size of precipitation events appeared to have a strong influence on the variations in old/new water percentages, and examples of both old and new water dominated events were observed. There were also large variations in the chemistry of lateral subsurface flow with time. For example, chloride and dissolved organic carbon concentrations were 10 and 70 times greater, respectively, under saturated conditions than under unsaturated conditions.
Yeast N‐glycanase distinguishes between native and non‐native glycoproteins
Authors: Hirsch Christian , Misaghi Shahram , Blom Daniël , Pacold Michael E , Ploegh Hidde L
Published: January 2004

N‐glycanase from Saccharomyces cerevisiae (Png1) preferentially removes N‐glycans from misfolded proteins. The ability of Png1 to distinguish between folded and misfolded glycoproteins is reminiscent of substrate recognition by UDP‐glucose glycoprotein glucosyl transferase, an enzyme that possesses this trait. The only known in vivo substrates of Png1 are aberrant glycoproteins that originate in the endoplasmic reticulum, and arrive in the cytoplasm for proteasomal degradation. The substrate specificity of Png1 is admirably suited for this task.
Health services research: what is it and what does it offer?
Authors: Scott I. , Campbell D.
Published: February 2004

Abstract

Over the last 20 years, clinical medicine has witnessed rapid expansion in its underlying evidence base, greater demand for accountability in clinicians’ use of limited resources and increasing societal expectation for health care that confers proven benefit at reasonable cost to all eligible recipients.

Health services research, also referred to as the clinical evaluative sciences, has grown in response to the need for objective empirical analysis of the modern health system’s ability to deliver effective, efficient, equitable and safe care and to further the health and well‐being of whole populations. In this article we provide an overview of the aims, methods and outputs of this burgeoning new discipline. (Intern Med J 2002; 32: 91–99)
Resection of residual pulmonary masses after chemotherapy in patients with metastatic non‐seminomatous germ cell tumours
Authors: Horvath L. G. , McCaughan B. C. , Stockler M. , Boyer M. J.
Published: February 2004

Abstract

Background: Resection of residual post‐chemotherapy pulmonary masses in patients with non‐seminomatous germ cell tumours gives therapeutic benefit and prognostic information.

Aim: This study was undertaken to review the experience of this intervention in a single teaching hospital.

Methods: The Germ Cell Database of the Sydney Cancer Centre was searched for all patients who had undergone excision of pulmonary metastases. These patient records were subsequently reviewed.

Results: Between 1976 and 1999, 15 patients underwent a combined total of 19 thoracotomies for resection of residual tumour mass after cisplatin‐based chemotherapy. The primary tumour histology included mature teratoma in 47% (7 of 15) of patients. Prior to chemotherapy, 73% (11 of 15) of patients had elevated serum levels of alpha‐fetoprotein (median 180 ng/mL) and 60% (9 of 15) of patients had elevated beta‐human chorionic gonadotropin (median 672 IU/L). The median length of hospital stay related to thoracotomy was 7 days. There were two surgical complications, a prolonged air leak and a residual pleural effusion. Pathology of residual pulmonary masses revealed necrosis alone in 37% (7 of 19) of procedures, mature teratoma alone in 32% (6 of 19) of procedures and viable tumour in 32% (6 of 19) of procedures. Of those with viable tumour, three achieved long‐term complete response (CR), two died of progressive disease (PD) and one is alive with PD. Of those with teratoma, two achieved CR and one relapsed. The long‐term CR rate was 80% (12 of 15 patients). The median follow up was 10 years (range 0.75–17.5 years). Four patients died, two of PD and two of cardiovascular disease while in CR.

Conclusion: At this institution, thoracotomy for residual pulmonary masses was well tolerated, with a high cure rate. (Intern Med J 2002; 32: 79–83)
Dipole interactions and electrical polarity in nanosystems: The Clausius–Mossotti and related models
Authors: Allen Philip B.
Published: February 2004

Point polarizable molecules at fixed spatial positions have solvable electrostatic properties in classical approximation, the most familiar being the Clausius–Mossotti (CM) formula. This paper generalizes the model and imagines various applications to nanosystems. The behavior is worked out for a sequence of octahedral fragments of simple cubic crystals, and the crossover to the bulk CM law is found. Some relations to fixed moment systems are discussed and exploited. The one-dimensional dipole stack is introduced as an important model system. The energy of interaction of parallel stacks is worked out, and clarifies the diverse behavior found in different crystal structures. It also suggests patterns of self-organization which polar molecules in solution might adopt. A sum rule on the stack interaction is found and tested. Stability of polarized states under thermal fluctuations is discussed, using the one-dimensional domain wall as an example. Possible structures for polar hard ellipsoids are considered. An idea is formulated for enhancing polarity of nanosystems by intentionally adding metallic coatings.
Grand canonical Monte Carlo simulation of argon adsorption at the surface of silica nanopores: Effect of pore size, pore morphology, and surface roughness
Authors: Coasne B. , Pellenq R. J.-M.
Published: February 2004

Argon adsorption (77 K) in atomistic silica nanopores of various sizes and shapes has been studied by means of grand canonical Monte Carlo simulations (GCMC). We discuss the effects of confinement (pore size), pore morphology (ellipsoidal, hexagonal, constricted pore), and surface texture (rough/smooth) on the thickness variation of the adsorbed film with pressure onto the disordered inner surface of porous materials (usually called t-plot or t-curve). We show that no confinement effect occurs when the diameter of the regular cylindrical pore is larger than 10 nm. For pores smaller than 6 nm, we find that the film thickness increases as the pore size decreases. We show that the adsorption isotherm in the rough pore can be described as the sum of an adsorbed amount similar to that found for a smooth pore (of the same radius) and a constant contribution due to atoms “trapped” in the infractuosities of the rough surface which act as a microporous texture. Simulation snapshots for Ar adsorption in hexagonal and ellipsoidal smooth pores indicate that at low pressures the gas/adsorbate interface retains memory of the pore shape and becomes cylindrical prior to the capillary condensation of the fluid in the pore. The film thickness in the hexagonal pore is close to that obtained for a cylindrical pore having a similar dimension. By contrast, we find that the film thickness for an ellipsoidal pore is always larger than that for an equivalent cylindrical pore (having the same length and volume but a circular section). We show that this effect strengthens as the pore size decreases and/or the pore asymmetry increases. Ar adsorption in a cylindrical constricted pore shows that the presence of the narrower part considerably modifies the adsorption mechanism. Finally, we report GCMC simulations of Ar adsorption (77 K) on a plane silica reference substrate for different intermolecular potentials. We discuss the effect of the interaction on the shape of the adsorption isotherm and compare our results with experiments.
Self-affine roughness influence on the friction coefficient for rubbers onto solid surfaces
Authors: Palasantzas George
Published: February 2004

In this paper we investigate the influence of self-affine roughness on the friction coefficient μf of a rubber body under incomplete contact onto a solid surface. The roughness is characterized by the rms amplitude w, the correlation length ξ, and the roughness exponent H. It is shown that with increasing surface roughening at short and/or long length scales (decreasing H and/or increasing ratio w/ξ, respectively), the maximum of the friction coefficient μf shifts to lower sliding velocities. The latter occurs only for conditions of incomplete contact for small contact length scales λ (<ξ). In all cases, the friction coefficient μf increases monotonically with decreasing roughness exponent H and/or increasing roughness ratio w/ξ and attains its maximum value for sufficiently large contact length scales (≫ξ).