bluuebunny/crossref_metadata_2025 huggingface dataset:
Non‐Gaussian Particle Tracking: Application to scaling of transport processes in heterogeneous porous media
Authors: Scheibe T. D. , Cole C. R.
Published: February 2004

Observation of scale‐dependent dispersion and non‐Gaussian concentration distributions under field‐scale transport in heterogeneous media motivates investigation of alternatives to the classical Fickian diffusion model. A numerical model of non‐Gaussian transport, employing a particle‐tracking approach, is examined here. Non‐Gaussian behavior is represented through modeling of the correlation between components of the random dispersive vector applied at each time step in the random walk particle‐tracking algorithm. This model is applied to a number of highly detailed, synthetic hydraulic conductivity fields. By simultaneously modeling particle movement at two different scales, the impact of the small‐scale heterogeneity on the correlation structure of the dispersive vectors at the larger scale can be observed directly. It is shown that accounting for the correlation structure of the dispersive vectors greatly improves prediction of arrival distributions (breakthrough curves) over the standard Fickian model. This improvement is especially evident when the model scale is not much larger than the correlation length of hydraulic conductivity but persists at relatively large transport distances. In particular, the non‐Gaussian model is better able to represent the late arrival tail, which has important implications for the design and implementation of aquifer remediation methods. It is also shown that the apparent scale dependence of field dispersivity results at least in part from neglecting the correlation structure of local velocity perturbations.
The 1978 Borden tracer experiment: Analysis of the spatial moments
Authors: Farrell D. A. , Woodbury A. D. , Sudicky E. A.
Published: February 2004

A reanalysis of the 1978 tracer experiment at Borden, Ontario, is performed using spatial moments. The tracer experiment involved the uniform injection of chloride ions into an aquifer, and the resulting plume was allowed to migrate under natural gradient conditions. Detailed monitoring of the evolving plume was then performed in three dimensions. The work of Sudicky et al. (1983) showed that the solute cloud evolved as two plumes owing to variations in hydraulic conductivity at the site. In this paper we compute the zeroth through the fourth spatial moments of the plume in the lower hydraulic conductivity zone. From these moments the solute mass of the plume, the velocity of the center of mass, the dispersion, the skew, and the kurtosis of the plume are calculated. These results are compared with the results of Sudicky et al. as well as with the theoretical results of Gelhar et al. (1979) and Naff (1990). Our computed plume velocity and apparent horizontal transverse macrodispersivity are in agreement with the estimates of Sudicky et al.; however, our apparent longitudinal macrodispersivity estimate exceeds that of Sudicky et al. by a factor of 2. The apparent horizontal transverse macrodispersivity obtained is also quite similar to that of Freyberg (1986). At early times, the plume is found to be positively skewed as well as platykurtic. This supports the findings of Gelhar et al. and contradicts those of Naff.
How water moves in a water repellent sandy soil: 2. Dynamics of fingered flow
Authors: Ritsema Coen J. , Dekker Louis W.
Published: February 2004

The dynamics of fingered flow in a water repellent sandy soil was studied in the field by sampling 10 vertical trenches in a 1‐year cycle. In dry soil, fingers were formed in those places in the top layer which have the lowest degree of potential water repellency. The finger diameters varied roughly between 10 and 50 cm depending on the sequence of weather conditions. The fingers were wet in the topsoil and increasingly drier with depth. In none of the trenches sampled were there any serious indications of finger merger. The actual water repellent soil volumes between fingers were excluded from the transport of water and solutes for at least several hours. The temporal and spatial variability of these actual water repellent soil volumes is illustrated and evaluated with respect to simulation model development.
The relationships between U.S. streamflow and La Niña Events
Authors: Dracup John A. , Kahya Ercan
Published: February 2004

In an earlier study of the teleconnection between streamflow and the warm (El niño) phase of the El Niño/Southern Oscillation (ENSO) cycle, we found a strong relationship evident in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. In this present study we have examined the same four regions for a relationship between streamflow and the cold (La niña) phase of the Southern Oscillation (SO). Invariably, we found evidence of strong and consistent streamflow responses to La niña events within the study regions. In each of the four regions, the strongest La niña signal occurred at the same time of year as had the El niño signal in their respective years. The sign of the seasonal streamflow anomaly associated with the La niña events is the opposite of that associated with the El niño events. This documents the existence of the biennial tendency related to the SO in the streamflow anomaly, which is expected, since La niña/El niño are opposite phases of the ENSO cycle. Finally, the relationships between streamflow and La niño/El niño were found to be statistically significant, based on the hypergeometric distribution. The results of this study demonstrate coherent, consistent, and significant midlatitude streamflow responses to the tropical SO phenomenon. This confirms the results of previous climatological studies that have examined the extratropical teleconnections from a hydrological and meteorological perspective.
The importance of pulse duration in pulse test analysis
Authors: Connell L. D.
Published: February 2004

The available approaches for analyzing pulse tests assume that the initiating pressure pulse is instantaneous. However, in practice this is difficult to achieve due to equipment response lags. A new model is derived which includes pulses of finite duration. This model is an extension of the instantaneous model of Bredehoeft and Papadopulos (1980), who presented an integral expression to explain behavior during a pulse test. In this paper, equations are derived for both instantaneous and finite duration cases in Laplace space. These equations are numerically inverted to real space using the algorithm of Stehfest (1970). The finite duration pulse test model differs from the instantaneous model in an integral term which acts as a correction to the flow response. In order to numerically invert the finite duration equation the integral term is first numerically evaluated. An approximation is introduced to enable the explicit numerical integration of this term. The use of the instantaneous pulse model to analyze pulse tests which involve a finite duration results in error in the estimation of formation hydraulic properties. In one example considered, where the pulse was 30 s, the error was near zero for a specific storage capacity less than 1 × 10−9 m−1, and the formation conductivity less than 1 × 10−9 m s−1. However, for the same test conditions, a conductivity of 1 × 10−8 m s−1 and a specific storage capacity of similar magnitude would result in an error of approximately 50% in conductivity.
A note on fractional dimension analysis of constant rate interference tests
Authors: Chakrabarty Chayan
Published: February 2004

This paper introduces a new approach for a fractional dimension analysis of constant rate interference (or cross‐hole) tests. The method does not require type curve matching and consists of a straight‐line analysis approach. The approach assumes infinitesimal source conditions at both pumping and monitoring wells. The hydraulic head solution at the monitoring well is differentiated with respect to time and rearranged so that a linear relation between the variables is obtained. A semilog plot of the derivative group against time−1 produces a straight line; the slope and intercept of this line may be combined with data from the pumping well for a fractional dimension analysis of the monitoring well data.
Benefits transfer in a random utility model of recreation
Authors: Parsons George R. , Jo Kealy Mary
Published: February 2004

We divide a data set on lake recreation in Wisconsin into two nonoverlappmg samples, Milwaukee residents and non‐Milwaukee residents. We then consider several hypothetical benefit transfers from a non‐Milwaukee‐based random utility model to Milwaukee residents. All transfers are for measuring water quality improvements. We consider transfers in which we assume no information on Milwaukee residents, limited (no behavioral) information, and some behavioral information. We consider simple transfers, model transfers, and updated transfers. In all cases we test the viability of the transfer by comparing it with benefits estimated from a random utility model estimated over the Milwaukee sample, which in effect is our “true” model. The values from the model and updated transfers typically deviated less than 10% from the true values. Confidence intervals are estimated by using the Krinsky‐Robb procedure for all of the benefit measures computed from the random utility model.
Retardation of sorbing solutes in fractured media
Authors: Wels Christoph , Smith Leslie
Published: February 2004

Sorption of a reactive solute during transport in a fractured geologic medium is analyzed under the assumption that within each fracture, retardation varies in proportion to the product of a surface distribution coefficient and the specific surface area of the fracture (i.e., its surface area‐to‐volume ratio). This approach is analogous to the KD model commonly adopted for granular porous media. Numerical migration experiments in discrete fracture networks show that at the plume scale, retardation is both nonuniform and anisotropic. Different segments of the plume, or equivalently different breakthrough fractions at a downstream boundary, are retarded to a different degree. The degree to which various breakthrough fractions are retarded varies as a function of the orientation of the mean hydraulic gradient relative to the orientation of the fracture sets. This variation can be described in the form of a retardation ellipse. The mean arrival time is the only breakthrough fraction that is consistently described by a uniform, isotropic retardation factor. The degree of nonuniformity and anisotropy in the retardation factor is controlled largely by the difference in the mean apertures of each fracture set, the standard deviation in fracture aperture, and the orientation of the fracture sets relative to the mean hydraulic gradient. A larger variability in fracture aperture, or in the orientation of fractures within each set, promotes nonuniform retardation while reducing the degree of anisotropy in the retardation factor. Our results suggest that a transport model based on the conventional advection‐dispersion equation, using a uniform retardation factor, may be conceptually incorrect, even for a dense fracture network. While the effects of nonuniform retardation modify the dispersive flux in a way that could be described by “effective” dispersion coefficients, it is our opinion that a model formulation is needed that clearly separates chemical effects from those due to hydrodynamic dispersion.
Flow path studies in forested watersheds of headwater tributaries of Brush Brook, Vermont
Authors: Ross Donald S. , Bartlett Richmond J. , Magdoff Frederick R. , Walsh Gregory J.
Published: February 2004

An investigation was undertaken into how headwater tributaries of Brush Brook, Vermont, could have average pH differences of almost two units (4.75 and 6.7). Sampling along four tributaries revealed that most of one tributary, below an area of seeps, had consistently higher pH, Ca2+, Mg2+, and K+, and lower Al than other sites. Bedrock mapping showed numerous fractures in vicinity of the seeps. A portion of this tributary's watershed and a portion of an acid tributary's watershed were intensively mapped for soil depth. Sampling showed the widespread existence of dense basal till in the watershed of the acid tributary but none in that of the near‐neutral stream. Lateral flow, found above the dense till, was chemically similar to that of the acid tributary and to solutions sampled from soil B horizons. There were no differences in the average pH of nonseep soils sampled from either watershed. Flow paths are hypothesized to be through the B horizons in the acid tributaries and from below the soil profile in the near‐neutral tributary. The acid catchment should be more sensitive to environmental change.
Transient analyses of an interceptor trench
Authors: Murdoch Lawrence C.
Published: February 2004

Unlike radial flow to a well the pattern of flow to a trench changes with time. Flow vectors at early times are normal to the surface of the trench and affect a small area, whereas after a long period of pumping they converge radially from large distances. This changing flow pattern controls performance, and ignoring the transient effects can result in errors when predicting flows in the vicinity of a trench. The method of instantaneous source functions is used to obtain transient analytical solutions to a variety of two‐ or three‐dimensional problems related to interceptor trenches. Drawdown in the vicinity of a trench that is pumped at constant rate and discharge from a trench that is held at constant drawdown are given as functions of time. Travel times to trenches in regional flow fields are also obtained and expressed in dimensionless form. Applications of the results include improved predictions of the performance of environmental applications of interceptor trenches and estimation of aquifer parameters from the analysis of pump tests from trenches.
An Analytic Approach to Modeling Land‐Atmosphere Interaction: 2. Stochastic Formulation
Authors: Entekhabi Dara , Brubaker Kaye L.
Published: February 2004

Feedback processes in land‐atmosphere interaction in a stochastic model of the soil and the turbulently mixed near‐surface atmosphere hydrothermal balance are identified. The model is forced by perturbations about the mean wind speed that advects air and water mass to the region. The fluctuations are routed within the system through the state dependence of the radiative and thermal fluxes. Variabilities and covariabilities that are evident in the output of the system are physically consistent and in proportion due to the hydrologic and micrometeorologic constraints. The partitioning of the atmospheric radiative and moisture forcing at the land surface is found to have significant influence on the variability and covariability of temperature and moisture in the soil and atmosphere. Through a series of experiments with the stochastic model of land‐ atmosphere interaction, the pathways and relative strengths of the feedbacks are identified.
Nonlocal properties of nonuniform averaged flows in heterogeneous media
Authors: Indelman P. , Abramovich B.
Published: February 2004

The properties of nonuniform average potential flows in media of stationary random conductivity are studied. The mathematical model of average flow is derived as a system of governing equations to be satisfied by mean velocity and mean head. The averaged Darcy's law determines the effective conductivity as an integral operator of the convolution type, relating the mean velocity to the mean head gradient in a nonlocal way. In Fourier domain the mean velocity is proportional to the mean head gradient. The coefficient of proportionality is referred to as the effective conductivity tensor and is derived by perturbation methods in terms of functionals of the conductivity spatial moments. It is shown that the effective conductivity cannot be defined uniquely for potential flows. However, this nonuniqueness does not affect the spatial distribution of the mean head and velocity. Analytical expressions of the effective conductivity tensor are derived for two‐ and three‐dimensional flows and for exponential and Gaussian correlations of isotropic conductivity. The fundamental solution of the governing equations in effective media (mean Green function) is calculated for the same cases. Two new asymptotic models of the averaged Darcy's law are developed to be applicable to large and small scales of heterogeneity. Several asymptotic expansions of tue two‐dimensional and three‐dimensional mean Green functions are derived for exponential and Gaussian correlations.
Modeling biodegradation of residual petroleum in a saturated porous column
Authors: Nicol Jean‐Paul , Wise William R. , Molz Fred J. , Benefield Larry D.
Published: February 2004

A framework for numerically modeling the biodegradation of petroleum hydrocarbons within a flooded column of porous media in which a residual saturation of petroleum is present is developed. The petroleum product of interest is considered to be representable, at a desired level of detail, as a mixture of a finite number of classes of hydrocarbon components. Of these classes, all but one are assumed to be biodegradable by indigenous microorganisms. A specific microbial population is associated with each degradable class. Microbes growing at the oil‐water interface take up the hydrocarbon components of the petroleum through the use of extracellular pseudosolubilization factors, which facilitate transport from the oil‐water interface into the cells. Minimum Monod kinetics is used to model many of the biological processes. The transport of dissolved oxygen, nitrogen, and phosphorus through the column provides the microorganisms with the required electron acceptor and nutrients for aerobic metabolism.
Effects of digital elevation model map scale and data resolution on a topography‐based watershed model
Authors: Wolock David M. , Price Curtis V.
Published: February 2004

The effects of digital elevation model (DEM) map scale and data resolution on watershed model predictions of hydrologic characteristics were determined for TOPMODEL, a topography‐based watershed model. The effects of topography on watershed hydrology are represented in TOPMODEL as the distribution of ln (a/tan B), where ln is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the gravitational gradient. The minimum, maximum, mean, variance, and skew values of the ln (a/tan B) distribution were computed from 1:24,000‐scale (24K) DEMs at 30‐ and 90‐m resolutions and from 1:250,000‐scale (250K) DEMs at 90‐m resolution for 71 areas in Pennsylvania, New York, and New Jersey. An analysis of TOPMODEL showed that model predictions of the depth to the water table, the ratio of overland flow to total flow, peak flow, and variance and skew of predicted streamflow were affected by both the DEM map scale and data resolution. Further TOPMODEL analyses showed that the effects of DEM map scale and data resolution on model predictions were due to the sensitivity of the predictions to the mean of the ln (a/tan B) distribution, which was affected by both DEM map scale and data resolution. DEM map scale affected the mean of the ln (a/tan B) distribution through its influence on the mean of the ln (a) distribution, which characterizes land‐surface shape, and the mean of ln (1/tan B) distribution, which characterizes land‐surface slope. DEM resolution, in contrast, affected the mean of the ln (a/tan B) distribution primarily by its influence on the mean of the ln (a) distribution.
When enough is enough: The worth of monitoring data in aquifer remediation design
Authors: James Bruce R. , Gorelick Steven M.
Published: February 2004

Given the high cost of data collection at groundwater contamination remediation sites, it is becoming increasingly important to make data collection as cost‐effective as possible. A Bayesian data worth framework is developed in an attempt to carry out this task for remediation programs in which a groundwater contaminant plume must be located and then hydraulically contained. The framework is applied to a hypothetical contamination problem where uncertainty in plume location and extent are caused by uncertainty in source location, source loading time, and aquifer heterogeneity. The goal is to find the optimum number and the best locations for a sequence of observation wells that minimize the expected cost of remediation plus sampling. Simplifying assumptions include steady state heads, advective transport, simple retardation, and remediation costs as a linear function of discharge rate. In the case here, an average of six observation wells was needed. Results indicate that this optimum number was particularly sensitive to the mean hydraulic conductivity. The optimum number was also sensitive to the variance of the hydraulic conductivity, annual discount rate, operating cost, and sample unit cost. It was relatively insensitive to the correlation length of hydraulic conductivity. For the case here, points of greatest uncertainty in plume presence were on average poor candidates for sample locations, and randomly located samples were not cost‐effective.
Aggregation‐disaggregation properties of a stochastic rainfall model
Authors: Bo Zhiquan , Islam Shafiqul , Eltahir E. A. B.
Published: February 2004

A statistical approach based on the modified Bartlett‐Lewis rectangular pulses model is presented to disaggregate rainfall statistics from daily data. Six model parameters are estimated from 24‐ and 48‐hour accumulated rainfall data. Based on these estimated parameters, in addition to reproducing 24‐ and 48‐hour statistics, the model is shown to infer 1‐, 2‐, 6‐, and 12‐hour historical statistics satisfactorily. An upper limit for disaggregation scale (about 2 days) for this model has been identified. This characteristic behavior of the model is related to the power law dependence of the power spectrum for timescales smaller than 2 days. A detailed comparison between observed and modeled statistics of rainfall data is presented for two rain gages, one from central Italy and the other from the midwestern United States.
Transport of bacteria in an aquifer sand: Experiments and model simulations
Authors: Tan Y. , Gannon J. T. , Baveye P. , Alexander M.
Published: February 2004

Experiments were carried out to determine the breakthrough of bacteria through a saturated aquifer sand at three flow velocities and three cell concentrations. Bacteria were either suspended in deionized water or 0.01 mol L−1 NaCl solution. Bacterial transport was found to increase with flow velocity and cell concentration but was significantly retarded in the presence of 0.01 mol L−1 NaCl. A mathematical model based on the advection‐dispersion equation was formulated to describe bacterial transport and retention in porous media. The transport equations for bacteria were solved using the finite difference Crank‐Nicolson scheme combined with Newton‐Raphson iterations. The best fit of the numerical model to the experimental data was obtained using the downhill simplex optimization technique to minimize the sum of the squares of deviations between model predictions and experimental data by varying three parameters. This numerical model was found to describe the experimental data very well under all the experimental conditions tested. An alternative model (also based on the advection‐dispersion equation) was tested against all the experimental data sets, but it did not represent the experimental data as well as the model proposed in this paper.
An Economic Analysis of Clean Water Act Issues
Authors: Lyon Randolph M. , Farrow Scott
Published: February 2004

This paper presents an economic analysis of the incremental benefits and costs of clean water programs. Benefit estimates are principally based on contingent valuation research on willingness to pay for clean freshwater. These data are supplemented with estimated benefits for municipal and industrial water withdrawals and saltwater fishing. Cost estimates are largely based on Environmental Protection Agency “needs” data and on studies of nonpoint source pollution control practices. The analysis suggests that Clean Water Act programs, as currently planned, may have incremental costs that exceed their incremental benefits. Several alternative scenarios, with different benefit and cost assumptions, support this conclusion. Importantly, the policy options examined in this study generally are not the least cost approach for attaining water quality goals. The analysis suggests that the focus on municipal treatment facilities, as opposed to nonpoint source practices, is unlikely to be cost effective as a national strategy. The analysis also suggests that the concept of “needs” has limitations when viewed in a benefit‐cost context. Finally, better information linking planned expenditures (costs) to expected ambient quality improvements (benefits) appears necessary to plan efficient programs.
Spatial Prediction of Soil Salinity Using Electromagnetic Induction Techniques: 1. Statistical Prediction Models: A Comparison of Multiple Linear Regression and Cokriging
Authors: Lesch Scott M. , Strauss David J. , Rhoades James D.
Published: February 2004

We describe a regression‐based statistical methodology suitable for predicting field scale spatial salinity (ECe) conditions from rapidly acquired electromagnetic induction (ECa) data. This technique uses multiple linear regression (MLR) models to estimate soil salinity from ECa survey data. The MLR models incorporate multiple ECa measurements and trend surface parameters to increase the prediction accuracy and can be fitted from limited amounts of ECe calibration data. This estimation technique is compared to some commonly recommended cokriging techniques, with respect to statistical modeling assumptions, calibration sample size requirements, and prediction capabilities. We show that MLR models are theoretically equivalent to and cost‐effective relative to cokriging for estimating a spatially distributed random variable when the residuals from the regression model are spatially uncorrelated. MLR modeling and prediction techniques are demonstrated with data from three salinity surveys.
Continuous Time Stochastic Analysis of Groundwater Flow in Heterogeneous Aquifers
Authors: Hantush Mohamed M. , Mariño Miguel A.
Published: February 2004

The problem of depth‐averaged groundwater flow in heterogeneous aquifers is looked at from a stochastic point of view. The Galerkin finite element version of the linearized stochastic equation governing the flow is solved analytically in time using an eigenvalue‐eigenvector technique. The stochastic solution, which is valid for small variance of aquifer log transmissivity, relates spatial and temporal variabilities of aquifer head to aquifer heterogeneity, stochastic recharge, and random initial head. The computational effort requires the evaluation of integrals of matrices whose elements are linear functions of the nodal mean heads. The solutions are obtained for the exact (i.e., continuous) and quasi‐steady approximation of the mean head. Aquifer‐head temporal covariances are evaluated using standard matrix operations once the storage matrix is inverted and the associated generalized eigenvalue problem is solved. Implication of the level of spatial discretization on the performance of the solution is examined, and the influence of aquifer heterogeneity on spatial and temporal variances of hydraulic heads is investigated in the presence of a semipervious boundary.
Ion dynamics of a freezing soil monitored in situ by time domain reflectometry
Authors: Lundin L.‐C. , Johnsson H.
Published: February 2004

In situ measurements of the electrical conductivity of the soil solution are cumbersome, especially under frozen conditions, and often involve destruction of the soil volume sampled. The objective of this work was to use time domain reflectometry (TDR) to study the dynamics of ions in the soil solution caused by freezing and thawing. The data set consisted of field‐measured TDR and temperature records in four soil profiles in a loamy soil. Bulk electrical conductivity and electrical conductivity were evaluated from the TDR traces. Freezing caused a temporary twofold to fourfold increase in electrical conductivity. Strong evidence of a downward transport of ions during freezing was found. The transport was attributed to thermally induced regelation. Infiltration resulted in an increased electrical conductivity. TDR was shown to be a promising technique for monitoring the electrical conductivity of the soil solution, in both frozen and unfrozen soils.
Bed Load Sediment Transport in an Ephemeral Stream and a Comparison with Seasonal and Perennial Counterparts
Authors: Reid Ian , Laronne Jonathan B.
Published: February 2004

Bed load sediment flux in an ephemeral channel, the Nahal Yatir, is shown to be a comparatively simple function of stream power and to reach levels that are several orders of magnitude higher than maxima measured at similar levels of stream power in perennial counterparts. Channel average submerged unit flux rates are recorded as high as 4.3 kg s−1 m−1, while at the center of the channel, the highest rate recorded is 6.5 kg s−1m−1. Transport efficiency is at least an order of magnitude higher than in other channels for which there are comparable data and, on average, as much as 400 times that of Oak Creek. These differences are explained by the fact that the bed of the Yatir is not armored. It is surmised that the unvegetated nature of this desert watershed provides ample supplies of sediment of all sizes and that this, together with the rapid recession of the flash flood hydrograph and the extended periods of no flow, discourages the development of an armor layer. The flux rates are not sediment supply‐limited, as they are in perennial channels. Nahal Yatir and Oak Creek represent two ends of a spectrum, between which come seasonal and less well armored perennial streams. Transport efficiency is shown to vary considerably for each stream and from one stream to another, suggesting that it may not be possible to incorporate it easily into bed load equations in order to improve levels of prediction.
Eulerian‐Lagrangian Analysis of Transport Conditioned on Hydraulic Data: 1. Analytical‐Numerical Approach
Authors: Zhang Dongxiao , Neuman Shlomo P.
Published: February 2004

Recently, a unified Eulerian‐Lagrangian theory has been developed by one of us for nonreactive solute transport in space‐time nonstationary velocity fields. We describe a combined analytical‐numerical method of solution based on this theory for the special case of steady state flow in a mildly fluctuating, statistically homogeneous, lognormal hydraulic conductivity field. We take the unconditional mean velocity to be uniform but allow conditioning on measurements of log hydraulic conductivity (or transmissivity) and/or hydraulic head. This renders the velocity field nonstationary. We solve the conditional transport problem analytically at early time and express it in pseudo‐Fickian form at later time. The deterministic pseudo‐Fickian equations involve a conditional, space‐time dependent dispersion tensor which we evaluate numerically along mean “particle” trajectories. These equations lend themselves to accurate solution by standard Galerkin finite elements on a relatively coarse grid. The final step is an explicit numerical computation of lower bounds on conditional concentration prediction variance‐covariance (and coefficient of variation), travel time distribution, cumulative mass release across a “compliance surface,” the associated error, and plume spatial moments. Our method also allows quantification of the uncertainty in the original source location of any solute “particle” located anywhere in the field, at any time. This paper describes the methodology and presents some unconditional results. Conditioning and more advanced computations are presented in the subsequent papers.
Pilot Point Methodology for Automated Calibration of an Ensemble of Conditionally Simulated Transmissivity Fields: 2. Application
Authors: LaVenue A. Marsh , RamaRao Banda S. , De Marsily Ghislain , Marietta Melvin G.
Published: February 2004

This paper, the second in a two‐part series, presents the application of a methodology to assess spatial variability of the transmissivities within a regional aquifer in the vicinity of the Waste Isolation Pilot Plant (WIPP), the Culebra dolomite. An innovative aspect of this methodology is the generation of an ensemble of conditional simulations of the transmissivity field which preserve the statistical moments and spatial correlation structure of the measured transmissivity field and honors the measured transmissivity values at their locations. Each simulation is then calibrated, using an iterative procedure, to match an exhaustive set of steady state and transient pressure data. A fully automated inverse algorithm using pilot points as parameters of calibration was employed. The application of this new methodology to the Culebra dolomite flow system produced 70 conditional simulations which were consistent with all the measured transmissivity and head data at the site. Based on an analysis of the calibrated transmissivity fields, the spatial variability of the transmissivity fields was increased as a result of the calibration process. This increase is in part due to the addition of a high‐transmissivity feature to each of the transmissivity fields which is needed to match both steady state and transient state head data.
On the vectorization of finite element codes for high‐performance computers
Authors: Zhang H. , Schwartz F. W. , Sudicky E. A.
Published: February 2004

This paper presents strategies for vectorizing finite element codes in simulating large groundwater flow and transport problems. The approaches take advantage of vector‐processing capabilities of the Cray Y‐MP by regulating the node‐element and node‐node relationships. Regularization is achieved by adding auxiliary nodes and elements around the simulation domain. Vectorization of the global matrix assembly is due solely to the regularity of incidence matrix definition, while the vectorization of the iterative solver takes advantage of the regularity of the node‐node relationship and the concepts of wavefronts. The vectorization schemes are illustrated using the code VapourT. Rectangular elements are also added to the VapourT in addition to its original triangular elements. Test runs of the vectorized code with the vector processor turned on (VT4) versus the original code with the vector processor turned off (VTl) for triangular elements indicate overall speedups of 6–10.93 times in terms of CPU seconds. Part of the speedup results from the ability to eliminate some addressing subroutines. However, most of the speedup is due to the vectorization scheme. The speedups purely due to vectorization for the vectorized code (from VT3 to VT4) are 3.51–5.81 times in terms of CPU seconds for triangular elements and 4.28–7.25 (from VT5 to VT6) for rectangular elements.
Multispecies Contaminant Plumes in Variable Density Flow Systems
Authors: Zhang Hubao , Schwartz Franklin W.
Published: February 2004

A finite element flow and transport code, highly optimized for a Cray Y‐MP, is used to investigate the complex character of multispecies contaminant plumes that can develop in variable density flow systems. The reference data set for the simulation is based on a study of dense leachate plumes developed at Babylon, New York. A shallow unconfined aquifer receives leachates generated by annual recharge throughout the unlined landfill. The reference simulation generally reproduces the distribution of a density‐determining species in the leachate for both intermittent and continuous sources. A sensitivity analysis illustrates how the initial source concentrations for both a density‐determining species and a trace organic compound, the style of loading, and the reactive character of the organic compound can profoundly influence concentration distributions. For relatively large density contrasts between the leachate and the ambient groundwater, pockets of denser water sink to or near the bottom of the aquifer. A trace organic compound that is chromatographically separating from a dense plume due to sorption may follow different pathways as compared to the dense species.
Description of Simazine Transport with Rate‐Limited, Two‐Stage, Linear and Nonlinear Sorption
Authors: Streck Thilo , Poletika Nicholas , Jury William A. , Farmer Walter J.
Published: February 2004

This study was conducted to reconcile an apparent inconsistency between the simazine laboratory sorption isotherm data and the field lysimeter transport experiment reported by Poletika et al. (this issue). In this investigation, linear and nonlinear one‐ and two‐stage simazine sorption models were fitted to the sorption and desorption isotherm laboratory data to obtain parameter estimates for use in the transport model. Once obtained, the calibrated sorption model was combined with the parameterized outflow concentration record from a mobile Br tracer to represent rate‐limited sorption and transport of the simazine added simultaneously with the Br. The calibrated model did an excellent job of representing the final simazine profile in the soil, particularly with the nonlinear model. This is in contrast to a single‐stage adsorption model tested by Poletika et al. (this issue), which reached poor agreement with the field profile when laboratory‐measured sorption parameters were used. The results demonstrate the compatibility of field and laboratory experiments on pesticide movement and also indicate that sorption isotherms may require substantially longer to reach equilibrium than is customarily allowed in current protocols.
Constant Rate Rainfall Infiltration in a Field of Variable α
Authors: Parkin Gary W. , Kachanoski R. Gary , Elrick David E.
Published: February 2004

Spatial variability of hydraulic parameters is an important factor in prediction of water flow and solute transport in unsaturated soils. An analytical solution for one‐dimensional constant rate rainfall infiltration in a homogeneous soil is incorporated into a model of a spatially variable field. The model is a composite of vertically homogeneous soil columns that differ in their hydraulic properties. In this study, horizontal variation in infiltration profiles is due to variation in the inverse macroscopic capillary length (α) of each column. The sill in travel time variance of wetting profiles with depth for the homogeneous column confirms the notion of a long‐time traveling wave of fixed shape. Variation in α slightly enhances the spread of wetting profiles and delays formation of the traveling wave. Identification of the mean α value is more critical than inclusion of variability in α for wetting profile simulations under the examined conditions.
Effects of Carbon and Nitrate on Denitrification in Bottom Sediments of an Effluent‐Dominated River
Authors: Bradley P. M. , McMahon P. B. , Chapelle F. H.
Published: February 2004

Nitrogen and carbon limitation of denitrification in the bed sediments of an effluent‐dominated stream were investigated by quantifying the effects of nitrate and glucose additions on the rate of sediment N2O production. Bed sediment samples were collected from a 30‐km stretch of the South Platte River where up to 95% of the base flow discharge consists of effluent from a water treatment plant in Denver, Colorado. The rate of denitrification in upstream sediment samples incubated under in situ nitrate and carbon conditions was primarily limited by nitrate supply. The stimulatory effect of nitrate additions on the rate of bed sediment denitrification decreased with increasing distance downstream of the treatment plant. Approximately 35 km downstream of the treatment plant, denitrification in the bed sediment samples was carbon limited. The observed decreases in the concentration of total inorganic nitrogen (as NH4 + NO3) dissolved in the river and the organic carbon content of the bed sediments with increasing distance downstream of the treatment plant suggest that bed sediment denitrification is a significant sink for nitrogen in this stretch of the river.
Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction
Authors: Francois Roger , Honjo Susumu , Manganini Steven J. , Ravizza Gregory E.
Published: February 2004

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio‐Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio‐Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio‐Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio‐Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio‐Ba fluxes in the deep sea. These data confirm Dymond et al.'s findings that settling particles have significantly higher Corg/bio‐Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio‐Ba to the seafloor. Instead, we propose that high Corg/bio‐Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio‐Ba ratios in settling particles could be used to estimate the input of continental or shelf‐derived refractory organic matter into the deep sea.
Nitrogen fixation: Anthropogenic enhancement‐environmental response
Authors: Galloway James N. , Schlesinger William H. , Levy Hiram , Michaels Anthony , Schnoor Jerald L.
Published: February 2004

In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90–130 Tg N yr−1 (Tg = 1012 g) on the continents. Human activities have resulted in the fixation of an additional ≈140 Tg N yr−1 by energy production (≈20 Tg N yr−1 ), fertilizer production (≈80 Tg N yr−1), and cultivation of crops (e.g., legumes, rice) (≈40 Tg N yr−1 ). We can only account for part of this anthropogenic N. N2O is accumulating in the atmosphere at a rate of 3 Tg N yr−1. Coastal oceans receive another 41 Tg N yr−1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr−1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO3−, N2). The remaining 80 Tg N yr−1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N2. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N‐fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil‐fuel combustion. About two‐thirds of the increase will occur in Asia, which by 2020 will account for over half of the global anthropogenic N fixation.
Ecological controls on methane emissions from a Northern Peatland Complex in the zone of discontinuous permafrost, Manitoba, Canada
Authors: Bubier Jill L. , Moore Tim R. , Bellisario Lianne , Comer Neil T. , Crill Patrick M.
Published: February 2004

Methane emissions were measured by a static chamber technique in a diverse peatland complex in the Northern Study Area (NSA) of the Boreal Ecosystem Atmosphere Study (BOREAS). Sampling areas represented a wide range of plant community and hydrochemical gradients (pH 3.9–7.0). Emissions were generally larger than those reported from other boreal wetland environments at similar latitude. Seasonal average fluxes from treed peatlands (including palsas) ranged from 0 to 20 mg CH4 m−2 d−1 compared with 92 to 380 mg CH4 m−2 d−1 in open graminoid bogs and fens (with maximum single fluxes up to 1355 mg CH4 m−2 d−1). Permafrost‐related collapse scars had similarly high CH4 emissions, particularly in the lag areas where continuous measurements of water table, peat surface elevation, and peat temperature showed that the peat surface adjusted to a falling water table in the abnormally dry 1994 season, maintaining warm, saturated conditions and high CH4 flux later into the season than nonfloating sites. A predictive model for CH4 flux and environmental variables was developed using multiple stepwise regression. A combined variable of mean seasonal peat temperature at the average position of the water table explained most of the spatial variability in log CH4 flux (r2 = 0.64), with height above mean water table (HMWT), water chemistry (Kcorr, pH, Ca), tree cover, and herbaceous plant cover explaining additional variance (r2 = 0.81). Canonical correspondence analysis (CCA) of combined vascular and bryophyte data with environmental variables showed that CH4 flux was negatively correlated with HMWT, the second axis of vegetation variability, and was only weakly correlated with chemistry, the first axis. Sedge and tree cover were correlated with high and low CH4 fluxes, respectively, while shrub cover was of less predictive value. Microtopographic groupings of hummocks and hollows were separated in terms of CH4 flux at the intermediate ranges of the moisture gradient. These data show that multivariate vegetation analyses may provide a useful framework for integrating the complex environmental controls on CH4 flux and extrapolating single point chamber measurements to the landscape scale using remote sensing. (Key words: CH4 flux, peatland, vegetation, and remote sensing.)