bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal Moment‐Generating Equations: Modeling Transport and Mass Transfer in Heterogeneous Aquifers
Authors: Harvey Charles F. , Gorelick Steven M.
Published: February 2004

We present an efficient method for determining temporal moments of concentration for a solute subject to first‐order and diffusive mass transfer in steady velocity fields. The differential equations for the moments of all orders have the same form as the steady state nonreactive transport equation. Thus temporal moments can be calculated by a solute transport code that was written to simulate nonreactive steady state transport, even though the actual transport system is reactive and transient. Higher‐order moments are found recursively from lower‐order moments. For many cases a small number of moments sufficiently describe the movement of a solute plume. The first four moments describe the accumulated mass, mean, spread, and skewness of the concentration histories at all locations. Actual concentration histories at any location can be approximated from the moments by applying the principle of maximum entropy, a constraint consistent with the physical process of dispersion. The forms of the moment‐generating equations for different mass transfer models provide insight into reactive transport through heterogeneous aquifers. For the mass transfer models we considered, the zeroth moment in a heterogeneous aquifer is independent of the mass transfer coefficients. Thus, if the velocity field is known, the mass transported past any point, or out any boundary, can be calculated without knowledge of the spatial pattern of mass transfer coefficients and, in fact, without knowledge of whether mass transfer is occurring. Also, for both first‐order and diffusive mass transfer models, the mean arrival time depends on the distribution coefficient but is independent of the values of the rate coefficients, regardless of the spatial variability of groundwater velocity and mass transfer coefficients.
The Pacific Rain Gage Rainfall Database
Authors: Morrissey Mark L. , Shafer Mark A. , Postawko Susan E. , Gibson Barbara
Published: February 2004

The Comprehensive Pacific Rainfall Database includes daily rain gage data from over 250 meteorological stations located on islands and atolls throughout the Pacific basin with records beginning in 1971 and extending up to the present. Included with these station records are data obtained from the Schools of the Pacific Rainfall Climate Experiment. The database includes a metadata file containing information pertaining to local island topography and other site characteristics. The data are constantly updated and are available to interested researchers through the Internet file transfer protocol system.
Solute Transport Through an Integrated Heterogeneous Soil‐Groundwater System
Authors: Destouni Georgia , Graham Wendy
Published: February 2004

The coupled transport process through an integrated soil‐groundwater system is quantified for kinetically sorbing solute that originates from a time dependent source at the soil surface and is transported by steady random velocity. The derived expressions of ensemble mean solute breakthrough at some arbitrary control plane normal to the mean flow direction involve probability density functions (pdfs) of advective solute travel time through the unsaturated and the saturated zone of the transport domain. A nonstationary travel time pdf is derived for the saturated zone, to account for possible effects of flow nonuniformity due to recharge of water from the unsaturated zone. Nonuniform mean flow in the saturated zone decreases the relative influence of spatial variability within that zone on the ensemble mean solute breakthrough curve. Factors such as the longitudinal extent of the solute source and the unsaturated zone variability become more important for the spreading of the expected solute breakthrough as the degree of flow nonuniformity in the saturated zone increases. This implies that possible far‐field simplifications based on the assumption that the transport process in an integrated soil‐groundwater system is dominated by the transport conditions in the saturated zone may not be valid in cases with significant groundwater recharge from the unsaturated zone.
Hydraulic Behavior of Quasi‐Saturated Soils in the Presence of Entrapped Air: Laboratory Experiments
Authors: Faybishenko Boris A.
Published: February 2004

Entrapped air in soils beneath the water table is one of the key factors controlling the hydraulic behavior under conditions of ponded infiltration, in perched waters, and in unconfined aquifers. The term quasi‐saturated soils defines the soils with entrapped air, and the term quasi‐saturated hydraulic conductivity defines the relationship between the hydraulic conductivity and entrapped air content. This paper focuses on an investigation of how entrapped air, along with other factors, affects the three‐stage temporal behavior of the quasi‐saturated hydraulic conductivity of soils. During the first stage the quasi‐saturated hydraulic conductivity of soils decreases by as much as 5–8 times, presumably because mobile entrapped air blocks the largest pores. During the second stage, as the mobile entrapped air is discharged from the core, the quasi‐saturated hydraulic conductivity of the soils slowly increases. When the mobile air is removed, the remaining immobile entrapped air is discharged as a dissolved phase, and the quasi‐saturated hydraulic conductivity increases rapidly by about 1–2 orders of magnitude, essentially reaching the value of the saturated hydraulic conductivity. During the third stage the hydraulic conductivity is decreased to minimum values. The effects of sealing at the soil surface and microbiological activities are assumed to be major factors in the final decrease of the hydraulic conductivity. This three‐stage temporal behavior of percolation in loam soils is repeatable. A new power law and an exponential relationship are proposed to describe the quasi‐saturated hydraulic conductivity of loams as a function of the entrapped air content.
Quasi‐Linear Geostatistical Theory for Inversing
Authors: Kitanidis Peter K.
Published: February 2004

A quasi‐linear theory is presented for the geostatistical solution to the inverse problem. The archetypal problem is to estimate the log transmissivity function from observations of head and log transmissivity at selected locations. The unknown is parameterized as a realization of a random field, and the estimation problem is solved in two phases: structural analysis, where the random field is characterized, followed by estimation of the log transmissivity conditional on all observations. The proposed method generalizes the linear approach of Kitanidis and Vomvoris (1983). The generalized method is superior to the linear method in cases of large contrast in formation properties but informative measurements, i.e., there are enough observations that the variance of estimation error of the log transmissivity is small. The methodology deals rigorously with unknown drift coefficients and yields estimates of covariance parameters that are unbiased and grid independent. The applicability of the methodology is demonstrated through an example that includes structural analysis, determination of best estimates, and conditional simulations.
Stochastic analysis of head spatial variability in bounded rectangular heterogeneous aquifers
Authors: Osnes Harald
Published: February 2004

Groundwater flow through bounded rectangular aquifers is analyzed in a stochastic framework. New analytical expressions of the head covariance (and variance) and the log transmissivity head cross covariance are derived for the simplified first‐order problem. Effects of different boundary conditions, the aquifer size, and the log transmissivity correlation scale upon head moments are studied. The results are compared to analytical half‐plane solutions, demonstrating significant differences in several cases. Finally, the problem is solved numerically using the Monte Carlo simulation method. The realizations of the log transmissivity field are generated in two different ways. Through comparisons with the analytical results the accuracy of the numerical methods is shown to be excellent.
Fractional packing model for hydraulic conductivity derived from sediment mixtures
Authors: Koltermann Christine E. , Gorelick Steven M.
Published: February 2004

Petrophysical relations are derived to predict porosity and hydraulic conductivity from grain size distributions considering particle packing in sediment mixtures. First, we develop a fractional packing model for porosity that considers the fraction of intrapore fines that occur as the fines content increases. Then, a fractional packing Kozeny‐Carman relation for hydraulic conductivity is developed by examining which particle sizes dominate the pore structure, and which averaging procedure best represents the mean grain diameter in any given sediment mixture. The relations developed here perform well for a wide range of sediment mixtures regardless of confining pressure. Graphs are presented that show hydraulic conductivity versus weight fraction of fines for mixtures of coarse‐ and fine‐grained sediment commonly observed in the field, such as clayey gravel and silty sand. These graphs show that the wide range of hydraulic conductivity values reported for sediment mixtures can display a 5 order of magnitude variation over a few percent fines. Finally, a field scale application using grain size distributions from a quantitative depositional model shows that these petrophysical relations successfully predict more than 90% of hydraulic conductivity values to within 1 order of magnitude over 7 orders of magnitude of spatial variability.
Modeling Facilitated Contaminant Transport by Mobile Bacteria
Authors: Corapcioglu M. Yavuz , Kim Seunghyun
Published: February 2004

Introduction of exogenous biocolloids such as genetically engineered bacteria in a bioremediation operation can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria are efficient contaminant carriers. In cases where contaminants have a low mobility in porous media because of their high partition with solid matrix, facilitated contaminant transport by mobile bacteria can create high contaminant fluxes. When metabolically active mobile bacteria are present in a subsurface environment, the system can be treated as consisting of three phases: water phase, bacterial phase, and stationary solid matrix phase. In this work a mathematical model based on mass balance equations is developed to describe the facilitated transport and fate of a contaminant and bacteria in a porous medium. Bacterial partition between the bulk solution and the stationary solid matrix and contaminant partition among three phases are represented by expressions in terms of measurable quantities. Solutions were obtained to provide estimates of contaminant and bacterial concentrations. A dimensional analysis of the transport model was utilized to estimate model parameters from the experimental data and to assess the effect of several parameters on model behavior. The model results matched favorably with experimental data of Jenkins and Lion (1993). The presence of mobile bacteria enhances the contaminant transport. However, bacterial consumption of the contaminant, which serves as a bacterial nutrient, can attenuate the contaminant mobility. The work presented in this paper is the first three‐phase model to include the effects of substrate metabolism on the fate of groundwater contaminants.
A Combined Laplace Transform and Streamline Upwind Approach for Nonideal Transport of Solutes in Porous Media
Authors: Xu Linlin , Brusseau Mark L.
Published: February 2004

A new numerical method for nonideal transport of solutes in porous media is developed by first using the Laplace transform to eliminate the time dependency and then the streamline upwind scheme to solve the spatial equations. The transient solution is ultimately recovered by an efficient quotient difference inversion algorithm. By introducing complex‐valued artificial dispersion in the weighting functions, characteristics of the transient solutions have been successfully addressed. The optimum of the complex‐valued artificial dispersion has been derived for one‐dimensional problems. In multidimensional cases the streamline upwind scheme is modified by adding complex‐valued artificial dispersion along the streamline. Within this numerical scheme the grid orientation problems have been successfully treated. The limitations on the cell Peclet number and on the Courant number were greatly relaxed. Both one‐dimensional and two‐dimensional numerical examples are used to illustrate applications of this technique.
Analytical Solution for One‐Dimensional Drainage: Burgers' and Simplified Forms
Authors: Warrick A. W. , Parkin Gary W.
Published: February 2004

Solutions are developed for Burgers' equation and for the surface water content subject to one‐dimensional drainage. These are found as limiting cases of Richards' equation using diffusivity and conductivity functions from Fujita as extended by Broadbridge and White. The solution from Burgers' equation can generally be expressed as a function of only a dimensionless time, depth, and initial reduced water content. For the more strongly nonlinear case, initially the surface water content decreases in proportion to the square root of time. For large times, the surface water content approaches the residual water content inversely with the square root of time.
Modeling Bacterial Detachment During Transport Through Porous Media as a Residence‐Time‐Dependent Process
Authors: Johnson William P. , Blue Karen A. , Logan Bruce E. , Arnold Robert G.
Published: February 2004

Bacterial transport through porous media was modeled using detachment functions that incorporate the dependence of detachment rate on bacterial residence time on the collector. Model parameters and the relative merit of alternative forms for the detachment function were evaluated on the basis of comparisons between model simulations and experimentally derived bacterial breakthrough and elution curves. Only detachment functions that provided an initial period in which bacteria were rapidly released, followed by slow bacterial detachment, were able to reproduce the elution portion of the breakthrough curves. In optimal simulations, 90% of the bacteria that were captured by the porous medium detached within 1 min of attachment. Experiments involving saturated flow through columns packed with sand indicated that the time to achieve complete breakthrough was inversely related to the influent bacterial concentration. On this basis and because of the relatively slow approach to breakthrough that was typically observed in transport experiments, it was hypothesized that the experimental medium contained a number of preferred attachment sites that must be essentially filled before breakthrough is achieved. Only when such (irreversible) sorption sites were included in the model formulations was it possible to produce transport simulations that matched both the breakthrough and elution portions of the empirically derived curves. It is concluded that both a time‐dependent detachment function and a degree of sorption site heterogeneity are required to describe bacterial attachment and detachment during transport as observed in our laboratory.
Hydrofacies distribution and correlation in the Miami Valley Aquifer System
Authors: Ritzi Robert W. , Dominic David F. , Brown Nathan R. , Kausch Kenneth W. , McAlenney Peter J. , Basial Michael J.
Published: February 2004

This study combines geostatistical analyses with geologic interpretations to further the quantitative understanding of physical heterogeneity within glaciofluvial aquifers. The stochastic simulation of aquifers requires quantitative measures of heterogeneity, including both cumulative distribution and spatial correlation functions. The heterogeneity in glaciofluvial aquifers is typified by low‐permeability facies (e.g., till or lacustrine clay) juxtaposed with high‐permeability facies (e.g., sand and gravel outwash). The Miami Valley aquifer system was examined at multiple sites for the spatial distribution and correlation of these two hydrofacies. Binary indicator geostatistics were used to quantitatively determine, at each site, the relative volume of each hydrofacies, their spatial distribution, the major principal direction of their spatial correlation, the minor principal direction, and the correlation range in these directions. The percent by volume of the system that is aquitard material decreases down the valley, from 31% to 12%. Each site has an elevation zone with more aquitard material relative to other elevations at that site. The percent aquitard material in these zones decreases down the valley from 45% to 22%. The maximum principal direction of spatial correlation in the aquitard zones generally is NE‐SW, subparallel to the trend of the bedrock valley, with a range of the order of 0.75 km and minimum/maximum anisotropy ratio of 0.4. The locations exceeding 0.5 probability of aquitard occurrence generally occur on the valley margins. Thus, among the sites investigated, there is a trend down the valley in the ratio of aquitard volume to aquifer volume, and the spatial correlation and distribution of aquitard material are similar within the aquitard zones. Furthermore, these findings are consistent with aspects of the subglacial and proglacial depositional environments responsible for the facies assemblage and thus are likely to be applicable to other parts of the aquifer system and other aquifer systems where similar geologic processes are inferred to have existed.
Use of sedimentological information for geometric simulation of natural porous media structure
Authors: Scheibe Timothy D. , Freyberg David L.
Published: February 2004

A geometric simulation method was used to develop a three‐dimensional, highly detailed synthetic representation of point bar sediments in the Wabash River system. Geometric simulation methods, in comparison to well‐known second‐order stochastic methods, offer the advantage of being more closely related to depositional processes, which are often similarly conceptualized (i.e., described in terms of shapes of discrete bed forms, trends in grain size, and spatial relationships of defined geologic facies). Multiple scales of geometric variation were defined within a sedimentologically prescribed framework, and shapes of discrete geometric elements were established at each scale. The selected shapes were based on published field studies including sedimentological bed form studies and trench studies in active point bar sediments. The parameterization of the shapes allowed for random variability of the shape descriptors; discrete shapes were then generated and assimilated by computer. Hydraulic conductivity values were assigned to the discrete elements based on reports of observed variations in grain size and field measurements of hydraulic conductivity. The synthetic model, referred to as a numerical aquifer, is being used as the basis for extensive numerical experimentation to study the relationship between natural spatial structure and subsurface flow and transport.
Eolian transport, saline lake basins, and groundwater solutes
Authors: Wood Warren W. , Sanford Ward E.
Published: February 2004

Eolian processes associated with saline lakes are shown to be important in determining solute concentration in groundwater in arid and semiarid areas. Steady state mass balance analyses of chloride in the groundwater at Double Lakes, a saline lake basin in the southern High Plains of Texas, United States, suggest that approximately 4.5 × 105 kg of chloride is removed from the relatively small (4.7 km2) basin floor each year by deflation. This mass enters the groundwater down the wind gradient from the lake, degrading the water quality. The estimates of mass transport were independently determined by evaluation of solutes in the unsaturated zone and by solute mass balance calculations of groundwater flux. Transport of salts from the lake was confirmed over a short term (2 years) by strategically placed dust collectors. Results consistent with those at Double Lake were obtained from dune surfaces collected upwind and downwind from a sabkha near the city of Abu Dhabi in the United Arab Emirates. The eolian transport process provides an explanation of the degraded groundwater quality associated with the 30–40 saline lake basins on the southern half of the southern High Plains of Texas and New Mexico and in many other arid and semiarid areas.
Identifying Soil Hydraulic Heterogeneity by Detection of Relative Change in Passive Microwave Remote Sensing Observations
Authors: Hollenbeck K. J. , Schmugge T. J. , Hornberger G. M. , Wang J. R.
Published: February 2004

The ability of passive microwave remote sensing (PMRS) to assess soil moisture has been amply demonstrated. Building on the expertise, this research is a first attempt at obtaining near‐surface soil hydraulic characteristics from PMRS. We show that relative change in surface soil moisture can be determined accurately from PMRS measurements of brightness temperature, given the commonly observed linear relationship between the two variables. Relative, rather than absolute, change within a series of PMRS images, obtained during a period of drying, is also a very useful statistic for filtering out the drydown heterogeneity caused by spatial variability in initial wetness rather than soil heterogeneity. We develop a test of soil heterogeneity based on the detection of deviations of relative change from the average rate in a series of images by more than the instrument error. With the test, the existence of soil heterogeneity for a pair of images showing a drydown during the 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX‐Sahel) experiment can be proven. Areas of relatively fast or slow drying can be mapped. These areas coincide with geomorphological features and soil‐type patterns that are expected to show the observed moisture dynamics. The variogram of relative change in the HAPEX images differs from those of brightness temperature, corroborating the rejection of the soil homogeneity hypothesis. Furthermore, the variogram indicates the existence of an effective correlation length much larger than that commonly observed in ground‐based soil surveys. These findings encourage the use of PMRS for assessing soil hydraulic characteristics that are valid at a scale appropriate for hydrometeorological models.
Vapor Transport of Trichloroethylene in the Unsaturated Zone: Field and Numerical Modeling Investigations
Authors: Conant Bernadette Hughes , Gillham Robert W. , Mendoza Carl A.
Published: February 2004

Vapor transport of chlorinated solvents in the unsaturated zone may be an important mechanism for the spread of contamination at spill sites and may be a significant factor controlling the extent of groundwater contamination. Two field experiments were carried out at the Canadian Forces Base Borden field site to provide detailed monitoring of the transport behavior of trichloroethylene vapors in the unsaturated zone. Experiments were conducted for both winter and summer conditions and under different surface boundary conditions. The observed results were simulated using a Fickian‐based numerical model with linear equilibrium phase partitioning. The model includes both diffusion and density‐induced advection and allows for the incorporation of spatial heterogeneities and nonisothermal conditions. Numerical sensitivity analyses were conducted to further evaluate the relative influence of various transport parameters on vapor migration. Use of measured field values as input parameters resulted in a very good match between the experimental results and numerical simulations. In both experiments, vapor plumes spread several meters from the source and downward to the capillary fringe within only a few days. Seasonal temperature variations were found to have a significant impact on the rate and total mass of vapor transport, and variations in organic carbon content, and to a lesser extent moisture content, exerted the greatest control on retardation of vapor migration. Transport was diffusion dominated, but density‐induced advection was an appreciable component of net transport under summer conditions, when vapor concentrations were higher. Geologic conditions at the site made overall transport relatively insensitive to the ground surface boundary condition.
Comparison of Hydrograph Deconvolutions using Residual Alkalinity, Chloride, and Oxygen 18 as Hydrochemical Tracers
Authors: Ribolzi O. , Vallès V. , Bariac T.
Published: February 2004

Hydrograph deconvolution using geochemical tracers is currently widely used for determining the hydrologic mechanisms occurring in watersheds. However, few chemical parameters can be used as tracers because their involvement in biogeochemical processes prevents them from behaving in a conservative way. The aim of this study was to combine several geochemically controlled parameters into a single tracer. Residual alkalinity is a combination of several controlled parameters and is conservative in a wide range of natural environments. It was used in this study for quantifying the contributions of surface runoff and of groundwater flow during a flood in a Mediterranean watershed underlain by sedimentary rock. A preliminary geochemical study revealed that interactions with calcite, dolomite, and the clay‐humus complex controlled calcium and magnesium concentrations as well as carbonate alkalinity (Alkc), which prevented using them as tracers. Nevertheless, although residual alkalinity (Alkresidual) is a combination of these three parameters (Alkresidual = Alkc ‐ 2[Ca2+]T‐2[Mg2+]T), it provided results that were highly comparable to those obtained using chloride and δ18O. Contrary to the most cases in the literature, the contribution of direct runoff was dominant (about 80% at peak discharge). Accuracy estimates, which took into account analytical errors, temporal variations in the isotopic signature of rainfall, and the spatial variability of chemical elements, supported this result and confirmed that residual alkalinity is a useful concept in hydrology.
An Improved Fractal Equation for The Soil Water Retention Curve
Authors: Perfect E. , McLaughlin N. B. , Kay B. D. , Topp G. C.
Published: February 2004

The water retention curve, θ(ψ), is important for predicting soil physical properties and processes. Until recently, equations for the θ(ψ) were empirical. Advances in fractal geometry have led to the derivation of physical models for the θ(ψ). However, both existing fractal equations have only two parameters and thus are relatively inflexible. We derived a new three‐parameter fractal model for the θ(ψ). This equation was fitted to 36 θ(ψ)'s for a silt loam soil with a wide range of structural conditions. The new equation fitted these data much better than the existing equations. The parameters of the new equation, ψa, ψd and D are physical entities, corresponding to the air‐entry value, tension draining the smallest pores, and fractal dimension, respectively. Estimates of the ψa, ψd and D were physically reasonable, with median values of 2.9 × 10−1 kPa, 1.6 × 104 kPa, and 2.87, respectively. In contrast, the existing equations yielded anomalous estimates of either ψa or D. The new equation was able to fit θ(ψ) for a variety of porous media, including sandstone, glass beads, sands, sieved soil, and undisturbed soils ranging from very fine sandy loam to heavy clay. The ψa and ψd were more sensitive to structural and textural variation than D. The new equation represents an improvement over existing models in terms of both goodness of fit and the physical significance of its parameters.
Effect of Horizontal Heat and Fluid Flow on the Vertical Temperature Distribution in a Semiconfining Layer
Authors: Lu Ning , Ge Shemin
Published: February 2004

By including the constant flow of heat and fluid in the horizontal direction, we develop an analytical solution for the vertical temperature distribution within the semiconfining layer of a typical aquifer system. The solution is an extension of the previous one‐dimensional theory by Bredehoeft and Papadopulos [1965]. It provides a quantitative tool for analyzing the uncertainty of the horizontal heat and fluid flow. The analytical results demonstrate that horizontal flow of heat and fluid, if at values much smaller than those of the vertical, has a negligible effect on the vertical temperature distribution but becomes significant when it is comparable to the vertical.
Using a Multiregion Model to Study the Effects of Advective and Diffusive Mass Transfer on Local Physical Nonequilibrium and Solute Mobility in a Structured Soil
Authors: Gwo J. P. , Jardine P. M. , Wilson G. V. , Yeh G. T.
Published: February 2004

Waste management problems for shallow land burial facilities in the humid eastern United States are usually complicated by slow but continuous movement of wastes through the soil matrix and discrete but rapid pulses of wastes through macropores and fractures. Multiple‐pore‐region models employed to describe flow and solute transport in the soils usually consist of multiple mass transfer coefficients that cannot be measured experimentally, and their effects on subsurface mass transport are poorly understood. The objective of this research was to study the individual and concurrent effects of interaggregate advection and diffusion on mass transport in a structured soil. The interactions of these two mass transfer processes and local solute concentration equilibrium are examined for a heterogeneous soil. Pore region water retention, hydraulic conductivity, and dispersivities, obtained from independent measurements and published calibration results, were used to test a novel three‐pore‐region, one‐dimensional numerical model. Advective and diffusive mass transfer coefficients were estimated using mass transfer equations and fracture spacings published in the literature. The mass transfer coefficients were then varied systematically, and the sensitivity of local fluid pressure and solute concentration nonequilibrium to interregion mass transfer were analyzed. Our results indicated that time‐dependent interaggregate advection and diffusion were important processes controlling solute mobility in heterogeneous media. Under transient flow conditions, interaggregate advection may reduce the significance of interaggregate diffusion that otherwise dominates interaggregate mass transfer under steady state conditions. Nonetheless, the equilibrium of local solute concentrations was 20 times more sensitive to diffusive mass transfer than to advective mass transfer, which suggests that site characterization efforts should be directed more toward the former process. Unfortunately, characterization efforts of this type are not commonplace and if available are frequently ignored because they add a difficult reality to complex waste management problems. Since advective and diffusive mass transfer may be important processes limiting the efficiency of cleanup activities such as pump and treat, it is perhaps time to include the characterization of these processes and quantification of the timescale of physical nonequilibrium in site remediation efforts.
Simulation of Preferential Flow in Three‐Dimensional, Heterogeneous Conductivity Fields with Realistic Internal Architecture
Authors: Webb Erik K. , Anderson Mary P.
Published: February 2004

Subsurface flow is primarily controlled by the distribution of hydrogeological properties. Spatially correlated random fields, currently the primary means of describing heterogeneity in these properties, underutilizes the existing knowledge of geological systems. A new computer code, the Braided Channel Simulator (BCS‐3D), is used to geometrically simulate the spatial heterogeneity of hydrogeological properties using representations of surface topography to estimate the three‐dimensional arrangement of subsurface units. Four methods of assigning conductivity were used to generate conductivity fields as a basis for saturated flow simulation. Variograms measured for each of the conductivity fields give no indication of underlying discrete structure. However, particle‐tracking simulations showed that flow occurred along paths of preferentially high hydraulic conductivity. Results indicate that in systems where the contrast in conductivity is sufficiently great, the location and magnitude of flow is constrained by discrete internal structure. Additionally, geometrical simulation techniques such as BCS‐3D can be used to produce property fields that embody these discrete geological structures.
Evaluation of Risk from Contaminants Migrating by Groundwater
Authors: Andričević Roko , Cvetković Vladimir
Published: February 2004

The general formulation of the environmental risk problem captures the entire process of identifying the source term of the risk agent, its fate and transport through porous media, estimation of human exposure, and conversion of such exposure into the risk level. The contaminant fate and transport is modeled using the solute flux formulation evaluated with its first two moments, which explicitly account for the spatial variability of the velocity field, sorption properties, and parametric uncertainty through the first‐order analysis. The risk level is quantified on the basis of carcinogenicity using the risk factor (which describes the risk per unit dose or unit intake) employed to the total doses for individuals potentially consuming radionuclide‐contaminated groundwater. As a result of the probabilistic formulation in the solute flux and uncertainty in the water intake and dose‐response functions, the total risk level is expressed as a distribution rather than a single estimate. The results indicate that the geologic heterogeneity and uncertainty in the sorption estimate are the two most important factors for the risk evaluation from the physical and chemical processes, while the mean risk factor is a crucial parameter in the risk formulation.
Modeling Overland Flow Contamination by Chemicals Mixed in Shallow Soil Horizons Under Variable Source Area Hydrology
Authors: Govindaraju Rao S.
Published: February 2004

Agricultural chemicals (fertilizers, nutrients, pesticides, etc.) are frequently mixed into the shallow soil layers so that they may be leached down during water applications (rainfall/irrigation). These events often generate overland flows which cause the release and migration of these chemicals into surface waters. In many such applications, overland flow does not immediately develop as a sheet over the whole plane, but gradually increases in extent in accordance with the variable source area (VSA) concept. This paper deals with the modeling of surface contamination under such circumstances. Results were obtained for a single hypothetical agricultural plot. Some comparisons were made regarding the relative amounts of solute lost to overland flow and to those leached into the soil as a function of time.
Toward a Generalization of the TOPMODEL Concepts: Topographic Indices of Hydrological Similarity
Authors: Ambroise Bruno , Beven Keith , Freer Jim
Published: February 2004

Preliminary studies of the application of TOPMODEL to the 36‐ha Ringelbach catchment suggested that the original form of exponential transmissivity function leading to the In (a/tan β) topographic index and first‐order hyperbolic base flow recession curve is not appropriate to this catchment. Two alternative forms of topographic index and soil‐topographic index are developed based on parabolic and linear transmissivity functions, leading to the more frequently observed second‐order hyperbolic and exponential recession curves, respectively. It is shown how these can be used in the same way as the original to relate catchment average water table depths to local water table depths so that patterns of saturation can be evaluated. Two companion [Ambroise et al., this issue; Freer et al., this issue] papers show how the new parabolic index is used in the prediction of Ringelbach discharges, and how the limitations of the model are reflected in the estimated predictive uncertainties using the Generalised Likelihood Uncertainty Estimation (GLUE) approach.
Operational Rainfall Prediction on Meso‐γ Scales for Hydrologic Applications
Authors: Lee Tim H. , Georgakakos Konstantine P.
Published: February 2004

Presented is a rainfall prediction methodology for application in operational hydrologic forecasting with forecast lead times of 1–6 hours and spatial‐resolution scales of 10–30 km. The essential elements of the prediction methodology are a mathematical model for precipitation prediction from surface and upper air meteorological variables; operational forecasts of temperature, pressure, humidity, and wind fields by large‐scale numerical weather prediction models; surface and upper air meteorological observations; remote and on‐site rainfall observations; and a state estimator for real‐time updating from local frequent rainfall observations and for probabilistic predictions. This paper formulates a class of rainfall models suitable for this prediction methodology. The models are based on the differential equation of conservation of cloud and rainwater equivalent mass and on a newly introduced advection equation for a parameter that determines updraft strength. The latter advection equation is a prognostic equation for the strength of convection in space and time. The innovative features of the model formulated and tested are the inclusion of the prognostic equation for the advection of regions of active convection, the formulation of the state estimator component for state updating and probabilistic forecasts, and the utilization of a numerical solution scheme which reduces artificial numerical diffusion and can be used with the state estimator because of its explicit form. Utilization of the prediction model formulated was exemplified in several case studies of summer convection in Oklahoma using data available during routine forecast operations. The case studies show that when verified with radar rainfall data, the model's hourly precipitation predictions over a 20,000 km2area with a 100–900 km2resolution are better than simple persistence and explain more than 60% of the observed hourly rainfall variance. Sensitivity studies quantify dependence of rainfall predictions to microphysical and state‐estimator parameters.
Minimum Relative Entropy Inversion: Theory and Application to Recovering the Release History of a Groundwater Contaminant
Authors: Woodbury Allan D. , Ulrych Tadeusz J.
Published: February 2004

In this paper we show that given prior information in terms of a lower and upper bound, a prior bias, and constraints in terms of measured data, minimum relative entropy (MRE) yields exact expressions for the posterior probability density function (pdf) and the expected value of the linear inverse problem. In addition, we are able to produce any desired confidence intervals. In numerical simulations, we use the MRE approach to recover the release and evolution histories of plume in a one‐dimensional, constant known velocity and dispersivity system. For noise‐free data, we find that the reconstructed plume evolution history is indistinguishable from the true history. An exact match to the observed data is evident. Two methods are chosen for dissociating signal from a noisy data set. The first uses a modification of MRE for uncertain data. The second method uses “presmoothing” by fast Fourier transforms and Butterworth filters to attempt to remove noise from the signal before the “noise‐free” variant of MRE inversion is used. Both methods appear to work very well in recovering the true signal, and qualitatively appear superior to that ofSkaggs and Kabala[1994]. We also solve for a degenerate case with a very high standard deviation in the noise. The recovered model indicates that the MRE inverse method did manage to recover the salient features of the source history. Once the plume source history has been developed, future behavior of a plume can then be cast in a probabilistic framework. For an example simulation, the MRE approach not only was able to resolve the source function from noisy data but also was able to correctly predict future behavior.
In situ production of 36Cl in uranium ore: A hydrogeological assessment tool
Authors: Cornett R. J. , Cramer J. , Andrews H. R. , Chant L. A. , Davies W. , Greiner B. F. , Imahori Y. , Koslowsky V. , McKay J. , Milton G. M. , Milton J. C. D.
Published: February 2004

In situ neutron activation of 35Cl within the rock and groundwater of geologic deposits that have elevated concentrations of uranium provides a hydrogeological tracer. We determine the production rate and mobility of 36Cl in the 1.3‐billion‐year‐old Cigar Lake uranium ore deposit. Accelerator mass spectrometry was used to map the concentrations of 36Cl in the ore and in the groundwater that were up to 100 times greater than those encountered in unmineralized portions of the host sandstone aquifer. The residence time of this mobile anion in groundwater within the mineralized zone ranged from 14 to 280 kyr. These residence times are consistent with the hydraulic and geochemical data, suggesting significant control of Cl− and groundwater movement by the clay‐rich matrix of the mineralized zone.
Contaminant Transport at a Waste Residue Deposit: 2. Geochemical Transport Modeling
Authors: Engesgaard Peter , Traberg Rikke
Published: February 2004

Contaminant transport in an aquifer at an incinerator waste residue deposit in Denmark is simulated. A two‐dimensional, geochemical transport code is developed for this purpose and tested by comparison to results from another code. The code is applied to a column experiment and to the field site. The ongoing geochemical processes and any unknown geochemical parameters are obtained through simulation of the column experiment that used soil and leachate from the field site. The application of the code to the field site, which has been monitored for more than 15 years, use this geochemical information along with the flow and nonreactive transport parameters obtained by the inverse modeling procedure described in the first paper [Sonnenborg et al., this issue] of this two‐paper series. The simulation results of the site model are compared with several measured component breakthroughs at monitoring wells. Contamination was first controlled by transport, and later by transport and ion exchange. In both the column and field site simulations the code is used to identify the controlling transport processes, physical or geochemical (ion exchange and mineral precipitation), and to estimate the involved parameters (primarily ion exchange selectivity coefficients).
Testing high‐resolution nitrous oxide emission estimates against observations using an atmospheric transport model
Authors: Bouwman A. F. , Taylor J. A.
Published: February 2004

Global monthly estimates of N2O emissions were used to prescribe a three‐dimensional atmospheric transport model. The simulated N2O surface concentrations in the northern hemisphere (NH) are in general agreement with atmospheric observations, about 1 ppb higher than in the southern hemisphere (SH). The results suggest that N2O concentrations over strong source regions in continental interiors are much higher than those over oceans. Simulated N2O concentrations in the NH vary strongly with the season, while the measured concentrations oscillate strongly without seasonality. The modelled results for the SH show much less seasonal variation, which is more consistent with atmospheric measurement data. Most atmospheric N2O monitoring stations are not well situated for verifying the simulated seasonality in atmospheric N2O. There is only one continental site. In addition, atmospheric N2O observations from different studies are sometimes contradictory. The difference between model results and observations may be the result of local scale pulses of N2O acting on a much shorter timescale than the time step of 1 month of the emission inventory. Such N2O pulses may cause oscillations of the N2O concentration at the monitoring sites which dominate the large‐scale seasonality. It is also possible that source and sink processes other than those included in this study are operating. The transport model does not include a description of the downward stratosphere‐troposphere flux of N2O which is greater in the NH than in the SH. The emission inventory does not account for soil N2O consumption in N‐limited temperate ecosystems, and N2O emissions during autumn, thaw periods, and from snow covered soils. These potential errors and possible underestimation of N2O emissions from combustion sources may exaggerate the simulated seasonal trends.
TURC: A diagnostic model of continental gross primary productivity and net primary productivity
Authors: Ruimy Anne , Dedieu Gérard , Saugier Bernard
Published: February 2004

TURC, a diagnostic model for the estimation of continental gross primary productivity (GPP) and net primary productivity (NPP), is presented. This model uses a remotely sensed vegetation index to estimate the fraction of solar radiation absorbed by canopies, and an original parameterization of the relationship between absorbed solar radiation and GPP, based on measurements of CO2 fluxes above plant canopies. An independent, uncalibrated model of autotrophic maintenance and growth respiration is parameterized from literature data, and uses databases on temperature, biomass, and remotely sensed vegetation index. This model results in global estimates of GPP and NPP of 133.1 and 62.3 Gt(C) per year, respectively, which is consistent with commonly admitted values. The ratio of autotrophic respiration to GPP is about 70% for equatorial rain forests and 50% for temperate forests, as a result the highest predicted NPP are in tropical savannas of Africa and South America, and in temperate, highly cultivated zones of North America, not in equatorial rain forest zones. Conversion efficiencies defined as the ratio of yearly integrated NPP to absorbed photosynthetically active radiation (PAR) compare relatively well with a previous compilation of literature values, except for ecosystems with probable reduction of conversion efficiency due to water stress. Several sensitivity studies are performed on some input data sets, model assumptions, and model parameters.
Sources and sinks of carbon monoxide in the mixed layer of the tropical South Pacific Ocean
Authors: Johnson James E. , Bates Timothy S.
Published: February 2004

The magnitude of the oceanic source of carbon monoxide (CO) to the atmosphere depends on the concentration of CO in ocean surface waters. To ascertain the relative importance of the processes controlling this concentration, depth profiles of CO concentrations in the oceanic mixed layer and upper thermocline were made at two time series stations in the tropical Pacific Ocean during 1993. The two stations were chosen to contrast an oligotrophic region of low CO water concentrations (RITS93) with a biologically productive region of high seawater CO concentrations (RITS94). The column burden of CO in the upper 75 m of the water column averaged 53 μmole m−2 at the RITS93 station and 240 μmole m−2 at the RITS94 station. From the observed diurnal cycle of the CO column burden, production and loss rates at each site were estimated. The factor of 4.5 difference in the column burden is primarily due to a smaller in situ oxidation rate constant at the RITS94 site.