bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogeochemistry of the shallow dutch groundwater: Interpretation of the National Groundwater Quality Monitoring Network
Authors: Frapporti G. , Vriend S. P. , van Gaans P. F. M.
Published: February 2004

Since 1979 the Dutch National Institute of Public Health and Environmental Protection (RIVM) has been developing the Dutch Groundwater Quality Monitoring Network (LMG). This network presently consists of about 350 monitoring sites. At each site, well screens are placed at two depths: 10 and 25 m below surface level. Samples are collected every year and are analyzed for all macrochemical parameters and some trace elements. Tritium contents were measured in the first sampling round. The geochemistry of Dutch groundwater is complex, due to the different sources (seawater, surface water and rainwater), complicated hydrogeology, and human impact on flow systems and pollution. Structuring or data analysis is required for the interpretation of the large number of hydrogeochemical data from such a monitoring network. An exploratory approach is to look within the data set for homogeneous groups, each with a typical (macro)chemistry. The selection criteria for the location of the monitoring sites of the LMG are mainly based on soil type and land use, and to some extent on the hydrogeological situation. However, a classification based on the two most reliable criteria, soil type and land use, does not result in chemically distinguishable homogeneous groups or water types. Fuzzycmeans clustering was successfully used to discern structure and natural groups in the LMG data for 1 year. A seven‐cluster model was adopted. The number of clusters was decided heuristically with the aid of nonlinear mapping, on the basis of the geographic distribution, the hydrogeochemical interpretability, and the unimodality of the distribution of the parameters per cluster. The consistency of the model is illustrated by the reproducibility of the clusters in different years. The clusters are related to geochemical processes, natural sources, and anthropogenic input and are designated as follows: (1) “seawater” in coastal areas, (2) “desalinization” in organic‐rich Holocene marine and peat deposits, (3) “surface water” for downward seeping river water or surface water near the main rivers, (4) “carbonate/reduction” in peat areas or old groundwater in seepage zones, (5) “carbonate” for carbonate‐dissolving “precipitation,” (6) acid “precipitation” water in sandy topographic highs, and (7) a “polluted” cluster characterized by agricultural contaminants. Although the influence of soil type and land use is noticeable in some of the clusters, the geochemical controls, which characterize the different clusters, appear to outweigh their anticipated influence on the hydrogeochemistry on the scale of the national groundwater network. Consequently, the homogeneous groups, obtained through the cluster analysis, present a better base for further statistical and hydrogeochemical evaluation than, e.g., a stratification of the data based on soil type and land use.
A generalized two‐dimensional analytical solute transport model in bounded media for flux‐type finite multiple sources
Authors: Batu Vedat
Published: February 2004

This paper presents a generalized two‐dimensional analytical solute transport model with unidirectional flow field from time‐ and space‐dependent sources in a bounded homogeneous medium using the flux‐type (or third‐type or Cauchy) boundary condition at the inlet location of the medium. The solute transport equation incorporates terms accounting for advection, dispersion, linear equilibrium sorption, and first‐order decay. General solutions were determined for arbitrary input boundary conditions with the help of Fourier analysis and Laplace transform techniques. After presenting a generally applicable solution, special solutions are given for a single and two finite sources having different inlet solute flux distributions. The expressions for convective‐dispersive flux components are also derived. Expressions for concentration distribution and convective‐dispersive flux components for steady state transport case are also presented. Comparison of the results of this analytical model and a finite difference numerical model showed very good agreement. The solutions may be used for predicting solute concentrations in a unidirectional porous media flow field as well as in rivers and canals. The solutions may also be used for verification of more comprehensive numerical models, and laboratory or field determination of solute transport parameters.
Boundary element solution for stochastic groundwater flow: Temporal weakly stationary problems
Authors: Cheng A. H.‐D. , Abousleiman Y. , Ruan F. , Lafe O. E.
Published: February 2004

This paper continues the development of boundary integral equation technique for solving stochastic groundwater flow problems. The case of aquifers under deterministic hydraulic conductivity, but subject to random boundary condition and domain recharge has been investigated in an earlier paper under the assumption of steady state governing equation. In the current paper the governing equation is transient, but the temporal variation of second‐order statistical moments is assumed to be weakly stationary. Several examples of fundamental importance to the understanding of stochastic process are examined both by the analytical and by the numerical (boundary element) techniques.
Stochastic analysis of the transport of kinetically sorbing solutes in aquifers with randomly heterogeneous hydraulic conductivity
Authors: Quinodoz Hernán A. M. , Valocchi Albert J.
Published: February 2004

One of the most promising approaches for modeling solute transport in heterogeneous aquifers describes the natural variability in hydraulic conductivity by means of a stationary random field. Therefore the solute concentration distribution at a given time also becomes a random field. For the case of an inert solute, Dagan (1982, 1984) found a solution for the temporal evolution of the spatial moments of the ensemble average concentration random field; this solution describes the impact of small‐scale heterogeneity upon large‐scale spreading of the plume (the so‐called macrodispersion phenomenon). In this paper we extend Dagan's (1982, 1984) results to the case of a reactive solute undergoing linear reversible adsorption. We assume spatially uniform kinetic rate coefficients. The solution obtained explicitly quantifies the combined effect of nonequilibrium adsorption reactions and macrodispersion, for a range of possible time scales of both processes. We show that for very large times the effects of macrodispersion and kinetics are additive, while for shorter times they are related in a nonlinear fashion. As a particular application of our solution, we demonstrate that the conditions necessary for the existence of local equilibrium depend not only on the kinetic rate coefficients but also upon the spatial structure of the conductivity field. The observed behavior of an organic solute used in the Borden field experiment appears to be consistent with the predictions of the new model.
Incorporating public preferences in planning urban water supply reliability
Authors: Howe Charles W. , Smith Mark Griffin
Published: February 2004

This study has two objectives: (1) to compare the attitudes of the water‐using public, water officials, and elected officials toward the risk of water supply shortage; and (2) to develop a methodology for incorporating water users' valuation of reliability in system design. Using contingent valuation techniques, we have measured the benefits and costs of different reliability levels in terms of water users' willingness to pay (WTP) for increases in reliability and in terms of their willingness to accept (WTA) compensation in the form of lower water bills for lower levels of reliability. Three cities in northern Colorado with diverse baseline levels of water supply reliability (Aurora, Boulder, and Longmont) are the study sites. Contrary to our hypothesis that water managers are unjustifiably risk averse, we find that water managers' preferences are consistent with customer WTP (WTA) values associated with the risk of water shortages and the system costs associated with reliability. Water managers in Boulder (high reliability) were willing to consider reductions in the level of system reliability while water managers in Aurora and Longmont (low reliability) favored the status quo or increased reliability. While these attitudes were sometimes contrary to a majority of customers' expressed interests in change, they were shown to be justified by comparison of supply system costs (savings) with aggregate WTP for additional reliability (WTA for less reliability).
A stochastic approach for assessing the effect of changes in synoptic circulation patterns on gauge precipitation
Authors: Hughes James P. , Lettenmaier Dennis P. , Guttorp Peter
Published: February 2004

A stochastic model is described that allows transfer of information from general circulation models to precipitation gauge locations using a weather state classification scheme. The weather states, which are based on present and previous day's sea level pressure, are related stochastically to gauge daily precipitation and temperature. Weather states are defined to give maximal separation of test station precipitation distributions using the Classification and Regression Trees procedure. Precipitation amounts are resampled from historical observations, conditional on the weather state and the previous day's rain state. Daily temperature maxima and minima are simulated by conditioning on the present and previous day's rain state, with the residual modeled as a first‐order autoregressive process. The model parameters are estimated using 9 years (1965–1973) of concurrent U.S. National Meteorological Center (NMC) gridded observations and four precipitation stations in the Columbia River Basin. The model is illustrated using both historical NMC gridded observations of sea level pressure and lower atmosphere temperature, as well as the same variables from the General Fluid Dynamics Laboratory general circulation model for present climate and CO2doubling. A 40‐year sequence of simulated precipitation and temperature is used to estimate seasonal streamflows and flood frequency distributions under present and doubled CO2climates.
Plume scale‐dependent dispersion in heterogeneous aquifers: 2. Eulerian analysis and three‐dimensional aquifers
Authors: Rajaram Harihar , Gelhar Lynn W.
Published: February 2004

An analytical approach is developed for describing the ensemble average of the second moment of a solute plume in three‐dimensional heterogeneous porous media. While existing approaches describe scale‐dependent dispersion in terms of a single scale, the plume displacement, the approach developed here presents an enhanced picture of scale‐dependent dispersion involving two scales: the plume displacement and the plume scale. The plume scale arises naturally in the formulation, permitting a distinction between the dispersive role of heterogeneity at scales smaller than the plume size and the variability in the plume location caused by larger scale heterogeneity. A physically consistent description of scale‐dependent dispersion is thus achieved. The growth of the ensemble average second moment is related to the product of concentration values at two points. The concept of the separation distribution function related to the latter is introduced. The separation distribution function physically describes the fraction of solute particles which have another solute particle at a given separation. An Eulerian partial differential equation based on a small perturbation approach is developed to describe the evolution of the separation distribution function. Simple analytical expressions for the second moment growth rates are presented. These expressions incorporate the influence of the plume size through a low wavenumber filter depending of the plume second moment. Asymptotic expressions for the second moment growth rate are presented which apply at large displacement. These expressions indicate that the longitudinal second moment growth rate depends on the transverse second moments of the plume. Comparison of predicted second moment evolution with results from earlier numerical simulations indicates excellent agreement. Application to the Borden tracer test indicates a significant reduction in the longitudinal second moment from that predicted by existing three‐dimensional theories and better agreement with the measured second moments.
Plume scale‐dependent dispersion in heterogeneous aquifers: 1. Lagrangian analysis in a stratified aquifer
Authors: Rajaram Harihar , Gelhar Lynn W.
Published: February 2004

An analytical approach is developed for describing the ensemble average of the second moment of a solute plume in heterogeneous porous media. The growth of the ensemble average second moment is related to the growth of the mean square separation of a pair of particles. Exact Lagrangian expressions are developed for the growth of the mean square separation in a perfectly stratified aquifer. These exact expressions are made possible by an exact relation between the Lagrangian and Eulerian velocity covariances in the perfectly stratified aquifer. The ensemble average second moment is shown to depend on the initial vertical dimensions of the concentration distribution. As the vertical size increases, the second moment growth rate is larger. The second moment expressions are contrasted with the expressions for the mean square displacement of a single particle. While all scales of heterogeneity contribute to the mean square displacement of a single particle, only scales of heterogeneity smaller than the plume size contribute to the second moment. Asymptotic large displacement expressions for the case of a Gaussian covariance function describing the hydraulic conductivity variations are derived. These expressions indicate that at large displacement, the second moment grows as the 3/2 power of displacement. However, for any finite source size no matter how large, the second moment growth rate approaches a different asymptote from the rate of growth of the mean square displacement of a single particle. This is in conflict with the traditional notion of ergodicity which leads to the expectation that for large source sizes, the dispersivity approaches the rate of growth of the mean square displacement of a single particle.
A fast and exact method for multidimensional gaussian stochastic simulations
Authors: Dietrich C. R. , Newsam G. N.
Published: February 2004

To generate multidimensional Gaussian random fields over a regular sampling grid, hydrogeologists can call upon essentially two approaches. The first approach covers methods that are exact but computationally expensive, e.g., matrix factorization. The second covers methods that are approximate but that have only modest computational requirements, e.g., the spectral and turning bands methods. In this paper, we present a new approach that is both exact and computationally very efficient. The approach is based on embedding the random field correlation matrixRin a matrixSthat has a circulant/block circulant structure. We then construct products of the square rootS1/2with white noise random vectors. Appropriate sub vectors of this product have correlation matrixR, and so are realizations of the desired random field. The only conditions that must be satisfied for the method to be valid are that (1) the mesh of the sampling grid be rectangular, (2) the correlation function be invariant under translation, and (3) the embedding matrixSbe nonnegative definite. These conditions are mild and turn out to be satisfied in most practical hydrogeological problems. Implementation of the method requires only knowledge of the desired correlation function. Furthermore, if the sampling grid is ad‐dimensional rectangular mesh containingnpoints in total and the correlation between points on opposite sides of the rectangle is vanishingly small, the computational requirements are only those of a fast Fourier transform (FFT) of a vector of dimension 2dnper realization. Thus the cost of our approach is comparable with that of a spectral method also implemented using the FFT. In summary, the method is simple to understand, easy to implement, and is fast.
The effects of acidic deposition on streams in the Appalachian Mountain and Piedmont Region of the Mid‐Atlantic United States
Authors: Herlihy A. T. , Kaufmann P. R. , Church M. R. , Wigington P. J. , Webb J. R. , Sale M. J.
Published: February 2004

Streams in the Appalachian Mountain area of the mid‐Atlantic receive some of the largest acidic deposition loadings of any region of the United States. A synthesis of the survey data from the mid‐Appalachians yields a consistent picture of the acid base status of streams. Acidic streams, and streams with very low acid neutralizing capacity (ANC), are almost all located in small (<20 km2), upland, forested catchments in areas of base‐poor bedrock. In the subpopulation of upland forested systems, which comprises about half the total stream population in the mid‐Appalachian area, data from various local surveys show that 6–27% of the streams are acidic, and about 25–50% have ANC less than 50 μeq L−1. After excluding streams with acid mine drainage, National Stream Survey estimates for the whole region show that there are 2330 km of acidic streams and 7500 km of streams with ANC less than 50 μeq L−1. Many of the streams with base flow ANC less than 50 μeq L−1become acidic during storm or snowmelt episodes. Sulfate from atmospheric deposition is the dominant source of strong acid anions in acidic mid‐Appalachian streams. Their lowpH (median, 4.9) and high levels of inorganic monomeric aluminum (median, 129 μg L−1) leached through soils by acidic deposition are causing damage to aquatic biota. Quantification of the extent of biological effects, however, is not possible with available data. Localized studies have shown that stream water ANC is closely related to bedrock mineralogy. Attempts to quantify this relationship across the mid‐Appalachians, however, were frustrated by the lack of adequate scale geologic mapping throughout the region. Sulfate mass balance analyses indicate that soils and surface waters of the region have not yet realized the full effects of elevated sulfur deposition due to watershed sulfate retention. Sulfur retention is likely to decrease in the future, resulting in further losses of stream ANC.
Integrating irrigation water demand, supply, and delivery management in a stochastic environment
Authors: Dudley Norman J. , Scott Bradley W.
Published: February 2004

Previously developed suites of models integrate irrigation water supply and demand management for simplified surface reservoir supply systems. A central stochastic dynamic programming model is supported by simulation models, including a soil water‐plant growth model. That modeling is extended herein to include decisions about timing and quantity of reservoir water releases into the delivery system, resulting in the integration of supply, demand, and delivery management. Irrigators have rights to percentages of reservoir capacity, reservoir inflows, and downstream tributary flows. Natural rivers are the supply channels in the study area. Large on‐farm water storages exist for storing regulated and unregulated river flows available to the irrigators. Farms can be many days flow from the reservoir, requiring orders for reservoir releases to be lodged before the arrival of a previous order. The probability of unregulated flows from tributaries downstream of the reservoir further complicates ordering decisions. Abandonment of irrigated area to rain‐fed status occurs at two levels, forced abandonment if insufficient water is available at the farm when the irrigation‐trigger soil water deficit is reached, and planned abandonment to save water for possible later use. The need for forced abandonment is determined by simulation models; planned abandonment decisions are derived by stochastic dynamic programming. Results show annual net revenue means and standard deviations as functions of different capacities of the on‐farm storages and water supplies from either regulated or unregulated flows, or both.
An approximation technique for stochastic environmental modeling
Authors: Zielinski Przemyslaw A.
Published: February 2004

This paper describes the outline of an approximate mathematical technique capable of including stochastic components in environmental models based on ordinary differential equations. The technique is based on the decomposition of the solution into additive components, with the first component being the solution of a simplified deterministic equation and each of the other components being found in terms of those preceding it. Moments of the solution process can then be calculated by performing appropriate averaging. The method provides natural statistical separability, that is, no truncations in the derivation stage or closure approximations are necessary. The random quantities involved can be either random variables or stochastic processes. No unrealistic assumptions, like white noise behavior or small randomness, are necessary. Solutions, which are in the form of infinite but convergent series, can be easily implemented into computer programs, providing convenient support in management and decision analysis. A demonstration application of a simple biochemical oxygen demand model for river waters illustrates the usefulness of the technique in environmental modeling.
Estimation of lake hydrologic budget terms using the simultaneous solution of water, heat, and salt balances and a Kalman Filtering Approach: Application to Lake Kinneret
Authors: Assouline S.
Published: February 2004

The Lake Kinneret hydrologic budget includes two unknown terms: evaporation losses Qe, and inflow from hot saline springs which represent the groundwater component, Qs. This last term affects also the salt budget as well as the energy budget of the lake, which is, in turn, the basis for evaporation estimates. A new approach consisting of the simultaneous solution of the water, heat, and salt balance equations is presented to evaluate Qe and Qs. This approach gives a better estimation of evaporation, since it accounts for the net energy advected to the lake. Neglecting this term led to overestimation of evaporation during the dry winter of 1991 and to underestimation during the whole year of 1992, which was particularly rainy. The maximal monthly error occurred for both years in February: +55% for 1991, and −29% for 1992. The application of the approach to the period of 1988–1991 shows that the annual hydrograph of the saline springs system is characterized by two distinct periods of high discharge during each year. The first period, in winter, seems to be related to the structure and timing of the rainfall season. The second occurs generally at the beginning of the summer, after the rainfall season has ended. The overall trend for the period is declining peaks and annual discharges, which is consistent with the hydrologic impact of a 3‐year drought since 1989. The measured, as well as the estimated, components of the water balance equation are subject to uncertainty. The formulation of the balance equation in terms of a state‐space system and the application of the Kalman filter (KF) allows quantifying the effect of this uncertainty on the different components. Therefore for given noise statistics and a priori data, it is possible to determine the optimal linear estimates of the estimated components and to compare them to the values resulting from the simultaneous solution approach. The method was applied on monthly water budgets for a period of 60 months. The effect on evaporation varies within a range of ±5%. The estimate of runoff from the ungauged part of the direct watershed was generally increased by the KF, especially during the rainy winter of 1988. The effect on this estimate varies within a range of ±12%. The component that was affected the most is the estimated discharge from the saline springs system (±15%). The KF increases by 15% the winter peak of 1988 and decreases all the discharges estimates for the dry years, especially those of 1990. Therefore the stochastic approach strengthens the agreement between the trends in the estimated values and in the climatic conditions during the period studied.
Numerical simulations of Twin Lake Natural‐Gradient Tracer Tests: A comparison of methods
Authors: Moltyaner G. L. , Klukas M. H. , Wills C. A. , Killey R. W. D.
Published: February 2004

The primary objectives of the Twin Lake experiments have been to study geologic heterogeneities and field‐scale dispersion and to provide data for developing and evaluating groundwater flow and transport models. The experiment reported here was undertaken in 1987–1988 to expand the studies of heterogeneity and dispersion from a 40‐m scale investigated in 1983 to a full 270‐m scale from the point of tracer injection to the groundwater discharge zone. The hydrogeologic setting, the experimental and analytical methods, and the interpretation of experimental data are the topics of discussion in this paper. The 1983 and 1987–1988 tracer tests were used to evaluate the predictive abilities of various numerical groundwater flow and transport models. The modeling is done along the mean flow direction in the vertical plane. The two‐dimensional finite‐element modeling was successful in simulating the hydraulic head distribution but not the groundwater motion or the tracer transport. In addition, owing to the advection‐dominated nature of the groundwater flow, the transport simulation suffered from numerical dispersion. Significant improvements in simulations were obtained using method‐of‐characteristics‐and random‐walk‐based computer models. The influence of dimensionality on the transport simulation was evaluated by comparing the two‐ and three‐dimensional random‐walk simulations. It was found that over the first 40 m along the mean flow path, the three‐dimensional simulation does not reproduce the plume migration any better than the two‐dimensional simulation does. Random‐walk simulations of the transport observed in the 1983 and 1987–1988 tests are also done on the basis of the advection‐dispersion equation and tracer test‐derived velocities without invoking the concept of hydraulic conductivity and Darcy's law. These simulations were found to be superior to simulations based on the coupled flow and transport models. It was also found that the simulated hydraulic head distribution is insensitive to spatial variations in hydraulic conductivity and the simulated groundwater velocity distribution is inappropriate for transport modeling. It was concluded that the advection‐dispersion models of the local‐scale transport perform better in predicting the tracer migration at the Twin Lake aquifer using measured velocities rather than velocities simulated by a flow model.
An analytical solution to the solute transport equation with rate‐limited desorption and decay
Authors: Fry V. A. , Istok J. D. , Guenther R. B.
Published: February 2004

An analytical solution is derived for the advection‐dispersion equation with rate‐limited desorption and first‐order decay, using an eigenfunction integral equations method. The model equations represent one‐dimensional solute transport in a homogeneous isotropic porous medium where the porous medium is saturated with the aqueous solution. The flow field is uniform. Rate‐limited desorption is described as a first‐order process where the rate is proportional to the difference in concentration between the sorbed phase and the aqueous phase. The solution was verified for the limiting case of equilibrium desorption using the solution of van Genuchten and Alves (1982). Example calculations are presented to show the effect of the desorption rate, decay rate, and distribution coefficient on the rate of contaminant removal from both the aqueous and sorbed phases of a groundwater aquifer. The solution quantifies the expected results, where the larger the desorption and decay rate and the smaller the distribution coefficient, the faster the rate of contaminant removal from the aqueous and sorbed phases.
Velocity dependence of dispersion for transport through a single fracture of variable roughness
Authors: Dronfield Douglas G. , Silliman Stephen E.
Published: February 2004

Several authors have recently expressed interest in chemical transport within fractured media. The majority of these efforts have been based on a linear relationship between the dispersion coefficient and the average fluid velocity within the fracture. It is not apparent that this relationship is fully justified in all applications. For the present study, it is assumed (as suggested by authors working in porous media) that the dispersion coefficient is proportional to the velocity raised to a power, n. Further, it is assumed that transport within the fracture follows classic advection‐dispersion behavior (e.g., Fickian dispersion). The present study focuses on the value of the power n in a series of artificial fractures. In particular, an experimental apparatus is utilized to run controlled tracer experiments through a single fracture. When the fracture consists of smooth parallel plates, the results from the experiments indicate that the dispersion coefficient is proportional to the velocity squared (consistent with the early work by Taylor (1953) for transport dominated by transverse diffusion). As the fracture roughness is increased through use of blockages within the fracture and/or addition of surface roughness along the fracture walls, the relationship between dispersion and velocity varied. For each fracture roughness, the results followed the general relationship in which dispersion is proportional to velocity raised to the power n. The power n, however, was strongly dependent on the fracture roughness, taking on a value of 2.0 for smooth parallel plates and decreasing to a value of approximately 1.3 for rough plates.
A stochastic model of flow duration curves
Authors: LeBoutillier David W. , Waylen Peter R.
Published: February 2004

The estimation of flow quantiles is a major focus of current hydrologic research. Probabilistic representations of a few individual quantiles, notably the annual flood and low flow, have been devised, and recently a few of the larger quantiles have been modeled. This research describes and demonstrates the use of a probability model for all flow quantiles in a river. A flow duration curve represents the annual flow frequency characteristics of rivers by depicting the cumulative frequencies for average ranked flows in a river. A model requiring only five parameters is developed, by combining the principles of order statistics and traditional flow frequency analyses, and applied to flow duration curves for rivers in the province of British Columbia, Canada. Streamflows in British Columbia are generated from a number of distinct physical processes operating in highly variable environments. The model presented incorporates the physical generating processes of streamflow in both the statistical representation of flow duration curves and their interpretation.
Robustness of the multiple realization method for stochastic hydraulic aquifer management
Authors: Chan Nathan
Published: February 2004

Hydraulic containment of a groundwater plume is modeled as a pump‐and‐treat system of extraction wells in a confined aquifer with uncertain transmissivity. The transmissivity is modeled as a random field with a specified covariance structure, conditioned at measurement locations. The response matrix method is used to form a linear program to obtain the optimal pumping strategy. High reliability is achieved by employing a multiple realization or “stacking” method which requires the solution to simultaneously satisfy a number of realizations of the random field. Empirical tests indicate that this reliability is a monotonically increasing function of the number of stacked realizations and appears to be relatively insensitive to moderate changes in the variance, correlation length, and covariance structure of the stochastic transmissivity and to changes in the boundary conditions and measurement data. Two simple statistical models based on Bayesian analysis and order statistics are formulated, leading to predicted reliabilities of (N+1)/(N+2) and N/(N+1), respectively, where N is the number of stacked realizations. These simple curves are shown to compare quite well with the empirical findings. These results indicate that it may be possible to determine the stack size necessary to achieve a prespecified reliability. The robustness of the reliability curve indicates that it may be useful as a preliminary design tool for aquifer management.
Simulating the volatilization of solvents in unsaturated soils during laboratory and field infiltration experiments
Authors: Cho H. Jean , Jaffé Peter R. , Smith James A.
Published: February 2004

This paper describes laboratory and field experiments which were conducted to study the dynamics of trichloroethylene (TCE) as it volatilized from contaminated groundwater and diffused in the presence of infiltrating water through the unsaturated soil zone to the land surface. The field experiments were conducted at the Picatinny Arsenal, which is part of the United States Geological Survey Toxic Substances Hydrology Program. In both laboratory and field settings the gas and water phase concentrations of TCE were not in equilibrium during infiltration. Gas‐water mass transfer rate constants were calibrated to the experimental data using a model in which the water phase was treated as two phases: a mobile water phase and an immobile water phase. The mass transfer limitations of a volatile organic compound between the gas and liquid phases were described explicitly in the model. In the laboratory experiment the porous medium was nonsorbing, and water infiltration rates ranged from 0.076 to 0.28 cm h−1. In the field experiment the water infiltration rate was 0.34 cm h−1, and sorption onto the soil matrix was significant. The laboratory‐calibrated gas‐water mass transfer rate constant is 3.3×10−4 h−1 for an infiltration rate of 0.076 cm h−1 and 1.4×10−3 h−1 for an infiltration rate of 0.28 cm h−1. The overall mass transfer rate coefficients, incorporating the contribution of mass transfer between mobile and immobile water phases and the variation of interfacial area with moisture content, range from 3×10−4 h−1 to 1×10−2 h−1. A power law model relates the gas‐water mass transfer rate constant to the infiltration rate and the fraction of the water phase which is mobile. It was found that the results from the laboratory experiments could not be extrapolated to the field. In order to simulate the field experiment the very slow desorption of TCE from the soil matrix was incorporated into the mathematical model. When desorption from the soil matrix was added to the model, the calibrated gas‐water mass transfer rate constant is 2 orders of magnitude lower than that predicted using the power law model developed for the nonsorbing laboratory soil.
Roughness of stable, armored gravel beds
Authors: Gomez Basil
Published: February 2004

The grain roughness of stable armored beds that formed in a laboratory flume under a range of steady flow conditions on rounded, flat and angular gravel is analyzed. Gravel roughness geometry is determined from bed surface profiles and vertical photographs. These techniques have been employed in field situations. Thus the methodology is potentially applicable to the analysis of grain roughness in natural gravel bed channels. The description of representative roughness geometry is also analogous to that used to characterize artificial roughness arrays. Armor roughness increases with increasing flow. Armored surfaces composed of angular gravel are roughest, and surfaces formed of flat gravel offer least resistance to the flow. Stable armored beds may exhibit a tendency to maximize the ratio of the shear due to drag on representative roughness elements to total shear. Roughness concentration is strongly correlated with the energy slope, and there is a linear increase in equivalent roughness height with increasing roughness concentration. The friction factor for an armored surface varies in a linear manner with representative roughness geometry. The equation defining this relation is probably similar to that used to characterize variations in the friction factor with artificial roughness geometry at low roughness concentrations. However, to reconcile the relations for artificial and natural roughness completely, it may be necessary to explicitly consider the contribution to flow resistance made by roughness shape, background roughness, and blocking in shallow flows.
Hydrologic response of freshwater watersheds to climatic variability: Model development
Authors: Nikolaidis Nikolaos P. , Hu Hsien‐Lun , Ecsedy Christopher , Lin J. D.
Published: February 2004

To evaluate the hydrologic and biogeochemical response of freshwater watersheds to climatic variability properly, a mathematical model with detailed parameterization in describing the hydrologic and thermal processes in a watershed is needed. For this purpose, the Enhanced Trickle Down model was modified to predict the hydrologic and thermal responses of freshwater watersheds to various climate change scenarios. Modifications of the model included the incorporation of an energy transfer submodel, an improved hydraulic conductivity scheme, and the coupling with a point source snowmelt model. The results of calibration and verification of the model using 8 years of field data collected at the Agricultural Research Service, W‐3 watershed, located near Danville, Vermont, are presented.
Thermodynamic basis of capillary pressure in porous media
Authors: Hassanizadeh S. Majid , Gray William G.
Published: February 2004

Important features of multiphase flow in porous media that distinguish it from single‐phase flow are the presence of interfaces between the fluid phases and of common lines where three phases come in contact. Despite this fact, mathematical descriptions of these flows have been lacking in rigor, consisting primarily of heuristic extensions of Darcy's law that include a hysteretic relation between capillary pressure and saturation and a relative permeability coefficient. As a result, the standard capillary pressure concept appears to have physically unrealistic properties. The present paper employs microscopic mass and momentum balance equations for phases and interfaces to develop an understanding of capillary pressure at the microscale. Next, the standard theories and approaches that define capillary pressure at the macroscale are described and their shortcomings are discussed. Finally, an approach is presented whereby capillary pressure is shown to be an intrinsic property of the system under study. In particular, the presence of interfaces and their distribution within a multiphase system are shown to be essential to describing the state of the system. A thermodynamic approach to the definition of capillary pressure provides a theoretically sound alternative to the definition of capillary pressure as a simple hysteretic function of saturation.
Response of a Channel with alternate bars to a decrease in supply of mixed‐size bed load: A Flume Experiment
Authors: Lisle Thomas E. , Iseya Fujiko , Ikeda Hiroshi
Published: February 2004

The response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand‐gravel mixture. After a series of quasi‐stationary alternate bars were formed under equilibrium sediment transport, feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant. The primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased. After the first feed rate reduction, the channel incised by twice the mean water depth, on average, and caused distal bar surfaces to emerge as terracelike features. Bar roughness decreased, and mean boundary shear stress exerted on bed surface particles increased. Little incision occurred after the second feed rate reduction.
Experimental studies of bed topography and flow patterns in large‐amplitude meanders: 1. Observations
Authors: Whiting Peter J. , Dietrich William E.
Published: February 2004

Large amplitude river meanders have been observed to contain multiple bars within a single loop and to exhibit planforms that are asymmetric or have subsidiary bends. Here we report experiments conducted in symmetric sine‐generated meanders of large amplitude which document the bed topography, flow patterns, and sediment transport fields in such bends. Multiple pools spaced at a distance of 3–4 widths develop successively along the outer concave bank. Downstream of each pool a distinct bar lies along the inside convex bank. The series of bars overlap and appear shingled downstream; the leading edge of each bar is oblique to the channel trace and curves toward the inside bank. The first and most pronounced pool, opposite the first inner bank bar, is consistently positioned where the projection of the inner bank tangent at the upstream crossing intersects the outer bank. Successive shingled bars in the bend migrate downstream if bars arriving at the persistent first pool deliver large enough pulses of sediment to the pool to form new bars. In experiments with an erodible bank, the banks near pools are sites of accelerated retreat. This suggests that the shear stress fields engendered by these multiple features may cause the planimetric distortion seen in larger rivers.
Determination of contributing areas for saturation overland flow from chemical hydrograph separations
Authors: Eshleman Keith N. , Pollard John S. , O'Brien Anne Kuebler
Published: February 2004

A recurring problem in watershed hydrology is the identification of flow paths along which water is routed in both surface and subsurface environments. In particular, the question of whether storm flow is composed primarily of water flowing over the land surface (overland flow) or beneath the land surface (subsurface flow) is significant, because the former flow path provides for little soil contact, while the latter allows for extensive interaction with subsurface materials. The Reedy Creek watershed (45.1 km2) in the Virginia Coastal Plain has been the subject of field hydrogeological, hydrochemical, and hydrological studies since 1989. Results from chemical separation of six storm flow hydrographs indicate that saturation overland flow (including both direct precipitation onto and return flow to the surface) from saturated contributing areas is the dominant storm flow generation mechanism in the watershed; new water composed 11–38% of creek discharge at the hydrograph peaks, and maximum 24‐hour rainfall intensity explained 96% of the total variation in the peak new water contribution among the six events. Estimates of new water‐contributing areas determined from the chemical separations were found to vary as a function of antecedent conditions and were also found to be consistent with estimates of areas of likely surface saturation (e.g., perennial channels, open water, and riparian wetland areas) based on field observations and topographic maps.
An analytic solution of the stochastic storage problem applicable to soil water
Authors: Milly P. C. D.
Published: February 2004

The accumulation of soil water during rainfall events and the subsequent depletion of soil water by evaporation between storms can be described, to first order, by simple accounting models. When the alternating supplies (precipitation) and demands (potential evaporation) are viewed as random variables, it follows that soil‐water storage, evaporation, and runoff are also random variables. If the forcing (supply and demand) processes are stationary for a sufficiently long period of time, an asymptotic regime should eventually be reached where the probability distribution functions of storage, evaporation, and runoff are stationary and uniquely determined by the distribution functions of the forcing. Under the assumptions that the potential evaporation rate is constant, storm arrivals are Poisson‐distributed, rainfall is instantaneous, and storm depth follows an exponential distribution, it is possible to derive the asymptotic distributions of storage, evaporation, and runoff analytically for a simple balance model. A particular result is that the fraction of rainfall converted to runoff is given by (1 ‐ R−1)/(eα(1−R−1) − R−1), in which R is the ratio of mean potential evaporation to mean rainfall and a is the ratio of soil water‐holding capacity to mean storm depth. The problem considered here is analogous to the well‐known problem of storage in a reservoir behind a dam, for which the present work offers a new solution for reservoirs of finite capacity. A simple application of the results of this analysis suggests that random, intraseasonal fluctuations of precipitation cannot by themselves explain the observed dependence of the annual water balance on annual totals of precipitation and potential evaporation.
Modeling of aerobic biodegradation and oxygen consumption in benthic sediments
Authors: Dolgonosov Boris M.
Published: February 2004

A brief overview of the existing chemical and biological models of biodegradation in porous media is given. A revised biological model of aerobic degradation of organic matter in benthic sediments is developed. The model is based on steady state balance equations for microbial mass, organic matter, and oxygen. The biomass balance equation involves a logistic trophic function (instead of the classical Malthusian one) describing the self‐regulation of microbial abundance. One of the corollaries of the model is the dependence of the decay rate upon the organic matter content S in the form dS/dt ∝ S2/(S + Ks)2 (where Ks is a constant) that implies two extreme cases: dS/dt is independent of S at small Ks (refractory organics) and dS/dt ∝S2 at large Ks (labile organics), instead of the commonly used law dS/dt ∝ S. The oxygen consumption rate J is expressed through the oxygen concentration O0 in the overlying water and three generalized parameters. This function demonstrates the effect of the plateau at large O0. The dependence J(O0) is calibrated for the three experimental systems of the literature, two of which have high oxygen consumption rates and one low. For the latter, the model parameters are estimated and their correspondence to the evidence available is discussed. Depth profiles of oxygen, organic matter, and biomass are computed and the aerobic zone thickness is determined.