bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic analysis of flow in a heterogeneous unsaturated‐saturated system
Authors: Zhang Dongxiao , Lu Zhiming
Published: October 2002

In this study we develop a stochastic model for transient unsaturated‐saturated flow in randomly heterogeneous media with the method of moment equations. We first derive partial differential equations governing the statistical moments of the flow quantities by perturbation expansions and then implement these equations under general conditions with the method of finite differences. The stochastic model developed is applicable to the entire domain of a bounded, multidimensional unsaturated‐saturated system in the presence of random or deterministic recharge and sink/source and in the presence of multiscale, nonstationary medium features. The unsaturated and saturated zones are coupled through the water table, whose position is random in randomly heterogeneous porous media. The presence of the water table renders the flow moments strongly nonstationary even in the absence of medium nonstationary features. This finding is confirmed with Monte Carlo simulations. The resulting first two moments of flow may be used to approximate confidence intervals for the flow quantities. In addition, the integrated stochastic flow model provides the prerequisite quantities for realistically studying contaminant transport in unsaturated‐saturated systems with a stochastic approach.
A multi‐isotope (B, Sr, O, H, and C) and age dating (3H–3He and 14C) study of groundwater from Salinas Valley, California: Hydrochemistry, dynamics, and contamination processes
Authors: Vengosh Avner , Gill Jim , Lee Davisson M. , Bryant Hudson G.
Published: October 2002

The chemical and isotope (11B/10B, 87Sr/86Sr, 18O/16O, 2H/H, 13C/12C, 14C, and 3He/3H) compositions of groundwater from the upper aquifer system of the Salinas Valley in coastal central California were investigated in order to delineate the origin and processes of groundwater contamination in this complex system. The Salinas Valley has a relatively deep, confined “400‐foot” aquifer, overlain by a “180‐foot” aquifer and a shallower perched aquifer, all made up of alluvial sand, gravel and clay deposits. Groundwater from the aquifers have different 14C ages: fossil (14C = 21.3 percent modern carbon (pmc) for the 400‐foot aquifer and modern (14C = 72.2–98.2 pmc) for the 180‐foot aquifer. Fresh groundwater in all aquifers is recharged naturally and artificially through the Salinas River. The two modes of recharge can be distinguished chemically. We identified several different saline components with distinguishable chemical and isotopic fingerprints. (1) Saltwater intrusion in the northern basin has C1 concentrations up to 1700 mg/L, a Na/Cl ratio less than seawater, a marine Br/Cl ratio, a Ca/Cl ratio greater than seawater, δ11B between +17 and +38‰ and 87Sr/86Sr between 0.7088 and 0.7096. Excess dissolved Ca, relative to the expected concentration for simple dilution of seawater, correlates with 87Sr/86Sr ratios, suggesting base exchange reaction with clay materials. (2) Agriculture return flow is high in NO3 and SO4, with a 87Sr/86Sr = 0.7082,δ11B =19‰and δ13C between −23 and −17‰. The 3H–3He ages (5–17 years) and 14C data suggest vertical infiltration rates of irrigation water of 3–10 m/yr. (3) Nonmarine saline water in the southern part of the valley has high total dissolved solids up to 3800 mg/L, high SO4, Na/Cl ratio >1, δ11B between +24 and +30‰, and 87Sr/86Sr = 0.70852. This groundwater may have acquired its geochemical signature from leaching of sedimentary rocks associated with the Coast Range marine deposits of Mesozoic to early Cenozoic age. The combination of different geochemical and isotopic fingerprints enables us to delineate the impact of salt sources in different areas of the valley and to reconstruct the origin of the SO4‐enriched NO3‐depleted saline plume that is located west of the city of Salinas. We suggest that the latter is derived from a mixture of different natural saline waters rather than from anthropogenic contamination.
A characteristic‐based finite analytic method for solving the two‐dimensional steady state advection‐diffusion equation
Authors: Lowry Thomas , Li Shu‐Guang
Published: October 2002

This paper develops an improved finite analytic (FA) solution method to the advection‐diffusion equation (ADE) for solving advection‐dominated steady state transport problems. The FA method solves the ADE analytically in localized, discrete elements, with each element linked through local boundary conditions. Previous FA methods have suffered from complex solution formulations as well as from numerical dispersion stemming from inaccuracies in the local boundary estimations. Here we use finite difference approximations of the dispersion terms to reduce solution complexity, implement an improved particle‐tracking scheme to account for velocity variations within each element, and use a Hermite interpolation scheme to estimate the local boundary conditions. The new FA method, previous FA methods, and a finite difference upwinding method are compared in both homogeneous and heterogeneous velocity fields at different Peclet numbers. Results show the new FA method exhibits little numerical dispersion without undue complexity or computational effort across all tested flow conditions.
Designing multidimensional environmental programs: Assessing tradeoffs and substitution in watershed management plans
Authors: Johnston Robert J. , Swallow Stephen K. , Allen Chad W. , Smith Lynn A.
Published: October 2002

Efforts to value particular dimensions of environmental quality may misrepresent true social values if dimensions of environmental quality interact and researchers nonetheless seek to value single dimensions in isolation. While prior work illustrates that substitution among environmental improvements may occur, questions remain regarding the practical implications of such interactions for microlevel policy evaluation and whether similar effects occur when policies unavoidably include improvements in some resources and degradations in others. This paper identifies potential effects of preference interactions among watershed management components, with an emphasis on whether resource interactions can create substantial shifts in willingness to pay for policy changes.
Multiscaling pulse representation of temporal rainfall
Authors: Veneziano Daniele , Iacobellis Vito
Published: October 2002

We develop a pulse‐based representation of temporal rainfall with multifractal properties in the small‐scale limit. The representation combines a traditional model for the exterior process at the synoptic scale with a novel hierarchical pulse model for the event interiors. For validation we apply the model to a temporal rainfall record from Florence, Italy. Although the model has only six parameters (four for the exterior process and two for the event interiors), it accurately reproduces a wide range of empirical statistics, including the distribution of dry and wet periods, the distribution of rainfall intensity up to extreme fractiles, the spectral density, the moment scaling function K(q), and the distribution of the partition coefficients for rainfall disaggregation. The model also reproduces observed deviations of physical rainfall from perfect scaling/multiscaling behavior.
Measuring nonaqueous phase liquid saturation in soil using time domain reflectometry
Authors: Persson Magnus , Berndtsson Ronny
Published: October 2002

Transport of nonaqueous phase liquids (NAPLs) in soils is becoming an increasingly serious threat to the environment. Detection and observation of these substances are thus an increasingly important issue. During recent years, studies have used the apparent dielectric constant (Ka) measured by time domain reflectometry (TDR) for determining the saturation of NAPLs (θNAPL) in soils (m3 m−3). Ka has been related to θNAPL using dielectric mixing models. In this approach for the unsaturated zone, the water content θw should be known beforehand (by assumptions or measured using other techniques). Here, unlike previous research, detailed laboratory experiments were conducted to investigate the relationship between the TDR measured Ka and bulk electrical conductivity σa and θNAPL. Calibration was made in homogeneous sand using three different NAPLs. It was shown that the mixing model used previously led to errors up to 0.05 m3 m−3 in saturated sand. Moreover, in unsaturated sand, measurements of Ka only cannot be used for estimation of θNAPL even if θw is known. Instead, TDR's capability of determining both Ka and σa was utilized to estimate θw and θNAPL. The approach presented in this study can be used for simultaneous observation of θw and θNAPL during NAPL transport experiments in both unsaturated and saturated sandy soils. Thus it is potentially helpful when developing new NAPL transport models.
Numerical simulations of radar rainfall error propagation
Authors: Sharif Hatim O. , Ogden Fred L. , Krajewski Witold F. , Xue Ming
Published: October 2002

The primary advantage of radar observations of precipitation compared with traditional rain gauge measurements is their high spatial and temporal resolution and large areal coverage. Unfortunately, radar data require vigorous quality control before being converted into precipitation products that can be used as input to hydrologic models. In this study we coupled a physically based atmospheric model of convective rainfall with an active microwave radiative transfer model to simulate radar observation of thunderstorms. We used the atmospheric model to simulate a well‐documented tornadic supercell storm that occurred near Del City, Oklahoma, on 20 May 1977. We then generated radar observations of that storm and used them to evaluate the propagation of radar rainfall errors through distributed hydrologic simulations. This physically based methodology allows us to directly examine the impact of radar rainfall estimation errors on land‐surface hydrologic predictions and to avoid the limitations imposed by the use of rain gauge data. Results indicate that the geometry of the radar beam and coordinate transformations, due to radar‐watershed‐storm orientation, have an effect on radar rainfall estimation and runoff prediction errors. In addition to uncertainty in the radar reflectivity versus rainfall intensity relationship, there are significant range‐dependent and orientation‐related radar rainfall estimation errors that should be quantified in terms of their impact on runoff predictions. Our methodology provides a tool for performing experiments that address some operational issues related to the process of radar rainfall estimation and its uses in hydrologic prediction.
On using the equivalent conductivity to characterize solute spreading in environments with low‐permeability lenses
Authors: Guswa Andrew J. , Freyberg David L.
Published: October 2002

Solute transport through highly heterogeneous geologic environments with connected pathways through high‐conductivity material and lenses of low permeability often is not described well by a macroscopic advection‐dispersion equation. An upscaled advection‐dispersion model with a uniform velocity and dispersion coefficient does not predict the significant plume asymmetry and extended tailing often observed over finite distances in such environments. We investigate the hydrogeologic conditions under which an upscaled model must incorporate another mechanism to describe the extended tailing arising from slow advection through and diffusion into and out of low‐permeability inclusions. We use high‐resolution simulations to determine ground truth transport results for 84 hydrogeologic scenarios comprising distinct low‐permeability lenses set into an otherwise homogeneous background. We compare the ability of two one‐dimensional, fitted, upscaled models to reproduce the arrival time curves from the fully resolved simulations. The first model uses a macrodispersion coefficient to describe the spreading due to the low‐permeability inclusions. The second model accounts for the effect of the geologic heterogeneity with a nonequilibrium mass transfer component. When the equivalent conductivity of a domain is less than or equal to the geometric mean conductivity, a macroscopic advection‐dispersion model matches the results well. When the equivalent conductivity is greater than the geometric mean, however, another model may be needed to describe the solute tailing.
Nonstationary stochastic analysis of flow in a heterogeneous semiconfined aquifer
Authors: Lu Guoping , Zhang Dongxiao
Published: October 2002

In this study, we investigate two‐dimensional flow through a heterogeneous, semiconfined aquifer. In the presence of leakage the mean flow varies in space and the fluctuations of the flow become nonstationary spatially. Such a situation calls for a nonstationary stochastic approach since the classical stationary stochastic approaches are no longer appropriate. We make use of a nonstationary spectral method to account for such nonstationarities in finite semiconfined aquifers. Analytical expressions are obtained for head and specific discharge covariances that account for the spatial variability in the mean flow but neglect the contributions of the finite boundaries. Closed‐form analytical expressions for the variances of hydraulic head and specific discharge are derived. The statistical structures of the head and specific discharge fields are investigated in terms of the leakage factor and the spatial structure of hydraulic conductivity field. Results based on the nonstationary approach show that the stationary assumption is inappropriate even for a small leakage.
Tide‐induced groundwater level fluctuation in coastal aquifers bounded by L‐shaped coastlines
Authors: Li Hailong , Jiao Jiu Jimmy , Luk Mario , Cheung Kayiu
Published: October 2002

This paper presents an analytical solution to describe tidal groundwater level fluctuations in an aquifer bounded by two water‐land boundaries that form a right angle. The aquifer configuration can represent the situation of a right‐angle corner in an island when the tidal boundaries are the same along both boundaries, or the estuary situation if the amplitude damps with distance along one boundary (river) and the amplitude along the other boundary (sea) does not change spatially. The two‐dimensional flow equation subject to periodic boundary conditions is changed into a time‐independent elliptic problem using complex transform. The elliptic problem is then solved using the Green's function method. Li et al. [2000] obtained a nonperiodic solution to the same problem with an additional hypothetical initial condition. Their solution includes a time‐dependent integral. The solution presented here is a significant improvement over theirs in the sense that the solution is periodic and computationally efficient since the integral involved is independent of time. An approximate solution without integral is also derived. An error analysis and a case study in Hong Kong demonstrate that the approximate solution is adequate for most field problems.
Comparison of alternative methods for deriving hydraulic properties and scaling factors from single‐disc tension infiltrometer measurements
Authors: Jacques Diederik , Mohanty Binayak P. , Feyen J.
Published: October 2002

Analysis of single‐disc tension infiltrometer data is commonly based on the interpretation of the steady state infiltration rate derived from large time measurements. In this study, three analytical models describing the cumulative infiltration curve were used to estimate the sorptivity and the steady state infiltration rates. Hydraulic conductivity was estimated from (1) two steady state infiltration rates, (2) a sorptivity value and a steady state infiltration rate, and (3) two sorptivity estimates. In total, seven different methods were used to calculate the hydraulic conductivity. We compared the spatial variability of sorptivity, steady state infiltration rates, unsaturated hydraulic conductivity, and scaling factors of these seven methods. Cumulative infiltration curves were measured at 50 locations and three pressure heads along a 40 m long transect on a silty loam Eutric Regosol. In a first step the amount of water and the time needed to wet the contact sand under the disc was successfully filtered from the raw data using nonlinear regression techniques. The analytical models described the infiltration curves very well but gave different estimates of the sorptivities and steady state infiltration rates. Method 2 using both sorptivity and steady state infiltration rate resulted in the largest estimates of the mean unsaturated hydraulic conductivity, whereas the other two methods (1 and 3) resulted in similar mean values. Linear scaling analysis described well the observed variability in the hydraulic conductivity. Overall, different methods produced quite different unsaturated hydraulic conductivity estimates at specific locations, while their field mean or scaled properties were better comparable. Results of this study may prove to be important for deciding appropriate disc infiltrometer data analysis procedure while addressing water flow and chemical transport behavior at different spatial scales.
Inverse modeling of partitioning interwell tracer tests: A streamline approach
Authors: Datta‐Gupta Akhil , Yoon Seongsik , Vasco D. W. , Pope Gary A.
Published: October 2002

Identifying the location and distribution of nonaqueous phase liquid (NAPL) in the subsurface constitutes a vital step in the design and implementation of aquifer remediation schemes. In recent years, partitioning interwell tracer tests (PITT) have gained increasing popularity as a means to characterize NAPL saturation distribution in situ. In this method a suite of conservative and partitioning tracers are injected into the contaminated site. The chromatographic separation between the conservative and the partitioning tracers can be used to infer NAPL saturation distribution. The conventional approach to the analysis of the tracer response uses a first‐order method of moments to compute average NAPL saturation in the tracer swept regions and cannot provide detailed spatial distribution of the NAPL. We propose a computationally efficient streamline‐based inverse method for analyzing partitioning interwell tracer tests to estimate three dimensional spatial variation of NAPL saturation in the subsurface. Our approach is based on an analogy between streamlines and seismic ray tracing and relies on an analytic sensitivity computation method that yields sensitivities of the partitioning tracer response to subsurface parameters such as porosity, hydraulic conductivity, and NAPL saturation in a single streamline simulation. The inversion of tracer response is carried out in a manner analogous to seismic waveform inversion whereby we first match the “first arrival” followed by matching the “amplitudes” of the tracer response. The power and utility of the method is illustrated using synthetic and field applications. The field example is from the Hill Airforce Base, Utah, where partitioning tracer tests were conducted in an isolated test cell with 4 injection wells, 3 extraction wells, and 12 multilevel samplers. Tracer responses from 51 sampling locations are analyzed to determine hydraulic conductivity variations and NAPL saturation distribution in the test cell. Finally, a performance comparison with simulated annealing shows that our proposed approach is faster by 3 orders of magnitude.
Numerical simulation of bank erosion and channel migration in meandering rivers
Authors: Darby Stephen E. , Alabyan Andrei M. , Van de Wiel Marco J.
Published: October 2002

A numerical model of river morphology for meander bends with erodible cohesive banks is herein developed and tested. The new model has three key features. First, it couples a two‐dimensional depth‐averaged model of flow and bed topography with a mechanistic model of bank erosion. Second, it simulates the deposition of failed bank material debris at and its subsequent removal from the toe of the bank. Finally, the governing conservation equations are implemented in a moving boundary fitted coordinate system that can be both curvilinear and nonorthogonal. This simplifies grid generation in curved channels that experience bank deformation, allowing complex planform shapes associated with irregular natural channels to be simulated. Model performance is assessed using data from two flume experiments and a natural river channel. Results are encouraging, but the model underpredicts the scour depth in pools adjacent to the outer bank and, consequently, underpredicts bank migration rates.
Annual hydroclimatology of the United States
Authors: Sankarasubramanian A. , Vogel Richard M.
Published: October 2002

An overview of the annual hydroclimatology of the United States is provided. Time series of monthly streamflow, temperature, and precipitation are developed for 1337 watersheds in the United States. This unique data set is then used to evaluate several approaches for estimating the long‐term water balance and the interannual variability of streamflow. Traditional relationships which predict either actual evapotranspiration or the interannual variability of streamflow from an aridity index are shown to perform poorly for basins with low soil moisture storage capacity. A water balance model is used to formulate new relationships for predicting actual evapotranspiration and the interannual variability of streamflow. These relationships depend on both the aridity index and a new soil moisture storage index. A physically based approach for estimating the soil moisture storage index is introduced which requires monthly time series of precipitation, potential evapotranspiration, and an estimate of maximum soil moisture holding capacity. The net results are improved expressions for the long‐term water balance and the interannual variability of streamflow which do not require either calibration or streamflow data.
Predicting hydraulic conductivity changes from aggregate mean weight diameter
Authors: Nemati M. R. , Caron J. , Gallichand J.
Published: October 2002

Rapid wetting of structurally unstable soils results in aggregate disintegration, soil densification, reduced porosity, and changes in the pore‐size distribution and intake hydraulic properties. Knowledge of changes in these properties is critical for use in hydrological, solute transport, or erosion models. Multiple determinations of these parameters as they evolve over time are needed for short time step models. Routine measurement of all these properties is time‐consuming, and it would therefore be practical if some of them could be used to predict others. A model is proposed to predict hydraulic conductivity changes based on aggregate size distribution, stability, and wetting rate information in order to determine the stability of the pore network during wetting. Two independent experiments were conducted in the laboratory on two different soil types to evaluate the proposed model. The decrease in mean weight diameter of aggregates exposed to different rates of wetting was used to predict the mean weight diameter after wetting, which was then used in a model to predict changes in hydraulic conductivity. The results show that the changes in predicted and measured hydraulic conductivities between the potentials of −0.5 and −4 kPa were significantly correlated for both soils, despite an overestimation of the measured changes. The close correlation suggests that this bias could be empirically estimated from measurements of aggregate size and that information on aggregate size and stability would be useful for predicting changes in hydraulic conductivity.
Long‐term slowdown of river tracer pebbles: Generic models and implications for interpreting short‐term tracer studies
Authors: Ferguson R. I. , Hoey T. B.
Published: October 2002

We propose generic approaches to modeling the reduction in mobility that occurs as tracer pebbles seeded on the surface are mixed to less active locations (e.g., beneath the bed or high on bars) or advected downstream to reaches with different shear stress or bed grain size. Advection slowdown, which may be negligible, is quantified using a regression equation relating virtual velocity to tracer and reach properties. Mixing slowdown, which is always present, is simulated using a Markov model of exchanges between three stores and assumptions about relative mean velocities of tracers in each store. The model can be calibrated from a full search on one date and at least a surface search on another date. Calibration to six reaches of Allt Dubhaig, Scotland, in which tracers were mapped after 2 and 8 years, suggests mixing took 3–5 years to approach equilibrium in five reaches and was still proceeding in one. Simulated slowdown through mixing is far greater than that through advection and goes much of the way to explaining the 50% observed reduction in virtual velocity over 8 years compared to 2 years. Implications for the design and comparison of tracer‐pebble experiments are considered.
Use of the geotechnical centrifuge as a tool to model dense nonaqueous phase liquid migration in fractures
Authors: Levy Laurent C. , Culligan Patricia J. , Germaine John T.
Published: October 2002

We describe a study undertaken to investigate the potential of the geotechnical centrifuge as an experimental tool to study dense nonaqueous phase liquid (DNAPL) behavior in fracture systems. Scaling laws are developed that link observations made in a centrifuge model of a fracture system to the full‐scale problem (prototype). Centrifuge experiments carried out in synthetic fractures, described by smooth‐walled vertical capillary tubes, demonstrate for the first time that the geotechnical centrifuge can be used to model DNAPL transport in a simplified system provided that inertial forces are negligible in both the model and the prototype. Experiments also confirm, as predicted by Kueper and McWhorter [1991], that DNAPL invades a smooth‐walled, vertical fracture when the DNAPL pool height at its entrance reaches its entry pressure. A simple mathematical model describing the displacement of the DNAPL‐water interface is also presented. The predictions of this model and the experimental data are in good agreement with each other in the lower half of an invaded fracture. However, at the early stages of DNAPL invasion into the fracture, our observed DNAPL‐water interface velocities are slower than predicted by the model. We attribute this to an apparent dependence of the local capillary pressure on the interface displacement velocity.
Bootstrap estimation of the mass transfer coefficient of a dissolving nonaqueous phase liquid pool in porous media
Authors: Chrysikopoulos Constantinos V. , Hsuan Pin‐Yi , Fyrillas Marios M.
Published: October 2002

A new method for confidence interval estimation of mass transfer coefficients suitable for dissolving dense nonaqueous phase liquid pools in homogeneous, water‐saturated porous media is developed. The method is based on the bootstrap resampling technique in conjunction with a least squares regression procedure. The method is successfully applied to experimental data collected from bench scale trichloroethylene pool dissolution experiments.
Denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota
Authors: Böhlke J. K. , Wanty R. , Tuttle M. , Delin G. , Landon M.
Published: October 2002

Recharge rates of nitrate (NO3−) to groundwater beneath agricultural land commonly are greater than discharge rates of NO3− in nearby streams, but local controls of NO3− distribution in the subsurface generally are poorly known. Groundwater dating (CFC, 3H) was combined with chemical (ions and gases) and stable isotope (N, S, and C) analyses to resolve the effects of land use changes, flow patterns, and water‐aquifer reactions on the distributions of O2, NO3−, SO4=, and other constituents in a two‐dimensional vertical section leading from upland cultivated fields to a riparian wetland and stream in a glacial outwash sand aquifer near Princeton, Minnesota. Within this section a “plume” of oxic NO3−‐rich groundwater was present at shallow depths beneath the fields and part of the wetland but terminated before reaching the stream or the wetland surface. Groundwater dating and hydraulic measurements indicate travel times in the local flow system of 0 to >40 years, with stratified recharge beneath the fields, downward diversion of the shallow NO3−‐bearing plume by semiconfining organic‐rich valley‐filling sediments under the wetland and upward discharge across the valley and stream bottom. The concentrations and δ15N values of NO3− and N2 indicate that the NO3− plume section was bounded in three directions by a curvilinear zone of active denitrification that limited its progress; however, when recalculated to remove the effects of denitrification, the data also indicate changes in both the concentrations and δ15N values of NO3− that was recharged in the past. Isotope data and mass balance calculations indicate that FeS2 and other ferrous Fe phases were the major electron donors for denitrification in at least two settings: (1) within the glacial‐fluvial aquifer sediments beneath the recharge and discharge areas and (2) along the bottom of the valley‐filling sediments in the discharge area. Combined results indicate that the shape and progress of the oxic NO3− plume termination were controlled by a combination of (1) historical and spatial variations in land use practices, (2) contrast in groundwater flow patterns between the agricultural recharge area and riparian wetland discharge area, and (3) distribution and abundance of electron donors in both the sand aquifer and valley‐filling sediments. The data are consistent with slow migration of redox zones through the aquifer in response to recharging oxic groundwater during Holocene time, then an order‐of‐magnitude increase in the flux of electron acceptors as a result of agricultural NO3− contamination in the late twentieth century, to which the redox zone configuration still may be adjusting. The importance of denitrification for NO3− movement through formerly glaciated terrains should depend on the source areas and depositional environments of the glacial sediments, as well as geomorphology and recent stream‐valley sediment history.
Determination of anisotropy of spatial correlation structure in a three‐dimensional permeability field accompanied by shallow faults
Authors: Nakaya Shinji , Yohmei Taiyoh , Koike Akihisa , Hirayama Tetsuhiro , Yoden Toshiaki , Nishigaki Makoto
Published: October 2002

Structural planes, such as fractures, bedding planes, schistosity, and so on, induce heterogeneity of permeability field in geologic bodies. One of the ways to demonstrate the heterogeneity is to geostatistically infer the spatial correlation structure. This paper presents a newly developed geostatistical technique for estimating the three‐dimensional heterogeneous permeability field. In the presented method, the plane‐directional anisotropy of spatial correlation structure of the three‐dimensional permeability field is determined by estimating semivariograms in multiplane directions, using limited in situ measurement data of basement rock permeability. A case study of a dam site shows that the spatial correlation of permeability in the direction parallel to the fault planes is greater than that perpendicular to it. Direction of the largest anisotropy of the spatial correlation of the permeability field is concordant with the strike of the major faults.
A first‐order analysis of solute flux statistics in aquifers: The combined effect of pore‐scale dispersion, sampling, and linear sorption kinetics
Authors: Fiori A. , Berglund S. , Cvetkovic V. , Dagan G.
Published: October 2002

We consider steady groundwater flow of uniform mean in aquifers of random, spatially variable, hydraulic conductivity. Analytical expressions for the statistical moments of mass fluxes of sorbing solutes in presence of pore‐scale dispersion are derived, where the reactive solutes undergo first‐order sorption kinetics. The developments which lead to the analytical formulation of the solute flux are rigorous in the first‐order analysis framework, and results obtained are valid for weakly heterogeneous formations. The methodology is exemplified for a two‐dimensional aquifer, assuming that the source is of small transverse extent compared to the heterogeneity length scales. The examples show that pore‐scale dispersion has a relatively small effect on the mean point flux, whereas the point flux variance shows much larger sensitivity to pore‐scale dispersion. The variance first decreases as the reaction rate departs from the nonreactive limit, but for equilibrium reactions it is of the same order as for nonreactive solutes. The effect of averaging the solute flux over a finite sampling area is also investigated. It is found that for the expected area‐averaged flux the mixing effect induced by sampling tends to supersede that caused by pore‐scale dispersion. On the contrary, pore‐scale dispersion may have a strong effect on the flux variance also when sampling effects are taken into account.
Hydrogeochemical modeling of enhanced benzene, toluene, ethylbenzene, xylene (BTEX) remediation with nitrate
Authors: Eckert Paul , Appelo C. A. J.
Published: October 2002

During a 5‐month field test, active remediation of a benzene, toluene, ethylbenzene, xylene (BTEX)‐contaminated aquifer was initiated by injecting water with varying amounts of KNO3. The experiment was performed prior to selecting bioremediation for full‐scale cleanup, particularly to evaluate the competing reaction of nitrate with hydrocarbons and reduced sulfur components. The nitrate oxidized sulfides that had precipitated earlier as a result of the natural degradation of BTEX with SO42− from groundwater. When the sulfides were exhausted, BTEX degradation was enhanced by nitrate. A hydrogeochemical model with kinetic oxidation reactions for Fe(II), FeS and BTEX by nitrate was developed to calculate the observed concentration patterns along a flow line in the aquifer. The rates for the kinetic model were based on published kinetic reaction equations for oxidation with oxygen. Nitrate was introduced in the equations in the same form as oxygen, with a premultiplier added to fit the observed concentration changes in the aquifer. The oxidation of Fe(II) with nitrate in the aquifer was 4 times slower than the abiological oxidation reaction with oxygen in water. Similar rates were found for oxidation of FeS with nitrate as for FeS2 with oxygen, but the specific surface area of FeS in the aquifer was larger. The reaction rate for degradation of BTEX compounds was about 107 times faster than for natural organic matter. BTEX release from pools in the aquifer was modeled with a linear driving force equation in which the pollutant/water interfacial area was linked to the mass of BTEX. The release rate and the denitrification rate were used to calculate the initial amounts of BTEX at the start of the KNO3 injection. This study shows that an assessment of the efficiency of nitrate addition for stimulating bioremediation has to consider possible reactions of nitrate with reduced sulfur components and ferrous iron.
Considering barometric pressure in groundwater flow investigations
Authors: Spane F. A.
Published: October 2002

Water level elevation measurements in wells are commonly used as a basis to delineate groundwater flow patterns (i.e., flow direction and hydraulic gradient). Barometric pressure fluctuations, however, can have a discernible impact on well water levels. These barometric effects may lead to erroneous indications of hydraulic head within the aquifer. Total hydraulic head within the aquifer, not well water level elevation, is the hydrologic parameter for determining groundwater flow direction and hydraulic gradient conditions. For low‐gradient, unconfined aquifer sites exhibiting variable vadose zone characteristics (e.g., thickness, pneumatic diffusivity), barometric pressure fluctuations can cause temporal changes in lateral flow direction and flow velocity. Discrete water level measurements used to determine the average or long‐term groundwater flow conditions, therefore, may provide nonrepresentative results. Calculation of the barometric response characteristics for individual wells provides the basis to account for the temporal effects of barometric pressure fluctuations from monitor well measurements, so that average, long‐term groundwater flow pattern behavior can be determined.
Upscaling of soil hydraulic properties for steady state evaporation and infiltration
Authors: Zhu Jianting , Mohanty Binayak P.
Published: October 2002

Estimation of effective/average soil hydraulic properties for large land areas is an outstanding issue in hydrologic modeling. The goal of this study is to provide flow‐specific rules and guidelines for upscaling soil hydraulic properties in an areally heterogeneous field. In this study, we examined the impact of areal heterogeneity of soil hydraulic parameters on soil ensemble behavior for steady state evaporation and infiltration. The specific objectives of this study are (1) to address the impact of averaging methods of shape parameters and parameter correlation on ensemble behavior of steady state flow in an areally heterogeneous field and (2) to investigate the effectiveness of the “average parameters” in terms of the degree of correlation between hydraulic property parameters for the steady state evaporation and infiltration in unsaturated soil. Using an analytical solution of Richards' equation, the ensemble characteristics and flow dynamics based on average hydraulic property parameters are studied for evaporation and infiltration. Using various flow and average scenarios, we illustrated the resulting differences among the various averaging schemes. For vertical evaporation and infiltration the use of a geometric mean value for the shape parameter α of Gardner‐Russo model and Brooks‐Corey model and arithmetic mean value for the saturated hydraulic conductivity Ks simulates the ensemble flow behavior the best. The efficacy of the “average parameters” depends on the flow condition and the degree of correlation between the hydraulic property parameters. With the α and Ks parameters perfectly correlated, the “average parameters” were found to be generally most effective. The correlation between the hydraulic conductivity Ks and the parameter α results in an ensemble soil behavior more like a sand.
A space‐time Neyman‐Scott model of rainfall: Empirical analysis of extremes
Authors: Cowpertwait P. S. P. , Kilsby C. G. , O'Connell P. E.
Published: October 2002

A spatial‐temporal model of rainfall, based on a Neyman‐Scott stochastic point process, is fitted to hourly data taken from nine sites in the Arno Basin, Italy. The stochastic model is an extension of the temporal Neyman‐Scott rectangular pulses model into two‐dimensional space and introduces a further parameter into the model. In the model, storms arrive in a Poisson process, where each storm consists of discs representing rain cells, with centers distributed over an area according to a spatial Poisson process. The cells have a random radius, lifetime, and intensity, with the intensity remaining constant over the area of the disc and cell lifetime. A fitting procedure is proposed which couples the results obtained in two preceding papers: the second‐order properties of the spatial‐temporal model and the third moment function of the single site model [Cowpertwait, 1995, 1998]. The model is validated by comparing extreme historical hourly data and equivalent data simulated using the fitted spatial‐temporal model. These comparisons are made using a regional frequency analysis, based on L moments, and log‐log plots of the upper distribution tail. The results indicate that the model is able to preserve regional extremes and support the use of the model in hydrological applications.
An implicit finite difference method for drainage basin evolution
Authors: Fagherazzi Sergio , Howard Alan D. , Wiberg Patricia L.
Published: October 2002

In drainage basin evolution models the implementation of sediment transport and sediment balance is constrained by numerical instabilities. In order to overcome these difficulties an implicit finite difference method is proposed. The method simultaneously adjusts the elevation at each node of the numerical grid, is unconditionally stable, and significantly reduces the computational time. The performance of the method is tested and briefly discussed with a numerical example.
Henry Darcy and the making of a law
Authors: Brown G. O.
Published: October 2002

Henry Darcy was a distinguished engineer, scientist, and citizen who is remembered for his many contributions in hydraulics, including Darcy's law for flow in porous media. While he has been given full credit for the finding, little insight has been available on the process of his discovery. It is shown that his discovery was the logical result of a lifetime of education, professional practice, and research. Darcy understood both its significance and its relationship to the broader fields of hydraulics and groundwater hydrology. Besides the discovery of Darcy's law, he was the first to show that significant flow resistance occurs within aquifers, the first to recognize the law's similarity to Poiseuille flow, and the first to combine the law with continuity to obtain a solution for unsteady flow.
Tree‐grass competition in space and time: Insights from a simple cellular automata model based on ecohydrological dynamics
Authors: van Wijk Mark T. , Rodriguez‐Iturbe Ignacio
Published: October 2002

Water is a key resource in determining the composition and structure of savanna ecosystems. In this study we present a simple cellular automata model in which death and reproduction chances of trees and grasses are based on the dynamical description of plant water stress by a probabilistic ecohydrological point model, using the parameterization for a Texas savanna. The results show that the model behavior, despite its simplicity, can be linked to ecological reality: the model yields a dynamic tree‐grass coexistence driven by the annual rainfall, and the space‐time behavior shows that both random and clustered tree distributions for periods up to 100 years can be observed. Both temporal and spatial model output display fractal characteristics suggesting the possibility of a self‐organized critical dynamics. Thus power law behavior is observed in both the spectral density function and the cluster size distribution. The presence of spatial fractal characteristic opens avenues for more thorough model testing.
Effect of flow‐induced exchange in hyporheic zones on longitudinal transport of solutes in streams and rivers
Authors: Wörman Anders , Packman Aaron I. , Johansson Håkan , Jonsson Karin
Published: October 2002

Temporary storage of solutes in streams is often controlled by flow‐induced uptake in hyporheic zones. This phenomenon accounts for the tails that are generally observed following the passage of a solute pulse, and such exchange is particularly important for the transport of reactive substances that can be subject to various biogeochemical processes in the subsurface. Advective pumping, induced by streamflow over an irregular permeable bed, leads to a distribution of pore water flow paths in the streambed and a corresponding distribution of subsurface solute residence times. This paper describes a modeling framework that couples longitudinal solute transport in streams with solute advection along a continuous distribution of hyporheic flow paths. Moment methods are used to calculate the shape of solute breakthrough curves in the stream based on various representations of hyporheic exchange, including both advective pumping and several idealized formulations. Basic hydrodynamic principles are used to derive the distribution of solute residence times due to pumping. The model provides an accurate representation of the breakthrough curves of tritium along a 30 km reach of Säva Brook in Uppland County in Sweden. Both hydrodynamic theory for pumping exchange and pore water samples obtained from the bed during the tracer experiment suggest that the residence time for solutes in the hyporheic zone is characterized by a log normal probability density function. Closed‐form solutions of the central temporal moments of solute breakthrough curves in the stream reveal a significant similarity between this new model and existing models of hyporheic exchange, including the Transient Storage Model. The new model is advantageous because its fundamentally derived exchange parameters can be expressed as functions of basic hydrodynamic quantities, which allows the model results to be generalized to conditions beyond those directly observed during tracer experiments. The utility of this approach is demonstrated by using the pumping theory to relate the spatial variation of hyporheic exchange rate along Säva Brook with the local Froude number, hydraulic conductivity and water depth.
Estimating groundwater recharge in fractured rock from environmental 3H and 36Cl, Clare Valley, South Australia
Authors: Cook P. G. , Robinson N. I.
Published: October 2002

Vertical profiles of 3H and 36Cl concentrations are obtained from piezometer nests installed in fractured metasedimentary aquifers in the Clare Valley, South Australia. Because 3H is lost during evapotranspiration with negligible fractionation, while 36Cl is retained within the soil, comparison of 3H and 36Cl concentrations allows estimation of the aquifer recharge rate. An analytical solution for the transport of 3H and 36Cl through planar, parallel fractures is used to investigate the effect of variations in matrix porosity, tortuosity, fracture aperture, fracture spacing and aquifer recharge rate on tracer profiles and then to reproduce observed profiles within piezometer nests. While the measured distributions of these tracers are not able to constrain most model parameters, they are able to tightly constrain the aquifer recharge rate. The broad nature of the 36Cl and 3H peaks measured at our sites is simulated using a constant fracture spacing, lognormal distributions of fracture apertures, and mean recharge rates of 60–75 mm yr−1.
Coupled vertical and lateral preferential flow on a forested slope
Authors: Buttle J. M. , McDonald D. J.
Published: October 2002

Coupling of vertical and lateral preferential flow paths was examined on a forested slope with thin soil cover during artificial irrigations. Point‐scale infiltration was measured at sites with differing soil macroporosities using vertical profiles of time domain reflectometry probes and suction samplers. Lateral fluxes of water and solutes from the slope were determined at a through flow trench. Sites with greater macroporosities tended to exhibit vertical preferential flow, while infiltration at sites with relatively small macroporosities was largely by vertical propagation of a well defined wetting front through the soil profile. Generation of vertical preferential flow at sites with relatively large macroporosities increased with input intensity. Lateral macropores made a minor contribution to slope runoff. Instead, runoff largely occurred in a thin saturated layer at the soil‐bedrock interface, both in a highly conductive zone at the bedrock surface and in the overlying saturated soil matrix. Some assumptions underlying the use of isotopic and geochemical tracers to study runoff generation are called into question by complex mixing of event and pre‐event water in this saturated layer. Soil depth, bedrock topography, and antecedent soil wetness determine the thickness, connectivity, and upslope extent of the pre‐event saturated layer above the bedrock surface. These, in turn, control whether vertical preferential and matrix flow reaching the bedrock surface participate in slope runoff.