bluuebunny/crossref_metadata_2025 huggingface dataset:
A cross section of the convergent Pacific margin of Nicaragua
Authors: Ranero César R. , von Huene Roland , Flueh Ernst , Duarte Manuel , Baca Dania , McIntosh Kirk
Published: September 2002

Prestack depth migration of multichannel seismic reflection lines across the Pacific margin of Nicaragua has yielded an accurate depth image to about a 9‐km depth from the deep ocean basin to the coast. The margin contains the Sandino forearc basin, probably underlain by oceanic igneous basement and fronted by a small prism accreted at the seaward end of the continental basement. Seismic stratigraphy and drill hole information indicate that sediment has been accumulating since Late Cretaceous. The margin configuration formed between late Cretaceous and Paleocene time and has endured since that time. Uplift of the outer high and slope was probably coeval with subsidence of a deep basin beneath the shelf. From middle‐late Eocene time to Oligocene time, the outer high was a barrier to sediment transport. A similar Late Cretaceous to Oligocene tectonic history has been described for the Guatemalan and Costa Rican segments of the Pacific margin. We speculate that the structure of the Pacific forearc basin formed by subduction initiation at the edge of the Caribbean igneous province. Since late Oligocene time, margin‐wide subsidence occurs in the Nicaraguan margin, perhaps related to subduction erosion of the upper plate. Coeval steep reverse and normal faulting along local structures in the forearc basin might occur by transpression along margin‐parallel strike‐slip faults. These faults have been active since the early development of the basin, but the greatest rate of vertical displacement along them was in early ‐ middle Miocene time, probably related to a plate kinematic reorganization involving the collision of Central and South America.
Formation and evolution of the Solander Basin, southwestern South Island, New Zealand, controlled by a major fault in continental crust and upper mantle
Authors: Sutherland Rupert , Melhuish Anne
Published: September 2002

Seismic reflection and refraction data from the Solander Basin, southern New Zealand, show that its structural evolution has been controlled by a major fault, named here the Tauru Fault, that cuts the entire crust and splays into a diffuse zone in the upper mantle. The tectonic setting of the Solander Basin has evolved from Eocene‐Oligocene extension and transtension to Miocene‐Quaternary transpression and subduction. The Tauru Fault is 100 km east of the active Puysegur subduction zone thrust and is part of the overriding plate. On the basis of lower crustal reflectivity, the base of the crust beneath the adjacent Stewart Island shelf is at ∼30 km depth (∼9 s two‐way time (TWT)), and rises to ∼20 km (∼8 s TWT) beneath the Solander Basin. This is consistent with gravity data. Prominent dipping reflections show that the Tauru Fault can be traced to ∼30 km depth (∼12 s TWT), where it merges with a zone of subhorizontal reflectors in the upper mantle. The Tauru Fault dips ∼30° northeast and appears to offset the Moho in a reverse sense. Stratigraphic relationships show that the Tauru Fault was active as a normal fault during Eocene extension, when Solander Basin crust was thinned and ocean crust was generated farther south in the Solander Trough. It has been reactivated as a reverse fault during at least two phases of Miocene‐Quaternary compression and is still active. The strike of the Tauru Fault, which is parallel to Paleozoic‐Mesozoic structures and was poorly oriented for the known Eocene extension direction, strongly suggests that it formed prior to Eocene time. The Tauru Fault significantly influenced the geometry of Eocene basin formation, producing a strongly asymmetric basin dominated by east dipping normal faults, with a single eastern boundary fault. Our data demonstrate that Miocene‐Quaternary simple shear associated with the Tauru Fault cuts the whole crust and continues into the upper mantle. We conclude that variations in strength of the lithosphere, particularly associated with inherited structures in the crust and upper mantle, may control many aspects of basin development, passive margin formation, and the kinematics of continental deformation zones.
Erosional control on the geometry and kinematics of thrust belt development in the central Andes
Authors: Horton Brian K.
Published: September 2002

Long‐term erosion rates in the central Andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt. A drastic along‐strike erosional gradient exists in the modern central Andes, from a high‐erosion region directly north of the 17.5°S bend in the Andes to a low‐erosion region south of the bend. This gradient has existed since ∼10–15 Ma based on fission track analyses of middle Miocene to Holocene denudation and qualitative evaluations of the preservation potential of middle‐late Tertiary volcanic edifices and synorogenic sediment. Global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17.5°S bend. Whereas present‐day shortening in the thrust belt north of the bend is distributed over much of the width of the belt, south of the bend it is concentrated near the eastern frontal margin. Structural data suggest a similar kinematic situation during late Miocene to Holocene shortening: an out‐of‐sequence chronology of thrusting in the narrow (200 km wide) thrust belt to the north versus a forward‐breaking sequence of thrusting in the relatively wide (350 km wide) thrust belt to the south. The long‐term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust‐wedge conditions, induced by rapid erosion rates since ∼10–15 Ma. Such conditions inhibit thrust‐front advance and favor distributed deformation within the thrust‐belt interior. In the thrust belt south of the bend, a progressive eastward migration of thrusting is interpreted to be the result of long‐term critical thrust‐wedge conditions promoted by extremely low rates of denudation since middle Miocene time.
The late exhumation history of the ultrahigh‐pressure Maksyutov Complex, south Ural Mountains, from new apatite fission track data
Authors: Leech Mary L. , Stockli Daniel F.
Published: September 2002

Apatite fission track samples were collected from the ultrahigh‐pressure (UHP) Maksyutov Complex, south Ural Mountains, in the foot wall of the Main Uralian fault (MUF) to constrain the low‐temperature cooling history and to establish the late stage exhumation rate for the complex. Fission track samples were taken along a 70‐km north‐south transect and a 5‐km east‐west traverse through the Maksyutov Complex, with two samples from the hanging wall of the MUF. Apparent age and track length modeling results indicate that the Maksyutov Complex was exhumed and cooled to 110°C en masse in the Early Permian (300 ± 25 Ma). The east‐west transect shows that no significant interunit movement occurred in the Maksyutov Complex after ∼315 Ma; on the basis of higher‐temperature thermochronometers, the entire Maksyutov Complex must have been assembled between 335 and 315 Ma. Modeling for the north‐south transect indicates that exhumation occurred contemporaneously in the north and south regions of the complex with cooling to 110°C between 375 and 315 Ma, coinciding with the onset of the Uralian orogeny. Comparison of modeling for Maksyutov samples and an Ordovician metasediment from the hanging wall of the MUF indicates that late movement on the MUF was minor and that the footwall and hanging wall had a similar cooling history after the late Carboniferous (∼300 Ma). Exhumation rates range from 0.3 to 1.5 mm yr¹ between a high‐pressure metamorphic event at 375 and 315 Ma using current heat flow data. Our calculated exhumation rate for the Maksyutov Complex is consistent with the complex being a UHP terrane, even though coesite and diamond are not preserved.
The arc of the western Alps in the light of geophysical data on deep crustal structure
Authors: Schmid S. M. , Kissling E.
Published: September 2002

Recently, two international deep seismic campaigns in the western and central Alps (Etude Continentale et Océanique par Reflexion et Refraction Sismique ‐ Progetto Strategico Crosta Profonda, ECORS‐CROP [Roure et al. 1996b]; and National Research Program 20, NRP‐20 [Pfiffner et al. 1997]) have been completed. Here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the Alps collected during the past 40 years, including the two above mentioned geophysical projects. The tectonic reinterpretation of the ECORS‐CROP high‐resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology. Combined with previously published interpretations regarding the central Alps [Schmid et al., 1996], this study reveals substantial differences in geometry and kinematics between transects across the western and central Alps, respectively. At depth the transition between the central and western Alps is marked by the western limits of an Adriatic lower crustal wedge‐shaped structure and the northeastern limits of a similar structure made of European lower crustal material. At the surface it coincides with a corridor of dextral strike‐slip along the Tonale and Simplon lines. In addition, the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the Penninic realm of the western Alps to less than 20 km in the central Alps. The formation of the western Alpine arc was initiated during convergence and collision before 35 Myr ago, when the Adriatic micro‐plate, moving northward with respect to the European foreland, caused sinistral transpression in the western Alps while the central and eastern Alps underwent head‐on convergence and collision. During the post‐collisional stage, i.e., after 35 Myr ago, the arcuate shape of the western Alps was accentuated by WNW‐directed movement and anticlockwise rotation of the Adriatic microplate, decoupled from the central and eastern Alps along the Tonale‐Simplon dextral shear zone. This led to wedging of lower crustal slices both in the western Alps and in the central Alps. The new tectonic interpretation of the ECORS‐CROP transect allows a kinematic model to be established for crustal shortening in the western Alps during the past 35 Myr. The Ivrea mantle plays the role of a backstop in our tectonic model. We distinguish three episodes of post‐collisional crustal shortening along the ECORS‐CROP transect: From 35 to 30 Ma the Brianconnais basement was back‐thrusted over the Gran Paradiso units, leading to 30 km of crustal shortening (first episode). In the early Miocene, movements concentrated mainly along the Penninic frontal thrust and resulted in about 60 km shortening (second episode). Post‐12 Ma shortening within the external massifs is associated with folding in the Jura mountains when the crust was shortened by an additional 30 km (third episode).
An early Pliocene uplift of the central Apenninic foredeep and its geodynamic significance
Authors: van der Meulen M. J. , Buiter S. J. H. , Meulenkamp J. E. , Wortel M. J. R.
Published: September 2002

van der Meulen et al. [1998] identified a pattern of lateral shifts of Apenninic foredeep depocenters, which was interpreted as the regional expression of lateral migration of slab detachment in the sense of Wortel and Spakman [1992]. In addition to being expressed in such a pattern of lateral reorganisations, slab detachment is expected to result in rebound. This study focuses on Plio‐Pleistocene vertical motions of the central Apenninic inner forearc. Here, a major late early Pliocene phase of uplift is identified, by means of (1) micropaleontology‐based paleobathymetry, (2) analysis of the compaction history, and (3) numerical modeling of the flexural response of the underlying lithosphere to sediment loading. The timing of the uplift event is consistent with that of the regional reorganizations. On the basis of this consistency and the fact that it can be demonstrated that the uplift is not related to accretionary processes, we interpret this event as rebound after slab detachment.
Extensional tectonics in the Caledonides: Synorogenic or postorogenic?
Authors: Fossen Haakon
Published: September 2002

Extensional tectonism may form during as well as after the contractional history of a collisional orogeny. Kinematic studies combined with various age constraints in the southern Scandinavian Caledonides have revealed an extensional history which started by hinterland‐directed transport of the orogenic wedge above the basal décollement zone (Mode I extension) and proceeded by the development of hinterland‐dipping shear zones (Mode II) and subsequent brittle faults (Mode III). The top‐to‐hinterland kinematics of the basal décollement zone during Mode I extension indicate that this extensional history was entirely postorogenic already from the start. Any synorogenic extensional deformation must have occurred prior to the onset of this extensional history, which is dated to circa 405 Ma. Although synorogenic extension is likely to have occurred, large‐scale synorogenic extensional collapse models in the Caledonides are at present difficult to prove, whereas impressive postorogenic multi‐stage extension is beautifully portrayed.
A probabilistic view of hershfield's method for estimating probable maximum precipitation
Authors: Koutsoyiannis Demetris
Published: September 2002

A simple alternative formulation of Hershfield's statistical method for estimating probable maximum precipitation (PMP) is proposed. Specifically, it is shown that the published Hershfield data do not support the hypothesis that there exists a PMP as a physical upper limit, and therefore a purely probabilistic treatment of the data is more consistent. In addition, using the same data set, it is shown that Hershfield's estimate of PMP may be obtained using the generalized extreme value (GEV) distribution with shape parameter given as a specified linear function of the average value of annual maximum precipitation series and for return period of about 60,000 years. This formulation substitutes completely the standard empirical nomograph that is used for the application of the method. The application of the method can be improved when long series of local rainfall data are available that support an accurate estimation of the shape parameter of the GEV distribution.
Comprehensive at‐site flood frequency analysis using Monte Carlo Bayesian inference
Authors: Kuczera George
Published: September 2002

In flood frequency applications where the design flood is required to have a specified exceedance probability, expected probability should be used. Its computation, however, presents formidable difficulties. This study presents a Monte Carlo Bayesian method for computing the expected probability distribution as well as quantile confidence limits for any flood frequency distribution using data on gauged flows, possibly corrupted by rating curve error, and on censored flows. This is achieved by a three‐step process: (1) Formulate the likelihood function for the given data; (2) approximate the likelihood function using a multinomial distribution; and (3) integrate the expected probability integral using importance sampling. The FLIKE software for performing this is described, and an example is given.
On the distribution of multicomponent mixtures over generalized exposure time in subsurface flow and reactive transport: Foundations, and formulations for groundwater age, chemical heterogeneity, and biodegradation
Authors: Ginn Timothy R.
Published: September 2002

The fate of materials undergoing transport and reactions in natural porous media sometimes depends on the time of exposure of the conveyed material to other materials present in the system. The distribution of groundwater age, the effects of mineral chemical heterogeneity on reactive solute transport, and the occurrence of lag in reaction systems are some areas of hydrogeology that involve exposure time in an important way. A general balance equation for accounting for such effects is provided through an extended transport operator that incorporates generalized exposure time as an additional independent coordinate. Evolution of material distributions over exposure time appears within this transport operator as a convective process that represents space‐ and time‐dependent generalized exposure of material constituents undergoing physical transport and nonequilibrium chemical and microbiological mass transformations. The general equation is derived from basic mass balance arguments by treating the constituents as a mixture of overlapping continua and developing evolution equations for the mixture material densities in the new dimensions of space, time, and exposure time. Example applications of the approach to each of the three examples above are described.
Multifractal modeling of anomalous scaling laws in rainfall
Authors: Deidda Roberto , Benzi Roberto , Siccardi Franco
Published: September 2002

The coupling of hydrological distributed models to numerical weather prediction outputs is an important issue for hydrological applications such as forecasting of flood events. Downscaling meteorological predictions to the hydrological scales requires the resolution of two fundamental issues regarding precipitation, namely, (1) understanding the statistical properties and scaling laws of rainfall fields and (2) validation of downscaling models that are able to preserve statistical characteristics observed in real precipitation. In this paper we discuss the first issue by introducing a new multifractal model that appears particularly suitable for random generation of synthetic rainfall. We argue that the results presented in this paper may be also useful for the solution of the second question. Statistical behavior of rainfall in time is investigated through a high‐resolution time series recorded in Genova (Italy). The multifractal analysis shows the presence of a temporal threshold, localized around 15–20 hours, which separates two ranges of anomalous scaling laws. Synthetic time series, characterized by very similar scaling laws to the observed one, are generated with the multifractal model. The potential of the model for extreme rainfall event distributions is also discussed. The multifractal analysis of Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) radar fields have shown that statistical properties of rainfall in space depend on time durations over which precipitation is accumulated. Further analysis of some rainfall fields produced with a meteorological limited area model exhibited the same anomalous scaling as the GATE fields.
A noncalibrated rainfall‐runoff model for large, arid catchments
Authors: Lange J. , Leibundgut C. , Greenbaum N. , Schick A. P.
Published: September 2002

A distributed, field‐based rainfall‐runoff model was developed for the 1400‐km2 arid catchment of Nahal Zin, Israel. No calibration with measured flow data was performed. The model used rainfall radar input applied over a catchment that was spatially disaggregated into different terrain types according to hydrologically relevant surface characteristics. Hortonian overland flow generation on each type was parameterized independently using values of initial loss and temporal decay of infiltration determined from existing field experiments. Delimited by topography, this catchment wide pattern of rainfall excess was distributed over 850 tributary catchments (model elements). Runoff delivery from the model elements to the adjoining channel segments was timed by applying a mean response function determined in an environmentally similar experimental catchment. Inside the channel network the MVPMC3 method of the Muskingum‐Cunge technique was used for streamflow routing, accounting for channel dimensions and roughness. For each channel segment a constant infiltration rate was applied to account for transmission losses and discontinued when the wetting front reached the bottom of the available alluvial storage. Within two model tests, one separate for the routing component (October 1979) and one for the complete model (October 1991), observed hydrographs and reconstructed peak discharges were successfully simulated. The spatially distributed model output showed that during the October 1991 test, tributaries produced preceding peaks that wetted the channel alluvium before the main flood had arrived and transmission losses lost their significance downstream. Total maximum model uncertainty was estimated including the uncertainty ranges of each model parameter. In general, this study shows that field‐based data on generation and losses of runoff may be incorporated into a distributed hydrologic model to overcome calibration with the poor data records of arid high‐magnitude events.
Spatial correlation structure estimation using geophysical and hydrogeological data
Authors: Hubbard Susan S. , Rubin Yoram , Majer Ernie
Published: September 2002

The large spatial variability of hydraulic properties in natural geologic systems over a wide range of scales, and the difficulty of collecting representative and sufficient hydraulic property measurements using conventional sampling techniques, render estimation of spatial correlation parameters difficult. Further compounding the estimation problem is the observation that the integral scale estimate is a function of the measurement support scale. To mitigate these problems, we investigate the use of tomographic geophysical data in combination with hydrogeological data in the spatial correlation estimation procedure. Two synthetic case studies were investigated where the scale of the geophysical measurements were varied relative to the scale of the hydrogeological properties. The spatial correlation structure parameter estimation procedure was performed in the spectral domain, where analysis of data having different support scales and spatial sampling windows was facilitated. Comparison of the spatial correlation structure parameters estimated from measured data with those of the synthetic aquifers revealed which type of data (tomographic, hydrogeological, or a combination of both) was most effective for recovering spatial correlation statistics under different sampling/heterogeneity conditions. These synthetic case studies suggest that collection of a few tomographic profiles and interpretation of these profiles together with limited well bore data can yield correlation structure information that is otherwise obtainable only from extensive hydrological sampling.
Implications of the distribution of δD in pore waters for groundwater flow and the timing of geologic events in a thick aquitard system
Authors: Hendry M. J. , Wassenaar L. I.
Published: September 2002

A detailed vertical profile of the stable isotope δD in pore water was measured through a thick aquitard system consisting of surficial Quaternary clay‐rich till (80 m thick) and an underlying Cretaceous marine clay (76 m thick). Numerical modeling was used to simulate one‐dimensional (vertical) groundwater flow and transport of δD. Best fit simulations to the data provided an independent estimate of long‐term groundwater velocity through the aquitard and estimates of the timing of late Pleistocene and Holocene events. Best fit simulations to the measured isotope profile across the till‐clay interface yielded a groundwater velocity of 0.75–1.0 m per 10 ka for a transport time of between 20 ka and 30 ka. The estimate of velocity agreed well with that calculated from hydraulic data and suggested that hydraulic conductivities of these aquitards are independent of volume tested. The 20–30 ka time frame required for the δD profile to develop across the till‐clay interface reflects the timing of till deposition and shows that the till is the Battleford Formation, a younger till than previously believed. Numerical transport modeling of δD in the upper 30 m of the profile yielded a nonunique fit. Assuming a similar groundwater velocity to that determined across the till‐clay interface, a best fit was obtained for a transport times of 7.5–10 ka. This range compared favorably with that reported for the start of the Holocene (about 10 ka B.P.). This study shows that the application of δD, and by analogy δ18O, to the study of thick aquitard systems not only can provide independent, long‐term estimates of very low groundwater velocities but can also provide insight into the timing of major geologic events such as glaciations.
Dual dynamic programing: A note on implementation
Authors: Velásquez Jesús , Restrepo Pedro J. , Campo Rafael
Published: September 2002

This paper presents a correction to the dual dynamic programing algorithm, which was initially published by Pereira and Pinto [1985]. We show that as originally published, the algorithm is correct only for two%stage problems. For problems with three or more stages, there is a term that must be added to the original equations of Pereira and Pinto in order to meet the Benders decomposition requirements. In addition to the mathematical derivation we include an updated description of the algorithm.
The influence of ionic strength on the facilitated transport of cesium by kaolinite colloids
Authors: Saiers James E. , Hornberger George M.
Published: September 2002

We report results of laboratory experiments on the co‐transport of137Cs by inorganic colloids composed of kaolinite. We find that under conditions of low pore water ionic strength, the kaolinite colloids significantly accelerate137Cs transport through columns packed with quartz sand, kaolinite mobility and the affinity of kaolinite for binding137Cs diminish with increasing ionic strength. As a result, kaolinite exerts a progressively smaller influence on137Cs transport as the ionic strength increases from 0.002 to 0.1 m. The137Cs breakthrough data are used to test a model that incorporates advection‐dispersion equations for the movement of kaolinite colloids, dissolved137Cs, and kaolinite‐associated137Cs and mass transfer equations for kaolinite deposition,137Cs adsorption by kaolinite, and137Cs adsorption by quartz sand; The partition coefficient for137Cs retention by kaolinite colloids and the first‐order rate coefficient for kaolinite deposition vary in a discernible fashion with changes in ionic strength. The adsorption rate coefficient and the sorption capacity term of the second‐order rate law taken to describe137Cs adsorption to the quartz sand are independent of ionic strength; however, the magnitude of the desorption coefficient varies logarithmically with ionic strength. This work indicates the need to account for enhanced movement of sorbing solutes by inorganic colloids and provides a basis for quantifying the response of colloid‐associated solute transport to changes in pore Water chemistry.
Application of inverse methods to contaminant source identification from aquitard diffusion profiles at Dover AFB, Delaware
Authors: Liu Chongxuan , Ball William P.
Published: September 2002

This paper presents a refinement and expansion of our previously described efforts to estimate contaminant plume history from observed contaminant concentrations within a low‐permeability aquitard at the site of a field‐scale groundwater remediation experiment at Dover Air Force Base. At this site, a two‐layer aquitard has been contaminated with tetrachloroethene and trichloroethene through diffusive mass transfer from an overlying contaminated aquifer. Measurements of contaminant concentrations in the aquitard are used, together with independently obtained information about the sorption and diffusion properties of the aquitard medium, to estimate the timewise variation of the boundary concentration at the interface between the aquitard and aquifer, thus providing evidence related to the overlying plume history. In our refined analysis, we assume the contaminant source history to be a function of time with unknown form, and we supplement our interpretations with a second coring result at a different location. The results demonstrate how “forensic” interpretations of this kind can provide useful and important information regarding the contaminant release history at sites of groundwater contamination and cleanup; however, the results also show that the forensic problem is a highly nonunique problem associated with potentially large uncertainty. Interpretation of the estimated results therefore requires careful consideration in the context of other available information.
Stochastic solute transport under unsteady flow conditions: Comparison of theory, Monte Carlo Simulations, and field data
Authors: Wood Brian D. , Kavvas M. Levent
Published: September 2002

In this, the second of two papers concerning the stochastic description of solute transport under unsteady flow conditions, we show how the ensemble‐averaged solute transport equation derived in the companion paper [Wood and Kavvas, this issue] can be solved. A two‐dimensional analysis is conducted under conditions that are representative of the Borden aquifer, and a solution to the ensemble‐averaged solute transport equation is found numerically. The analytical model suggested by Dagan et al. [1996] is adopted for the velocity field mean and Lagrangian covariance functions. The numerical solution provides the ensemble‐averaged concentration field under transient flow conditions; from this concentration field the first‐ and second‐order moments of the ensemble‐averaged solute plume are calculated. The ensemble‐averaged plume moments compare favorably with the moments calculated using the approach of Dagan et al. [1996], with the plume moments from a Monte Carlo analysis, and with plume moments measured in the field. In our approach the Darcy‐scale dispersion is not neglected, and it is shown that this dispersion term has a small but significant influence on the resulting solutions.
Natural attenuation of chlorinated volatile organic compounds in a freshwater tidal wetland: Field evidence of anaerobic biodegradation
Authors: Lorah Michelle M. , Olsen Lisa D.
Published: September 2002

Field evidence collected along two groundwater flow paths shows that anaerobic biodegradation naturally attenuates a plume of chlorinated volatile organic compounds as it discharges from an aerobic sand aquifer through wetland sediments. A decrease in concentrations of two parent contaminants, trichloroethylene (TCE) and 1,1,2,2‐tetrachloroethane (PCA), and a concomitant increase in concentrations of anaerobic daughter products occurs along upward flow paths through the wetland sediments. The daughter products 1,2‐dichloroethylene, vinyl chloride, 1,1,2‐trichloroethane, and 1,2‐dichloroethane are produced from hydrogenolysis of TCE and from PCA degradation through hydrogenolysis and dichloroelimination (reductive dechlorination) pathways. Total concentrations of TCE, PCA, and their degradation products, however, decrease to below detection levels within 0.15–0.30 m of land surface. The enhanced reductive dechlorination of TCE and PCA in the wetland sediments is associated with the naturally higher concentrations of dissolved organic carbon and the lower redox state of the groundwater compared to the aquifer. This field study indicates that wetlands and similar organic‐rich environments at groundwater/surface‐water interfaces may be important in intercepting groundwater contaminated with chlorinated organics and in naturally reducing concentrations and toxicity before sensitive surface‐water receptors are reached.
Nonlocal and localized analyses of conditional mean steady state flow in bounded, randomly nonuniform domains: 1. Theory and computational approach
Authors: Guadagnini Alberto , Neuman Shlomo P.
Published: September 2002

We consider the effect of measuring randomly varying hydraulic conductivitiesK(x) on one's ability to predict numerically, without resorting to either Monte Carlo simulation or upscaling, steady state flow in bounded domains driven by random source and boundary terms. Our aim is to allow optimum unbiased prediction of hydraulic heads h(x) and fluxes q(x) by means of their ensemble moments, 〈h(x)〉c and 〈q(x)〉c, respectively, conditioned on measurements of K(x). These predictors have been shown by Neuman and Orr [1993a] to satisfy exactly an integrodifferential conditional mean flow equation in which 〈q(x)〉c is nonlocal and non‐Darcian. Here we develop complementary integrodifferential equations for second conditional moments of head and flux which serve as measures of predictive uncertainty; obtain recursive closure approximations for both the first and second conditional moment equations through expansion in powers of a small parameter σY which represents the standard estimation error of ln K(x); and show how to solve these equations to first order in σY2 by finite elements on a rectangular grid in two dimensions. In the special case where one treats K(x) as if it was locally homogeneous and mean flow as if it was locally uniform, one obtains a localized Darcian approximation 〈q(x)〉c ≈ −Kc(x)∇〈h(x)〉c in which Kc(x) is a space‐dependent conditional hydraulic conductivity tensor. This leads to the traditional deterministic, Darcian steady state flow equation which, however, acquires a nontraditional meaning in that its parameters and state variables are data dependent and therefore inherently nonunique. It further explains why parameter estimates obtained by traditional inverse methods tend to vary as one modifies the database. Localized equations yield no information about predictive uncertainty. Our stochastic derivation of these otherwise standard deterministic flow equations makes clear that uncertainty measures associated with estimates of head and flux, obtained by traditional inverse methods, are generally smaller (often considerably so) than measures of corresponding predictive uncertainty, which can be assessed only by means of stochastic models such as ours. We present a detailed comparison between finite element solutions of nonlocal and localized moment equations and Monte Carlo simulations under superimposed mean‐uniform and convergent flow regimes in two dimensions. Paper 1 presents the theory and computational approach, and paper 2 [Guadagnini and Neuman, this issue] describes unconditional and conditional computational results.
Karst aquifer evolution in fractured rocks
Authors: Kaufmann Georg , Braun Jean
Published: September 2002

We study the large‐scale evolution and flow in a fractured karst aquifer by means of a newly developed numerical method. A karst aquifer is discretized into a set of irregularly spaced nodal points, which are connected to their set of natural neighbors to simulate a network of interconnected conduits in two dimensions. The conduits are allowed to enlarge by solutional widening. The geometric flexibility of this method, along with a simplified model for the dissolution kinetics within the system water‐carbon dioxide‐calcite, enables us to study both laminar and turbulent flow in a karst aquifer during its early phase of evolution. A sensitivity analysis is conducted for parameters such as conduit diameter, hydraulic pressure differences, and recharge conditions along the surface of the aquifer and shows that passage evolution depends strongly on the recharge condition and the amount of water available. Under fixed hydraulic head boundary conditions an early single‐passage system develops under laminar conditions and is transformed into a maze‐like passage system after the onset of turbulence. Fixed recharge boundary conditions are more likely to result in a branchwork‐like passage system, although the addition of distributed recharge may lead to a maze‐like system of secondary passages.
The effect of water content on solute transport in unsaturated porous media
Authors: Padilla Ingrid Y. , Yeh T.‐C. Jim , Conklin Martha H.
Published: September 2002

The effect of water content on NaCl transport in unsaturated porous media was investigated under steady state flow conditions for water contents ranging between full saturation and 15% by volume. The experiments were conducted in a 25 cm column packed with homogeneous sand. Results of the experiments indicate that solute transport in unsaturated porous media is subject to greater velocity variations and slower solute mixing than one in saturated media. As a result, NaCl breakthrough curves (BTCs) show earlier initial arrival and greater tailing and variance as the average water content decreases. These results suggest that transport processes in our experiments have not fully developed to the Fickian regime at lower water contents. Because the classical convection‐dispersion equation does not adequately describe the movement of solutes under the pre‐Fickian regime, a mobile‐immobile model was employed to reproduce the BTCs obtained under unsaturated conditions. In general, the results indicate that at lower water contents the medium has a greater fraction of immobile water, higher dispersion, and slower mass transfer between the mobile and immobile regions. A power law relationship between dispersion and water content‐normalized velocities was found to exist for our experiments and other experiments reported in the literature using different porous media. Thus we suggest dispersivity is not only a function of properties of the media but also of water content.
Estimating unsaturated soil hydraulic properties from laboratory tension disc infiltrometer experiments
Authors: Šimůnek Jiří , Wendroth Ole , van Genuchten Martinus T.
Published: September 2002

Four tension disc infiltration experiments were carried out on a loamy soil in the laboratory for the purpose of estimating the unsaturated soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of −20, −10, −5, and −1 cm, with the experiment lasting for ∼5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parameter estimation method combined a quasi three‐dimensional numerical solution of the Richards equation with the Marquardt‐Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Wind's [1968] method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.
Recreational damages from reservoir storage level changes
Authors: Huszar Eric , Shaw W. Douglass , Englin Jeffrey , Netusil Noelwah
Published: September 2002

Several extreme events affecting recreation have occurred in the Humboldt River Basin of northern Nevada. In 1992, agricultural users completely drained Rye Patch Reservoir killing millions of fish. Additionally, since 1990 gold mines located in the basin have pumped and discharged water into the Humboldt River; in recent years, discharges have equaled approximately 60% of the river's annual flow. In this paper we develop and estimate a joint model of fish catch and recreation demand, both of which depend on water levels, to assess the losses and gains from water level changes tied to events in the basin.
Tidal networks: 1. Automatic network extraction and preliminary scaling features from digital terrain maps
Authors: Fagherazzi Sergio , Bortoluzzi Annalisa , Dietrich William E. , Adami Attilio , Lanzoni Stefano , Marani Marco , Rinaldo Andrea
Published: September 2002

We propose a method of automatic extraction of the tidal channel network from topographic data of marsh and tidal flat lands that uses a combination of a threshold elevation and threshold curvature. Not only the location but also the area of the channel bed is identified. This method differs substantially from that used to identify terrestrial channel networks, and it successfully predicts all of the main channels and nearly all of the smaller tributaries of the channel networks derived from SPOT imagery of the northern Venice Lagoon. Channel network maps of Venice and other sites (Petaluma Marsh in the San Francisco Bay and Barnstable Marsh in Massachusetts) were examined for scaling properties. Because of the large width of the channels relative to a characteristic length of their drainage area, we had to develop procedures for automatically delineating channel width and then for identifying the skeleton of the channel network (the pattern connecting the loci of the channel centerlines) for box‐counting analysis. Box‐counting dimensions of the network skeleton proved site‐dependent and showed finite‐size effects. Because of the large widths we also performed a scaling analysis based on the proportions of the total channel bed area occupied by the tidal networks (i.e., a “fat” fractal analysis). This analysis showed a strong break in scaling between large and small channels. These analyses suggest that tidal channels differ significantly in their scaling relationships from terrestrial systems. In subsequent papers [Rinaldo et al., this issue (a), (b)] we pursue this point much further.
Head variance and macrodispersivity tensor in a semiconfined fractal porous medium
Authors: Zhu Jianting , Sykes Jonathan F.
Published: September 2002

In this paper, the head variance and macroscopic dispersion are developed for one‐ and two‐dimensional flows through a semiconfined aquifer with isotropic and anisotropic fractal hydraulic conductivity distribution. It is assumed that the spatial distribution of log hydraulic conductivity can be described as a fractional Brownian motion. The resulting head variance and macrodispersivity tensor, obtained from stochastic fluctuation equations of the steady flow and solute transport, are studied in terms of the hydraulic conductivity process and the leakage in a semiconfined aquifer bounded by a leaky layer above and an impervious stratum below. The impact of the fractal dimension of this process, the leakage factor, and the characteristic length scale on the macrodispersivity is investigated. The results show that the head variance and the transverse macrodispersivity decrease, while the longitudinal macrodispersivity slightly increases with increasing leakage factor. The head variance and longitudinal macrodispersion increase significantly, and the transverse macrodispersion increases slightly as the maximum length scale increases. The increasing fractal dimension generally reduces the head variance and longitudinal macrodispersion and enhances the transverse macrodispersion. The influence of statistical anisotropy of conductivity field on both the head variance and macrodispersivities is also investigated.
Geophysical (Time Domain Electromagnetic Model) delineation of a shallow brine beneath a freshwater lake, the Sea of Galilee, Israel
Authors: Hurwitz Shaul , Goldman Mark , Ezersky Mikhail , Gvirtzman Haim
Published: September 2002

The Sea of Galilee is a freshwater lake, into which saline water emerges through onshore and offshore springs and through flux from the lake's sediments. The novel surface marine modification of the time domain electromagnetic method was used to map the spatial distribution of brines in the sediments below the lake. Results indicate that electrical resistivities of 1.0 and 0.5 ohm‐m are detected at depths of ∼10 m below the lake bottom in most of the lake area, which are equivalent to ∼11,000 and 22,000 mgCl/L, respectively. Relatively fresh groundwater was detected beneath most of the shoreline. Faulting controls the vertical interfaces between saline and fresh groundwaters. It is hypothesized that salt transport is dominated by molecular diffusion in the central part of the lake and by advection from regional aquifers in the margins.
Bed material transport in the Virgin River, Utah
Authors: Andrews E. D.
Published: September 2002

Detailed information concerning the rate and particle size distribution of bed material transport by streamflows can be very difficult and expensive to obtain, especially where peak streamflows are brief and bed material is poorly sorted, including some very large boulders. Such streams, however, are common in steep, arid watersheds. Any computational approach must consider that (1) only the smaller particle sizes present on the streambed move even during large floods and (2) the largest bed particles exert a significant form drag on the flow. Conventional methods that rely on a single particle size to estimate the skin friction shear stress acting on the mobile fraction of the bed material perform poorly. Instead, for this study, the skin friction shear stress was calculated for the observed range of streamflows by calculating the form drag exerted on the reach‐averaged flow field by all particle sizes. Suspended and bed load transported rates computed from reach‐averaged skin friction shear stress are in excellent agreement with measured transport rates. The computed mean annual bed material load, including both bed load and suspended load, of the East Fork Virgin River for the water years 1992‐1996 was approximately 1.3×10 5 t. A large portion of the bed material load consists of sand‐sized particles, 0.062–1.0 mm in diameter, that are transported in suspension. Such particles, however, constituted only 10% of the surface bed material and less than 25% of the subsurface bed material. The mean annual quantity of bed load transported was 1060 t/yr with a median size of 15 mm.
On the relationship of transient storage and aggregated dead zone models of longitudinal solute transport in streams
Authors: Lees Matthew J. , Camacho Luis A. , Chapra Steven
Published: September 2002

The relationship between the distributed transient storage (TS) and lumped aggregate dead zone (ADZ) models of longitudinal solute transport in rivers and streams is examined by a parallel application to tracer data and through an investigation of parameter relationships. Both models accurately describe observed solute transport in a stream where the effects of storage or dead zones significantly affect longitudinal dispersion. A moment matching technique, based on theoretical temporal moments, is used to develop parameter relationships. Tests using the previously calibrated parameters, in addition to simulation experiments, show that the moment matching procedure allows ADZ model parameters to be reliably derived from TS model parameters and vice versa. An investigation of these parameter relationships reveals an important difference between the effective solute transport velocity and the average reach flow velocity in streams subject to transient storage or dead zone processes. A number of practical uses for the derived relationships are suggested, including the ability to utilize powerful methods of system identification in the estimation of TS model parameters.