bluuebunny/crossref_metadata_2025 huggingface dataset:
Post‐Alleghanian unroofing history of the Appalachian Basin, Pennsylvania, from apatite fission track analysis and thermal models
Authors: Blackmer Gale C. , Omar Gomaa I. , Gold David P.
Published: February 2004

Results of apatite fission track analyses on 29 Ordovician through Permian sandstones from the Appalachian Basin in Pennsylvania are presented. Ages range from 111±17 to 184±10 Ma. Mean track lengths of 10.71±0.29 to 13.10±0.17 μm with unimodal, negatively skewed length distributions are indicative of slow cooling. The data separate into two groups on an age versus mean length plot. The younger group (111–144 Ma) is found in the structural depressions of the Anthracite Basin and the Broad Top Basin and adjacent Appalachian Plateau. The older group (144–184 Ma) is found in the structurally higher southwestern Appalachian Plateau and Juniata Culmination and adjacent central plateau. Fission track data suggest that the basin cooled slowly after the Alleghanian Orogeny, with culminations cooling earlier than depressions. Cooling histories modeled from apatite fission track data, with maximum temperatures constrained by vitrinite reflectance, indicate cooling beginning soon after the Alleghanian Orogeny except in the Juniata Culmination, which apparently experienced synorogenic cooling and unroofing during formation of the underlying duplex. Model cooling histories and available geologic information indicate that the foreland basin did not experience Mesozoic reheating. Unroofing histories were modeled from fission track cooling histories using heat flow estimates and burial depths derived from vitrinite reflectance profiles. The models suggest that the unroofing history of the Appalachian Basin in Pennsylvania can be divided into three episodes. An initial episode of relatively rapid cooling and unroofing (Late Permian‐Early Jurassic) is attributed to flexural rebound of the foreland in response to erosional removal of Alleghanian topographic load. Initial unroofing rates are higher in eastern Pennsylvania than in the west, consistent with a flexural model. An episode of little to no unroofing (Middle Jurassic‐late Oligocene) began contemporaneously with the inception of drift at the Atlantic continental margin. At this time, unloading of the orogen was replaced by subsidence and sedimentation on the new margin. Without flexural rebound the driving force for unroofing of the basin was removed and unroofing slowed greatly. An episode of rapid unroofing over the full width of the basin occurred from the Miocene to the present. Although the driving mechanism for unroofing at this time has not been identified, it is consistent with increased sedimentation rates in the middle Atlantic offshore basins for the same period.
Rapid uplift and rotation of mylonitic rocks from beneath a detachment fault: Insights from potassium feldspar 40Ar/39Ar thermochronology, northern Snake Range, Nevada
Authors: Lee Jeffrey
Published: February 2004

The thermal histories of mylonitic rocks from the footwall of the northern Snake Range decollement (NSRD), Nevada, were calculated using multiple diffusion domain analyses of potassium feldspar Arrhenius data and 40Ar/39Ar age spectra in order to characterize the cooling (exhumation) history of these mylonitic rocks and thereby constrain the origin and movement history of the NSRD. The calculated thermal histories, along with reported apatite fission track ages, indicate the three following episodes of rapid cooling (10–55°C/m.y.) related to extensional denudation: (1) middle Eocene (48–41 Ma), (2) late Oligocene (30–26 Ma), and (3) early Miocene (20–16 Ma). The first episode of rapid cooling may be related to the earliest period of normal faulting in east central Nevada and initiation of slip along the NSRD. The second episode signals a pulse of slip along the NSRD, and the third episode marks the cessation of lower plate mylonitic deformation and the onset of the major pulse of uplift and rotation of lower plate rocks from beneath the NSRD and/or younger east dipping normal faults. The thermal history data provide evidence for a temperature difference across the range within the same structural horizon; the northwestern flank of the range dropped below 300°C by 46 Ma and reached 115°C by 20 Ma, whereas lower plate rocks in the eastern parts of the range were at temperatures above 300°C until 19 Ma. If isotherms were subhorizontal, these relations suggest that either lower plate units had an eastward dip prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward as a result of top‐to‐the‐east shear within an east dipping shear zone. Either case results in differential cooling (uplift) of lower plate rocks from beneath the NSRD. A temperature‐distance‐age graph, constructed based upon the calculated thermal histories, shows differential uplift and rotation of moderately to steeply east dipping lower plate units from beneath the NSRD. Furthermore, this reconstruction shows that the dip of lower plate units increases and then decreases in time and space. This reconstruction implies that the NSRD initiated at moderate to steep angles that shallowed at depth, listric style, into a shallow dipping shear zone. These reconstructions provide a spectacular example of the rolling hinge/isostatic rebound model.
Do microplates in subduction zones leave a geological record?
Authors: Stock Joann M. , Lee Jeffrey
Published: February 2004

Active microplate boundaries in ocean‐continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc. We discuss two modern cases. The first is the South Gorda‐Juan de Fuca plate boundary in the Cascadia subduction zone, where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation. The second case is the Cocos‐Rivera plate boundary in the Middle America trench, where the overlying Colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub‐duction‐related volcanism is spatially and geochemically complex. We apply these observations to boundaries of the Arguello, Monterey, Guadalupe, and Magdalena microplates, which existed in the subduction zone west of Baja California at various times from 20 to 12.5 Ma. The past positions of these boundaries relative to Baja California are constrained by global plate reconstructions, closure of the Gulf of California, and an estimate of extension in the Mexican Basin and Range province. Existing regional mapping and our additional reconnaissance mapping show that Paleocene to Eocene fluvial and marine sedimentary rocks south of Ensenada along the western Baja California peninsula and eastward to the mid‐Miocene volcanic arc are undeformed. Limited available data reveal no major spatial or geochemical variations in the mid‐Miocene volcanic arc that might correlate with the past positions of the microplate boundaries. Thus these microplate boundaries had little to no effect on the overriding continental plate. The nature of Guadalupe and Magdalena interactions with North America may have been closer to the South Gorda‐Juan de Fuca example, with possible internal deformation of the microplates. The Monterey and Arguello microplates may have behaved like the modern Explorer plate, with largely strike‐slip motion relative to North America during their last stages of existence. Tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone (e.g., the Aluk plate in the trench beneath West Antarctica in early Tertiary time). Whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries.
Late syndepositional deformation and detachment tectonics in the Barberton Greenstone Belt, South Africa
Authors: Heubeck Christoph , Lowe Donald R.
Published: February 2004

The thick‐bedded, competent quartzose sandstones, conglomerates, and siltstones of the Moodies Group (<3,225 Ma) influence structural style in the ∼3.1–3.5 Ga Barberton Greenstone Belt. Moodies Group rocks are folded into a train of tight, subparallel, doubly plunging, overturned synclines separated by narrow, strike‐parallel fault zones. Structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic‐sedimentary greenstone sequence was buckle‐folded above a detachment within altered ultramafic rocks located at a depth of 3–6 km. Early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones. Penetrative strain during folding was low. The sense of displacement along faults, the original fold vergence, and the provenance of Moodies Group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins. Orogenic contraction due to folding alone was at least −56% and subsequent ductile shortening strain accounted for an additional contraction of approximately −10%. Buoyant rise of the Kaap Valley Tonalite to the north at approximately 3.22 Ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3.1 and 3.0 Ga, respectively, imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold‐and‐thrust belts. The syn‐and post‐Moodies tectonic history of the central Barberton Greenstone Belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable “protocontinental” crust. The probable extensional origin of at least some of these basins and their subsequent shortening, buckle folding, and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late‐stage deformation of this Archean greenstone belt.
Correlation of the Tonga Formation and the Chiwaukum Schist, North Cascades, Washington: Implications for Late Cretaceous orogenic mechanisms
Authors: Duggan K. M. , Brown E. H.
Published: February 2004

The Tonga Formation of the North Cascades, Washington, consists of weakly deformed, bedded feldspathic graywacke and shale metamorphosed at chlorite to staurolite grade. Analysis of protolith lithology, Rb‐Sr isotopic signature, and plutonic, metamorphic, and deformational evolution in the Tonga Formation suggests that this unit is correlative with the Chiwaukum Schist of the Cascades crystalline core. The Tonga Formation occurs on the flank of the Late Cretaceous orogen, whereas high‐grade Chiwaukum Schist is exposed in the core of the orogen. Metamorphic pressure, as reflected by the distribution of metamorphic minerals and thermobarometry, increases continuously northward in the Tonga Formation from the chlorite zone (<4 kbar) to the staurolite zone (≈ 7 kbar) and then to the kyanite zone (≈ 7 kbar) in the Chiwaukum Schist. The findings reported herein do not support the previously advanced concept that the Cascades crystalline core represents the Easton blueschist terrane that was converted into higher‐temperature metamorphic rock by thermal relaxation after cessation of subduction. The results of this study suggest a relatively simple two‐stage metamorphic history for the Chiwaukum Schist represented by 90–93 Ma shallow contact metamorphism, overprinted by Harrovian metamorphism that increased sharply in pressure from south–west to northeast and reached peak conditions after 90 Ma, all events occurring in a plutonic/magmatic arc setting.
Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps
Authors: von Blanckenburg Friedhelm , Davies J. Huw
Published: February 2004

Slab breakoff is the buoyancy‐driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low‐degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike‐slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high‐pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high‐pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.
Synkinematic(?) intrusion of the “anorogenic” 1425 Ma Beer Bottle Pass pluton, southern Nevada
Authors: Duebendorfer Ernest M. , Christensen Clint
Published: February 2004

The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (e.g., Windley, 1993).
Proterozoic extensional tectonics of the central Fennoscandian Shield: Results from the Baltic and Bothnian Echoes from the Lithosphere experiment
Authors: Korja A. , Heikkinen P. J.
Published: February 2004

A three‐dimensional crustal structure of the Bothnian Sea is deduced from the near‐vertical reflection data of the crosscutting lines 1, 6, 7, and C of the BABEL (BAltic and Bothnian Echoes from the Lithosphere) experiment. For this study the common depth point stacked sections have been migrated with a velocity 6.5 km/s and displayed as stacked envelope sections to show their enhanced large‐scale crustal structures. The seismic sections show listric shear zones which dip toward SE and flatten at major detachment zones 35–40 km and 48 km in depth. The detachment zones are associated with changes in P wave velocity from 6.8 km/s to 7.0 km/s and from 7.4 km/s to 8.2 km/s and coincide with the middle and lower crustal boundary and the Moho boundary, respectively. The geometry and crosscutting relationship of the detachment zone are well developed on the E–W directed line 7, from where the Moho detachment zone can be interpreted as to be younger. Different shear zone systems operated in the parallel N–S directed lines 1 and 6, and therefore it is not easy to correlate crustal structures between lines 1 and 6. In the thinned southern part of study area, the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal Moho. Increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the Subjotnian diabase dykes and rapakivi magmatism. In the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures. The distribution of the bimodal association seems to be controlled by the low‐angle listric shear zones. The spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation. The first stage of deformation, which concentrated in the upper and middle crust, records the collapse of the overthickened Svecofennian crust. The second stage involving the entire crust, may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period. The seismic structures predict major post‐Svecofennian deformation/faulting and tilting in the central Fennoscandian Shield. The extensional tectonics operating in the Fennoscandian Shield in the Proterozoic is comparable to modern extensional tectonics.
Tectonic evolution of the Fundy rift basin, Canada: Evidence of extension and shortening during passive margin development
Authors: Withjack Martha Oliver , Olsen Paul E. , Schlische Roy W.
Published: February 2004

Seismic and field data show that the Fundy rift basin of southeastern Canada experienced two distinct episodes of deformation during Mesozoic time. The first episode, during Middle Triassic to Early Jurassic time, was extensional. Rifting associated with NW–SE extension reactivated NE trending Paleozoic compressional structures as normal faults, forming the northwestern boundary faults of the Fundy basin. Displacements on the low‐angle boundary faults locally exceeded 10 km. Rifting also reactivated east trending Paleozoic compressional structures as oblique‐slip faults with normal and sinistral strike‐slip components, forming the northern boundary faults of the Fundy basin. Several kilometers of sediments and lava flows filled the basin during rifting. The second deformational episode occurred during or after Early Jurassic time and probably before or during Early Cretaceous time. Inversion associated with NW–SE shortening occurred along all faulted margins of the Fundy basin. The northwestern boundary faults experienced several kilometers of reverse displacement, broad anticlines developed within their hanging walls, and the Fundy basin acquired its synclinal form. The northern boundary faults of the Fundy basin became oblique‐slip faults with reverse and dextral strike‐slip components. Gentle synclines, tight anticlines, and faults with reverse separation deformed the synrift strata near the northern margin of the Fundy basin. Neither collision nor subduction zones existed near the Fundy basin during Mesozoic time. Hence we believe that tectonic processes associated with seafloor spreading (e.g., incipient ridge push forces, continental resistance to plate motion) produced the shortening in the Fundy basin. Shortening occurred during the transition from rifting to drifting as North America separated from northern Africa and/or during the early stages of drifting as the seafloor‐spreading centers of the North Atlantic propagated northward.
Transport modeling in heterogeneous aquifers: 2. Three‐dimensional transport model and stochastic numerical tracer experiments
Authors: Jussel Peter , Stauffer Fritz , Dracos Themistocles
Published: February 2004

Statistical information on the coherent sedimentary structures of highly heterogeneous gravel deposits (Jussel et al., this issue) is used to investigate numerically the transport of conservative tracers. The data are the basis for a numerical generation of synthetic aquifer models, whose statistical distributions of the sedimentary structures and their hydraulic conductivity and porosity correspond to the findings in the investigated deposits. A three‐dimensional finite element flow model and a corresponding random‐walk transport model were developed for this purpose. Because of the large number of finite elements needed, the solution algorithm is optimized for applications on vector‐type computers. In order to minimize the discretization errors a special interpolation technique is applied to the determination of the local velocity vector. Ten stochastic, numerical transport experiments over a transport distance of 100 m were carried out with synthetic gravel aquifer models. They allow the mean tracer velocity, the effective hydraulic conductivity, and the dispersion parameters to be estimated. These estimates are compared with estimates of these parameters from current theories for the flow and the tracer transport in random, correlated, anisotropic hydraulic conductivity fields.
A moving window semivariance estimator
Authors: Li Dachang , Lake Larry W.
Published: February 2004

Use of geostatistics in hydrology, reservoir engineering, soil physics, and environmental science has expanded vastly in the last few years. Since the semivariance is the backbone of geostatistics, it is very important to estimate it accurately. Building on Matheron's classical formula, there are several improved semivariance estimators, but they are only reliable at small lag distances. For large lag distances all estimators can produce erratic results that may lead to a misinterpretation of the spatial structure of a given data set. The lack of precision is primarily caused by a shrinking number of observation pairs as lag distance increases. This paper presents a new semivariance estimator that uses all given data at all lag distances. In addition, the new estimator is not restricted to samples taken along lines, and it easily accounts for missing data. We compare the new estimator to existing ones on simulated data sets as well as three outcrop data sets. The results show that the new estimator is unbiased, robust, and resistant to contamination.
Using linear approximations to rank realizations in groundwater modeling: Application to worst case selection
Authors: Gómez‐Hernández J. Jaime , Carrera Jesús
Published: February 2004

Cumulative distribution functions (cdf) of groundwater model responses are generally determined using Monte Carlo analysis. The procedure consists of (1) generating a number of realizations of the parameters controlling groundwater flow, (2) solving the groundwater flow equation in each of the realizations to obtain the model responses, (3) ranking the model responses, and (4) assigning a probability to each model response as a function of its rank and the total number of realizations. When one is only interested in determining one of the tails of the cdf, e.g., to determine model responses with a small probability of being exceeded, it would be more appropriate to try to reverse steps 2 and 3 above, so that the realizations are ranked first and then the groundwater flow equation is solved only for those realizations leading to responses in the tail of the cdf. Because the ranking of the realizations must be done in terms of their model responses, which calls for the solution of the groundwater flow equation, we propose to use a linear approximation of the flow equation to approximate the ranks. The proposal is based on the conjecture that first‐order approximations are more robust for computing the ranks of the piezometric heads than for computing the heads themselves. The conjecture is demonstrated in two two‐dimensional confined flow problems comparing the results of the approximation to the results of full Monte Carlo analyses on several sets of 200 realizations with varying standard deviations of log10 T.
A multicriterion approach to groundwater management
Authors: El Magnouni Samir , Treichel Wiktor
Published: February 2004

A new approach to groundwater quantity (hydraulic) management, based on a multicriterion decision aid methodology, is presented. The method couples a hydrodynamic groundwater flow simulation model with the decision aid one. It takes into account multiple criteria used by the decision maker (DM) in evaluating a management scenario as well as the hydrodynamic behavior of the groundwater system. The finite element and embedding methods are used to integrate the groundwater flow model into a multiobjective linear programming (LP) problem. Constraints on head, pumping rates, hydraulic gradient and velocity vector may be included in the management model. The piecewise linear utility function is assessed for modeling the DM's preferences. The best compromise solution is determined from the continuous Pareto set by solving a piecewise LP problem. User friendly software was developed to realize this methodology which is able to treat real scale problems. An illustrative example of an unconfined aquifer management is presented. Nonlinearities resulting from functional dependence of aquifer parameters on hydraulic head are handled iteratively.
Climate, soil water storage, and the average annual water balance
Authors: Milly P. C. D.
Published: February 2004

This paper describes the development and testing of the hypothesis that the long‐term water balance is determined only by the local interaction of fluctuating water supply (precipitation) and demand (potential evapotranspiration), mediated by water storage in the soil. Adoption of this hypothesis, together with idealized representations of relevant input variabilities in time and space, yields a simple model of the water balance of a finite area having a uniform climate. The partitioning of average annual precipitation into evapotranspiration and runoff depends on seven dimensionless numbers: the ratio of average annual potential evapotranspiration to average annual precipitation (index of dryness); the ratio of the spatial average plant‐available water‐holding capacity of the soil to the annual average precipitation amount; the mean number of precipitation events per year; the shape parameter of the gamma distribution describing spatial variability of storage capacity; and simple measures of the seasonality of mean precipitation intensity, storm arrival rate, and potential evapotranspiration. The hypothesis is tested in an application of the model to the United States east of the Rocky Mountains, with no calibration. Study area averages of runoff and evapotranspiration, based on observations, are 263 mm and 728 mm, respectively; the model yields corresponding estimates of 250 mm and 741 mm, respectively, and explains 88% of the geographical variance of observed runoff within the study region. The differences between modeled and observed runoff can be explained by uncertainties in the model inputs and in the observed runoff. In the humid (index of dryness <1) parts of the study area, the dominant factor producing runoff is the excess of annual precipitation over annual potential evapotranspiration, but runoff caused by variability of supply and demand over time is also significant; in the arid (index of dryness >1) parts, all of the runoff is caused by variability of forcing over time. Contributions to model runoff attributable to small‐scale spatial variability of storage capacity are insignificant throughout the study area. The consistency of the model with observational data is supportive of the supply‐demand‐storage hypothesis, which neglects infiltration excess runoff and other finite‐permeability effects on the soil water balance.
Optimal design of water distribution networks
Authors: Eiger Gideon , Shamir Uri , Ben‐Tal Aharon
Published: February 2004

Optimal design of a water distribution network is formulated as a two‐stage decomposition model. The master (outer) problem is nonsmooth and nonconvex, while the inner problem is linear. A semi‐infinite linear dual problem is presented, and an equivalent finite linear problem is developed. The overall design problem is solved globally by a branch and bound algorithm, using nonsmooth optimization and duality theory. The algorithm stops with a solution and a global bound, such that the difference between this bound and the true global optimum is within a prescribed tolerance. The algorithm has been programmed and applied to a number of examples from the literature. The results demonstrate its superiority over previous methods.
Causes of episodic acidification in five Pennsylvania streams on the Northern Appalachian Plateau
Authors: DeWalle David R. , Swistock Bryan R.
Published: February 2004

Causes of episodic stream acidification and acid‐neutralizing capacity (ANC) declines were investigated in five streams on forested, unglaciated catchments on the northern Appalachian Plateau of Pennsylvania, during October 1988 to April 1990. Increased SO4 concentrations during episodes were the primary cause of ANC declines on three of the streams. Episodes on the remaining two streams were dominated by base cation dilution and/or SO4 increases. Base cation dilution was most important in a stream with initially high preepisode ANC caused by seeps from limestone outcrops. Bedrock layers on this basin were dipping toward the channel, which promoted more rapid movement of low‐ANC subsurface storm flow water to the channel. A stream with even higher preepisode ANC, but on a basin with geologic structure that shunted low‐ANC subsurface storm flow away from the channel, exhibited only minor base cation dilution. Organic acidity contributed to ANC decreases on all five streams, although it was a minor source compared with SO4 and dilution. NO3 and Cl did not contribute to ANC decreases on any of the study streams. Aluminum increases acted to reduce episodic ANC declines especially in the more acidic streams. Episodic acidification on the northern Appalachian Plateau in Pennsylvania was similar to that found in mid‐Atlantic streams where SO4 increases play a dominant role, but unlike glaciated regions in the Northeast where NO3 can be important.
Modeling of snow melting and uniform wetting front migration in a layered subfreezing snowpack
Authors: Tseng Peng‐Hsiang , Illangasekare Tissa H. , Meier Mark F.
Published: February 2004

A method of modeling freeze‐thaw cycles of naturally deposited snowpacks is presented. The model involves the Stefan condition as an independent governing equation on the exterior moving boundary to calculate snowpack thinning, flow of water through a variably saturated layered porous medium as described by the Richards equation, and heat conduction with a phase change. The heat conduction problem was treated in two ways. Local heat conduction between a snow grain and its surrounding water film was treated by using a simple energy balance. Global heat conduction with a phase change (the Stefan problem) was introduced to calculate the space‐time temperature distribution. In order to handle multiple interior moving boundaries, a specific form of the enthalpy formulation was used for heat conduction with a phase change. Changing material properties were considered according to the calculated meltwater refreezing.
Inference for a generaliaed gibbsian distribution channel networks
Authors: Troutman Brent M. , Karlinger Michael R.
Published: February 2004

We introduce a two‐parameter (β0and β1) Gibbsian probability model to characterize the spatial behavior of channel networks. This model is defined for trees draining basins on a square lattice. The probability of a treesis proportional to exp [−β0H(s, β1)[, whereH(s,β1) is taken to be a summation over lattice points ν in the basin ofA(ν,s)β1, lettingA(ν,s) be the area upstream from (but not including) point ν. Procedures are developed for estimating the parameters given an actual network obtained, for example, from digital elevation data and for using bootstrapping to obtain confidence intervals for these estimates. We also develop several goodness of fit tests for the model. These procedures are applied to a set of data for 50 subnetworks from Willow Creek in Montana. The estimation algorithm converged successfully for 34 of these subnetworks. For these 34 subnetworks the two‐parameter model gives in most cases a fit significantly better than a one‐parameter model (with β1constrained to be 1) studied in a previous paper. There is still, however, statistically significant lack of fit for as many as 13 subnetworks (depending on which test is applied). The parameter β1averages 0.75 for the 34 subnetworks. The functionH(s, β1) in the Gibbsian model may be given a statistical mechanical interpretation of “energy” associated with a configurations. Other researchers have employed a similar definition of energy expenditure in drainage systems but have given physical arguments that the exponent β1should be 0.50. We finally discuss the idea of defining “optimal channel networks” as those which minimize free energy in the Gibbsian model; it is shown that minimization of energy (rather than free energy) as other researchers have proposed is valid only under the assumption of a “low temperature” (i.e., large β0) system. Our results indicate that this assumption would not be justified for Willow Creek. We devote some discussion to the discrepancies between our approach and results and those of other researchers.
Susceptibility of soils to preferential flow of water: A field study
Authors: Flury Markus , Flühler Hannes , Jury William A. , Leuenberger Jörg
Published: February 2004

Flow pathways of water and solutes in soils form distinct patterns, which are not a priori predictable. Macropore structure is a prime cause, but other factors, such as differing initial or boundary conditions, may also predispose a soil to produce bypassing of infiltrating water. This study was conducted to assess the flow pathways of water in different soils and to investigate the effect of initial water content on the flow pattern. Dye‐tracing experiments were carried out at 14 different field sites. The sites represent a good portion of soils used for agricultural crop production in Switzerland. Each site consisted of two 1.4 by 1.4 m plots, one of which had been covered with a plastic roof for two months before the experiment to achieve different initial water contents. Forty millimeters of water containing the dye Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto the plots with a sprinkling apparatus. One day after irrigation the plots were excavated, and the stained pattern was examined on a vertical 1 by l m soil profile. The spatial structure of flow patterns showed remarkable differences. In most soils, water bypassed the soil matrix. In some soils, dye penetrated beyond l m depth, whereas in others it remained in the top 50 cm. Structured soils were more prone to produce bypass flow, deep dye penetration, and pulse splitting than nonstructured soils. The initial water content had a less pronounced effect in some soils and no effect in others.
First‐ and second‐order kinetics approaches for modeling the transport of colloidal particles in porous media
Authors: Saiers James E. , Hornberger George M. , Liang Liyuan
Published: February 2004

We present results from experiments on the migration of inorganic colloids through laboratory columns containing clean quartz sand. Particle retention on the quartz collectors was found to be substantially less in experiments using negatively charged silica (SiO2) colloids than in experiments using positively charged anatase (TiO2) or boehmite (AlOOH) colloids. Analysis of these data with respect to two different advection‐dispersion models indicates that deposition of colloidal silica follows a first‐order, reversible kinetics process, while deposition of both anatase and boehmite is more closely depicted by second‐order kinetics. Fitted values of the rate constant used to describe particle attachment vary consistently with the mean grain size of the sand and, for anatase and boehmite, are within a factor of 2 of the values predicted on the basis of colloid filtration theory.
Mass transfer in soils with local stratification of hydraulic conductivity
Authors: Li L. , Barry D. A. , Cuiligan‐Hensley P. J. , Bajracharya K.
Published: February 2004

The two‐region model was developed originally to describe nonsorbing chemical transport in soils with dead‐end pores based on the concept of mobile and immobile regions in the soil. It has been shown that the model can simulate solute transport in soils with local stratification, or inhomogeneity, of hydraulic conductivity. However, the physical basis of the model becomes questionable, since the mobile‐immobile region concept does not apply in stratified soils. In both soil types the nonequilibrium effect is caused by an apparent mass transfer process within the soil, as distinct from advection and diffusion. Where there are immobile regions, the mass transfer is due to solute interregion diffusion alone. In stratified soils the nonequilibrium mass transfer process is affected also by local flow variations. A conceptual model, numerical simulations, and laboratory experiments are presented to analyze these effects. For a given soil with fixed local stratification of hydraulic conductivity, it is shown that in the low‐velocity range, the apparent mass transfer rate parameter, α, scales as V2/D (V is pore water velocity in the two‐region model and D is the longitudinal dispersion coefficient), which implies that the mass transfer process is predominantly affected by local flow variations. When the velocity is relatively high, α ∝ DT/h2 (DT is the interregion diffusion coefficient and h is the characteristic thickness of the stratified layers) and the mass transfer process is dominated by interregion diffusion. These scaling relations for α reflect the two mechanisms controlling the mass transfer process in locally stratified soils. They have implications for scaling of time‐dependent mass transfer from laboratory models to prototype soils. In particular, the relationship α ∝ V2/D leads to the conclusion that exact physical modeling of nonsorbing chemical transport coupled with apparent mass transfer in locally stratified soils may be viable.
Mass conservative numerical solutions of the head‐based Richards equation
Authors: Rathfelder Klaus , Abriola Linda M.
Published: February 2004

Numerical procedures for efficient mass conservative solutions of the head‐based form of the Richards equation are presented. Mass conservative solutions are shown to result when the capacity coefficient, C, is formulated by equating the storage term and its chain rule expansion in their discretized forms. Equivalence in the storage term expansion is maintained in finite difference models when C is evaluated with a standard chord slope approximation. This scheme is shown to produce excellent global mass balance accuracy in simulations of vertical moisture infiltration. An analogous approach to the expansion of the storage term using finite elements results in element dependent expressions of C. Application of this approach produces mass balance accuracy with errors less than 1%, but also exhibits slow convergence in the consistent form. A nontraditional finite element procedure is presented which maintains equivalence in the storage term expansion when C is evaluated with the standard chord slope approximation. This scheme exhibits excellent mass balance accuracy, in either the consistent or lumped forms, without significant loss in computational efficiency.
Some exact solutions of convection‐diffusion and diffusion equations
Authors: Philip J. R.
Published: February 2004

We examine solutions of the convection‐diffusion equation during steady radial flow in two and three dimensions with diffusivity proportional to Pn, with P the Péclet number. In two dimensions, closed‐form solutions of the Laplace‐transformed equation occur only for n = 1 and n = 2; and in three dimensions only for n = 5/4 and n = 2. The inversions for n = 1 (two dimensions) and 5/4 (three dimensions) cannot be expressed in closed form. On the other hand, the value n = 2, proposed by de Gennes (1983), leads to simple exact solutions, both in two dimensions and in three. In both cases a change of variable gives a one‐dimensional equation with constant coefficients. Exact instantaneous and continuous point source solutions for radial flow in two and three dimensions are thus established. In general, finite difference methods are needed for arbitrary n. Relevant instantaneous and continuous point source diffusion solutions are developed for radial two‐ and three‐dimensional systems. These represent small‐time solutions useful for initiating the finite difference solutions for convection‐diffusion with arbitrary n.
A kinetic model for cell density dependent bacterial transport in porous media
Authors: Lindqvist Roland , Cho Jong Soo , Enfield Carl G.
Published: February 2004

A kinetic transport model with the ability to account for variations in cell density of the aqueous and solid phases was developed for bacteria in porous media. Sorption kinetics in the advective‐dispersive‐sorptive equation was described by assuming that adsorption was proportional to the aqueous cell density and the number of available sites on the solid phase, whereas desorption was proportional to the density of sorbed cells. A numerical solution to the model was tested against laboratory column data, and the performance was compared with that of a two‐site model. The kinetic model described the column data as well as the two‐site model did, but the highest efficiencies of both models were associated with experiments with the smallest sorption. Furthermore, the kinetic model accounted for cell density dependent sorption, as demonstrated by fair predictions of bacterial transport at one cell density when using parameters obtained at another cell density.
Parameter uncertainty and interaction in complex environmental models
Authors: Spear Robert C. , Grieb Thomas M. , Shang Nong
Published: February 2004

Recently developed models for the estimation of risks arising from the release of toxic chemicals from hazardous waste sites are inherently complex both structurally and parametrically. To better understand the impact of uncertainty and interaction in the high‐dimensional parameter spaces of these models, the set of procedures termed regional sensitivity analysis has been extended and applied to the groundwater pathway of the MMSOILS model. The extension consists of a tree‐structured density estimation technique which allows the characterization of complex interaction in that portion of the parameter space which gives rise to successful simulation. Results show that the parameter space can be partitioned into small, densely populated regions and relatively large, sparsely populated regions. From the high‐density regions one can identify the important or controlling parameters as well as the interaction between parameters in different local areas of the space. This new tool can provide guidance in the analysis and interpretation of site‐specific application of these complex models.
Transient flow to open drains: Comparison of linearized solutions with and without the Dupuit assumption
Authors: van de Giesen N. C. , Parlange J.‐Y. , Steenhuis T. S.
Published: February 2004

A solution of the Laplace equation with linearized boundary conditions is presented for unsteady flow through a rectangular aquifer. The results are compared with those of the linearized Boussinesq equation. The linearized solution based on the Laplace equation is as, easy to use as Boussinesq's solution, has a wider range of application, and allows accurate computation of streamlines.
Conductive heat flow and thermally induced fluid flow around a well bore in a poroelastic medium
Authors: Wang Yarlong , Papamichos Euripides
Published: February 2004

Transient analytical solutions for temperature and pore pressure changes near a circular borehole under instantaneous temperature and fluid pressure changes inside the borehole are presented. The solutions couple conductive heat transfer with Darcy fluid flow, and a borehole under a nonhydrostatic far‐field stress state is simulated. The heat conduction equation is decoupled from the coupled system of isothermal governing equations, and the complete solution is obtained by superimposing this decoupled solution on the isothermal one. The solution is therefore applicable to low‐permeability media, where heat transfer is dominated by conduction only. Both cold and warm injection processes are studied, and the applications to hydraulic fracture initiation and thermally induced fluid flow are discussed. Taking Westerly granite as an example, it is concluded that the maximum thermally induced pore pressure inside the rock formation can be 30% higher than the isothermal pore pressure, with a borehole temperature and fluid pressure change ratio (ΔT/Δp) = 1°C/MPa. It is emphasized that the thermally induced pore pressure change can be significant inside a low‐permeability porous medium, and a coupled solution must be obtained to address the mechanical, hydraulic, and thermal responses appropriately.