bluuebunny/crossref_metadata_2025 huggingface dataset:
Activation of the Nyainqentanghla Shear Zone: Implications for uplift of the southern Tibetan Plateau
Authors: Harrison T. Mark , Copeland Peter , Kidd W. S. F. , Lovera Oscar M.
Published: February 2004

Neogene extension of the Tibetan plateau is manifested as a series of north–south trending graben, the most prominent of which is the Yadong‐Gulu rift. The Nyainqentanghla Shan, a NE–SW trending mountain range ∼100 km NW of Lhasa, bounds the western margin of the Yangbajian graben, the central segment of the Yadong‐Gulu rift. The eastern edge of the Nyainqentanghla massif is marked by a low angle (∼25°) detachment fault shear zone of amphibolite grade mylonites. The 40Ar/39Ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from ∼8 Ma in the core of the range to ∼4 Ma within the high strain zone at the eastern boundary. Assuming that faulting initiated at high angle (∼60°), thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history. The form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8±1 Ma in the southern valley (Goring‐la) and proceeded at an average slip rate of ∼3 mm/yr between ∼8 and 3 Ma. A more complex history is required to fit the data from the northern valley (Balum Chun), but the timing of initiation and average slip rate are similar to the Goring‐la result. Numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle (<40°). Because the extension direction of the Yangbajian graben is representative of most rifts on the southern Tibetan plateau, our data suggest that crustal thickness and elevation reached close to their present values by 8±1 Ma. A carbon isotopic shift in pedogenic carbonates from the Siwalik Formation at about 7.5 Ma appears to reflect intensification of the Asian monsoon and, by inference, that the plateau had attained an important threshold elevation by that time. Formation of a diffuse plate boundary in the Indian oceanic lithosphere beginning at 7.5–8.0 Ma is also consistent with this history. We suggest that the plateau had attained a threshold area and elevation by 8±1 Ma sufficient to trigger these three independent manifestations.
Correlation between seismic anisotropy and major geological structures in SW Iberia: A case study on continental lithosphere deformation
Authors: Abalos Benito , Cusí Jordi Díaz
Published: February 2004

In this paper we postulate a correlation between the lithospheric structure and seismic anisotropy inferred from geophysical data and the major structures and tectonic history (past tectonothermal features) of the outcropping basement rocks in SW Iberia. Oblique plate convergence, orogenesis, and terrane accretion have been documented for pre‐Mesozoic times in this area. However, the deviations and areal variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust indicate that the tectonic model of orogen‐parallel lithospheric deformation does not appear to succeed in relating Hercynian surface geology and the structural geology of the subcontinental lithospheric mantle. The model of polyorogenic lithosphere deformation proposed accounts for this divergence and is supported from additional perspectives including the manifestation of thermal effects induced in the crust by lithospheric mantle shear zones, the incidence of crust‐mantle coupling and/or decoupling, and the scales of subcrustal heterogeneity.
The Gem Lake shear zone: Cretaceous dextral transpression in the Northern Ritter Range pendant, eastern Sierra Nevada, California
Authors: Greene David C. , Schweickert Richard A.
Published: February 2004

The Gem Lake shear zone is a northwest striking, steeply dipping, dextral transpressional shear zone that provides the first direct evidence for dextral deformation in wall‐rock pendants in the central part of the Sierra Nevada batholith. The Gem Lake shear zone is a minimum of 30 km in length and extends at least from the north end of the Northern Ritter Range pendant to the southeast edge of the Ritter Range pendant. The amount of displacement on the zone is uncertain, but matching fault slivers of Pennsylvanian(?) marble in the Northern Ritter Range pendant to similar exposures north of the Mount Morrison pendant suggests a minimum dextral offset of 20 km. The Gem Lake shear zone was active in early Late Cretaceous time, from before 91 Ma (the age of the syntectonic granodiorite of Kuna Crest) to at least 80 Ma (40Ar/39Ar age of syndeformational mica from the shear zone in the Ritter Range pendant). Deformation in the Gem Lake shear zone is characterized by combined dextral simple shear and subvertical stretching, which are variably partitioned in anastomosing high‐strain zones. In the shear zone at Gem Lake, predominantly dextral deformation is indicated by porphyroclast asymmetries, S‐C fabric, and asymmetric crenulations; a component of subvertical stretching is indicated by a moderately to steeply plunging stretching lineation. In a segment of the shear zone at Kuna Crest, a strongly developed stretching lineation indicates predominantly subvertical stretching, with a lesser component of dextral strike slip. The Gem Lake shear zone is considered to be part of a proposed regional system of shear zones in the eastern Sierra Nevada, the Sierra Crest shear zone system. This dextral transpressional system was active prior to, and synchronous with, intrusion of the Late Cretaceous Sierra Nevada batholiths; deformation ceased shortly after pluton emplacement. The Sierra Crest shear zone system includes possibly related shear zones as far north as Saddlebag Lake pendant and at least as far south as Oak Creek pendant and the Owens Valley, indicating a possible strike length greater than 150 km.
Jurassic‐Cretaceous paleomagnetism and paleogeography of the Pontides (Turkey)
Authors: Channell J. E. T. , Tüysüz O. , Bektas O. , Sengör A. M. C.
Published: February 2004

Published paleomagnetic data from the Pontides indicate anomalously low Jurassic and Early Cretaceous paleolatitudes compatible with the southern Neo‐Tethyan (African) continental margin and significantly different from paleolatitudes predicted for the northern (Eurasian) Neo‐Tethyan margin. We present a new set of paleomagnetic data from 50 Late Cretaceous sites mainly from the Western Pontides and from the Eastern Pontides, and 11 Early Jurassic sites mainly from the Eastern Pontides. Fold tests indicate that the characteristic magnetization components predate Eocene folding. Late Cretaceous site mean declinations are northerly in the Eastern Pontides and rotated to the west by a few tens of degrees in the Western Pontides. Late Cretaceous site mean declinations are affected by local clockwise rotation in one sampling region close to the North Anatolian Fault. The mean Late Cretaceous inclinations and resulting paleolatitudes are 41.1° (23.5°N) and 43.7° (25.5°N) for the Western and Eastern Pontides, respectively. For the Eastern Pontides, the Early Jurassic (Liassic) mean inclination is 60.5°, yielding a paleolatitude of 41.4°N, considerably further north than previous paleolatitude estimates for the Eastern Pontides at this time. The paleolatitude estimates for the Western and Eastern Pontides are consistent with these units being close to the Eurasian continental margin during Liassic and Late Cretaceous time.
Large‐scale right‐slip displacement on the East San Francisco Bay Region fault system, California: Implications for location of late Miocene to Pliocene Pacific plate boundary
Authors: McLaughlin R. J. , Sliter W. V. , Sorg D. H. , Russell P. C. , Sarna‐Wojcicki A. M.
Published: February 2004

A belt of northwardly younging Neogene and Quaternary volcanic rocks and hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits about 160 to 170 km of cumulative dextral offset across faults of the East San Francisco Bay Region (ESFBR) fault system. The offset hydrothermal veins and volcanic rocks range in age from .01 Ma at the northwest end to about 17.6 Ma at the southeast end. In the fault block between the San Andreas and ESFBR fault systems, where volcanic rocks are scarce, hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 Ma. The age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the San Andreas transform to the opening of an asthenospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80–110 km of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfield‐Tolay‐Franklin‐Concord‐Sunol‐Calaveras faults. An additional 50–70 km of cumulative slip are taken up ≤ 6 Ma by the Rogers Creek‐Hayward and Concord‐Franklin‐Sunol‐Calaveras faults. An alternative model in which the Permanente terrane is offset about 80 km by pre‐Miocene faults does not adequately restore the distribution of 8–12 Ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend. If 80–110 km of slip was taken up by the ESFBR fault system between 8 and 6 Ma, dextral slip rates were 40–55 mm/yr. Such high rates might occur if the ESFBR fault system rather than the San Andreas fault acted as the transform margin at this time. Major transpression across the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of significant slip back to the San Francisco Peninsula segment of the San Andreas fault. Since that time, the ESFBR fault system has continued to slip at rates of 11–14 mm/yr. If this interpretation is valid, the ESFBR fault system was the Pacific‐North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward with time, in response to changing plate margin geometry and plate motions.
The geometry of a deformed carbonate slope‐basin transition: The Ventoux‐Lure fault zone, SE France
Authors: Ford Mary , Stahel U.
Published: February 2004

The Ventoux‐Lure fault zone (VLFZ) is a 70 km‐long, E–W trending triangle zone of folds and thrusts in the Alpine foreland of SE France. The VLFZ corresponds to the site of a Lower Cretaceous carbonate slope‐basin transition and it provides a good example of a deformed basin margin where, (1) compression was at a high angle to the basin margin; (2) deformation was mainly controlled by the mechanical stratigraphy and not by fault reactivation; and (3) inversion was a gradual process (from Middle Cretaceous) with deformation concentrated mainly in the basin to the north (as evidenced by growth strata) until the last (post‐Burdigalian) stages when the slope carbonates to the south were thrust northward on the Ventoux‐Lure Thrust (VLT). Within the eastern half of this zone structural geometries become increasingly complex from east to west, showing a progression from triangle zone to tectonic wedging geometries in which erosion of the emergent thrust sheets played an important role. This lateral variation was due to the obliquity of the eastern VLT to the Vocontian folds and the increase in displacement westward from a tip point south of Sisteron. The western sector of the VLFZ shows less N–S shortening and evidence of strike slip. On a regional scale, Late Cretaceous N–S shortening, contemporaneous with reactivation of NE–SW faults, may have been caused by the eastward migration of the Iberian‐Briançonnais plate to the south of the European plate. The post‐Burdigalian displacement of the VLT is correlated with the late Alpine SW emplacement on the Digne Thrust to the east. Within the French Alpine foreland the dextral NE–SW Durance Fault separated a zone where SW directed displacement was accommodated principally on the Digne Thrust from an area to the west, including the VLFZ, of more diffuse SW–NE shortening.
The Amazon‐Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile
Authors: Tosdal Richard M.
Published: February 2004

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Perú form the Arequipa‐Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U‐Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U‐Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite‐ and granulite‐facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U‐Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belén Schist in northern Chile generally have present‐day low 206Pb/204Pb (<18.0), elevated 207Pb/204Pb (>15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time‐averaged Th/U values. The Middle Proterozoic rocks in general have higher present‐day 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa‐Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa‐Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa‐Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa‐Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa‐Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa‐Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America‐Scotland to the oroclinal bend in western South America.
Tertiary tectonics of the Sea of Okhotsk, Russia: Far‐field effects of the India‐Eurasia collision
Authors: Worrall D. M. , Kruglyak V. , Kunst F. , Kuznetsov V.
Published: February 2004

A new structural map of the western Sea of Okhotsk, based on grids of multichannel seismic data, provides several new insights into the Tertiary deformation of Asia, especially in regard to the extension of “extrusion” tectonics, linked to the India‐Eurasia collision, into northeastern Asia. The sedimentary basins in this offshore region are the result of two regional shear systems. In the west, the north trending Sakhalin‐Hokkaido dextral fault system transects Sakhalin Island. Accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts. The accessory normal faults are predominantly Eocene to lower Miocene and indicate transtensional shear, whereas the folds and thrusts are mostly younger and indicate late Miocene and Pliocene transpression. In the north, an east‐west trending sinistral shear zone enters the sea at its northwest corner; this sinistral shear system links farther west to extensional faults of the early Tertiary Baikal rift. Extending northeastward from the terminus of this sinistral shear zone are a series of long, predominantly northwest dipping listric normal faults that form a large “lazy‐S” shaped pull‐apart basin, the Shantar‐Liziansky basin (SLB). Normal faults in this 500 × 140 km basin accommodate 15 to 20% extension. Extension and related sinistral shear appear to be largely Eocene to Oligocene in age, with lesser later activity. To the northeast of SLB, a further extension of the sinistral shear zone appears to bend northeastward, transecting Pustorets and Penzhina basins and following the course of the older, Mesozoic Mongol‐Okhotsk‐Chukotsk active margin and suture; it possibly reaches as far as the Arctic. As suggested by previous workers, the Baikal rift and its associated sinistral shear zone may be interpreted as by‐products of the India‐Eurasia collision in Eocene time; mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast. The sinistral shear zone is transected by the dextral Sakhalin‐Hokkaido fault zone in the Kashevarov region of central SLB, where a fan of north to northwest striking late Tertiary transpressional fault splays marks the northern termination of the dextral fault zone. Older SLB normal faults are locally deformed, and half grabens are partly inverted, by the slightly younger dextral splays. The two regional fault systems are essentially conjugate, and the pattern of intersection closely resembles that predicted by recent modelling studies of India/Eurasia collision tectonics.
Tectonic evolution of the Paleoproterozoic Torngat Orogen: Evidence from pressure‐temperature‐time‐deformation paths in the North River map area, Labrador
Authors: Van Kranendonk Martin J.
Published: February 2004

The study of ancient, deeply eroded orogenic belts can provide information about tectonic processes in the deep crust that is unobtainable from upper crustal sections in younger belts. In this light, the Paleoproterozoic Torngat Orogen in northeastern Canada is ideal because it is well exposed and eroded to maximum depths of 28 km, revealing information about transpressional deformation mechanisms during continent‐continent collision between the Archean Nain and southeastern Rae provinces. Evidence from detailed structural mapping, metamorphic petrology, and geothermobarometry of a 115‐km transect of the orogen is combined with previously published geochronology to construct pressure‐temperature‐time‐deformation (P‐T‐t‐d) paths for rocks from the granulite‐facies core and amphibolite‐facies Foreland zone of the orogen. Results show that rocks followed a clockwise P‐T‐t path, but different parts of the orogen reached peak conditions at different times. The orogen evolved through three temporally and kinematically distinct events over ∼80 m.y., rather than continuous deformation. Events were separated by gaps of 15–30 m.y. and include (1) crustal thickening during continental collision at ca.1860 Ma under peak metamorphic conditions of 9.5 ± 0.5 kbar and 950° ± 50°C in the core of the orogen, and prethermal peak conditions of 4.5 kbar and 550°C in the Foreland zone; (2) sinistral transpressional shear at ca. 1845–1822 Ma under post‐peak conditions of 7.3 ± 0.5 kbar and 750° ± 50°C in the core of the orogen, but at thermal peak conditions of ∼5 kbar and 700°C in the Foreland zone; and (3) west‐directed exhumation of the orogen at 1794–1740 Ma, under lower‐grade conditions of up to ∼5 kbar and 600°C. The overall transpressional features of the orogen are shown to be a cumulative result of three distinct tectonic events which can be related to changes in plate/subduction configurations across the rest of NE Laurentia.
Hillslope and Climatic Controls on Hydrologic Fluxes
Authors: Salvucci Guido Daniel , Entekhabi Dara
Published: February 2004

The position and shape of the hillslope water table has long been considered to be a useful diagnostic both for field analysis of the spatial structure of hydrologic behavior and for initializing hydrologic forecasting models. Water table positions near the surface often indicate areas which contribute to surface runoff generation, yield evaporation at the climate‐demanded rate, and produce a net discharge of saturated zone groundwater. Deeper water tables typically indicate drier areas where evaporation is suppressed and infiltration enhanced, therefore promoting net groundwater recharge. The long time integral of these fluxes over an area forms the local climatic hydrologic cycle. Continuity (throughout a hillslope) of modeled long‐term mean moisture fluxes at the land‐atmosphere interface within the unsaturated and saturated zones provides closure to the climate‐hillslope system, and it is used to determine the equilibrium water table position and the corresponding spatial structure of mean recharge, discharge, evaporation, and surface runoff production. This method of analysis provides a tool for the systematic study of how characteristics of the atmospheric forcing (e.g., mean precipitation, evaporative demand, and storm intermittency and intensity), soil properties, and geologic features (e.g., topography) interact to yield observed spatial patterns of distinct hydrologic behavior at various scales. Application of the methodology to simplified and idealized hillslope geometries reveals spatial patterns in good qualitative agreement with field observations. Case studies demonstrate that the wide range of behavior observed in nature is reproducible within an observed range of climatic, geologic, and soil parameters. Partial analysis of the full system yields a few dimensional parameter groups, the relative values of which are indicative of transitions between geologic‐, soil‐, and climate‐controlled conditions and the presence or absence of characteristic hydrologic zones such as seepage faces, recharge areas, hinges, midline zones, discharge areas, and partial areas of surface runoff production. Under conditions of limited lateral transmissivity and slope, the midline region (over which the long‐term mean net exchange between the saturated and unsaturated zones is zero) can dominate the hillslope and lead to an invariance of area‐averaged surface runoff and evaporation with respect to hillslope length scale.
A Semianalytical Analysis of Compressible Electrophoretic Cake Formation
Authors: Kambham Kiran K. R. , Tuncay Kagan , Corapcioglu M. Yavuz
Published: February 2004

Leaks in geomembrane liners of waste landfills and liquid impoundments cause chemical contaminants to leak into the subsurface environment. A mathematical model is presented to simulate electrophoretic sealing of impoundment leaks. The model describes the formation of a compressible clay cake because of electrical and gravitational forces. The model includes mass balance equations for the solid particles and liquid phase, modified Darcy's law in an electrical field, and Terzaghi's definition of effective stress. The formulation is presented in the Eulerian coordinates. The resulting second‐order, nonlinear partial differential equation and the lower boundary condition are linearized to obtain an analytical solution for time‐dependent settlement. After discretizing in time the analytical solution is applied to simulate compression of an accreting sediment. In the simulation of an accreting sediment, solid fluxes on either side of suspension/sediment interface are coupled using a no‐jump condition. The velocity of a discrete particle in the suspension zone is assumed to be equal to the algebraic sum of electrophoretic and Stoke's settling velocities. An empirical relationship available in the literature is used to account for the effect of concentration on the velocity of solid particles in the suspension zone. The validity of the semianalytical approach is partially verified using an exact steady state solution for self‐weight consolidation. The simulation results obtained for a set of material parameters are presented graphically. It is noted that the electrokinetic consolidation of sediment continues even after the completion of electrophoretic settling of all clay particles. An analysis reveals that the electrophoretic cake formation process is quite sensitive to voltage gradient and the coefficient of compressibility.
Mechanisms Controlling Vacuum Extraction Coupled With Air Sparging for Remediation of Heterogeneous Formations Contaminated by Dense Nonaqueous Phase Liquids
Authors: Unger A. J. A. , Sudicky E. A. , Forsyth P. A.
Published: February 2004

The numerical model CompFlow is used to study the mechanisms controlling vacuum extraction, coupled with air sparging, as a means for remediation of heterogeneous formations contaminated with dense nonaqueous phase liquids (DNAPLs). Two dominant mechanisms are demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing relatively rapid transfer of contaminant from the nonaqueous phase to the gas phase and subsequent removal by the vacuum extractor. Second, at later times, remediation is controlled by the transfer of contaminant from the nonaqueous phase to the aqueous phase below the water table. During this time the vacuum extractor pumps both liquid and vaporized water in the aqueous and gas phases. This causes the contaminant that is dissolved in the aqueous phase to migrate vertically upward across the permeability layers toward the vacuum extractor where it is removed. This intermediate to late time removal mechanism is shown to be controlled by contaminant dissolution, which is a slower transfer process than the direct DNAPL vaporization that occurs at early time. Our analysis indicates that as long as both air and water are actively flushed through the DNAPL zone, both early‐time vaporization and intermediate‐ to late‐time dissolution are effective mechanisms leading to the removal of the DNAPL. We show that it may be possible to design the remedial system so as to reduce its performance sensitivity to geologic heterogeneity. A lack of sensitivity of a remedial design to heterogeneity is highly desirable because a robust design implies that the degree of site characterization required for reasonable success will be less than that needed for a less robust scheme.
Partitioning Tracer Test for Detection, Estimation, and Remediation Performance Assessment of Subsurface Nonaqueous Phase Liquids
Authors: Jin Minquan , Delshad Mojdeh , Dwarakanath Varadarajan , McKinney Daene C. , Pope Gary A. , Sepehrnoori Kamy , Tilburg Charles E. , Jackson Richard E.
Published: February 2004

In this paper we present a partitioning interwell tracer test (PITT) technique for the detection, estimation, and remediation performance assessment of the subsurface contaminated by nonaqueous phase liquids (NAPLs). We demonstrate the effectiveness of this technique by examples of experimental and simulation results. The experimental results are from partitioning tracer experiments in columns packed with Ottawa sand. Both the method of moments and inverse modeling techniques for estimating NAPL saturation in the sand packs are demonstrated. In the simulation examples we use UTCHEM, a comprehensive three‐dimensional, chemical flood compositional simulator developed at the University of Texas, to simulate a hypothetical two‐dimensional aquifer with properties similar to the Borden site contaminated by tetrachloroethylene (PCE), and we show how partitioning interwell tracer tests can be used to estimate the amount of PCE contaminant before remedial action and as the remediation process proceeds. Tracer tests results from different stages of remediation are compared to determine the quantity of PCE removed and the amount remaining. Both the experimental (small‐scale) and simulation (large‐scale) results demonstrate that PITT can be used as an innovative and effective technique to detect and estimate the amount of residual NAPL and for remediation performance assessment in subsurface formations.
Analytical Solutions for Transport of Linearly Adsorbing Solutes in Heterogeneous Formations
Authors: Bellin Alberto , Rinaldo Andrea
Published: February 2004

A new first‐order analytical solution for stochastic transport of reactive solutes in physically and chemically heterogeneous porous formations is derived and discussed. Physical and chemical heterogeneities are assumed partially correlated. The derived solution complements previous results obtained by the authors (Bellin et al., 1993). The solution proposed relies on the assumption of chemical activity described by the local linear equilibrium assumption postulating the existence of a (spatially variable) retardation factor. Retardation factors, R(x) and log permeabilities (Y(x)) modeling heterogeneities are described statistically by random space functions with assigned correlation structure. The analytical expressions derived reduce to Dagan's (1984) classic solution for the case of nonreactive solute transport and to Bellin et al.'s (1993) solutions for perfectly correlated physical and chemical heterogeneities. The main conclusion is that longitudinal dispersion may be significantly affected by different degrees of correlation between chemical and physical heterogeneities. However, such an impact vanishes progressively as the geometric mean of the partition coefficient increases and the plume behaves as in perfectly correlated cases. Transverse dispersion is not affected by either perfect or partial correlation between R and Y.
Buoyancy‐Induced Flow of a Tracer in Vertical Conduits
Authors: Ronen Daniel , Yechieli Yoseph , Kribus Avi
Published: February 2004

Buoyancy‐induced flow (BIF) of both 18O‐enriched (δ18O, +160 to +5195‰) and 18O‐depleted (δ18O, −500‰) water has been studied in a 10‐cm‐diameter tube that simulates a section of a well. BIF was detected in all the experiments for a density contrast (Δρ) between 0.03 and 1.0 σ units (equivalent to a total dissolved solids contrast of 30−1000 mg/L). The magnitude of the Rayleigh number suggests that the natural convection flow is turbulent. A mathematical model is presented to predict vertical tracer propagation inside the cavity. The model assumes a local mixing length scale proportional to the tube diameter. A possible transition to the laminar natural convection regime, with a significant reduction in mixing rate compared to that predicted by the model, is detected when the density contrast is reduced to Δρ = 0.03 σ units. This smaller mixing rate is still orders of magnitude larger than diffusion alone. BIF may be an important groundwater transport and mixing mechanism in open conduits in aquifers. Its effects should also be considered when performing tracer experiments in the field.
Bacterial Sedimentation Through a Porous Medium
Authors: Wan Jiamin , Tokunaga Tetsu K. , Tsang Chin‐Fu
Published: February 2004

Numerous previous studies of bacterial transport in groundwaters and to deep aquifers and sediments have either neglected, or regarded as insignificant, the potential contribution of bacterial sedimentation. This study examines the potential significance of sedimentation as a mechanism for bacterial transport. A simple model is developed to predict the behavior of particles (bacterial or inorganic colloids) sedimenting through granular porous media under hydrostatic conditions. The model indicates that tortuosity‐limited sedimentation velocities through porous media consisting of large, well‐rounded grains can proceed at velocities close to (≈90% that of) free sedimentation in water columns when particle‐grain interactions involve only tortuosity. The assumption of neutral buoyancy of bacteria was demonstrated to be invalid through buoyant density measurements on 25 subsurface bacterial strains (using Percoll density gradient centrifugation). An average buoyant density of 1.088 Mg m−3 was obtained (range from 1.040 to 1.121 Mg m−3). The two nonmotile bacterial strains selected for sedimentation experiments were Arthrobacter globiformis B672 (isolated from the Middendorf aquifer, 259‐m depth), and OYS3, a streptomycin‐resistant strain isolated from shallow groundwaters at Oyster, Virginia. All experiments were carried out under nongrowth conditions. Stokes' law sedimentation velocities for the two bacterial strains calculated from measurements of buoyant densities and characteristic sizes were 5.8 and 40 mm d−1, respectively. Direct measurements of free sedimentation of Arthrobacter B672 and OYS3 through water columns (21°C) yielded median velocities of 7.1 and 42 mm d−1, respectively, in good agreement with Stokes' law calculations. The Arthrobacter B672 and OYS3 strains sedimented through saturated sand columns (quartz sand, 300–420 μm diameter) under hydrostatic conditions at median velocities of 7 and 17 mm d−1. Thus the sedimentation model is consistent with sand column observations on Arthrobacter B672 and too simplistic in the case of OYS3. Bacterial breakthrough by sedimentation exhibited trends consistent with first‐order attenuation with distance. Bacterial deposition coefficients for this first‐order model were in the range of 0.008–0.012 mm−1. Surface physical‐chemical interactions, grain and pore size distributions, and grain surface microtopography can be very important in controlling the effectiveness of bacterial sedimentation as a transport mechanism. This research suggested that if timescales are sufficiently long, spanning many generations, sedimentation can become a significant mechanism for bacterial transport.
Modeling Transport of Multiple Organic Compounds: Segregated Transport‐Sorption/Solubilization Numerical Technique
Authors: Adeel Zafar , Luthy Richard G. , Edwards David A.
Published: February 2004

The transport of multiple organic contaminants in the subsurface may be affected by their mutual interactions. Conventional modeling approaches cannot simulate the transport of multiple organic species whose sorption and/or solubilization is interdependent. This paper describes a numerical modeling approach for characterizing interactive solute sorption and solubilization reactions coupled with one‐dimensional advective‐dispersive transport in porous media. This numerical approach, referred to as segregated transport‐sorption/solubilization (STSS), allows for interphase mass transfer at each time step after solute advection and dispersion have occurred. The overall capabilities of the STSS technique include modeling equilibrium or nonequilibrium transport of individual and multiple solutes with linear or nonlinear sorption isotherms. Application of this modeling approach is illustrated for the transport of a conservative tracer, a surfactant, a hydrophobic organic compound (HOC), and a polychlorinated biphenyl (PCB) mixture. The coupling of sorption modules for phenanthrene and surfactant is used to reasonably describe surfactant‐enhanced flushing of sorbed phenanthrene from a sand column.
Active Channel Geometry and Discharge Relations of U.S. Piedmont and Midwestern Streams: The Variable Exponent Model Revisited
Authors: Kolberg Frederick J. , Howard Alan D.
Published: February 2004

Morphometric, discharge, and sediment data were gathered for 51 streamflow gaging locations in the Piedmont and nontidal coastal plain of Virginia and North Carolina. The Piedmont data are compared to those obtained in the Midwestern United States to calibrate a variable exponent channel geometry model. Least squares regression of the Midwestern data demonstrates that estimated discharge‐width exponents for high silt and clay, gravel, or cobble bed channels deviate significantly from those estimated for alluvial beds with 30% or greater sand content. However, no significant trend is apparent for the estimated exponents among other sand‐silt‐clay channel categories. Analyses of the Piedmont data indicate no significant departure of the discharge‐width relations if sediment categories are used to group stream types. This may be due in part to the relative homogeneity of the sediments that compose the bed and banks of the Piedmont stream channels sampled for this study. Least squares regression of the Midwestern and Piedmont data indicate that estimated width‐discharge exponents for groups of streams with width:depth ratios less than 45 range from 0.35 to 0.46 but are statistically indistinguishable. For groups of streams having width:depth ratios greater than 45, the width‐discharge exponents decrease toward values less than 0.15. This trend suggests systematic variation in the exponents and a diminished influence of channel shape on width‐discharge functions for streams with width:depth ratios greater than 45. These findings do not support the results of a previous investigation that used envelope curves to estimate width‐discharge exponents for Midwestern streams, and they suggest that the assumptions regarding the variable exponent channel geometry model should be reevaluated.
Simulation of Braided Channel Topology and Topography
Authors: Webb Erik K.
Published: February 2004

A computer code, BCS, was developed to simulate the three‐dimensional topographical expression of braided channel systems. The model uses a random walk approach to simulate channel braiding and incorporates the hydraulic geometry of stream channels to assign width, depth, and velocity to each channel segment. Input parameters are directly related to observable geometric characteristics of a braided system such as channel system width, depth, and sinuosity. Thirty‐one measures of the degree or type of braiding were taken from the literature and used as calibration targets for the model. All targets were matched for a portion of the Ohau River, New Zealand. This simulation technique can be used to assess quantitatively the distribution of channels with their associated hydraulic properties and visualize topographic surfaces associated with braided channel systems. This information provides a foundation for probabilistic (multiple realizations), geometric simulation of the three‐dimensional distribution of braided stream sediments.
Ponded Infiltration into Soils Bounded by a Water Table
Authors: Salvucci Guido Daniel , Entekhabi Dara
Published: February 2004

A simple model of ponded infiltration into homogenous soils bounded by, and at steady state equilibrium with, a water table is presented. The model parameterizes the wetting front profile as a tension‐saturated zone above a zone in which the moisture content decreases linearly with depth. Application of the model requires numerical evaluation of an integral relating the time since the onset of ponding to the infiltration rate or wetting front position. Wetting fronts predicted by the simplified model compare favorably with laboratory measurements of Vachaud and Thony (1971) and, for a wide range of soil types, with those predicted by finite‐element integration of the governing flow equations. The presence of a shallow water table causes the infiltration rate to decrease faster than it would under semi‐infinite conditions with uniform initial moisture content. Infiltration equations for soils bounded by water table may be used in distributed hydrologie models and other applications where the shallow position of the water table significantly affects the partitioning of precipitation forcing during storms.
Step‐Pool Streams: Adjustment to Maximum Flow Resistance
Authors: Abrahams Athol D. , Li Gang , Atkinson Joseph F.
Published: February 2004

Steep headwater streams are often characterized by alternating steps and pools, which may be described by mean step heightand mean step length. A conceptual model is developed based on the notion that the largest floods are just capable of moving the largest debris in the channel. The model suggests that step pools evolve toward a condition of maximum flow resistance because maximum resistance implies maximum stability and that this condition is achieved when steps are regularly spaced and the mean step steepnessis slightly greater than the channel slopeS. To test this conceptual model, four series of flume experiments were performed. These experiments show that the relation between resistance to flow andis convex upward with maximum flow resistance occurring when steps are regularly spaced and havevalues between 1 and 2. Field measurements reveal that 18 natural step‐pool streams also satisfy the inequality, strongly suggesting that the form of such streams is adjusted to maximize resistance to flow. The results of the flume experiments are inconsistent with the proposition that step pools form as antidunes, as Froude numbers for the flume step pools at which flow resistance was maximized fall well below those values usually associated with these bed forms.
Field Scale Vertical Transport from Horizontally Variable Local Scale Transport Parameters
Authors: Salzmann T. , Richter J.
Published: February 2004

In field soils, transport properties vary vertically and horizontally. With experimental measurements it is distinctly easier to consider the vertical than the horizontal variability. We present a simple approach for considering the horizontal variability of the transport behavior of the soil in a one‐dimensional transport model. The model approach assumes independence on depth of the vertical dispersion coefficient, which is justified by the relatively small effects of the vertical dispersion in comparison to those of the horizontal dispersion only. The approach is based on numerical experiments using different parameter statistics. An analytical expression of the dependence of effective parameters, describing the transport at the field scale, on the statistical lognormal distributions of transport parameters at the local scale is derived heuristically by careful inspection of the numerical experimental results. The model was tested in (1) a reevaluation of a one‐dimensional field experiment with chloride tracer in a heavy loam soil of Roth (1989), and (2) a two‐dimensional leaching field experiment with the same tracer in a sandy soil plot of 5600 m2. For both experiments the model using lognormal local parameter distributions provides a satisfactory representation of field averages of the vertical transport derived from local measurements.
Modeling Solute Leaching in Soils by Diffusion‐Limited Aggregation: Basic Concepts and Application to Conservative Solutes
Authors: Flury Markus , Flühler Hannes
Published: February 2004

The diffusion‐limited aggregation (DLA) model introduced by Witten and Sander (1981) was modified to describe leaching of conservative solutes through soils. In the DLA concept, particles that move by random walk (diffusion) form aggregates. This model was adapted by modifying the walking probabilities and by introducing layers. The model does not describe the actual random walk of solutes in a heterogeneous flow field, but it can describe results of observed experimental outcomes with a minimal set of parameters. The modified model is versatile: homogeneous distributions of chemicals can be simulated, such as those produced with piston flow on the one hand, and complicated distributions arising from macropore flow and fingering on the other hand. The model was successfully applied to the results of a field experiment where preferential flow of bromide occurred.
Stochastic‐Convective Transport with Nonlinear Reaction: Biodegradation With Microbial Growth
Authors: Ginn T. R. , Simmons C. S. , Wood B. D.
Published: February 2004

The representation of subsurface flow and reactive transport as an ensemble of one‐dimensional stream tubes is extended to account for nonlinear biodegradation with coupled microbial growth. The Stochastic‐convective reaction (SCR) model is derived for bioreaction of a single solute by a single class of microorganisms coupled with dynamic microbial growth. A new global variable, the integral of the solute degraded per unit length of system traversed, accounts for degradation. Dimensionless scaling and the method of characteristics are used to reduce the model, written for a single convecting reactor (stream tube), to a pair of coupled nonlinear functional equations for solute concentration and microbial biomass. Existence of a solution to the stream tube system is shown, both numerical and approximate analytical approaches to the solution are given, and example computations using both methods are presented. Conditions under which the stream tube solution is “canonical,” or scalable to fit any permissible stream tube travel time function, arise from requirements for invariance (over the stream tube ensemble) of effective one‐dimensional stream tubes used to represent transport along real stream tubes in three‐dimensional space. Averaging of the stream tube solution over travel time and reaction properties representative of physical and chemical heterogeneities is described as a way to separate and upscale the processes of macrodispersion and microbiological reaction. The approach is exercised to simulate Monte Carlo average behavior of bioreactive transport in physically heterogeneous two‐dimensional media. Results show that the method captures the ensemble average large‐scale effects of the nonlinear reactions more accurately than done in the classical reactive convection‐dispersion equation (CDR), even when the appropriate scale dependent dispersion coefficient is afforded to the CDR.
Computer construction of fractal soil structures: Simulation of their hydraulic and shrinkage properties
Authors: Perrier Edith , Mullon Christian , Rieu Michel , de Marsily Ghislain
Published: February 2004

To study relationships between soil hydraulic properties and soil structural properties, a computer micro model of soil is constructed. We present first a general method of building a two‐dimensional porous structure, including both pores and particles, with different levels of aggregation resulting from a fragmentation process. A fractal structure is obtained when self similarity is imposed over the successive scales of fragmentation. Emphasis is put upon the modeling of the retention curve. A classical capillary model and methods taken from percolation theory enable us to simulate qualitatively the primary and secondary loops of this hysteretic curve. In the fractal case, theoretical analytical expressions proposed for adjusting retention data are tested. The unsaturated hydraulic conductivity is also calculated on the same simulated soil by analogy with an electrical network. The soil structures are deformable and simulation proves to be a useful tool to investigate the behavior of swelling soils.