bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calcium and phosphorus content of roots exposed to the oral environment
Authors: Cohen Mark , Garnick Jerry J. , Ringle Robert D. , Hanes Philip J. , Thompson William O. 
Published: December 2005

AbstsractThis study analyzed the calcium and phosphorus content of extracted tooth roots exposed to the in vivo oral environment. 20 teeth were obtained from 16 patients and divided into two groups of 10 teeth each. In group 1, the teeth had gingival probing depths of 5 mm or more, and teeth of group 2 had gingival recessions of 3 mm or more. Prior to extraction, the gingival margin location was recorded by placing a groove on the tooth surface. After extraction, the teeth were sectioned coronatapically, air dried and coated with carbon. Energy dispersive X‐ray spectra, excited in a scanning electron microscope, were analyzed to measure relative calcium and phosphorus contents and for calculation of their ratios. X‐rays were collected from two positions on the sectioned root. Experimental positions were selected within the exposed portion of the roots of groups 1 and 2, and unexposed positions were selected from that portion of the same root with attached periodontal membrane. At each position, calcium and phosphorus content was measured at 4 depths into the root surface: in cementum, in dentin three‐quarters of the distance to the pulp chamber, and at 2 locations in between on either side of the cemento‐dentinal junction. Analysis of data demonstrated large variations in calcium and phosphorus content from surface to surface of individual teeth and from tooth to tooth in a subject. No statistically significant differences were found between experimental and unexposed locations. Calcium and phosphorus contents were greater in roots exposed to pockets when compared to roots exposed by recession at both experimental and unexposed locations. The mean calcium/phosphorus ratio of 1.8 was increased over reported values for hydroxyapatite. Trend analysis across combined data from groups 1 and 2 showed both calcium and phosphorus were significantly higher toward locations nearer to the ptilp chamber. This study showed that the oral environment had little effect on either calcium or phosphorus content or ratios.
Sorption and Intraparticle Diffusion of Fluorescent Dyes with Consolidated Aquifer Media
Authors: Sabatini David A. 
Published: December 2005

AbstractSorption of two fluorescent dyes (fluorescein and sulforhodamine B) was evaluated with two oppositely charged, consolidated aquifer materials (sandstone and limestone). Fluorescein, which has an anionic carboxylic group, experienced negligible sorption onto negatively charged sandstone and sorbed much less than sulforhodamine B, with its two sulfonic groups, onto positively charged limestone. The cationic charge on sulforhodamine B caused it to adsorb onto negatively charged sandstone. These results validate and extend previous research with pure mineral surfaces (alumina and silica). Sorption kinetic rates decreased with increasing particle size, which is consistent with diffusion‐limited intraparticle sorption. Tortuosity factors were determined by fitting a diffusion ‐limited intraparticle sorption model to kinetic data for one particle size (all other parameters being determined independently). This tortuosity factor, which was consistent with Archie's law‐based estimates and other independently determined parameters, was subsequently used to predict kinetic adsorption results for a second particle size. Close agreement between experimental data and model predictions corroborated intraparticle diffusion‐limited sorption as an important process. Diffusion‐limited sorption can impact dye transport in unconsolidated grains having internal porosity or consolidated or fractured media having dual porosity. These results thus demonstrate the importance of understanding both the equilibrium and kinetics of dye sorption when designing and interpreting tracer studies.
Effects of Heterogeneity in Aquifer Permeability and Biomass on Biodegradation Rate Calculations—Results from Numerical Simulations
Authors: Scholl Martha A. 
Published: December 2005

AbstractNumerical simulations were used to examine the effects of heterogeneity in hydraulic conductivity (K) and intrinsic biodegradation rate on the accuracy of contaminant plume‐scale biodegradation rates obtained from field data. The simulations were based on a steady‐state BTEX contaminant plume undergoing biodegradation under sulfate‐reducing conditions, with the electron acceptor in excess. Biomass was either uniform or correlated with K to model spatially variable intrinsic biodegradation rates. A hydraulic conductivity data set from an alluvial aquifer was used to generate three sets of 10 realizations with different degrees of heterogeneity, and contaminant transport with biodegradation was simulated with BIOMOC. Biodegradation rates were calculated from the steady‐state contaminant plumes using decrease in concentration with distance downgradient and a single flow velocity estimate, as is commonly done in site characterization to support the interpretation of natural attenuation. The observed rates were found to underestimate the actual rate specified in the heterogeneous model in all cases. The discrepancy between the observed rate and the “true” rate depended on the ground water flow velocity estimate, and increased with increasing heterogeneity in the aquifer. For a lognormal K distribution with variance of 0.46, the estimate was no more than a factor of 1.4 slower than the true rate. For an aquifer with 20% silt/clay lenses, the rate estimate was as much as nine times slower than the true rate. Homogeneous‐permeability, uniform‐degradation rate simulations were used to generate predictions of remediation time with the rates estimated from the heterogeneous models. The homogeneous models generally overestimated the extent of remediation or underestimated remediation time, due to delayed degradation of contaminants in the low‐K areas. Results suggest that aquifer characterization for natural attenuation at contaminated sites should include assessment of the presence and extent of, and contaminant concentrations in, low‐permeability areas of an aquifer.
Recharge and Preservation of Laurentide Glacial Melt Water in the Canadian Shield
Authors: Clark Ian D. , Douglas Malcolm , Raven Kenneth , Bottomley Dennis 
Published: December 2005

AbstractGround water inflows to drifts ranging from 700 to 1615 m below ground surface at the Con Mine, Yellowknife, Northwest Territories, Canada, were used to study deep hydrogeological flow regimes in Shield terrain. Salinity trends are due to mixing between low‐TDS ground water and deep Ca(Na)‐C1 brines (>290 g/L) likely derived from Devonian sea water. C1‐−δ18O relationships demonstrate that all inflows are a mixture of three distinct components: modern meteoric ground water (δ18O ∼−18.9 ± 0.1%o), brine (δ18O ∼−10%o), and an isotopically depleted water (δ18O ∼−28%o). The origin of this third endmember is attributed to glacial melt water injected into the subsurface during ablation of the Laurentide Ice Sheet at ca. 10 ka. A mechanism is proposed where high hydrostatic pressure in the ablation zone imposes strong downward gradients beneath the ice sheet margin. Numerical simulation with the SWIFT II finite‐difference code recreates the observed salinity gradients within a modeled 50‐year interval, corresponding with the rate of retreat of the ice sheet across the landscape at this time. The persistence of this melt water in the subsurface for some 10,000 years following retreat of the ice and decay of the steep hydraulic gradients highlights the importance of gradient, in addition to permeability, as a major control on ground water flow and transport in deep crystalline settings.
Impacts of Unsaturated Zone Properties on Oxygen Transport and Aquifer Reaeration
Authors: Neale C. Nelson , Hughes Joseph B. , Ward C.H. 
Published: December 2005

AbstractThis study investigated the unsaturated zone properties that affect ground water reaeration (i.e., the diffusive flux of oxygen through the unsaturated zone and into an aquifer system). Laboratory column experiments were undertaken to quantify oxygen flux into anaerobic ground water as a function of soil type, soil water content, soil oxygen demand, and unsaturated zone thickness. Soils used in these studies included coarse sand, sand, loamy fine sand, fine sandy loam, silt, silty clay loam, bentonite, kaolin, and peat. The results showed that reaeration flux rates ranging from 11,000 to 12,000 mg/m2‐day were possible under conditions typical of the natural environment. Soil water content provided the greatest resistance to oxygen transport in the unsaturated zone while the remaining factors of soil type, soil oxygen demand, and unsaturated zone thickness did not significantly inhibit reaeration flux into ground water. An unsaturated zone transport model based on Fick's second law and gas‐liquid interfacial mass transport was developed and showed good agreement with the experimental results derived from the column studies. The results of this study suggest that unsaturated zone reaeration of anaerobic ground water may be a contributing factor in controlling the steady–state size and shape of hydrocarbon plumes and incorporating reaeration into ground water models—based on unsaturated zone properties—may allow for a more accurate representation of bioattenuation reactions taking place in subsurface environments.
Geostatistical Inversion of Cross‐Hole Pumping Tests for Identifying Preferential Flow Channels Within a Shear Zone
Authors: Meier Peter M. , Medina Agustin , Carrera Jesús 
Published: December 2005

AbstractCross‐hole pumping tests within a subvertical shear zone in granite were interpreted using a geostatistical inverse approach. The objective was to identify flow channels that influence tracer experiments. The spatial correlation structure of transmissivity (T) was unknown. Therefore, a wide variety of geostatistical hypotheses were made for estimating a total of 40 T fields, that honor equally well the pumping tests, the static head measurements, and the point T data. All estimated T fields reveal channels with similar topology and with a preferred horizontal orientation, possibly due to vertical displacements within the subvertical shear zone. This shows that inverse geostatistical modeling of abundant hydraulic measurements can be useful to reveal a coarse heterogeneity structure, even when data are insufficient to identify the geostatistical structure. However, numerical simulations of a field tracer test were highly sensitive to the geostatistical hypotheses. In summary, T fields leading to successful hydraulic data fitting do not necessarily lead to successful transport predictions, even when they appear to reproduce the coarse heterogeneity structure.
Risk‐Cost Analysis in Ground Water Contaminant Transport: The Role of Random Spatial Variability and Sorption Kinetics
Authors: Andersson Charlotta , Destouni Georgia 
Published: December 2005

AbstractWe outline a methodology for quantifying the risk (probability) to exceed environmental regulation standards for ground water quality and coupling this risk quantification with an analysis of abatement costs for attaining an acceptable risk level. The methodology accounts for the uncertainty that results from random spatial variability in the subsurface, and for sorption‐desorption kinetics. Random spatial variability implies that, with a deterministic regulating framework, the risk (probability) of exceeding given water quality targets may remain high even after abatement measures have been taken. We show how this risk can be set in focus by use of a probabilistic regulating framework, which enables better risk management. Both the environmental risk and the associated abatement costs will depend on the actual combination of subsurface variability, the ground water sampling method, and sorption kinetics, through the underlying quantification of pollutant concentration statistics. Our results emphasize the importance of a relevant account of the kinetics of the sorption‐desorption process, by illustrating that neglect, or equilibrium simplification of that process, may result in highly misleading risk and abatement cost estimates.
Geochemical Evolution of Ground Water in the Great Plains (Dakota) Aquifer of Nebraska: Implications for the Management of a Regional Aquifer System
Authors: Gosselin David C. , Harvey F. Edwin , Frost Carol D. 
Published: December 2005

AbstractThe Great Plains (Dakota) aquifer system is one of the most extensive in North America extending from the Arctic Circle to New Mexico, and underlies approximately 94% of Nebraska. In Nebraska, we do not have the physical ground water monitoring data at the scale that is necessary to manage ground water flow systems. However, first‐order management strategies for this regional aquifer can be developed by understanding the geochemical evolution of the ground water. Using major‐ion water chemistry data from 203 wells in 19 counties in eastern Nebraska, reconnaissance δ180, δD, and δ87Sr data, and two geochemical models, PHREEQC and SNORM, we determine that modern meteoric water, NaCl brines from underlying formations, and cold glacial melt water are the primary sources for the water in the Dakota Aquifer. Based on these three water sources and the geochemical evolution of the various water types, the following first‐order management strategies are suggested. In areas where CaSO4 and Ca‐Na SO4 type water occur, Pleistocene‐age glacial meltwater is the source. This water supply is not easily renewable. It is recommended that detailed water resource evaluation be conducted before extensive development occurs. The source of Ca (± Mg) HCO3 type water is from recharge by local precipitation and should be managed to maintain them as a renewable resource. In mixed ground water type areas, the ground water chemistry reflects the interaction of two distinct water types, one of which is meteoric water and the other is either CaSO4 and Ca‐Na SO4‐type water or NaCl‐type water. If the relatively fresh ground water is extracted at a rate that changes the location of the interface between the endmembers, then monitoring changes in water chemistry in a well over time could be used as an early warning system for the onset of potential problems related to overpumping.
Stress‐Strain Analyses for Aquifer‐System Characterization
Authors: Burbey Thomas J. 
Published: December 2005

AbstractSubsidence data historically have been collected using costly extensometers. Recent advances in InSAR and GPS technology may provide noninvasive methods for collecting these data, which can be used in conjunction with hydrograph information to estimate the skeletal specific storage and hydraulic conductivity values of compressible units. Numerical analysis is used to evaluate the conditions affecting nonrecoverable (inelastic) and recoverable (elastic) skeletal specific storage and to investigate the affects of horizontal strain on the extensometer record. Stress‐strain diagrams represented as subsidence‐drawdown plots are one‐dimensional in scope, yet are typically used for estimating specific storage. Simulation results indicate that nonrecoverable specific can be estimated from these diagrams even when the storage contribution from horizontal strain is significant. Hysteresis effects are common during recovery and recompression of cyclically pumped confined aquifers. Several factors were found to affect the shape of the hysteresis loops; these include (a) the hydraulic diffusivity of the confining unit, (b) the distance from the pumped well, (c) length of the recovery cycle, (d) the ratio of confined aquifer to confining unit hydraulic conductivities, and (e) the thickness of the confining unit. Stress‐strain analyses can supplement classical time‐drawdown data to yield valuable parameter estimation of the confining unit that would be otherwise difficult to quantify.
A Stream Depletion Field Experiment
Authors: Hunt Bruce , Weir Julian , Clausen Bente 
Published: December 2005

AbstractA field experiment was carried out to measure drawdowns in observation wells and stream depletion flows that occurred when water was abstracted from a well beside a stream. The field data is analyzed herein to determine the aquifer transmissivity, T, the aquifer storage coefficient, S, and a streambed leakage parameter, λ, by comparing measurements with a solution obtained by Hunt (1999). The analysis uses early time drawdowns with a match‐point method to determine T and S, and stream depletion measurements at later times are used to determine λ. The final results are reasonably consistent for measurements taken in four observation wells. The advantages and disadvantages of this approach are discussed, and two alternative ways of estimating λ are also discussed.
Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, Nevada
Authors: Painter S. , Cvetkovic V. , Turner D.R. 
Published: December 2005

AbstractThe U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high‐level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate‐limited mass transfer between mobile and immobile zones. The latter process is caused by small‐scale heterogeneity and is thought to control the macroscopic‐scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site‐specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations.
Coefficient of Permeability Determined By Measurable Parameters
Authors: Barr Douglas W. 
Published: December 2005

AbstractSelection of a coefficient of permeability or the hydraulic conductivity in practical situations has required a fair amount of judgment and the expectation of accuracy can range over an order of magnitude. A determination of the coefficient based on measurable characteristics would greatly reduce the judgment aspect. The coefficient of permeability of a porous media can be derived from the following measurable parameters: the density and viscosity of the permeating fluid, the porosity of the media, the average hydraulic radius of the pores, and the gravitation constant. The hydraulic radius is calculated from the grain size distribution analysis of the porous media assuming spherical particles and a factor to account for the shape of the particles. The shape factor ranges from 1.0 to 1.4 in the extreme but, for common porous media, ranges only from 1.0 to about 1.1. All of the variables, except the shape factor, are measurable by standard test procedures. An important advantage of using this procedure for determining permeability is that the factors can be visualized as part of the physical flow process. The procedure assumes laminar flow and applies equally to liquid or gas fluids.
Travel‐Time Based Model of Bioremediation Using Circulation Wells
Authors: Cirpka Olaf A. , Kitanidis Peter K. 
Published: December 2005

AbstractVertical circulation wells can efficiently provide microorganisms with substrates needed for enhanced bioremediation. We present a travel‐time based approach for modeling bioreactive transport in a flow field caused by a series of circulation wells. Mixing within the aquifer is due to the differences in sorption behavior of the reactants. Neglecting local dispersion, transport simplifies to a single one‐dimensional problem with constant coefficients for each well. Recirculation is characterized by the discharge densities over travel time. We apply the model to the stimulation of cometabolic dechlorination of trichloroethene (TCE) by alternate injection of oxygen and toluene into the circulation wells. Mixing within the wells can be minimized by interposing sufficiently long breaks between the oxygen and toluene pulses. In our simulation, the proposed injection scheme stimulates biomass growth without risking biofouling of the aquifer.
Numerical Simulation of a Natural Gradient Tracer Experiment for the Natural Attenuation Study: Flow and Physical Transport
Authors: Julian Hank E. , Boggs J. Mark , Zheng Chunmiao , Feehley C. Erin 
Published: December 2005

AbstractResults are presented for numerical simulations of ground water flow and physical transport associated with a natural gradient tracer experiment conducted within a heterogeneous alluvial aquifer of the Natural Attenuation Study (NATS) site near Columbus, Mississippi. A principal goal of NATS is to evaluate biogeochemical models that predict the rate and extent of natural biodegradation under field conditions. This paper describes the initial phase in the model evaluation process, i.e., calibration of flow and physical transport models that simulate conservative bromide tracer plume evolution during NATS. An initial large‐scale flow model (LSM) is developed encompassing the experimental site and surrounding region. This model is subsequently scaled down in telescopic fashion to an intermediate‐scale ground water flow model (ISM) covering the tracer‐monitoring network, followed by a small‐scale transport model (SSM) focused on the small region of hydrocarbon plume migration observed during NATS. The LSM uses inferred depositional features of the site in conjunction with hydraulic conductivity (K) data from aquifer tests and borehole flowmeter tests to establish large‐scale K and flow field trends in and around the experimental site. The subsequent ISM incorporates specified flux boundary conditions and large‐scale K trends obtained from the calibrated LSM, while preserving small‐scale K structure based on some 4000 flowmeter data for solute transport modeling. The configuration of the ISM‐predicted potentiometric surface approximates that of the observed surface within a root mean squared error of 0.15 m. The SSM is based on the dual‐domain mass‐transfer approach. Despite the well‐recognized difficulties in modeling solute transport in extremely heterogeneous media as found at the NATS site, the dual‐domain model adequately reproduced the observed bromide concentration distributions. Differences in observed and predicted bromide concentration distributions are attributed to aquifer heterogeneity at the decimeter (dm) and smaller scales. The calibrated transport parameters for the SSM (i.e., 1:7 for the ratio of mobile‐to‐total porosity; 2.5 × 10−3 day−1 for the mass‐transfer coefficient; 1 m for longitudinal dispersivity; and 0.1 m for transverse dispersiv‐ity) are consistent with separate numerical simulations of two earlier tracer experiments at the site. The multiscale modeling approach adopted in this study permits the incorporation of both large‐scale geologic features important for flow simulation and small‐scale heterogeneities critical for transport simulation. In addition, the dual‐domain transport model provides a foundation for multispecies reactive transport modeling studies of natural attenuation of hydrocarbons during NATS.