bluuebunny/crossref_metadata_2025 huggingface dataset:
Lead isotopes and the sources of the Columbia River Basalt Group
Authors: Chamberlain V. E. , Lambert R. St. J.
Published: February 2004

A detailed study of 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios suggests that the number of Pb isotopic reservoirs required for the Columbia River Basalt Group (CRBG) (as currently defined) must be increased from the presently accepted four to at least six. The identities of the six reservoirs are two of mid‐ocean ridge basalt (MORB) slightly contaminated with sediment (Rl and R2, Rl being “Cascades” Pb); one a probable plume component (R3); two of unspecified mantle material but probably of Rl type, contaminated with local crustal Pb (R4 and R5); and the sixth a complex enriched reservoir of 2150 Ma age (R6). Rl is an end‐member for all CRBG, except the Saddle Mountains Basalts. R2 is a second end‐member for the Picture Gorge Basalt and appears to be unique to it. Both Rl and R2 are related to the Juan de Fuca subduction zone and the back arc spreading associated with it. R3 is the principal source for most of the Wanapum Basalt, as well as for most of the chemically evolved portions of the Grande Ronde Basalt. Volumetric and tectonic considerations require that one source of the CRBG be a plume or, at least, a nonlocal crustal or uppermost mantle source, and R3 is the ideal candidate for that role. R4 is the second source for the Imnaha Basalt, and R5 is the source for the 206Pb rich varieties of the Grande Ronde Basalt. R4 and R5 are identified with contamination by local Phanerozoic crust because of their similarity to local crustal Pb and because the rocks which contain them also have Cu contents correlated with their Pb isotopic compositions. R6 is the parent for Pb in all the Saddle Mountain Basalts. This last source appears to have been homogeneous 2150 m.y. ago but has since split into a number of separate, discrete pockets, each with its own characteristic Pb isotopic signature. Some of these appear in individual Saddle Mountain Basalt flows, while other sources have mixed with R4 to produce individual Saddle Mountain Basalt flows of the Ice Harbor set. R6 is similar in all its isotopic properties to the Precambrian basement that is exposed to the immediate north of the Columbia River Basalt Group. The overall model for Columbia River Basalt Group genesis is termed “plume reinforced back arc magmatism”.
Atmospheric holes and small comets
Authors: Frank L. A. , Sigwarth J. B.
Published: February 2004

Global images of Earth's ultraviolet dayglow as gained with an imaging photometer on board Dynamics Explorer 1 exhibit transient decreases, or atmospheric holes, in the dayglow intensities over areas with diameters ∼50 km. Features of these atmospheric holes include (1) preferential motion in the east‐to‐west direction across the sunlit face of Earth, (2) similar diurnal variations in occurrence rates as those for radar meteors, (3) correlation of the occurrence rates with the nonshower rates as determined with forward scatter radar, and (4) larger angular diameters for these atmospheric holes when the spacecraft approaches Earth during its perigee passes. A significantly less extensive series of images of Earth's ultraviolet dayglow with the Viking spacecraft also provides evidence of these atmospheric holes. The atmospheric holes are interpreted in terms of obscuration of the dayglow by water clouds from the disruption and subsequent vaporization of small comets at low altitudes above the atmosphere. Supporting evidences for the existence of these small comets are given by telescopic sighting of these objects at greater altitudes before disruption and the detection of water bursts in Earth's upper atmosphere. We summarize the current status of this small‐comet hypothesis and its relationship to conventional wisdom concerning geophysical, lunar, and interplanetary phenomena.
The formation factor of reconstructed porous media
Authors: Adler Pierre M. , Jacquin Christian G. , Thovert Jean‐François
Published: February 2004

The porosity and the autocorrelation function of the pore space are measured on thin sections of Fontainebleau sandstones. This information is used to generate artificial porous media which share these statistical properties. The Laplace equation is numerically solved to determine the formation factor, or equivalently the electrical conductivity. With no adjustable constant, the predicted formation factors were found to be in acceptable agreement with the experimental ones.
Simulation of a two‐pollutant, two‐season pollution offset system for the Colorado River of Texas below Austin
Authors: Letson David
Published: February 2004

A pollution offset system is a discharge permit system in which transfers are made subject to a restriction that no violations of water quality standards occur at any location. Simulation of a pollution offset system with seasonal variation and multiple pollutants allows for comparison of the savings possible from these design features. A simulation model (Qual‐TX) developed by the Texas Water Commission is applied to a case study region near Austin, Texas, yielding impact coefficients for an economic optimization model without investment whose least cost solution represents the theoretical equilibrium of a pollution offset system. The optimization model finds short‐run savings of 17.5% for a pollution offset system, as compared to a command and control policy that would also achieve the dissolved oxygen standard. Seasonal variation in permit design produces minimal effects; virtually all savings come from allowing pollution offsets for the two different pollutants.
Mapping permeability in heterogeneous aquifers using hydrologic and seismic data
Authors: Rubin Yoram , Mavko Gary , Harris Jerry
Published: February 2004

A new method is presented for identification of the permeability distribution in near‐surface aquifers. In addition to using the usual sparsely sampled pressure and permeability data, the method incorporates densely sampled seismic data, as obtained from a reflection or tomography survey, along with empirical relationships between seismic and hydraulic properties. The procedure is to first estimate by hydrologic inversion a pressure field. Then the velocity‐permeability‐pressure relationship is used to map the inverted pressure and measured seismic data to multivalued estimates of the permeability. Of those, the most probable value, based on the hydrologic inversion, is selected. In synthetic case studies the tremendous increase in coverage offered by the seismic data leads to dramatically better results in terms of both accuracy and resolution. An appealing feature is the use of relatively easy to acquire pressure data; a second is the incorporation of geophysical data which can sample an entire aquifer remotely without the need for an expensive and invasive drilling program.
Usage of “Equivalent apertures” for rock fractures as derived from hydraulic and tracer tests
Authors: Tsang Y. W.
Published: February 2004

In the literature during the past several years there appear numerous references to the “equivalent aperture” of a rough‐walled rock fracture as derived from various hydraulic and tracer tests. However, the similar or even identical terms used by different researchers for “equivalent aperture” often do not have the same meaning. This has led to some confusion in the comparison of their results. In particular, there is a serious apparent contradiction in the claims of some authors that “equivalent apertures” derived from tracer tests are much larger than those derived from hydraulic tests, and the findings of others that apertures estimated from tracer tests are consistently smaller than those estimated from hydraulic tests. This apparent contradiction of the field results in fact arises from the different definitions of the so‐called “tracer aperture” as employed by different researchers. In this short technical note I have attempted to sort out the different definitions, denotations and usage of the various “equivalent apertures” and show that there are mainly three alternative definitions used in the literature. The meaning of each as related to experimental measurements is explained and their interrelationship discussed. It is shown that once the specific definition of “equivalent aperture” referred to by each researcher is identified, then the relative magnitudes of these “equivalent apertures” as reported by different groups of researchers are perfectly consistent with each other.
Suitability of different flow equations and hydraulic resistance parameters for flows in surface irrigation: A review
Authors: Maheshwari B. L.
Published: February 2004

A suitable equation for describing overland flow and the values for parameters of hydraulic resistance are required in surface irrigation models. The approaches to describe the flows and to estimate hydraulic resistance parameters in the past varied from simple empirical studies to complex semiempirical and theoretical studies. The Manning equation and Manning n are widely used for shallow flows in surface irrigation although their suitability for such flows is questionable. In this article the various flow equations are reviewed for their suitability to surface irrigation flows. It is revealed that there are only a few equations which are relevant to these flows. A need for a dependable method of estimating the hydraulic resistance parameters in surface irrigation is highlighted.
Analyses of mass transport in a nuclear waste repository in salt
Authors: Hwang Y. , Pigford T. H. , Chambré P. L. , Lee W. W.‐L.
Published: February 2004

Salt is the proposed host rock for geologic repositories of nuclear waste in several nations because it is “dry” and probably “impermeable”. To evaluate the safety of nuclear waste disposal in salt, it is necessary to calculate the rate of release of nuclides from solidified waste form and to predict the transport of released radionuclides. Our objective is to develop analytic methods for such prediction, based on mass transfer theory. First, we present an analysis of pressure‐driven brine migration. After consolidation, brine migration analyzed by these equations and using the particular set of parameter values is of a small magnitude, with Darcy velocities of the order of micrometers per year. Brine migration in consolidated salt is very localized, within a few meters from the waste package, and highly transient. We have analyzed diffusion of radionuclides near waste packages in a salt repository, using analytic equations for low Peclet systems. According to this analysis it is unlikely that any low‐solubility species will have difficulty meeting the U.S. Nuclear Regulatory Commission release rate requirement at the bare waste/salt interface. We have also analyzed the role that an interbed might play in being a preferential pathway for radionuclide migration. We calculate the diffusive mass fluxes from an infinitely long bare waste cylinder in salt, facing an interbed. The fractional release rates of solubility‐limited species into the interbed are low for the parameter values used in the numerical illustrations.
Distributions of fractional order statistics in hydrology
Authors: Durrans S. Rocky
Published: February 2004

A critical issue in parametric methods of frequency analysis, regardless of the phenomenon being modeled, is that of selection of a form of probability distribution to be applied. When one is interested in continuous distributions there exists little theoretical guidance, other than perhaps that provided by the central limit theorem or the (asymptotic) results of extreme value theory, upon which one may base a choice. This paper, in a very general way, introduces a whole new class of probability models which are referred to as distributions of fractional order statistics. The potential efficacies of various member distributions within the class for hydrologic data analysis are also rationalized in a very intuitive way. Considered in some detail is an application of the theory of fractional order statistics to generalize the Gaussian distribution. Monte Carlo results comparing the performance of the generalized distribution with other common hydrologic models are also set forth.
A critical study of the linear programming gradient method for optimal design of water supply networks
Authors: Bhave Pramod R. , Sonak Vilas V.
Published: February 2004

The linear programming gradient (LPG) method originally proposed by Alperovits and Shamir (1977) and the subsequent publications based on it are briefly reviewed. The optimal solutions obtained by different investigators for a simple two‐loop network, originally taken by Alperovits and Shamir as an illustrative example, are compared and it is shown that they are not even the local optimum solutions. The latest modification, introduced by Kessler and Shamir (1989), which incorporates the projected gradient of the objective function, is also briefly reviewed and is shown to lead to a truly local optimum solution as claimed by Kessler and Shamir. It is shown that it is preferable initially to suppress the minimum link flow constraint, necessary for reliability purposes, and introduce it later after obtaining the optimal solution for a branching configuration. The philosophy of the LPG method is analyzed. It is shown that the LPG method is inefficient, at least for optimization of the illustrative two‐loop network, as compared to a heuristic method that initially identifies logically good branching configurations for obtaining the optimal solution.
Solute transport with multisegment, equilibrium‐controlled, classical reactions: Problem solvability and feed forward method's applicability for complex segments of at most binary participants
Authors: Rubin Jacob
Published: February 2004

The feed forward (FF) method derives efficient operational equations for simulating transport of reacting solutes. It has been shown to be applicable in the presence of networks with any number of homogeneous and/or heterogeneous, classical reaction segments that consist of three, at most binary participants. Using a sequential (network type after network type) exploration approach and, independently, theoretical explanations, it is demonstrated for networks with classical reaction segments containing more than three, at most binary participants that if any one of such networks leads to a solvable transport problem then the FF method is applicable. Ways of helping to avoid networks that produce problem insolvability are developed and demonstrated. A previously suggested algebraic, matrix rank procedure has been adapted and augmented to serve as the main, easy‐to‐apply solvability test for already postulated networks. Four network conditions that often generate insolvability have been identified and studied. Their early detection during network formulation may help to avoid postulation of insolvable networks.
Simulation of dispersion in heterogeneous porous formations: Statistics, first‐order theories, convergence of computations
Authors: Bellin Alberto , Salandin Paolo , Rinaldo Andrea
Published: February 2004

This paper discusses the results of numerical analysis of dispersion of passive solutes in two‐dimensional heterogeneous porous formations. Statistics of flow and transport variables, the accuracy and the role of approximations implicit in existing first‐order theories, and the convergence of computational results are investigated. The results suggest that quite different rates of convergence with Monte Carlo runs hold for different spatial moments and that over 1000 realizations are required to stabilize second moments even for relatively mild heterogeneity (σY2 < 1.6). This has implications for the extent of the spatial domain for single‐realization numerical studies of the same type. A comparison of the variance of plumes with the results of linear theories (0.05 < σY2 < 1.6) shows an unexpectedly broad validity field for the theoretical solution obtained from a suitable linearization of flow and transport. Reformulation of the same problem linearizing in turn the flow or the transport equations shows opposite deviations from the linear theory. The interesting consequence is that the errors induced by linearizations in the flow or the transport equations have different signs, and their effects on the moments of dispersing plumes are compensating, thereby yielding consistent formulations. Unexpected features of the statistics of probability distributions of longitudinal and transverse velocities and travel times are also computed and discussed.
Demodulation‐remodulation revisited: Theory and application
Authors: Humphries John W. , Restrepo Pedro J. , Valdés Juan B.
Published: February 2004

In modeling periodic hydrologic series, the amplitudes and phase angles of the harmonics reflect the magnitudes and timing of the seasonal peaks. Demodulation‐remodulation captures the temporal variations in these stochastic parameters, thereby providing useful information and more accurate models than traditional Fourier series models. Unfortunately, this technique has been neglected by hydrologic modelers. In this paper, application of the technique to a synthetic series validates the theory. The technique is then applied to the streamflow record of the Uda Walawe River which exhibits a strong bimodal annual pattern. Fourteen models are developed, providing a comparison between moving average filters commonly used in demodulation studies, the Fourier filter used in Fourier series analysis, and composite models which incorporate both filters. In addition, the development of a prototype forecasting filter demonstrates the viability of using demodulation‐remodulation for forecasting.
A critical review of data on field‐scale dispersion in aquifers
Authors: Gelhar Lynn W. , Welty Claire , Rehfeldt Kenneth R.
Published: February 2004

A critical review of dispersivity observations from 59 different field sites was developed by compiling extensive tabulations of information on aquifer type, hydraulic properties, flow configuration, type of monitoring network, tracer, method of data interpretation, overall scale of observation and longitudinal, horizontal transverse and vertical transverse dispersivities from original sources. This information was then used to classify the dispersivity data into three reliability classes. Overall, the data indicate a trend of systematic increase of the longitudinal dispersivity with observation scale but the trend is much less clear when the reliability of the data is considered. The longitudinal dispersivities ranged from 10−2 to 104 m for scales ranging from 10−1 to 105 m, but the largest scale for high reliability data was only 250 m. When the data are classified according to porous versus fractured media there does not appear to be any significant difference between these aquifer types. At a given scale, the longitudinal dispersivity values are found to range over 2–3 orders of magnitude and the higher reliability data tend to fall in the lower portion of this range. It is not appropriate to represent the longitudinal dispersivity data by a single universal line. The variations in dispersivity reflect the influence of differing degrees of aquifer heterogeneity at different sites. The data on transverse dispersivities are more limited but clearly indicate that vertical transverse dispersivities are typically an order of magnitude smaller than horizontal transverse dispersivities. Reanalyses of data from several of the field sites show that improved interpretations most often lead to smaller dispersivities. Overall, it is concluded that longitudinal dispersivities in the lower part of the indicated range are more likely to be realistic for field applications.
Hillslope infiltration and lateral downslope unsaturated flow
Authors: Jackson C. Rhett
Published: February 2004

Lateral downslope vadose flow occurs in hillslopes with deep homogeneous soils. When and why this happens has been partially answered by past experimental, numerical, and quasi‐analytical studies. What has been overlooked, however, is that lateral downslope flow in unlayered soils is largely a drainage phenomenon driven by the changing character of the boundary condition at the soil surface. Numerical simulations show that when rainfall ceases, the near‐surface head gradient in a hillslope becomes nearly parallel to the slope as a simple consequence of the surfaces becoming a no‐flow boundary, neglecting evaporation. During rainfall, infiltration is nearly vertical for most soils except those with high constant anisotropy.
A stochastic inverse solution for transient groundwater flow: Parameter identification and reliability analysis
Authors: Sun Ne‐Zheng , Yeh William W‐G.
Published: February 2004

In this paper, a stochastic approach is developed for solving the inverse problem of parameter identification for transient groundwater flow. Adjoint state equations are derived for the stochastic partial differential equation (SPDE) relating transient head and log hydraulic conductivity perturbations. The derived equations can be used to calculate the covariance matrix of head observations and the cross‐covariance matrix between head observations and log hydraulic conductivity measurements for each observation time, as well as the covariance and cross‐covariance matrices between different observation times. The number of simulation runs required for obtaining these matrices is equal to the number of head observations. The reliability of model prediction is evaluated through the variance estimate method using adjoint sensitivity analysis and the cokriging estimate. The reliability evaluation of model prediction can be used as an aid in the optimization of experimental design for sampling strategies. A numerical example is given to illustrate the stochastic inverse procedure for a general transient flow problem. The example shows that using head observations of all observation times simultaneously produces much better results than using them sequentially (commonly known as the quasi‐steady approach). The example also shows that it is possible to obtain a reliable stochastic inverse solution for meeting a given prediction objective based on a limited number of observations.
Minimum energy and fractal structures of drainage networks
Authors: Rinaldo Andrea , Rodriguez‐Iturbe Ignacio , Rigon Riccardo , Bras Rafael L. , Ijjasz‐Vasquez Ede , Marani Alessandro
Published: February 2004

This paper explores the similarities of digital elevation maps (DEMs) of natural river basins and optimal channel network (OCN) configurations obtained minimizing the total rate of energy expenditure in the system as a whole and in its parts. Striking similarities are observed for natural and optimal networks in their fractal aggregation structure and in certain multifractal structures found to be characteristic of river basins. Our results suggest, upon critical assessment of the reliability of the identification of the attractor of the underlying dynamics implied by our optimality concepts, that fractal structures are indeed possibly a product of least energy dissipation. Power laws emerging in the description of the distribution of aggregated quantities from both DEMs and OCNs suggest a link with the framework of self‐organized criticality in the dynamics of natural channel network formation. Also, the geomorphological description of OCNs reveals surprising analogies with well‐known empirical or experimental results, A comparison of Peano's basins with OCNs suggests that nature seems to reject the type of strict self‐similarity exhibited by Peano's construct in favor of different shapes implying statistical self‐similarity not only because of chance acting through random conditions but also because of necessity as reflected by least energy expenditure considerations.
On the stream function for variable‐density groundwater flow
Authors: Evans David G. , Raffensperger Jeffrey P.
Published: February 2004

When the stream function is used to describe variable‐density ground water flow, it should be defined in terms of mass flux. The historic use of the stream function describes volume flux and implicitly invokes the Boussinesq assumption. A comparison of the flow equations for mass‐based and volume‐based stream functions indicates that this assumption can lead to significant errors in the calculated flow field, especially when flow parallels large density gradients.
Determination of gas exchange rate constants for a small stream on Walker Branch Watershed, Tennessee
Authors: Genereux David P. , Hemond Harold F.
Published: February 2004

The steady state tracer gas method was used to determine gas exchange rate constants (k) for a small stream (annual average flow ≤1 m3/min) draining the West Fork of Walker Branch watershed in eastern Tennessee. Chloride was used as a conservative tracer to account for dilution by lateral inflow, and propane and ethane were used as volatile tracers. Gas exchange rate constants for propane (kp) were about 100 d−1 over a wide range of flow conditions, while those for ethane (ke) were about 117 d1; an equivalent rate constant for O2 (KO2) would be about 118–139 d−1, depending on the method used for its calculation. These rate constants are much larger than those typically found in rivers and large streams. Much lower kp values (about 50d−1) were found during one experiment conducted at low flow with much of the stream surface covered with floating leaves. Nineteen previously published empirical equations were used to predict kO2 values for one 72m stream reach; agreement between the predicted and measured values was generally very poor, underscoring the importance of field‐measured gas exchange rates in studies of the transport and fate of volatile compounds. Because ethane and propane have similar gas exchange rates and similar aqueous diffusion coefficients (ke/kp and De/Dp are both close to 1, where D is the compound's diffusion coefficient), accurate determination of the exponent n in the relationship ke/Kp− (De/Dp)n was not possible. The ratio ke/kp (1.17) is much closer to De/Dp (1.24) than to He/Hp (0.82, where H is the compound's Henry's law constant), suggesting that stripping of dissolved volatiles by air bubbles was not a dominant mode of gas exchange for the study stream.
Use of chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age‐dating tools: The alluvium and terrace system of central Oklahoma
Authors: Busenberg Eurybiades , Plummer L. Niel
Published: February 2004

The use of Chlorofluorocarbons (CFCs) as an age‐dating tool and tracer in shallow groundwaters has been investigated. New methodology for field sampling and preserving groundwaters containing parts per trillion concentrations of the CFCs, F‐1l and F‐12, is presented. Samples are analyzed by purge‐and‐trap gas chromatography with an electron capture detector. Physical and chemical processes that can alter natural concentrations (air‐water equilibrium) of CFCs were investigated to assess dating uncertainties. CFC model recharge ages appear to be defined within 2 years under optimum conditions. The method was applied to central Oklahoma to demonstrate the usefulness of CFCs as (1) an age‐dating tool of shallow groundwaters, (2) a tracer of sewage effluent in surface and shallow groundwaters, and (3) a tracer of shallow groundwater. Results of dating indicate two primary recharge periods in central Oklahoma over the past 45 years that correspond to the wet periods 1945–1960 and 1967–1975.
Cumulative storage of water under constant flux infiltration: Analytical solution
Authors: Parkin Gary W. , Elrick David E. , Kachanoski R. Gary
Published: February 2004

Recently, two exact analytical solutions of Richards' flow equation in Fokker‐Planck form using empirical nonlinear diffusivity and hydraulic conductivity functions were published. A direct application of the solutions is the modeling of cumulative water storage in a fixed depth under constant rainfall infiltration. Analytical solutions for storage require a change of variable of integration and a numerical inversion technique. The solutions contain three independent parameters including saturated hydraulic conductivity, the α parameter, and a parameter which defines the shape of the wetting front. Sensitivity analyses indicate that changes in parameter values give significant changes in cumulative storage curves. Cumulative storage can be measured by vertical parallel time domain reflectometry probes which are capable of measuring an evolving volumetric moisture content within a fixed depth. A nonlinear least squares fitting procedure may be used to evaluate the three parameters.
Estimating paleorecharge and paleoclimate from unsaturated zone profiles
Authors: Cook P. G. , Edmunds W. M. , Gaye C. B.
Published: February 2004

Unsaturated zone solute and isotope profiles may be interpreted in terms of past changes in recharge rate in cases where water movement can be approximated by piston flow. A model is presented which describes water movement in terms of piston flow below a surface mixing layer. Below the mixing layer, diffusion and dispersion occur in the liquid phase in the case of solutes, and in both the liquid and gas phases in the case of isotopically labeled water. Climatic events of 4–5 years duration may be preserved in the unsaturated zone for more than 50 years if the recharge rate is greater than approximately 20 mm yr−1. Longer‐term fluctuations in climatic conditions (century scale) might be preserved for over 1000 years, provided the recharge rate is greater than approximately 2 mm yr−1, and the unsaturated zone is sufficiently deep. Profiles from Cyprus and northern Senegal appear to record changes in recharge rate over periods up to 400 years. The profiles are well correlated with known variations in rainfall, and fluctuations in lake levels. The model provides limiting conditions to be applied to the interpretation of profiles.
Short‐pulse radar detection of groundwater in the Sagavanirktok River Floodplain in early spring
Authors: Arcone Steven A. , Chacho Edward F. , Delaney Allan J.
Published: February 2004

Short‐pulse radar operating in the UHF band was used to search for unfrozen water beneath ice blisters and within the gravels of the Sagavanirktok River floodplain near Prudhoe Bay, Alaska. The investigations were carried out in early April 1991, when daily mean air temperatures still were below −20°C. The radar pulse spectrum was centered near 400 MHz and the radar time range was set to cover about 10 m depth. The flat, snow‐covered surface allowed good antenna‐ground coupling, and the ice and gravels provided a low‐loss propagation medium that allowed detail of the gravel structure to be seen in the data. Grids of several traverses were profiled at three sites, all within an area of about 0.5 km2. Unfrozen water was inferred from the strength and polarization of the radar reflections. One site contained a large icing blister beneath which an extensive reservoir was mapped and seen to extend several tens of meters beyond the mound. Data from two other sites indicated taliks 5–7 m deep near a frozen mound and within the gravels. One of the profiles extended 700 m across the floodplain and revealed other high‐reflectivity targets, which were presumed to be water, within about 7 m of the surface; a deeper sounding confirmed this depth limit. It is concluded that taliks exist in the floodplain throughout the winter and are a probable source for the numerous icing mounds seen along most Arctic rivers east of Prudhoe Bay.
A helicopter‐borne electromagnetic survey to delineate groundwater recharge rates
Authors: Cook P. G. , Kilty S.
Published: February 2004

Groundwater recharge is one of the most difficult components of the water balance to measure. For this reason, electromagnetic methods have been used to infer its variability from measurements of apparent electrical conductivity. In this study, groundwater recharge was estimated at 20 sites using unsaturated zone chloride methods. Interpolation between drill sites was accomplished with the aid of a helicopter‐borne electromagnetic survey (DIGHEMIV). Correlations between recharge and apparent electrical conductivity were only significant (R2 = 65%) at the highest frequency (56,000 Hz). Using these single‐frequency data, variations in recharge were mapped over an area of 32 km2. Recharge, as inferred from the electromagnetic data, appears to be lognormally distributed, and varies from less than 1 to more than 50 mm yr−1. Within the study region, spatially averaged recharge can be estimated from the electromagnetic data, with an accuracy of approximately −60%, +140% (90% probability). This is comparable to the estimation accuracy when surface electromagnetic methods are used. Aerial electromagnetic methods appear very useful for identifying areas of high and low recharge over large regions.
Transient free‐surface groundwater flow due to an array of circular drainage ditches
Authors: Gjerde Kjell M. , Tyvand Peder A.
Published: February 2004

The present paper is a numerical study of a transient free‐surface flow into an array of circular drainage ditches in a porous medium. By the boundary integral equation method and a semi‐Lagrangian description of motion, the groundwater table is tracked as a function of time. Primarily, we consider the case of atmospheric pressure along the ditch contour. An analytical small‐time expansion is presented in an appendix. In contrast to the case of constant flux into a sink (Papatzacos, 1989), there is no cusp formation at the free surface. The flux into a ditch is approximately a linearly decreasing function of time. We also include a second version of the boundary condition at the ditch; parts of its contour are impermeable.
An exact peak capturing and Oscillation‐Free Scheme to solve advection‐dispersion transport equations
Authors: Yeh Gour‐Tsyh , Chang Jing‐Ru , Short Thomas E.
Published: February 2004

An exact peak capturing and essentially oscillation‐free (EPCOF) algorithm, consisting of advection‐dispersion decoupling, backward method of characteristics, forward node tracking, and adaptive local grid refinement, is developed to solve transport equations. This algorithm represents a refinement of LEZOOM, developed earlier by the senior author. In LEZOOM, a predetermined number of evenly spaced, hidden nodes was zoomed for a sharp front element, while in the EPCOF scheme, a subset of forwardly tracked nodes is zoomed. The number and location of this subset were automated. As a result, the peaks and valleys are captured exactly; and the ancillary problems of spurious oscillation, numerical dispersion, and phase errors are alleviated. Means of checking accumulated mass balance errors are provided. Application of the algorithm to two one‐dimensional benchmark problems under a variety of conditions indicated that it completely eliminated peak clipping, spurious oscillation, phase error, and numerical dispersion. It yielded identical results, within the error tolerance, to exact solutions for all 19 test cases. Accumulated mass balance errors are extremely small for all 19 cases. The EPCOF scheme could solve the advective transport problems exactly, within any prescribed error tolerance, using mesh Peclet numbers ranging from 0 to infinity and very large mesh Courant numbers. The size of mesh Courant number is limited only by the accuracy requirement of the dispersion solver. Extension of this approach to multidimensional problems does not pose any conceptual difficulty and should alleviate the grid orientation trouble associated with such problems.
Field study of dispersion in a heterogeneous aquifer: 1. Overview and site description
Authors: Boggs J. Mark , Young Steven C. , Beard Lisa M. , Gelhar Lynn W. , Rehfeldt Kenneth R. , Adams E. Eric
Published: February 2004

Results are presented for a large‐scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near Columbus, Mississippi. The study was initiated with a 48‐hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers (pentaflourobenzoic acid, o‐trifluoromethylbenzoic acid, and 2,6‐diflourobenzoic acid). Over a 20‐month period, seven comprehensive samplings of the tracer plume were performed at approximately 1‐ to 4‐month intervals using an extensive three‐dimensional sampling well network. The dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction. This asymmetry was produced by accelerating groundwater flow along the plume travel path that, in turn, resulted from an approximate 2‐order‐of‐magnitude increase in the mean hydraulic conductivity between the near‐field and far‐field regions of the site. The Columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer, the large‐scale spatial variations in groundwater velocity, and the extensive set of hydraulic conductivity measurements for the aquifer.
Field study of dispersion in a heterogeneous aquifer: 3. Geostatistical analysis of hydraulic conductivity
Authors: Rehfeldt Kenneth R. , Boggs J. Mark , Gelhar Lynn W.
Published: February 2004

Observations of the spatial variability of hydraulic conductivity at a tracer test site, located at Columbus Air Force Base in Mississippi, are presented. Direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging, slug tests, and a laboratory permeameter to test undisturbed soil cores. Indirect methods estimating hydraulic conductivity were also evaluated, including soil grain size analyses, surface geophysical surveys, and mapping of sediment facies. The spatial covariance of the 2187 hydraulic conductivity values obtained with the borehole flowmeter method was examined. The log hydraulic conductivity variance (σln2K) and the horizontal and vertical correlation scales (λh and λv) of 4.5, 12.8 m, and 1.6 m, respectively, were estimated assuming second‐order stationarity of the conductivity field. The covariance parameters are uncertain with bounding values that are 24–76% above or below the estimate. Covariance parameters estimated with more limited nonflowmeter data were within the same magnitude as those obtained using the extensive flowmeter data, suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters. Nonstationarity of the ln K field was examined by removing three‐dimensional polynomial trend surfaces and calculating variograms of the residuals. Significantly lower estimates for σln2K, λh and λv of 2.7, 4.8 m, and 0.8 m, respectively, were obtained from the third‐order log conductivity residuals. After trend removal, the bounding parameter values differ 15–44% from the estimated values. Accounting for unsteady flow and the uncertainty in the covariance parameters of the third‐order log conductivity residuals, the calculated longitudinal and horizontal transverse macrodispersivities ranged from 1.5 m to 1.8 m and 0.3 m to 0.6 m, respectively.
The role of groundwater transport in aquatic mercury cycling
Authors: Krabbenhoft David P. , Babiarz Christopher L.
Published: February 2004

Mercury, which is transported globally by atmospheric pathways to remote aquatic environments, is a ubiquitous contaminant at very low (nanograms Hg per liter) aqueous concentrations. Until recently, however, analytical and sampling techniques were not available for freshwater systems to quantify the actual levels of mercury concentrations without introducing significant contamination artifacts. Four different sampling strategies were used to evaluate ground water flow as a mercury source and transport mechanism within aquatic systems. The sampling strategies employ ultraclean techniques to determine mercury concentrations in groundwater and pore water near Pallette Lake, Wisconsin. Ambient groundwater concentrations are about 2–4 ng Hg L−1, whereas pore waters near the sediment/water interface average about 12 ng Hg L−1, emphasizing the importance of biogeochemical processes near the interface. Overall, the groundwater system removes about twice as much mercury (1.5 g yr−1) as it contributes (0.7 g yr−1) to Pallette Lake. About three fourths of the groundwater mercury load is recycled, thought to be derived from the water column.
Uptake of inorganic carbon and nitrate by marine plankton and the Redfield Ratio
Authors: Banse K.
Published: February 2004

Published experiments are reevaluated regarding the uptake ratio of dissolved inorganic carbon (DIC) and nitrate in two plankton communities and the resulting elemental ratio of particulate organic matter (POM). The ratios are lower than Redfield values of 6.6 or 7.6 (by atoms); the uptake ratios of dissolved moieties are not measures for the composition of the newly formed POM; and uptake of DIC and nitrate may be entirely uncoupled. The use of closed systems of several 100 L to 1 m−3 content is suggested for studies of the underlying mechanisms.
Dissolution kinetics of calcium carbonate in equatorial Pacific sediments
Authors: Berelson William M. , Hammond Douglas E. , McManus James , Kilgore Tammy E.
Published: February 2004

Benthic chambers were deployed in the equatorial eastern Pacific Ocean on a transect along the equator between 103°W and 140°W and on a transect across the equator at 140°W in order to establish the rate of calcium carbonate dissolution on the seafloor. Dissolution was determined from the rate of alkalinity increase within an incubation chamber, measured over an 80–120 hour incubation period. Dissolution rates were lowest at eastern Pacific sites (0.2‐0.4 mmol CaCO3/m2/d) and highest at the equatorial, 140°W sites (0.5‐0.7 mmol/m2/d). Both oxygen consumption rates and the degree of bottom water saturation govern dissolution rates. Measured dissolution and oxygen consumption rates are used with a numerical model to constrain the value of the dissolution rate constant k, formulated according to the equation developed by Keir [1980]: dissolution rate = kγ(1‐Ω)n. The observed dissolution fluxes are predicted by the model when k = 5 to 100%/d and n = 4.5. This range of k values has important implications regarding the type of carbonate dissolving and its location within the sediment column. At low values of k, organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution. At higher values of k, the degree of bottom water undersaturation becomes more important. Dissolution of carbonate within equatorial Pacific sediments can be adequately described with k = 20 ± 10%/d, a rate constant much lower than some previously used values. Dissolution rates do not vary significantly over chamber boundary layer thicknesses between 200 and 800 μm, indicating that dissolution is not controlled by hydrodynamic conditions. Chambers acidified with HCl yield very large dissolution rates, but for a given degree of acidification the dissolution rate was constant for sites ranging from water depths of 3300–4400 m. This implies that there are not more and less easily dissolved forms of CaCO3 arriving on the seafloor between these depths. A budget for alkalinity in the deep Pacific, predicted by the dissolution model and based on the assumption that all carbonate dissolution takes place in the sediments, is within 85% of the input required by a published box model alkalinity budget based on the distribution of nutrients and the water mass transport rates.
Effect of altitude on the carbon‐isotope composition of forest and grassland soils from Papua New Guinea
Authors: Bird M. I. , Haberle S. G. , Chivas A. R.
Published: February 2004

The carbon‐isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3 vegetation (forests and alpine grasslands above 4000 m) show an increase in δ13C value with altitude paralleling the increase in δ13C value observed in plant leaves by Körner et al. [1988]. Grassland soils show a decrease in δ13C value above about 1000 m, from maximum values which are close to pure C4 values (−12 to −13‰ vs. PDB) to minimum values which are indistinguishable from pure C3 values at 3500–4000 m (∼−26‰). Within this general framework, several factors can influence the soil δ13C value at individual locations. In local forest settings, soil δ13C values will be influenced by the degree to which respired CO2 is re‐utilized during photosynthesis, the proportions of leaf and wood litter, and the degree of decomposition. In grassland settings the primary factor controlling the observed δ13C variability at any specific altitude is the amount of nongrass C3 carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3 and C4 grasses at any given altitude, although further work would be required to substantiate such a link. The results provide a framework within which to more accurately constrain the carbon‐isotope composition of terrestrial carbon pools and to interpret variations in the isotopic composition of riverine particulate organic carbon.
Inductive source EM sounding of the Sudbury Structure
Authors: Boerner D. E. , Kellett R. , Mareschal M.
Published: February 2004

A controlled source electromagnetic survey conducted in the South Range of the Sudbury Igneous Complex has revealed a south dipping conductive zone at depths of three to five kilometers. The zone has a conductance of about seven siemens whereas the overlying rocks total less than one siemens, indicating the feature is well resolved by the data. The anomalous conductivity may be derived from graphitization of the carbonaceous Black member of the Onaping Formation. Presence of a graphitic conductor is compatible with a model of tight folding or thrusting of the southern portion of the original Sudbury structure over the northern part along a basal plane defined in the Onaping Formation. Graphite could provide the mechanical lubrication to decouple the North Range from the tectonic deformation of the South Range.
ozone loss in the lower stratosphere over the United States in 1992–1993: Evidence for heterogeneous chemistry on the Pinatubo aerosol
Authors: Hofmann D. J. , Oltmans S. J. , Komhyr W. D. , Harris J. M. , Lathrop J. A. , Langford A. O. , Deshler T. , Johnson B. J. , Torres A. , Matthews W. A.
Published: February 2004

Ozone profiles obtained at Boulder, Colorado and Wallops Island, Virginia indicate that ozone was about 25% below normal during the winter and spring of 1992–93 in the 12–22 km region. This large ozone reduction in the lower stratosphere, though sometimes partially compensated by higher than normal ozone above 24 km, was responsible for the low total column ozone values observed across the United States during this period. Normal temperatures throughout the low ozone region suggest that transport‐related effects are probably not the most important cause of the ozone deficits. The region of low ozone at Boulder corresponds closely with the location of the enhanced H2SO4/H2O aerosol from the Pinatubo eruption of 1991 as measured near Boulder and at Laramie, Wyoming. Trajectory analyses suggest that except at low altitudes in spring, air parcels on the days of the ozone measurements generally arrived at Boulder from higher latitude, although seldom higher than 60°N, and hence may have been subjected to heterogeneous chemical processing on the surface of Pinatubo aerosol droplets resulting in chlorine‐catalyzed ozone destruction, a process which is believed to be more effective under the lower winter temperatures and sunlight levels of higher latitudes.