bluuebunny/crossref_metadata_2025 huggingface dataset:
Endogenous Lobby Formation and Endogenous Protection: A Long-Run Model of Trade Policy Determination
Authors: Mitra Devashish
Published: July 2010

This paper provides a theory of lobby formation within a framework in which trade policy is determined through political contributions. Under certain conditions, free trade turns out to be an equilibrium outcome either when the government has a high affinity for political contributions or when it cares a great deal about social welfare. Moreover, greater inequality in asset distribution results in a greater number of lobbies and, in most cases, more protection for each of these lobbies. Furthermore, industries with higher levels of capital stock, fewer capitalists, more inelastic demand, and smaller geographical dispersion are the ones that get organized. (JEL F10, F13)
On the Driving Forces Behind Cyclical Movements in Employment and Job Reallocation
Authors: Davis Steven J , Haltiwanger John
Published: July 2010

Theory restricts short-run job creation and destruction responses and cumulative employment and job reallocation responses to allocative and aggregate shocks. We formulate these restrictions and implement them for postwar data on U.S. manufacturing. Allocative shocks are the main driving force behind cyclical movements in job reallocation, but their contribution to employment fluctuations varies greatly across alternative identification assumptions. Also, the data compel one or both of the following inferences: aggregate shocks greatly alter the shape and not just the mean of the cross-sectional density of employment growth rates; allocative shocks cause short-run reductions in aggregate employment. (JEL C32, E32, J63)
Labor-Market Integration, Investment in Risky Human Capital, and Fiscal Competition
Authors: Wildasin David E
Published: July 2010

This paper presents a general-equilibrium model where human capital investment increases specialization and exposes skilled workers to region-specific earnings risk. Interjurisdictional mobility of skilled labor mitigates these risks; state-contingent migration of skilled labor also improves efficiency. With perfect capital markets, labor-market integration raises welfare and reduces ex post earnings inequality. If instead human capital investment can only be financed through local taxes, labor-market integration leads to interjurisdictional fiscal competition, shifting the burden of taxation to low-skilled immobile workers. Decentralized public provision of human capital investment creates earnings inequalities and is inefficient. (JEL H00)
Habit Formation in Consumption and Its Implications for Monetary-Policy Models
Authors: Fuhrer Jeffrey C
Published: July 2010

This paper explores a monetary-policy model with habit formation for consumers, in which consumers' utility depends in part on current consumption relative to past consumption. The empirical tests developed in the paper show that one can reject the hypothesis of no habit formation with tremendous confidence, largely because the habit-formation model captures the gradual hump-shaped response of real spending to various shocks. The paper then embeds the habit-consumption specification in a monetary-policy model and finds that the responses of both spending and inflation to monetary-policy actions are significantly improved by this modification. (JEL D12, E52, E43)
Habit Formation in Consumer Preferences: Evidence from Panel Data
Authors: Dynan Karen E
Published: July 2010

This paper tests for the presence of habit formation using household data. A simple model of habit formation implies a condition relating the strength of habits to the evolution of consumption over time. When the condition is estimated with food consumption data from the Panel Study on Income Dynamics (PSID), the results yield no evidence of habit formation at the annual frequency. This finding is robust to a number of changes in the specification. It also holds for several proxies for nondurables and services consumption created by combining PSID variables with weights estimated from Consumer Expenditure Survey data. (JEL D12, D91, E21)
What Inventory Behavior Tells Us About Business Cycles
Authors: Bils Mark , Kahn James A
Published: July 2010

The countercyclical pattern of inventory-sales ratios is a striking feature of inventory behavior. In a model where inventories are productive for sales, both the markup of price over marginal cost and expected changes in marginal cost are key determinants of that ratio. This paper argues that costly variation in factor utilization gives rise to countercyclical markups in production-to-stock manufacturing industries. The markup turns out to be more important than intertemporal substitution in explaining the behavior of inventory-sales ratios. (JEL E22, E32)
Job Destruction and Propagation of Shocks
Authors: den Haan Wouter J , Ramey Garey , Watson Joel
Published: July 2010

This paper considers propagation of aggregate shocks in a dynamic general-equilibrium model with labor-market matching and endogenous job destruction. Cyclical fluctuations in the job-destruction rate magnify the output effects of shocks, as well as making them much more persistent. Interactions between capital adjustment and the job-destruction rate play an important role in generating persistence. Propagation effects are shown to be quantitatively substantial when the model is calibrated using job-flow data. Incorporating costly capital adjustment leads to significantly greater propagation. (JEL E24, E32)
Wage Shocks and North American Labor-Market Integration
Authors: Robertson Raymond
Published: July 2010

This study uses household-level data from the United States and Mexico to examine labor-market integration. I consider how the effects of shocks and rates of convergence to an equilibrium differential are affected by borders, geography, and demographics. I find that even though a large wage differential exists between them, the labor markets of the United States and Mexico are closely integrated. Mexico's border region is more integrated with the United States than is the Mexican interior. Evidence of integration precedes the North American Free Trade Agreement (NAFTA) and may be largely the result of migration. (JEL F15, F20, J61)
A Reassessment of the Relationship Between Inequality and Growth
Authors: Forbes Kristin J
Published: July 2010

This paper challenges the current belief that income inequality has a negative relationship with economic growth. It uses an improved data set on income inequality which not only reduces measurement error, but also allows estimation via a panel technique. Panel estimation makes it possible to control for time-invariant country-specific effects, therefore eliminating a potential source of omitted-variable bias. Results suggest that in the short and medium term, an increase in a country's level of income inequality has a significant positive relationship with subsequent economic growth. This relationship is highly robust across samples, variable definitions, and model specifications. (JEL O40, O15, E25)
Intelligence, Social Mobility, and Growth
Authors: Hassler John , Mora José V. Rodríguez
Published: July 2010

We develop a model where the allocation of human resources, intergenerational social mobility, and technological growth are jointly determined. High growth endogenously increases the equilibrium return to innate cognitive ability and makes the allocation of individuals depend more on innate ability and less on social background. Individuals with a higher level of innate cognitive ability can deal better with less known, but more productive, technologies and thus choose a higher rate of technological growth. A social allocation based on innate ability and high growth will thus reinforce each other, implying the possibility of multiple endogenous growth equilibria. (JEL J62, O1)
Does Exchange-Rate Stability Increase Trade and Welfare?
Authors: Bacchetta Philippe , van Wincoop Eric
Published: July 2010

This paper develops a simple general-equilibrium framework to study the effect of the exchange-rate system on trade and welfare. An important feature of the model is deviations from purchasing-power parity, caused by rigid price setting in buyers' currency. In a benchmark model with separable preferences and only monetary shocks, trade is unaffected by the exchange-rate system, consistent with most evidence. In general, both trade and welfare can be higher under either exchange-rate system, depending on preferences and on the monetary-policy rules followed under each system. There is no one-to-one relationship between the levels of trade and welfare across exchange-rate systems. (JEL F31, F33, F41)
Monetary Aggregates and Output
Authors: Freeman Scott , Kydland Finn E
Published: July 2010

We ask whether the following observations may result from endogenously determined fluctuations in the money multiplier rather than a causal influence of money on output: (i) M1 is positively correlated with real output; (ii) the money multiplier and deposit-to-currency ratio are positively correlated with output; (iii) the price level is negatively correlated with output; (iv) the correlation of M1 with contemporaneous prices is substantially weaker than the correlation of M1 with real output; (v) correlations among real variables are essentially unchanged under different monetary-policy regimes; and (vi) real money balances are smoother than money-demand equations would predict. (JEL E300, E510)
Does Competition Among Public Schools Benefit Students and Taxpayers?
Authors: Hoxby Caroline M
Published: July 2010

Tiebout choice among districts is the most powerful market force in American public education. Naive estimates of its effects are biased by endogenous district formation. I derive instruments from the natural boundaries in a metropolitan area. My results suggest that metropolitan areas with greater Tiebout choice have more productive public schools and less private schooling. Little of the effect of Tiebout choice works through its effect on household sorting. This finding may be explained by another finding: students are equally segregated by school in metropolitan areas with greater and lesser degrees of Tiebout choice among districts. (JEL H70, I20)
The Determinants of Equilibrium Unemployment
Authors: Yashiv Eran
Published: July 2010

The paper takes the search and matching model of the aggregate labor market to the data. It tests the model's empirical validity and employs structural estimation to generate a characterization of the optimal behavior of firms and workers. The model is applied to Israeli data that are uniquely suited for this kind of empirical investigation. The structural estimates are used to quantify the frictions embodied in the model, including the costs of search, the congestion and trading externality effects, and the matching process. A calibration-simulation analysis then studies the effect of several key variables on equilibrium unemployment. (JEL E24, E32, J63, J64)
Aggregate Employment Fluctuations with Microeconomic Asymmetries
Authors: Campbell Jeffrey R , Fisher Jonas D. M
Published: July 2010

We provide a simple explanation for the observation from the U.S. manufacturing sector that the job destruction rate fluctuates more than the job creation rate. In our model, proportional plant-level costs of creating and destroying jobs cause shrinking plants to be more sensitive to aggregate shocks than growing plants. We describe circumstances in which this microeconomic asymmetry is preserved in the aggregate and show that it can account for much of the observed asymmetries in gross job flows. This is so even though we abstract from job matching frictions, incomplete contracts, and aggregate congestion effects. (JEL E00, L00, J00)
Preventing Crime Waves
Authors: Bond Philip , Hagerty Kathleen
Published: July 2010

We study the design of enforcement mechanisms when enforcement resources are chosen ex ante and are inelastic ex post. Multiple equilibria arise naturally. We identify a new answer to the old question of why non-maximal penalties are used to punish moderate actions: “marginal” penalties are much more attractive in the Pareto inferior crime wave equilibrium. Specifically, although marginal penalties have both costs and benefits, the net benefit is strictly positive in the crime wave equilibrium. In contrast, marginal penalties frequently have a net cost in the noncrime wave equilibrium. We also show that increasing enforcement resources may worsen crime. (JEL D82, K42)
Ascending Prices and Package Bidding: A Theoretical and Experimental Analysis
Authors: Kagel John H , Lien Yuanchuan , Milgrom Paul
Published: July 2010

We use theory and experiment to explore the performance of multi-round, price-guided, combinatorial auctions. We define efficiency-relevant and core-relevant packages and show that if bidders bid aggressively on these and losing bidders bid to their limits, then the auction leads to efficient or core allocations. We study the theoretically relevant behaviors and hypothesize that subjects will make only a few significant bids, and that certain simulations with auto-bidders will predict variations in performance across different environments. Testing the combinatorial clock auction (CCA) design, we find experimental support for these two hypotheses. We also compare the CCA to a simultaneous ascending auction. (JEL D44)
Why Tie a Product Consumers Do Not Use?
Authors: Carlton Dennis W , Gans Joshua S , Waldman Michael
Published: July 2010

We provide an explanation for tying not based on any of the standard arguments: efficiency, price discrimination, or exclusion. In our analysis a monopolist ties a complementary good to its monopolized good, but consumers do not use the tied good. The tie is profitable because it shifts profits from a complementary good rival to the monopolist. We show such tying is socially inefficient, but arises only when the tie is socially efficient in the absence of the rival. We relate this form of tying to several examples, discuss how it can also arise under competition, and explore its antitrust implications. (JEL D42, K21, L12, L25, L40)
FPGA Fault Tolerance in Particle Physics Experiments
Authors: Gebelein Jano , Engel Heiko , Kebschull Udo
Published: July 2010

The behavior of matter in extreme physical conditions is in focus of many high-energy-physics experiments. For this purpose, high energy charged particles (ions) are collided with each other and energy- or baryon densities are created similar to those at the beginning of the universe or to those which can be found in the center of neutron stars. In both cases a plasma of quarks and gluons (QGP) is present, which decomposes to hadrons within a short period of time. At this process, particles are formed, which allow statements about the beginning of the universe when captured by large detectors, but which also lead to the massive occurrence of hardware failures within the detector´s electronic devices. This article is about methods to mitigate radiation susceptibility for Field Programmable Gate Arrays (FPGA), enabling them to be used within particle detector systems to directly gain valid data in the readout chain or to be used as Detector-Control-System (DCS).
DFG Priority Programme 1305: Control Theory of Digitally Networked Dynamical Systems
Authors: Lunze Jan , Stöcker Christian
Published: July 2010

The application of modern means of communication, like wireless LAN or Ethernet, enables novel concepts and structures for control engineering. Data can be transmitted between sensors, actuators, and controllers over long distances. Information can even be exchanged between moving objects which was impossible in conventional point to point wired realizations of control loops. However, besides these advantages the use of networks within a control loop also brings up new challenges due to nondeterministic effects like delays and dropouts of data packets. But for yet, there does not exist a theory for the modeling, analysis and design of such complex systems. The connection of several components of a control loop over a communication network represents a digitally networked dynamical system, which is subject of the Priority Programme 1305 “Control Theory of Digitally Networked Dynamical Systems”. The goal of this priority programme, which is founded by the German Research Foundation (DFG) over a period of six years, is to develop a comprehensive theory for this new class of systems. Here, three defined cross-sectional issues are worked out by 13 research groups and their solutions are experimentally proved in demonstration and benchmark processes. Within this priority programme several research groups from the disciplines of control engineering, mathematics and information technology cooperate.
Playing Across Borders: Transnational Sports and Identities in Southern California and Mexico, 1930–1945
Authors: Alamillo José M.
Published: July 2010

This article examines the local and transnational dimensions of sports in Southern California through the activities of the Mexican Athletic Association of Southern California (MAASC) from the Great Depression to the end of World War II. This amateur athletic organization promoted sports in the barrios and colonias throughout Southern California and forged transnational ties with the Mexican government and its sports federation. MAASC and its related activities reflected two competing historical trajectories that have been subjects of debate in Chicano historiography. MAASC sports simultaneously reinvigorated transnational ties with Mexico that emphasized a México de afuera identity and contributed to the making of a Mexican American identity that connected immigrants to Southern California and American society in general. Ultimately, both impulses helped to instill a new political confidence among MAASC members to challenge the Los Angeles Department of Playground and Recreation's paternalistic approach toward the Mexican community.
Li(Al1–zZnz) alloys as anode materials for rechargeable Li-ion batteries
Authors: Chumak I. , Dmytriv G. , Pavlyuk V. , Oswald S. , Eckert J. , Trill H. , Eckert H. , Pauly H. , Ehrenberg H.
Published: July 2010

The cycling behavior of anode materials based on alloys from the Li(Al1–zZnz) continuous solid solution has been studied. The performance of the most promising composition Li(Al0.8Zn0.2) was tested in half-cells against metallic Li with three different electrolytes and in full Li-ion cells against a V2O5cathode. The underlying structure evolution during cycling and the most relevant fatigue mechanisms are elucidated by x-ray diffraction, nuclear magnetic resonance, and x-ray photoelectron spectroscopy, and reveal a loss of mobile Li due to the ongoing formation of solid electrolyte interfaces. An enhanced stability for Li(Al1–zZnz) electrodes withz˜0.2 results from a peculiar microstructure due to the decomposition of Al and Zn in the Li-poor state and their intermixing in the Li-rich state.
Easy preparation of SnO2@carbon composite nanofibers with improved lithium ion storage properties
Authors: Yang Zunxian , Du Guodong , Guo Zaiping , Yu Xuebin , Chen Zhixin , Zhang Peng , Chen Guonan , Liu Huakun
Published: July 2010

SnO2@carbon nanofibers were synthesized by a combination of electrospinning and subsequent thermal treatments in air and then in argon to demonstrate their potential use as an anode material in lithium ion battery applications. The as-prepared SnO2@carbon nanofibers consist of SnO2 nanoparticles/nanocrystals encapsulated in a carbon matrix and contain many mesopores. Because of the charge pathways, both for the electrons and the lithium ions, and the buffering function provided by both the carbon encapsulating the SnO2 nanoparticles and the mesopores, which tends to alleviate the volumetric effects during the charge/discharge cycles, the nanofibers display a greatly improved reversible capacity of 420 mAh/g after 100 cycles at a constant current of 100 mA/g, and a sharply enhanced reversible capacity at higher rates (0.5, 1, and 2 C) compared with pure SnO2 nanofibers, which makes it a promising anode material for lithium ion batteries.
Da pátria da seringueira à borracha de plantação: Jacques Huber e seus estudos sobre a cultura das heveas no Oriente (1911-1912)
Authors: Castro Anna Raquel de Matos , Sanjad Nelson , Romeiro Doralice dos Santos
Published: July 2010

O texto aborda a viagem do botânico suíço Jacques Huber (1867-1914) aos principais países produtores de borracha no Oriente, entre os anos de 1911 e 1912, com o intuito de desenvolver estudos acerca do cultivo de Hevea brasiliensis naquela região. Tal viagem foi determinada pelo então governador do estado do Pará, João Coelho, que tentava encontrar soluções para a crise da economia regional, baseada no extrativismo do látex e seriamente ameaçada pela borracha comercializada pelos ingleses a partir de grandes plantações no Oriente. Ao retornar da viagem, Huber publicou um relatório com informações sobre a sociedade, a economia e o ambiente onde a seringueira era cultivada. O relatório condensa não apenas as observações científicas sobre o assunto, mas também pode ser lido como um resumo das preocupações das elites regionais naquele contexto. Juntamente com uma breve análise desse relatório, constam fotografias tiradas por Huber no Oriente, retratando as plantações de seringueiras, o processo de sangramento das árvores e os trabalhadores locais.
Mercado e produção de terras na Amazônia: avaliação referida a trajetórias tecnológicas
Authors: Costa Francisco de Assis
Published: July 2010

A dinâmica da economia rural na Amazônia tem sido observada por meio do fenômeno do desmatamento e avaliada basicamente pelos riscos ambientais e pelas desigualdades distributivas a isso associadas. Este artigo indica possibilidades de mudanças, ao tempo que demonstra os obstáculos a superar. Partindo da observação da economia agrária da região como totalidade em movimento, situa os fundamentos dessa evolução em seis trajetórias tecnológicas, explicitando as assimetrias de capacidade demonstradas entre elas no contexto de suas relações com as instituições presentes. Entre estas, dispensará foco especial nas mediações que garantem a produção e o mercado de terras.
Erwin Heinrich Frank: o antropólogo alemão que escolheu a Amazônia para viver
Authors: Frank Nelita
Published: July 2010

Nota biográfica sobre Erwin Heinrich Frank (1950-2008), antropólogo alemão que trabalhou no Peru, Equador e Brasil. Neste último país, Frank foi professor na Universidade Federal do Pará e na Universidade Federal de Roraima. Escrita pela esposa do cientista, a nota apresenta elementos da formação, do trabalho de campo e da obra do antropólogo, incluindo o projeto de pesquisa que deu origem ao texto inédito publicado neste mesmo número da revista, sobre a etnografia de Theodor Koch-Grünberg.
Making "Uirapuru": a musical quest in the Brazilian Rain Forest
Authors: Zebba Sam
Published: July 2010

Sixty years ago the author, an Israeli film student at the University of California, Los Angeles, set out to make a film based on a Brazilian Indian legend which had been set to music by Heitor Villa-Lobos (1887-1959). Filming was carried out among Urubú-Ka'apor Indians in the state of Maranhão. In Belém, capital of Pará, he was joined by German anthropologist Peter Paul Hilbert (1914-1989) of the Goeldi Museum on an adventurous and creative expedition, culminating in a prize-winning art-documentary film and a life-long friendship between the two.
Infiltration into an Analog Fracture: Experimental Observations of Gravity‐Driven Fingering
Authors: Nicholl M. J. , Glass R. J.
Published: July 2010

The infiltration of water into unsaturated geologic media is an immiscible displacement process that is unstable with respect to gravity and can thus lead to the formation of gravity‐driven fingers. Where the geologic media (e.g., rock, soil) is fractured, gravity‐driven fingers within the fractures may lead to extremely rapid vertical migration of waterborne contaminants. We designed analog fractures to facilitate the competition between viscous, gravity, and capillary forces that is expected to control finger behavior, then conducted an extended experimental investigation to observe and measure finger behavior. Results show that the spatially variant two‐dimensional nature of fracture geometry leads to different behavior than is reported for the related problem of gravity‐driven fingers in porous media. Observations of finger behavior are presented, along with a simple scale analysis used to relate the key measures of finger velocity, finger width, and fingertip length. We also present a series of illustrative experiments designed to guide future research.
Colloids and Colloid‐Facilitated Transport of Contaminants in Soils: An Introduction
Authors: de Jonge L. W. , Kjaergaard C. , Moldrup P.
Published: July 2010

Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid‐Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid‐bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid‐facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.
Colloid Movement in Unsaturated Porous Media: Recent Advances and Future Directions
Authors: DeNovio Nicole M. , Saiers James E. , Ryan Joseph N.
Published: July 2010

Investigations of colloid movement through geologic materials are driven by a variety of issues, including contaminant transport, soil‐profile development, and subsurface migration of pathogenic microorganisms. In this review, we address recent advances in understanding of colloid transport through partially saturated porous media. Special emphasis is placed on features of the vadose zone (i.e., the presence of air–water interfaces, rapid fluctuations in porewater flow rates and chemistry) that distinguish colloid transport in unsaturated media from colloid transport in saturated media. We examine experimental studies on colloid deposition and mobilization and survey recent developments in modeling colloid transport and mass transfer. We conclude with an overview of directions for future research in this field.
Comparison of Hanford Colloids and Kaolinite Transport in Porous Media
Authors: Zhuang Jie , Jin Yan , Flury Markus
Published: July 2010

Understanding colloid transport at the Hanford site in Washington State is critical in assessing migration of radionuclides because colloid transport is a potential means for facilitated off‐site migration of radioactive wastes. In this study, eight saturated column experiments were conducted to investigate transport of Hanford colloids and a model colloid (kaolinite) through two types of porous media (Hanford sediments characteristic of 2:1 clay minerals and silica Accusand). Experiments were conducted at a pH value of 10 to mimic the conditions at the Hanford site. The Hanford colloids used were obtained by reacting Hanford sediments with simulated tank waste solutions. The results show that factors influencing transport of Hanford colloids and kaolinite include flow velocity, solution ionic strength, medium type, and colloid properties. Hanford colloids exhibited higher deposition rates than kaolinite in both Hanford sediments and Accusand. Likewise, Hanford sediments retained more colloids than did the silica Accusand. Comparison of transport behaviors of the two colloids through two sands supports the assumption that chemical heterogeneity is important in controlling particle–particle and particle–collector interactions in colloid retention and transport.
Water‐Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate
Authors: Kjaergaard Charlotte , de Jonge Lis W. , Moldrup Per , Schjønning Per
Published: July 2010

The fraction of clay that disperses in water, water‐dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low‐energy input measurement of WDC (LE‐WDC). These results were referenced by WDC obtained by a conventional, high‐energy input measurement based on air‐dried soil (HE‐WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE‐WDC increased with clay content, while the amount of LE‐WDC decreased with increasing clay content. The decrease in LE‐WDC was explained by an increase in cohesive strength, reflected by the increase in water‐stable aggregates (≥4 mm). A strong dependency of IMP on LE‐WDC was observed, with maximum release of LE‐WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE‐WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE‐WDC independent of clay content. The content of particulate organic C (POC) in the LE‐WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE‐WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.
Visualization and Modeling of Polystyrol Colloid Transport in a Silicon Micromodel
Authors: Baumann Thomas , Werth Charles J.
Published: July 2010

A new experimental approach and complementary model analysis are presented for studying colloid transport and fate in porous media. The experimental approach combines high precision etching to create a controlled pore network in a silicon wafer (i.e., micromodel), with epifluorescent microscopy. Two different sizes of latex colloids were used; each was stained with a fluorescent dye. During an experiment, water with colloids was purged through a micromodel at different flow rates. Flow paths and particle velocities were determined and compared with flow paths calculated using a two‐dimensional (2D) lattice Boltzmann (LB) model. For 50% of the colloids evaluated, agreement between measured and calculated flow paths and velocities were excellent. For 20%, flow paths agreed, but calculated velocities were less. This is attributed to the parabolic velocity profile across the micromodel depth, which was not accounted for in the 2D flow model. For 12%, flow paths also agreed, but calculated velocities were less. These colloids were close to grain surfaces, where model errors increase. Also, particle–surface interactions were not accounted for in the model; this may have contributed to the discrepancy. For the remaining 18% of colloids evaluated, neither flow paths nor velocities agreed. The majority of colloids in this last case were observed after breakthrough, when concentrations were high. The discrepancies may be due to particle–particle interactions that were not accounted for in the model. Filtration efficiencies for all colloid sizes at different flow rates were calculated from filtration theory. Attachment rates were obtained from successive images during an experiment. With these, attachment efficiencies were calculated, and these agreed with literature values. The study demonstrates that excellent agreement between experimental and model results for colloid transport at the pore scale can be obtained. The results also demonstrate that when experimental and model results do not agree, mechanistic inferences and system limitations can be evaluated.
Pyrene Sorption to Water‐Dispersible Colloids: Effect of Solution Chemistry and Organic Matter
Authors: Laegdsmand M. , de Jonge L. W. , Moldrup P. , Keiding K.
Published: July 2010

Chemical sorption to mobile soil colloids is a controlling factor for colloid‐facilitated chemical transport in the vadose zone and groundwater. We investigated sorption of pyrene to soil colloid suspensions originating from soils differing in organic matter content for different solution chemistries. Colloids were obtained from two soils with different organic matter contents but similar geological histories by three different methods: (i) chemical dispersion, (ii) mechanical dispersion in water, and (iii) spontaneous release in water. Batch sorption experiments were conducted at five pyrene concentrations, in either pure water or at two different concentrations of K+ and Ca2+ Generally, K+ addition enhanced pyrene sorption, whereas Ca2+ addition decreased sorption. The chemically dispersed colloids exhibited the highest pyrene sorption capacity and had the most nonlinear sorption isotherms, whereas whole soil had the most linear isotherm. Model calculations of the potential amounts of leachable pyrene illustrated the importance of including both colloid‐ and dissolved organic matter (DOM)‐facilitated transport in risk assessment models when dealing with pyrene transport. The leaching potential of dissolved pyrene (with no DOM‐ and colloid‐facilitated transport) was 5% of the leaching potential when both DOM‐ and colloid‐sorbed pyrene was included.
Neutron Log Measurement of Moisture in Unsaturated Basalt: Progress and Problems
Authors: Helm‐Clark Catherine M. , Smith Richard P. , Rodgers David W. , Knutson Carroll F.
Published: July 2010

The research described here examines whether neutron logs can be used in unsaturated basalt to quantitatively estimate moisture content, and, in tandem with other wireline logs, to determine the relative contribution to neutron log response by minerals containing chemically bound H. Our results show that it should be possible to quantitatively correlate neutron log response to the amount of both bound and unbound H regardless of saturated or unsaturated conditions. Such a correlation is possible only if bulk density, saturated porosity and neutron log response are already known in saturated conditions. It is not yet known, however, what the exact form of this correlation should be. To evaluate the candidate correlation equations, we compare measured permeability in unsaturated basalts with the calculated H content derived from these correlations. As the result of this evaluation, we identify intervals of clay‐free impermeable, unsaturated basalt with apparently high H content. The neutron log response in these intervals may be caused by localized variations of the neutron slowing‐down length, Ls, due to low concentration of H in the formation and borehole environment. When Ls is greater than the source‐detector spacing of a neutron logging tool, the neutron log response can invert, resulting in potentially faulty interpretation of neutron logs. These measurement uncertainties in unsaturated basalts might be overcome by using a neutron logging tool with both a higher‐flux neutron source and increased source‐detector spacing, or a neutron tool with several detectors placed at different spacing from the source.
Constraining the Inferred Paleohydrologic Evolution of a Deep Unsaturated Zone in the Amargosa Desert
Authors: Walvoord Michelle A. , Stonestrom David A. , Andraski Brian J. , Striegl Robert G.
Published: July 2010

Natural flow regimes in deep unsaturated zones of arid interfluvial environments are rarely in hydraulic equilibrium with near‐surface boundary conditions imposed by present‐day plant–soil–atmosphere dynamics. Nevertheless, assessments of water resources and contaminant transport require realistic estimates of gas, water, and solute fluxes under past, present, and projected conditions. Multimillennial transients that are captured in current hydraulic, chemical, and isotopic profiles can be interpreted to constrain alternative scenarios of paleohydrologic evolution following climatic and vegetational shifts from pluvial to arid conditions. However, interpreting profile data with numerical models presents formidable challenges in that boundary conditions must be prescribed throughout the entire Holocene, when we have at most a few decades of actual records. Models of profile development at the Amargosa Desert Research Site include substantial uncertainties from imperfectly known initial and boundary conditions when simulating flow and solute transport over millennial timescales. We show how multiple types of profile data, including matric potentials and porewater concentrations of Cl−, δD, δ18O, can be used in multiphase heat, flow, and transport models to expose and reduce uncertainty in paleohydrologic reconstructions. Results indicate that a dramatic shift in the near‐surface water balance occurred approximately 16000 yr ago, but that transitions in precipitation, temperature, and vegetation were not necessarily synchronous. The timing of the hydraulic transition imparts the largest uncertainty to model‐predicted contemporary fluxes. In contrast, the uncertainties associated with initial (late Pleistocene) conditions and boundary conditions during the Holocene impart only small uncertainties to model‐predicted contemporaneous fluxes.
Impact of the Capillary Fringe on Local Flow, Chemical Migration, and Microbiology
Authors: Berkowitz Brian , Silliman Stephen E. , Dunn Andrea M.
Published: July 2010

We critically examine historical and recent studies of flow and transport at the local scale within the capillary fringe (CF). The characterization of subsurface pathways traveled by water—and the impact of these pathways on the movement of chemicals and the viability of subsurface microbial populations—has been the subject of intensive investigation in the last 50 yr in the fields of soil science and hydrology. However, consideration of the complexity of local pathways within the CF has been largely ignored. Recent studies, as well as some historical ones, have reported observations of fluid flow and chemical transport within the CF, emphasizing the impact of physical heterogeneity and exchange of water and chemicals between the CF and the region below the water table. These observations lead to the conclusion that the CF may affect, at the local scale, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated groundwater flow far more significantly than is usually assumed. Here, we examine underlying physical factors that control local flow and transport behaviors in and across the CF and illustrate these factors through examination of images collected in the laboratory. We also provide a brief survey of the literature on the CF. It is argued that existing definitions of the CF are inadequate to describe flow and transport behavior in the vicinity of the water table. We suggest replacing the concept of the CF with the concept of a partially saturated fringe, which involves multiphase transport in the immediate vicinity of (both above and below) the water table.
Unstable Flow during Redistribution: Controlling Factors and Practical Implications
Authors: Wang Zhi , Jury William A. , Tuli Atac , Kim Dong‐Ju
Published: July 2010

Unstable flow causes major uncertainties in the characterization of drainage in the vadose zone by inducing finger‐like flow paths in soils with or without macropores. Recent studies have identified the major factors governing fingered flow to be the combined effects of capillary hysteresis, the existence of a threshold water‐entry value in a porous medium, and a positive matric potential gradient behind the wetting front. This situation typically occurs during redistribution following high‐rate infiltration, a common occurrence in hydrology. The conditions favoring instability can also develop during infiltration into a fine‐over‐coarse layered soil, into hydrophobic or air‐entrapped soils, or even in a homogeneous coarse‐textured soil if the infiltration rate is low. An analysis of the conditions necessary for the onset of unstable flow in a uniform soil is provided in this paper. We demonstrate that if the matric potential gradient (dh/dz) becomes positive during redistribution, a perturbation at the wetting front will cause finger flow. However, if dh/dz remains negative, the perturbation will be dissipated. The analysis is used to predict a critical depth of irrigation (Ic) beyond which the flow should become unstable. A series of point‐source and line‐source infiltration experiments were conducted using a slab‐box filled with uniform sands. The results confirmed that as soon as Ic is exceeded, a finger was formed at the bottom of the wetting front, channeling the flow and stopping water movement in the surrounding areas. We discuss this phenomenon's implications for practical irrigation and leaching designs.
Application of a Darcian Approach to Estimate Liquid Flux in a Deep Vadose Zone
Authors: Hubbell J. M. , Nicholl M. J. , Sisson J. B. , McElroy D. L.
Published: July 2010

Approaches for estimating liquid flux in the shallow (0–2 m) vadose zone are hindered by the high degree of spatial and temporal variability present near the land surface. It is hypothesized that high‐frequency variations in flux will be damped with depth. This study was conducted to estimate deep liquid flux using the Darcian approach at a waste disposal site in south‐central Idaho that is underlain by a complex sequence of unsaturated basalt flows intercalated with thin sedimentary layers. Flux is estimated by combining in situ water potential measurements from sedimentary interbeds located at depths of 34 and 73 m below land surface (bls) with laboratory estimates for the unsaturated hydraulic conductivity. Tensiometer data at seven locations indicated nearly constant conditions for 30 mo, while nine of the other 10 sites showed small gradual trends. Assumption of a unit hydraulic gradient led to flux estimates ranging from 0.2 to 10000 cm yr−1 Estimates in the 34‐m interbed ranged across four orders of magnitude while flux estimates for the 73‐m interbed ranged three orders of magnitude. While the tensiometer data appear to reflect in situ conditions and are a sensitive indicator of hydrologic conditions in the deep vadose zone, the laboratory‐developed hydraulic properties introduce a high degree of uncertainty, potentially affecting predictions by orders of magnitude. There is a need to develop techniques for assessing flux rates for the range of applicable field conditions to improve the confidence in deep flux estimates.
Multiple Tracing Experiments in Unsaturated Fractured Clayey Till
Authors: Mortensen Annette P. , Jensen Karsten H. , Nilsson Bertel , Juhler René K.
Published: July 2010

Current monitoring and sampling techniques in unsaturated fractured clay often fail to characterize fast preferential flow. To circumvent these problems, an isolated block (3.5 by 3.5 by 3.3 m) of unsaturated fractured clayey till was used for multiple tracing experiments. The setup allowed full control of the water balance in the block. Experiments at three different steady‐state flow rates were performed. Multiple tracers with different diffusion coefficients were applied in each experiment to evaluate the influence of diffusive exchange between fractures and the matrix. The tracers included two halogen anions (Cl− and Br−), two fluorobenzoic acids (FBA) (2,3‐DFBA and 2,6‐DFBA), two fluorescent dyes (uranine and sulforhodamine B), and one colloidal tracer (0.5‐μm latex particles). At high flow rates, the obtained tracer breakthrough showed a traditional asymmetrical behavior where a fast peak was followed by a long tailing period. At low flow rates, two of the applied tracers revealed a double peak breakthrough curve, whereas the tracer with the lowest molecular diffusion coefficient showed only one peak. The separation of the tracers was hypothesized to be influenced by extensive diffusion into stagnant areas by the tracers with the high diffusion coefficients. This was supported by the breakthrough curve obtained for the colloidal tracer, which showed earlier breakthrough and only one tracer peak.
Volatilization of Binary Nonaqueous Phase Liquid Mixtures in Unsaturated Porous Media
Authors: Abriola Linda M. , Bradford Scott A. , Lang John , Gaither Charles L.
Published: July 2010

This study examines the volatilization behavior of binary nonaqueous phase liquid (NAPL) mixtures consisting of styrene, and toluene or tetrachloroethylene (PCE). Residual NAPL saturations were emplaced in unsaturated (residual water saturation) soil columns packed with Wagner 50‐80 sand. Initial column effluent concentrations were measured for the NAPL mixtures at several pore gas phase velocities. Rate‐limited volatilization occurred at higher gas phase pore velocities, and mass transfer coefficients could be reasonably predicted with a correlation developed from single component NAPL volatilization data. Long‐term volatilization studies for the binary NAPL mixtures were also conducted. The effluent concentrations for both NAPL components were observed to be initially proportional to their mole fractions. After the more volatile component became depleted, a rapid drop in the effluent concentration of this component was accompanied by an increase in the mole fraction and effluent concentration of the remaining constituent to near saturated values until the free phase NAPL was volatilized. The final stage of removal was associated with a dramatic decrease in effluent concentration, attributed to reduction in the gas‐NAPL interfacial area, followed by low concentration tailing. The tailing and subsequent flow interruption behavior are likely a consequence of rate‐limited desorption. Equilibrium and rate‐limited simulations of the long‐term volatilization experiments did not provide a satisfactory description of the data. A simulation that included a fixed concentration gradient and fitted activity coefficients provided a better characterization of the volatilization data. Various potential explanations for this “fixed gradient” volatilization behavior were considered, but additional research is needed to test these hypotheses.