bluuebunny/crossref_metadata_2025 huggingface dataset:
Bioreactors for bone tissue engineering
Authors: El Haj A J , Cartmell S H
Published: July 2010

Engineering bone tissue for use in orthopaedics poses multiple challenges. Providing the appropriate growth environment that will allow complex tissues such as bone to grow is one of these challenges. There are multiple design factors that must be considered in order to generate a functional tissue in vitro for replacement surgery in the clinic. Complex bioreactors have been designed that allow different stress regimes such as compressive, shear, and rotational forces to be applied to three-dimensional (3D) engineered constructs. This review addresses these considerations and outlines the types of bioreactor that have been developed and are currently in use.
Experimental versus Computational Analysis of Micromotions at the Implant—Bone Interface
Authors: Tarala M , Janssen D , Telka A , Waanders D , Verdonschot N
Published: July 2010

In total hip arthroplasty, micromotions at the implant—bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant—bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM).

The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an ‘experimental’ method to measure micromotions. This ‘experimental’ method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone.

No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface.
Scale Economies and Industry Agglomeration Externalities: A Dynamic Cost Function Approach
Authors: Paul Catherine J. Morrison , Siegel Donald S
Published: July 2010

Scale economies and agglomeration externalities are alleged to be important determinants of economic growth. To assess these effects, we outline and estimate a microfoundations model based on a dynamic cost function specification. This model provides for the separate identification of the impacts of externalities and cyclical utilization on short- and long-run scale economies and input substitution patterns. We find that scale economies are prevalent in U.S. manufacturing, cost savings and scale effects often attributed to internal inputs may be due to external factors, and supply-side agglomeration effects are greater than demand-side, especially in the long run. (JEL O47, E23)
Voluntary Export Restraints on Automobiles: Evaluating a Trade Policy
Authors: Berry Steven , Levinsohn James , Pakes Ariel
Published: July 2010

We evaluate the voluntary export restraint (VER) that was initially placed on exports of automobiles from Japan in 1981. We evaluate the impact this policy had on U.S. consumer welfare, firm profits, and forgone tariff revenue from its initiation through 1990. (JEL F13)
A Dynamic Economy with Costly Price Adjustments
Authors: Danziger Leif
Published: July 2010

This paper studies a general-equilibrium model of a dynamic economy with menu costs. Each firm's productivity is exposed to idiosyncratic and aggregate productivity shocks around a trend, and the money supply to monetary shocks around a trend. All consumption, pricing, and production decisions are based on optimizing behavior. There exists a staggered Markov perfect equilibrium with prices determined by a two-sided (s, S) markup strategy. The paper analyzes the optimal markup strategy and investigates the dynamics of the price index and the aggregate output. The welfare consequences of the uncertain aggregate productivity and money supply are also examined. (JEL E31, E32)
Adverse Selection in Durable Goods Markets
Authors: Hendel Igal , Lizzeri Alessandro
Published: July 2010

We present a dynamic model of adverse selection to examine the interactions between new and used goods markets. We find that the used market never shuts down, the volume of trade can be large, and distortions are lower than previously thought. New cars prices can be higher under adverse selection than in its absence. An extension to several brands that differ in reliability leads to testable predictions of the effects of adverse selection. Unreliable brands have steeper price declines and lower volumes of trade. We contrast these predictions with those of a model where brands physically depreciate at different rates. (JEL D82, L15)
Monopoly Rights: A Barrier to Riches
Authors: Parente Stephen L , Prescott Edward C
Published: July 2010

Our thesis is that poor countries are poor because they employ arrangements for which the equilibrium outcomes are characterized by inferior technologies being used, and being used inefficiently. In this paper, we analyze the consequences of one such arrangement. In each industry, the arrangement enables a coalition of factor suppliers to be the monopoly seller of its input services to all firms using a particular production process. We find that eliminating this monopoly arrangement could well increase output by roughly a factor of 3 without any increase in inputs. (JEL D58, O11, O41)
Optimal Adoption of Complementary Technologies
Authors: Jovanovic Boyan , Stolyarov Dmitriy
Published: July 2010

When a production process requires two extremely complementary inputs, conventional wisdom holds that a firm would always upgrade them simultaneously. We show, however, that if upgrading each input involves a fixed cost, the firm may upgrade them at different dates, “asynchronously.” This insight helps us understand why productivity rises with the age of a plant, why investment in structures is more spiked than equipment investment, and why plants have spare capacity. The bigger point of the paper is that complementarity does not necessarily imply comovement—not even for a single decision maker. (JEL E22, O31, P11)
The Choice Between Market Failures and Corruption
Authors: Acemoglu Daron , Verdier Thierry
Published: July 2010

Because government intervention transfers resources from one party to another, it creates room for corruption. As corruption often undermines the purpose of the intervention, governments will try to prevent it. They may create rents for bureaucrats, induce a misallocation of resources, and increase the size of the bureaucracy. Since preventing all corruption is excessively costly, second-best intervention may involve a certain fraction of bureaucrats accepting bribes. When corruption is harder to prevent, there may be both more bureaucrats and higher public-sector wages. Also, the optimal degree of government intervention may be nonmonotonic in the level of income. (JEL D23, H40)
Tax Policy and Aggregate Demand Management Under Catching Up with the Joneses
Authors: Ljungqvist Lars , Uhlig Harald
Published: July 2010

This paper examines the role for tax policies in productivity-shock driven economies with catching-up-with-the-Joneses utility functions. The optimal tax policy is shown to affect the economy countercyclically via procyclical taxes, i.e., “cooling down” the economy with higher taxes when it is “overheating” in booms and “stimulating” the economy with lower taxes in recessions to keep consumption up. Thus, models with catching-up-with-the-Joneses utility functions call for traditional Keynesian demand-management policies but for rather unorthodox reasons. (JEL E21, E62, E63)
What Do a Million Observations on Banks Say About the Transmission of Monetary Policy?
Authors: Kashyap Anil K , Stein Jeremy C
Published: July 2010

We study the monetary-transmission mechanism with a data set that includes quarterly observations of every insured U.S. commercial bank from 1976 to 1993. We find that the impact of monetary policy on lending is stronger for banks with less liquid balance sheets—i.e., banks with lower ratios of securities to assets. Moreover, this pattern is largely attributable to the smaller banks, those in the bottom 95 percent of the size distribution. Our results support the existence of a “bank lending channel” of monetary transmission, though they do not allow us to make precise statements about its quantitative importance. (JEL E44, E52, G32)
A Representative Consumer Theory of Distribution
Authors: Caselli Francesco , Ventura Jaume
Published: July 2010

This paper introduces various sources of consumer heterogeneity in one-sector representative consumer (RC) growth models and develops tools to study the evolution of the distribution of consumptions, assets, and incomes. These tools are applied to the Ramsey-Cass-Koopmans model of optimal savings and the Arrow-Romer model of productive spillovers. The RC property per se places very few restrictions on the nature of observed distributions, and a wide range of distributive dynamics and income mobility patterns can arise as the equilibrium outcome. An example illustrates how to use these tools to generate quantitative predictions and compare them to the data. (JEL E13, O41)
Does Schooling Cause Growth?
Authors: Bils Mark , Klenow Peter J
Published: July 2010

A number of economists find that growth and schooling are highly correlated across countries. A model is examined in which the ability to build on the human capital of one's elders plays an important role in linking growth to schooling. The model is calibrated to quantify the strength of the effect of schooling on growth by using evidence from the labor literature on Mincerian returns to education. The upshot is that the impact of schooling on growth explains less than one-third of the empirical cross-country relationship. The ability of reverse causality to explain this empirical relationship is also investigated. (JEL I2, J24, O4)
The Political Economy of Debt Bondage
Authors: von Lilienfeld-Toal Ulf , Mookherjee Dilip
Published: July 2010

What are the effects of restricting bonded labor clauses in tenancy or debt contracts? While such restrictions reduce agents' ability to credibly commit ex ante to repay principals in states where they default on their financial obligations, they also generate a pecuniary externality on other principal-agent pairs by reducing the equilibrium profit earned by principals. This turns out to imply that on both political and normative grounds, restrictions on bonded labor become more attractive when borrowers become wealthier or the range of collateral instruments widens. (JEL D82, D86, J82, K12)
The Effects of Information Transparency on Suppliers, Manufacturers, and Consumers in Online Markets
Authors: Zhou Zach Zhizhong , Zhu Kevin Xiaoguo
Published: July 2010

This paper looks into the effects of information transparency on market participants in an online trading environment. We study these effects in business-to-business electronic markets with firms competing in both upstream and downstream industries. The prior literature generally assumes that either the downstream firm (buyer) or the upstream firm (seller) is a monopoly. It is not clear whether information transparency would still create value if both buyers and sellers face oligopolistic competition, where the benefits of information transparency could be competed away. To answer this question, we first develop a simple two-echelon e-market model and then extend the model to more general settings. We find that information transparency can create value for the overall e-market, yet it affects buyers and sellers very differently: one side will be hurt, depending on the competition mode (Cournot or Bertrand) in the downstream. This suggests that a manufacturer-owned, a supplier-owned, and a neutral e-market will have different preferences for information transparency. Finally, we find that information transparency can hurt consumers when the downstream industry engages in Bertrand competition. This is a surprising result given the expectation that online markets create substantial value for consumers.
Na Primeira República, Bulhões Carvalho legaliza a atividade estatística e a põe na ordem do Estado
Authors: Senra Nelson de Castro
Published: July 2010

Recriada nos primórdios da República (jan./1890), como órgão central, a Diretoria Geral de Estatística (DGE) enfrentou dificuldades para consolidar a atividade estatística brasileira. Um novo rumo só foi ensaiado, e razoavelmente alcançado, quando José Luiz Sayão de Bulhões Carvalho (1866-1940), médico e sanitarista, dedicado aos estudos e pesquisas demográficas, assumiu sua direção (em dois períodos) por quase 17 anos. Em várias frentes de atuação, aplicou-se em facilitar o convívio da DGE com seus similares na federação (extremada) então vigente. Idealizou acordos com órgãos estaduais; imaginou conselhos para tomada de decisões coletivas; imaginou uma conferência estatística, baseada na experiência de participação em dois congressos do International Statistical Institute (1S1); ajudou a formar uma comunidade estatística, pensando a formação do profissional especializado, para tanto tendo estimulado e patrocinado a tradução de livros; entre outras medidas. O Censo de 1920, que idealizou e conduziu, sendo o único grande censo na Primeira República, pautou-se em relações amistosas com os governos federal e estaduais e com a sociedade organizada (Igreja, imprensa, sindicatos, clubes etc.). Realizou, então, por assim dizer, um censo moderno, utilizando máquinas de apuração e o concluindo em tempo hábil. Na divulgação dos resultados, inovou na feitura de gráficos (pictóricos), seja para os volumes impressos, seja para o Pavilhão da Estatística, também chamado de Pavilhão da Ciência da Certeza, que marcou a presença da DGE na Exposição Internacional Comemorativa do Centenário da Independência. Com sua vivência, antecipou as bases do Instituto Brasileiro de Geografia e Estatística (1BGE), obra de um seu discípulo, e que pode ser visto como a medida do seu sucesso (a longo prazo).
Ciência e política durante o regime militar (1964-1984): a percepção dos físicos brasileiros
Authors: Freire Junior Olival , Videira Antonio Augusto Passos , Ribeiro Filho Aurino
Published: July 2010

Essa nota de pesquisa apresenta resultados parciais da análise de uma fonte de história oral - a qual ainda encontra-se em processo de elaboração - com físicos brasileiros que tiveram atuação destacada no período do regime militar brasileiro (1964-1984). Quatorze físicos já foram entrevistados e o projeto prevê um total de vinte e cinco. O projeto de pesquisa - do qual essa nota resulta - visa construir uma narrativa unificada dos processos conflitantes (autoritarismo político e apoio ao desenvolvimento científico), capaz de descrever as transformações que eles causaram na comunidade de físicos.
Colloid Transport in the Subsurface: Past, Present, and Future Challenges
Authors: McCarthy John F. , McKay Larry D.
Published: July 2010

This paper attempts to introduce the work described in this special section on colloid transport within a more general perspective of the evolution of our understanding of the importance of colloids in subsurface systems. The focus will be on the transport of colloidal particles in natural (i.e., chemically and physically heterogeneous) geological settings because the complexity imposed by these situations represents the greatest challenge to current and future understanding. Great progress has been made in addressing many of the key questions related to colloid transport. However, as in most areas of science, increased knowledge also serves to reveal new and more complex challenges that must be addressed.
Mobile Organic Sorbent Affected Contaminant Transport in Soil: Numerical Case Studies for Enhanced and Reduced Mobility
Authors: Totsche Kai Uwe , Kögel‐Knabner Ingrid
Published: July 2010

Mobile organic sorbents (MOS) such as dissolved and colloidal phase organic matter control flow of water and transport of solutes in soils. We studied the effect of MOS on contaminant fate by systematic numerical case studies. The scenarios considered were (i) enhanced mobility, (ii) reduced mobility due to cosorption, and (iii) reduced mobility due to cumulative sorption. The enhanced mobility and cosorption scenario require contaminant sorption to the MOS. The cosorption and cumulative sorption scenario require sorption of MOS to the immobile sorbent. Simulations were run for physicochemically different fractions of dissolved organic matter and two model contaminants. Mobile organic sorbents mobility is characterized by a wide range of retardation parameters. Continued import of MOS to subsoil material high in MOS‐specific sorption sites will increase the solid phase organic matter content. Enhanced mobility was observed for soils without MOS‐specific sorption sites or for situations where MOS are in sorptive equilibrium with the immobile sorbent. Cosorption resulted in reduced contaminant mobility. The extent to which reduced mobility was observed depended on the ratio of the affinities of the free contaminant and the MOS‐bound contaminant. As the sorption of the MOS‐bound contaminants is controlled by the properties of the MOS, the characterization of these properties is a crucial step for the estimation of the effect of MOS on contaminant mobility. Cumulative sorption resulted in reduced contaminant mobility as well. However, this result is the consequence of the increase of the sorption capacity due to the sorption of MOS, a long‐term process that may last for years to decades.
Coupled Microbial and Transport Processes in Soils
Authors: Rockhold M. L. , Yarwood R. R. , Selker J. S.
Published: July 2010

This paper reviews methods for modeling coupled microbial and transport processes in variably saturated porous media. Of special interest in this work are interactions between active microbial growth and other transport processes such as gas diffusion and interphase exchange of O2 and other constituents that partition between the aqueous and gas phases. The role of gas–liquid interfaces on microbial transport is also discussed, and various possible kinetic and equilibrium formulations for bacterial cell attachment and detachment are reviewed. The primary objective of this paper is to highlight areas in which additional research may be needed—both experimental and numerical—to elucidate mechanisms associated with the complex interactions that take place between microbial processes and flow and transport processes in soils. In addition to their general ecological significance, these interactions have global‐scale implications for C cycling in the environment and the related issue of climate change.
Pore‐Scale Visualization of Colloid Transport and Retention in Partly Saturated Porous Media
Authors: Crist John T. , McCarthy John F. , Zevi Yuniati , Baveye Philippe , Throop James A. , Steenhuis Tammo S.
Published: July 2010

In unsaturated porous media, sorption of colloids at the air–water (AW) interface is accepted as a mechanism for controlling colloid retention and mobilization. However, limited actual pore‐scale observations of colloid attachment to the AW interface have been made. To further investigate these processes, a real‐time pore‐scale visualization method was developed. The method builds on the light transmission technique for fingered flow studies in packed‐sand infiltration chambers and combines it with high‐resolution, electro‐optical hardware and public domain imaging software. Infiltration and drainage of suspensions of hydrophilic negatively charged carboxylated latex microspheres provides compelling visual evidence that colloid retention in sandy porous media occurs via trapping in the thin film of water where the AW interface and the solid interface meet, the air–water–solid (AWS) interface. With this modified theory of trapped colloids at the AWS interface, we are able to explain the apparent discrepancy between previous experimental evidence of hydrophilic colloids seemingly partitioning to the AW interface and more recent findings that suggest this type of colloid does not adsorb at the AW interface.
Particle Leaching and Particle‐Facilitated Transport of Phosphorus at Field Scale
Authors: de Jonge L. W. , Moldrup P. , Rubæk G. H. , Schelde K. , Djurhuus J.
Published: July 2010

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle‐facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle‐facilitated transport of P at the field scale, and investigate which soil‐physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of −20 cm and irrigated at a rate of 10 mm h−1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle‐facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.
Cyanide Leaching from Soil Developed from Coking Plant Purifier Waste as Influenced by Citrate
Authors: Mansfeldt Tim , Leyer Heike , Barmettler Kurt , Kretzschmar Ruben
Published: July 2010

Soils in the vicinity of manufactured gas plants and coal coking plants are often highly contaminated with cyanides in the form of the compound Prussian blue, FeIII4FeIICN63 The objective of this study was to investigate the influence of citrate on the leaching of iron–cyanide complexes from an extremely acidic soil (pH 2.3) developed from gas purifier waste near a former coking plant. The soil contained 63 g kg−1 CN, 148 g kg−1 Fe, 123 g kg−1 S, and 222 g kg−1 total C. Analysis of the soil by X‐ray diffraction (XRD) and Fourier transform infrared (FT‐IR) spectroscopy revealed the presence of Prussian blue, gypsum, elemental sulfur, jarosite, and hematite. For column leaching experiments, air‐dried soil was mixed with purified cristabolite sand at a ratio of 1:3 and packed into chromatography columns. The soil was leached with dilute (0.1 or 1 mM) CaCl2 solutions and the effluent was collected and analyzed for total and dissolved CN, Ca, Fe, SO4, pH, and pe. In the absence of citrate, the total dissolved CN concentration in the effluent was always below current drinking water limits (<1.92 μM), indicating low leaching potential. Adding citrate at a concentration of 1 mM had little effect on the CN concentrations in the column effluent. Addition of 10 or 100 mM citrate to the influent solution resulted in strong increases in dissolved and colloidal CN concentrations in the effluent, which was due to ligand‐controlled dissolution of Prussian blue, desorption of FeIICN4−6 or FeIIICN3−6 by sorption competition with citrate, and mobilization of colloidal particles by citrate. However, our results indicate that relatively high concentrations of citrate are necessary to significantly increase CN leaching from the strongly acidic soil.
Small‐Scale Features of Gravity‐Driven Flow in Unsaturated Fractures
Authors: Su Grace W. , Geller Jil T. , Hunt James R. , Pruess Karsten
Published: July 2010

Liquid flow through unsaturated fractures often proceeds as fingers or preferential flow paths. During the invasion of liquid fingers into an initially dry, nonhorizontal fracture, fingers may drain, forming a narrow thread of liquid called a rivulet that connects to a wider portion of liquid at the advancing front, defined as a blob. Experimental studies using idealized fractures were performed to investigate the effects of wettability, surface roughness, and aperture size on several important features of gravity‐driven flow in fractures: liquid drainage, blob migration, and rivulet flow. The experiments demonstrate that the critical length of the blob before drainage occurred was significantly longer on surfaces with intermediate wettability and on surfaces with roughness on the order of 100 μm than on a smooth, flat water‐wetting surface. However, drainage did not occur on surfaces with smaller‐scale roughness on the order of 10 μm. Blob velocities were also measured and were always less than the saturated gravity‐driven flow velocity, even when a liquid with a static contact angle of zero was used. This reduction in velocity was attributed to contact angle hysteresis. Rivulet widths measured as a function of flow rate between glass and acrylic parallel plates were generally larger on the acrylic plates than the glass plates for a particular flow rate, demonstrating the sensitivity of rivulet flow to wettability. In addition, the cubic law overpredicted the measured rivulet widths, except for the widths measured between the acrylic plates at 20°. The effect of aperture variability on rivulet flow was also examined. At a critical aperture ranging between 0.25 and 0.37 mm, the liquid in the rivulet did not completely span the aperture, forming two streamlets of liquid on either side of the fracture.
Flow Focusing in Unsaturated Fracture Networks: A Numerical Investigation
Authors: Zhang Keni , Wu Yu‐Shu , Bodvarsson G. S. , Liu Hui‐Hai
Published: July 2010

A numerical modeling study is presented to investigate flow–focusing phenomena in a large‐scale fracture network, constructed using field data collected from the unsaturated zone of Yucca Mountain, Nevada, the proposed repository site for high‐level nuclear waste. The two‐dimensional fracture network for an area of 100 by 150 m contains more than 20000 fractures. Steady state unsaturated flow in the fracture network is investigated for different boundary conditions and rock properties. Simulation results indicate that flow paths are generally vertical, and that horizontal fractures mainly provide pathways between neighboring vertical paths. In addition to fracture properties, flow–focusing phenomena are also affected by rock–matrix permeability, with lower matrix permeability leading to a high degree of flow focusing. The simulation results further indicate that the average spacing between flow paths in a layered system tends to increase and flow tends to become more focused, with depth.
Measurement of the Solid Dielectric Permittivity of Clay Minerals and Granular Samples Using a Time Domain Reflectometry Immersion Method
Authors: Robinson D. A.
Published: July 2010

Both porosity and water content of rocks, sediments, and soils can be estimated from measurements of the effective dielectric permittivity. To achieve accurate modeling to obtain either water content or porosity the permittivity of the solid phase must be known. Until recently the most common method of obtaining the permittivity of the solid phase of granular materials relied on packing samples in air and using a mixing model to estimate the permittivity of the solid. This approach preassumes the correctness of the mixing models that we want to test. This work develops a recently proposed immersion methodology suitable for fine‐grained mineral samples and clay minerals. Measured permittivity values were 4.4 for ground quartz, 9.1 for Iceland Spar calcite, 6.0 for biotite mica, 5.8 for phlogopite mica, 5.3 for talc, 5.1 for kaolin, 5.8 for illite, and 5.5 for montmorillonite. The methodology was found to work well other than for the high surface montmorillonite sample where the immersion polar fluid adsorbed to the clay.
The Role of the Unsaturated Zone in Artificial Recharge at San Gorgonio Pass, California
Authors: Flint Alan L. , Ellett Kevin M.
Published: July 2010

The hydrogeology of the unsaturated zone plays a critical role in determining the suitability of a site for artificial recharge. Optimally, a suitable site has highly permeable soils, a capacity for horizontal flow at the aquifer boundary, a lack of impeding layers, and a thick unsaturated zone. The suitability of a site is often determined by field and laboratory measurements of soil properties, field experiments, and numerical modeling. An artificial recharge site in the San Gorgonio Pass area in southern California, USA was studied to better understand the role of the unsaturated zone in artificial recharge by surface spreading. Field measurements and observations were used to characterize the site and to develop a conceptual model of the unsaturated zone. A numerical model was developed based on the conceptual model and calibrated using data from a 50‐d artificial recharge experiment conducted in 1991 and borehole data collected between 1997 and 2002. Results indicate that an impeding layer exists 70 m below land surface that will cause lateral diversion of artificially recharged water, which would spread out and delay recharge to the water table 185 m below land surface.
Application of Nonisothermal Multiphase Modeling to In Situ Soil Remediation in Söderkulla
Authors: Kling Terhi , Korkealaakso Juhani , Saarenpää Jukka
Published: July 2010

Nonisothermal multiphase modeling with the T2VOC numerical simulator was used as an integration tool in an in situ remediation project in which trichloroethylene (TCE)‐contaminated soil was treated with thermally (steam) enhanced soil vapor extraction. Numerical simulation studies were employed in the iterative conceptualization of the migration of the contaminants and the effects of different treatment operations, in the planning of optimal system design and control of the remediation processes, as well as for integration of information obtained from field investigations during different phases of the project. Modeling proved to be a valuable tool in planning and decision making. It was, however, difficult to forecast the costs or the duration of the project, since the actual TCE mass and distribution was poorly known. The power of modeling lies in its ability to compare the effect of different strategies or measures on the cleanup efficiency, and in studying the associated effects during the treatments.
Numerical Simulation of the Radius of Influence for Landfill Gas Wells
Authors: Vigneault Harold , Lefebvre René , Nastev Miroslav
Published: July 2010

In North America, most domestic waste produced is disposed in landfills. These sites generate leachate and gas, mainly CH4 and CO2, which are harmful for the environment if not properly controlled. The design of active landfill gas recovery systems is based in large part on the radius of influence of vertical pumping wells. This parameter is commonly estimated empirically. This study presents results of numerical simulations of the radius of influence of gas recovery wells for different site conditions. The simulations were performed with the TOUGH2‐LGM simulator. In the simulation scenarios, the radius of influence was defined in relation to several factors: the waste thickness, the generation rate of CH4 gas in the waste, and CH4 concentration in the recovered landfill gas. The results are presented in the form of general graphs that are not site‐specific. The adequacy of the results still needs to be validated against field measurements. On the basis of simulation results, a systematic approach is proposed for the design of landfill gas recovery systems, and this approach is illustrated with a hypothetic example. This approach should guide landfill managers and engineers in the design of landfill gas recovery systems. The simulations only considered cases where landfills are open to the atmosphere, which are representative of most operating conditions. The results thus do not apply to post‐closure conditions usually involving an impermeable cover built on top of the waste.
Using High‐Resolution Soil Moisture Data to Assess Soil Water Dynamics in the Vadose Zone
Authors: Starr James L. , Timlin Dennis J.
Published: July 2010

Infiltration and water flow in soils are highly transient processes, but may be estimated from high frequency measurements of soil water content. The objectives of our study were to assess the impact of vadose zone soil water dynamics from real‐time and near‐continuous soil water content measurements and to assess the interactive effects of tillage, row position, and season on water infiltration, storage, drainage, and crop water uptake. Multisensor capacitance probes were installed at row and interrow positions at four soil depths in plow tillage (PT) and no tillage (NT) corn (Zea mays L.) plots, with volumetric water contents values recorded every 10 min for a 30‐mo period. Three water parameters (cumulative water storage, net water storage, and drainage below 55 cm) were grouped for statistical analysis by rainfall amounts and “seasons” of primary periods of groundwater recharge and evapotranspiration. Crop water uptake amounts during extended dry‐down periods were also quantified. We found more significant differences between tillage and row position treatments for high rainfalls (>17 mm) than for low rainfalls. When significant differences in the three water parameters existed between positions, these were due either to greater values in the row position or smaller values at the traffic interrow position. In general, more water uptake took place under PT than under NT and from row more than interrow or traffic interrow positions. These results show both the value and limitation of near‐continuous real‐time soil water data for quantifying soil water dynamics in varying management and weather conditions.
Inverse Estimation of the Unsaturated Soil Hydraulic Properties from Column Outflow Experiments Using Free‐Form Parameterizations
Authors: Bitterlich S. , Durner W. , Iden S. C. , Knabner P.
Published: July 2010

Inverse methods are increasingly used to estimate the hydraulic properties of unsaturated soils. The method generally uses a weighted least‐squares approach in which numerically simulated data are fitted to measured data. In this study we used inverse methods to estimate the unsaturated soil hydraulic properties from continuous and multistep column outflow experiments. The method employs piecewise polynomial functions to obtain a free‐form parameterization of the hydraulic properties, rather than fixed functional forms typical of the van Genuchten–Mualem and Brooks–Corey–Burdine models. For the polynomial functions we used quadratic B‐splines and piecewise cubic Hermite interpolation. The method leads to local parameterizations that can also be hierarchic, depending on the invoked number of degrees of freedom. Since a suitable number of degrees of freedom cannot be defined a priori, we embedded the estimation method into a multilevel procedure, which also included a stability analysis, based on singular value decomposition of the sensitivity matrix. The optimization procedure was made more stable by imposing monotonicity constraints on the hydraulic functions. Tests with synthetic and measured data from column outflow experiments show the validity and robustness of the method.
Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot
Authors: Rosen Michael R. , Reeves Robert R. , Green Steven , Clothier Brent , Ironside Nigel
Published: July 2010

Nitrate contamination of groundwater by a sheep feedlot in Hawke's Bay, New Zealand led to closure of the feedlot in 1998. However, knowledge of the processes controlling how long the contamination will remain and an analysis of whether the current land use (vineyard) will also impact groundwater quality are required to assess long‐term groundwater quality issues after feedlot operations cease. To determine the fate of NO3 following land use changes and the rate of reduction of contamination that may be expected under natural conditions following such changes, we compared the chemical concentrations of NO3–N, Cl, and alkalinity (HCO3) in the groundwater and rivers from surveys conducted in 1994 and 1995 with sampling conducted in 2001. Profile sampling of total N and C of the <2‐mm size fraction in the vadose zone from two sites was used in a one‐dimensional soil–plant–atmosphere system model (SPASMO) program to predict N loading in future years and predict how long it would take to improve the groundwater quality. Groundwater sampling in 1994 and 1995 determined that the highest NO3–N concentrations were under the feedlot (>140 g m−3 NO3–N) and down gradient. In 2001, 3 yr after the feedlot closed, the Cl concentrations had increased in down‐gradient wells but remained similar to the 1994 survey in other wells. There has been a decrease in NO3–N concentrations in most wells, compared with the peak NO3–N concentrations recorded in the 1995 survey, but an increase compared with 1994. Alkalinity concentrations in wells located within the influence of the feedlot are approximately 150 g m−3 lower than in surrounding wells. This indicates that nitrification reactions are affecting the HCO3 concentrations in the feedlot‐influenced wells. However, the HCO3 concentrations of some of these wells are increasing, indicating that nitrification could be slowing down and the aquifer is beginning to recover. SPASMO modeling indicates that NO3 contamination from the site will continue for the next 3 to 5 yr. The impact of NO3 leaching due to current land use practices is likely to be much less than the feedlot. The model predicts there will be an improvement in groundwater quality in the next 3 to 5 yr as NO3 from the feedlot eventually leaves the vadose zone profile and mixes into the unconfined aquifer.
Two Numerical Methods for Modeling Variably Saturated Flow in Layered Media
Authors: Lima‐Vivancos Violeta , Voller Vaughan R.
Published: July 2010

Two alternative methods for numerical modeling of variably saturated flow in layered media are developed. The first method is based on a space and time finite difference discretization of the mixed form of Richards' equation. The solution of the resulting equations follows the scheme introduced by Celia et al. The unique features in the proposed solution are treatments of (i) the saturated–unsaturated transition and (ii) the interface between material layers. In the second method, following work by Marinelli and Durnford, Richards' equation is discretized in time. At each time step a system of two first‐order equations in space is obtained. These equations are solved with an ordinary differential equation (ODE) shooting method. The ODE solution uses a fixed space grid and first order Euler integration. Both the finite difference and shooting methods are used to model the drainage of an initially saturated one‐dimensional three‐layer soil column. Grid converged results indicate a close agreement between the finite difference and shooting method predictions.
Analytic Element Modeling of Cylindrical Drains and Cylindrical Inhomogeneities in Steady Two‐Dimensional Unsaturated Flow
Authors: Bakker Mark , Nieber John L.
Published: July 2010

The analytic element method, first developed for modeling flow in saturated porous media, is adapted to the solution of the governing equation for steady flow in unsaturated porous media. The governing equation for steady‐state unsaturated flow is made amenable to analytic element solution through transformation with the Kirchhoff integral, representation of the hydraulic conductivity by an exponential function of the pressure head, and use of a coordinate transformation. A number of analytic elements are available for the resulting modified Helmholtz equation; analytic element equations are presented for uniform flow, cylindrical drains, and cylindrical inhomogeneities. The analytic element solution allows for the analytic evaluation of the pressure head, saturation, and Darcy flux at any point in the vadose zone. The applicability of the analytic element method to simulate unsaturated flow is demonstrated by solving for several cases of steady flow in a region containing arbitrarily located cylindrical inhomogeneities, and for flow in a region containing one cylindrical inhomogeneity and two cylindrical drains.
Measuring the Soil Water Content Profile of a Sandy Soil with an Off‐Ground Monostatic Ground Penetrating Radar
Authors: Lambot S. , Rhebergen J. , van den Bosch I. , Slob E. C. , Vanclooster M.
Published: July 2010

We explore the possibility of measuring a continuously variable soil moisture profile by inversion of a ground penetrating radar (GPR) signal. Synthetic experiments were conducted to demonstrate the well‐posedness of the inverse problem for the specific case of identifying a soil moisture profile in hydrostatic equilibrium with a water table. In this case, the profile agrees with the water retention curve of the soil. The analysis subsequently extends to an actual case study in controlled outdoor conditions on a large tank filled with sand. Due to the presence of a discontinuity in the actual dielectric profile, inversion of the continuous model (Model 1) led to poor results. Only the surface soil moisture was well estimated. Including the observed discontinuity in the model (Model 2) led to a good estimation of the water content profile. Finally, we observed that the surface water content can be accurately estimated using a simplified three‐layer model (Model 3). Generally, the observed confidence intervals on the estimated parameters are large, which denotes a lack of model sensitivity to the soil parameters. We attributed the low sensitivity to the high operating frequency range. Lower frequencies would have been required to obtain more information from the larger depths. Nevertheless, high frequencies allowed for an accurate estimation of the surface soil moisture, which offers particularly promising perspectives in humanitarian demining and agricultural applications.
Imaging of Water Content Distributions inside a Lysimeter using GPR Tomography
Authors: Schmalholz Jürgen , Stoffregen Heiner , Kemna Andreas , Yaramanci Ugur
Published: July 2010

In a study to investigate water content distributions inside a lysimeter in a noninvasive manner, we used ground penetrating radar (GPR) tomography. Our main objective was to evaluate the temporal changes and spatial distributions of the volumetric water content after a short but intensive irrigation of part of the lysimeter. High frequency GPR antennas of 1‐GHz nominal frequency were used because of the small spatial dimensions of the investigated lysimeter (cylinder of 1.5‐m height and 1.2‐m diameter) and the desired spatial resolution in the range of decimeters. To ensure a relatively steady distribution of water inside the lysimeter for the time‐consuming tomographic survey, simple parallel transmission measurements were used to track the water dynamics. Water contents and water content changes were calculated by means of a mixing formula describing the relation between electromagnetic wave propagation velocity and the water content. The transmission measurements indicate a diffusive process following the irrigation for a duration of several hours. The tomographic measurements clearly show the area of increased water content associated with the irrigation.
Electrical Response of Flow, Diffusion, and Advection in a Laboratory Sand Box
Authors: Maineult Alexis , Bernabé Yves , Ackerer Philippe
Published: July 2010

Self‐potential monitoring (SPM) is one of the most promising geophysical methods for hydrologic applications, since any change in subsurface water flow, chemistry, or thermodynamics can induce an electrical response. However, major difficulties may arise because different couplings (e.g., electrokinetics and electrodiffusion) can occur simultaneously. We performed laboratory experiments to isolate the electric response of flow during conditions of constant composition, of ionic diffusion of NaCl in stagnant fluid, and of advective transport of NaCl and KCl. For this purpose, fluid flow and/or salt diffusion were generated in a rectangular sand box, and the resulting electric potential differences were measured between custom‐made, small, unpolarizable electrodes. In pure electrokinetic experiments (i.e., flow of water with constant salinity), the electric signal was proportional to the hydraulic gradient and to the salinity, in agreement with previous experimental and theoretical results. The other experiments showed that diffusive and advective transport of salt (i.e., in stagnant and flowing fluid conditions, respectively) can generate significant electric potential differences. Monitoring these potential differences allows determination of the motion of the concentration front in the sand box.
Transport and Time Lag of Chlorofluorocarbon Gases in the Unsaturated Zone, Rabis Creek, Denmark
Authors: Engesgaard Peter , Højberg Anker L. , Hinsby Klaus , Jensen Karsten H. , Laier Troels , Larsen Flemming , Busenberg Eurybiades , Plummer L. Niel
Published: July 2010

Transport of chlorofluorocarbon (CFC) gases through the unsaturated zone to the water table is affected by gas diffusion, air–water exchange (solubility), sorption to the soil matrix, advective–dispersive transport in the water phase, and, in some cases, anaerobic degradation. In deep unsaturated zones, this may lead to a time lag between entry of gases at the land surface and recharge to groundwater. Data from a Danish field site were used to investigate how time lag is affected by variations in water content and to explore the use of simple analytical solutions to calculate time lag. Numerical simulations demonstrate that either degradation or sorption of CFC‐11 takes place, whereas CFC‐12 and CFC‐113 are nonreactive. Water flow did not appreciably affect transport. An analytical solution for the period with a linear increase in atmospheric CFC concentrations (approximately early 1970s to early 1990s) was used to calculate CFC profiles and time lags. We compared the analytical results with numerical simulations. The time lags in the 15‐m‐deep unsaturated zone increase from 4.2 to between 5.2 and 6.1 yr and from 3.4 to 3.9 yr for CFC‐11 and CFC‐12, respectively, when simulations change from use of an exponential to a linear increase in atmospheric concentrations. The CFC concentrations at the water table before the early 1990s can be estimated by displacing the atmospheric input function by these fixed time lags. A sensitivity study demonstrates conditions under which a time lag in the unsaturated zone becomes important. The most critical parameter is the tortuosity coefficient. The analytical approach is valid for the low range of tortuosity coefficients (τ = 0.1–0.4) and unsaturated zones greater than approximately 20 m in thickness. In these cases the CFC distribution may still be from either the exponential or linear phase. In other cases, the use of numerical models, as described in our work and elsewhere, is an option.