Significant Transport Mechanism Research Articles

Excited State Dynamics and Transport of Self-Trapped Excitons in Bi-Doped Cs2Na0.4Ag0.6In(1-y)BiyCl6 Double Perovskites.

Lead-free double perovskites (DPs) have become notable in white light emission applications due to the self-trapped exciton (STE) formation in the excited state. However, the mechanism understanding of the excited state dynamics and transport of STE remains ambiguous. Here, we demonstrate a new STE (Bi-STE) forming in tiny Bi-doped Cs2Na0.4Ag0.6InCl6, alongside its intrinsic STE (i-STE), resulting in the DPs photoluminescence quantum yield (PLQY) increasing to as high as >90%. The i-STE exhibits faster formation (60 fs) and slower relaxation dynamics (2.8 μs) compared to the Bi-STE. Moreover, we unveil that the Bi doping can augment the i-STE diffusion properties to attain a diffusion coefficient (diffusion length) of 0.012 cm2 s-1 (1.7 μm) at room temperature, indicating their promise in photovoltaic applications. Our results shed light on significant STE dynamics and transport mechanisms in DPs, providing a new roadmap for advancing existing and crafting new DPs in light emission applications.

Ge–GaAs–Ge Heterojunction MOSFETs for Mixed-Signal Applications

A lattice matched heterojunction intraband tunnel (HJIBT) FET is proposed. The performance dependence of the device on conduction band (CB) discontinuity at source-channel and drain-channel interface is addressed using numerical simulation. Various mechanisms governing transport phenomena in the HJIBT FET are investigated in detail for different CB offsets (CBOs). For low gate to source voltage ( ${V}_{\text {GS}}$ ), thermionic emission is found to be the most significant transport mechanism. For moderate ${V}_{\text {GS}}$ , intraband tunneling phenomenon dominates over thermionic emission and continues to remain so. At high ${V}_{\text {GS}}$ , band-to-band tunneling occurs in HJIBT FETs. The proposed device shows improved figures of merit such as drain-induced barrier lowering (DIBL), ON-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) to OFF-current ( ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) ratio ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ), subthreshold slope (SS), gate capacitance ( ${C}_{\text {G}}$ ), ${g}_{m}$ (transconductance), and ${f}_{T}$ (cut-off frequency), with respect to conventional MOSFET. Also, the design of a high-performance hybrid 6T-static random access memory (SRAM) is proposed.

Charge-state independent anomalous transport for a wide range of different impurity species observed at Wendelstein 7-X

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges (Z = 12–44) and atomic masses (M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X.

Novel technique to measure mutual bulk fluid diffusion using NMR 1-D gradient

Many modelling and theoretical studies have shown that diffusion can be a significant transport mechanism in low-permeability porous media. Understanding the process allows engineers to better predict reservoir performance during both primary production and enhanced recovery in unconventional reservoirs. Direct measurement of effective diffusion in tight rocks is difficult, due to small pore volumes and the lack of techniques to actually monitor the process. Conventional diffusion measurements generally require fluid sampling, which induces a pressure transient which changes the mass transfer mechanism. Previously, we introduced a novel technique to measure tortuosity in nano-porous media by simultaneously monitoring methane versus nitrogen concentrations at high pressure using transmission Infrared Spectroscopy (IR). To complete the estimation of effective diffusion, bulk fluid diffusion coefficient also needs to be measured. In this study, we demonstrate the usage of Nuclear Magnetic Resonance (NMR) 1-D imaging to examine the dynamic change of Hydrogen Index (HI) across the interface between two bulk fluids. The experiment was conducted between a crude oil sample and methane; fluid samples were pressurized within an NMR transparent ZrO2 pressure cell which operates at pressures up to 10,000 psi. The Hydrogen Index (HI) profile was continuously measured and recorded for 7 days. The results provided oil the swelling factor and the concentration profile as a function of both time and distance. These data then were fitted with Maxwell-Stefan equation to precisely back calculate the diffusion coefficient between oil and gas samples at high pressure. Accurate estimation of tortuosity and fluid diffusion is critical for the gas injection strategy in a shale formation. Greater tortuosity and smaller fluid diffusion rate lead to longer injection and production times for desirable economic recovery.

The wind-driven formation of cross-shelf sediment plumes in a large lake.

Wind-driven turbidity plumes frequently occur in the western arm of Lake Superior and may represent a significant cross shelf transport mechanism for sediment, nutrient and biota. Here we characterize a plume that formed in late April 2016 using observations from in situ sensors and remote sensing imagery, and estimate the volume of cross shelf transport using both the observations and an idealized analytical model of plume formation. The steady-state, barotropic model is used to determine a relationship between the intensity and duration of a wind event and the volume of water transported from nearshore to offshore during the event. The model transport is the result of nearshore flow in the direction of the wind and a pressure-gradient-driven counter flow in the deeper offshore waters, consistent with observations. The volume of offshore transport associated with the 2016 plume is estimated by both methods to have been on the order of 1010 m3. Analysis of similar events from 2008-2016 shows a strong relationship between specific wind impulse and plume volume. Differences in the intensity and duration of individual events as well as ice cover, which prevents plume formation, lead to interannual variability of offshore transport ranging over an order of magnitude and illustrates how wind-driven processes may contribute to interannual variability of ecosystem functioning.

A wind-driving disc model for the mm-wavelength polarization structure of HL Tau

The recent advent of spatially resolved mm- and cm-wavelength polarimetry in protostellar accretion discs could help clarify the role of magnetic fields in the angular momentum transport in these systems. The best case to date is that of HL~Tau, where the inability to produce a good fit to the 1.25-mm data with a combination of vertical and azimuthal magnetic field components was interpreted as implying that centrifugally driven winds (CDWs) are probably not a significant transport mechanism on the $\sim 10^2\,$au scale probed by the observations. Using synthetic polarization maps of heuristic single-field-component discs and of a post-processed simulation of a wind-driving disc, we demonstrate that a much better fit to the data can be obtained if the radial field component, a hallmark of the CDW mechanism, dominates in the polarized emission region. A similar inference was previously made in modelling the far-infrared polarization map of the pc-scale dust ring in the Galactic centre. To reconcile this interpretation with theoretical models of protostellar discs, which indicate that the wind is launched from a comparatively high elevation above the mid-plane, we propose that most of the polarized emission originates -- with a high ($\ga 10$\%) intrinsic degree of polarization -- in small ($\la 0.1\,$mm) grains that remain suspended above the mid-plane, and that the bulk of the mm-wavelength emission is produced -- with low intrinsic polarization -- by larger grains that have settled to the mid-plane.

Modeling biological tissue growth: Discrete to continuum representations

There is much interest in building deterministic continuum models from discrete agent-based models governed by local stochastic rules where an agent represents a biological cell. In developmental biology, cells are able to move and undergo cell division on and within growing tissues. A growing tissue is itself made up of cells which undergo cell division, thereby providing a significant transport mechanism for other cells within it. We develop a discrete agent-based model where domain agents represent tissue cells. Each agent has the ability to undergo a proliferation event whereby an additional domain agent is incorporated into the lattice. If a probability distribution describes the waiting times between proliferation events for an individual agent, then the total length of the domain is a random variable. The average behavior of these stochastically proliferating agents defining the growing lattice is determined in terms of a Fokker-Planck equation, with an advection and diffusion term. The diffusion term differs from the one obtained Landman and Binder [J. Theor. Biol. 259, 541 (2009)] when the rate of growth of the domain is specified, but the choice of agents is random. This discrepancy is reconciled by determining a discrete-time master equation for this process and an associated asymmetric nonexclusion random walk, together with consideration of synchronous and asynchronous updating schemes. All theoretical results are confirmed with numerical simulations. This study furthers our understanding of the relationship between agent-based rules, their implementation, and their associated partial differential equations. Since tissue growth is a significant cellular transport mechanism during embryonic growth, it is important to use the correct partial differential equation description when combining with other cellular functions.

Water Vapor Transport in Soils from a Pervaporative Irrigation System

AbstractA novel method for irrigation with saline water uses a polymer membrane, formed into a tube, to treat and distribute the water simultaneously. The flux of water across the membrane occurs by the process of pervaporation, during which a phase change from liquid to vapor occurs. Thus, water arrives in the soil in the vapor phase. The experimental results presented in this paper demonstrate that, contrary to previous assumptions, soil vapor flows are a significant transport mechanism during pervaporative irrigation in dry soils. The soil water sorption properties affect the rate of condensation in the soil, which in turn affects both the water distribution in the soil and the loss of water vapor to the atmosphere. The flux from the tube becomes limited by high humidities adjacent to the external surface of the membrane. Thus, enhancing condensation in the soil or increasing diffusion through the soil increases flux from the system. These findings highlight the need to consider how plants might intera...

Evaluación de mejores prácticas de manejo para reducir las pérdidas de nutrientes contaminantes en escorrentía desde suelos enmendados con carnada de pollos parrilleros. II. Cobertura de hierbas.

Se evaluó el impacto de la extensión de la cobertura del pasto Bermuda [Cynodon dactylon (L.)] en la pérdida de sedimentos y nutrientes en escorrentía en suelos enmendados con residuos de carnada de pollos parrilleros (con alum). La escorrentía generada de los suelos completamente descubiertos reflejó valores significativamente mayores en los volúmenes de escorrentía y en las concentraciones de sólidos suspendidos, fósforo total (TP) y nitrógeno total Kjeldahl (TKN), que en suelos con ≥80% de cobertura de pasto. Además, el suelo descubierto exhibió un tiempo de inicio de escorrentía menor (lapso de tiempo entre inicio de la lluvia y el comienzo de la escorrentía) que los suelos con pasto. Las concentraciones promedio de P total y TKN en la escorrentía de bandejas experimentales con a80% de cobertura de hierba exhibieron una reducción de 93% y 89.5%, respectivamente, comparado con aquellas en la escorrentía de las bandejas con suelo al descubierto. Por otra parte, la concentración de sólidos suspendidos mostró una reducción de 99% sobre la del suelo al descubierto. Las pérdidas en fósforo disuelto estuvieron bien por debajo del límite de 1 mg/L sugerido como el nivel para el control de contaminación de fósforo de tierras agrícolas, incluyendo suelos enmendados con residuos de carnada de pollos parrilleros. En el caso de estos residuos, el resultado es un reflejo de la efectividad del alum en la reducción de la pérdida de fósforo disuelto en la escorrentía. En general, tanto las concentraciones de P total como TKN siguieron las tendencias establecidas por las pérdidas de sedimentos. Una disminución significativa en la concentración de sedimentos suspendidos fue observada con el tiempo en ambos tipos de coberturas del suelo, con pérdidas significativamente más altas en suelos al descubierto que en suelos con cobertura vegetal. En contraste, el nitrato exhibió una correlación negativa con las concentraciones de sólidos suspendidos. Aplicaciones de residuos de carnada de pollos parrilleros causaron un aumento significativo en las concentraciones de los constituyentes disueltos (i.e., P disuelto y N03-N) en la escorrentía, pero no así de otros constituyentes (i.e., P total y TKN) para los cuales las pérdidas de sedimentos representan un mecanismo significativo de transporte. La aplicación de residuos de carnada de pollos parrilleros redujo significativamente la pérdida en la concentración de sólidos suspendidos para cada categoría de cubierta vegetal, aunque una pérdida mucho mayor fue observada para el suelos al descubierto. La combinación de aplicación de alum y una buena cobertura de hierba (≥80%) reduce los estimados de carga de exporte de sedimentos suspendidos, P total y TKN en la escorrentía por más de un 95% comparado con los de los suelos al descubierto.

Vadose zone attenuation of organic compounds at a crude oil spill site — Interactions between biogeochemical reactions and multicomponent gas transport

Contaminant attenuation processes in the vadose zone of a crude oil spill site near Bemidji, MN have been simulated with a reactive transport model that includes multicomponent gas transport, solute transport, and the most relevant biogeochemical reactions. Dissolution and volatilization of oil components, their aerobic and anaerobic degradation coupled with sequential electron acceptor consumption, ingress of atmospheric O 2, and the release of CH 4 and CO 2 from the smear zone generated by the floating oil were considered. The focus of the simulations was to assess the dynamics between biodegradation and gas transport processes in the vadose zone, to evaluate the rates and contributions of different electron accepting processes towards vadose zone natural attenuation, and to provide an estimate of the historical mass loss. Concentration distributions of reactive (O 2, CH 4, and CO 2) and non-reactive (Ar and N 2) gases served as key constraints for the model calibration. Simulation results confirm that as of 2007, the main degradation pathway can be attributed to methanogenic degradation of organic compounds in the smear zone and the vadose zone resulting in a contaminant plume dominated by high CH 4 concentrations. In accordance with field observations, zones of volatilization and CH 4 generation are correlated to slightly elevated total gas pressures and low partial pressures of N 2 and Ar, while zones of aerobic CH 4 oxidation are characterized by slightly reduced gas pressures and elevated concentrations of N 2 and Ar. Diffusion is the most significant transport mechanism for gases in the vadose zone; however, the simulations also indicate that, despite very small pressure gradients, advection contributes up to 15% towards the net flux of CH 4, and to a more limited extent to O 2 ingress. Model calibration strongly suggests that transfer of biogenically generated gases from the smear zone provides a major control on vadose zone gas distributions and vadose zone carbon balance. Overall, the model was successful in capturing the complex interactions between biogeochemical reactions and multicomponent gas transport processes. However, despite employing a process-based modeling approach, honoring observed parameter ranges, and generally obtaining good agreement between field observations and model simulations, accurate quantification of natural attenuation rates remains difficult. The modeling results are affected by uncertainties regarding gas phase saturations, tortuosities, and the magnitude of CH 4 and CO 2 flux from the smear zone. These findings highlight the need to better delineate gas fluxes at the model boundaries, which will help constrain contaminant degradation rates, and ultimately source zone longevity.

Free‐surface visualization of streamwise vortices in a channel flow

Visualization of the free surface in an open channel can provide insight into certain characteristics of the flow field, such as the occurrence of low‐frequency, large‐scale eddies. This paper presents some experimental results obtained in a moving‐bed flume constructed for research on free‐surface transport without significant shear at the free surface (without wind). Free‐surface velocities were measured using particle tracer velocimetry (PTV) at bulk Reynolds numbers (UH/ν) between 8500 and 45,500. The PTV streaks were digitized in order to perform a quantitative analysis, allowing determination of spatial velocity autocorrelations. Large, vertical oriented vortices and lateral movement are apparent on the water surface, with a lateral length scale of between 1 and 1.5 times the depth. A relationship between these largest scales of turbulence, large streamwise vortices, and secondary currents in the flow is hypothesized. The large‐scale turbulent eddies with streamwise vorticity that form secondary currents across a movable‐bed, open‐channel flow also exist in a smooth‐bed open‐channel flow but are not fixed in space. They oscillate laterally at low frequencies and form permanent secondary currents only near the wall, where lateral migration is limited. They appear to be a significant transport mechanism in the outer layer, transporting mass, energy, and outer region bursts to the free surface.

A theoretical and experimental analysis of a membrane bioreactor performance in recycle configuration

The behavior of membrane bioreactors operating in recycle configuration is characterized from both a theoretical and an experimental point of view. The theoretical model is based on the unsteady-state balance equations governing momentum and mass transfer within the regions that can be identified in a membrane reactor with immobilized enzyme, coupled to the unsteady-state mass balance on the components in feed tank. The resulting system of non-linear partial differential equations has been numerically solved by Finite Elements Method (FEM), implemented by a commercial package called FEMLAB. The available CFD code allowed determining the time evolution of both velocity and concentration profiles without resorting to considerable simplification hypotheses. As far as the reaction term in the mass balance equations is concerned, reference has been made to lactose hydrolysis catalyzed by immobilized β-galactosidase, also carried out in the experimental part of this work. The chemical reaction is supposed to take place solely in the membrane dense and spongy layers where biocatalyst is actually confined and distributed according to its concentration ratio E 3/ E 2. A set of intrinsic kinetic constants, obtained from the literature, has been used to evaluate the actual reaction rate thus evaluating the effects of immobilization on enzyme activity. The contribution of the most significant transport mechanisms affecting reactor performance has been determined analyzing the dynamic evolution of three representative parameters, i.e. the effectiveness factor, the degree of conversion within the membrane and the bioreactor performance index, as functions of the operating conditions. The proposed transport model is very general as it may describe the behavior of different types of heterogeneous reacting systems by just adapting either the reaction rate expression or the set of intrinsic kinetic constants. An experimental analysis of system behavior has validated the theoretical predictions, showing a good agreement between experimental and theoretical results.

Spin injection across (110) interfaces: Fe∕GaAs(110) spin-light-emitting diodes

We report electrical spin injection from an Fe contact into a (110)-oriented light-emitting diode (LED) structure, and compare results with data obtained from (001)-oriented structures to address the dependence of spin injection on interface and orientation. Fe∕AlGaAs∕GaAs LEDs were grown by molecular-beam epitaxy, and processed to form surface emitting structures. Electroluminescence results obtained using a reverse-biased Fe Schottky tunnel barrier injector show that a 13% electron spin polarization is achieved in the GaAs(110) quantum well due to injection across the Fe∕AlGaAs(110) interface. Analysis of the transport data indicates that tunneling is a significant transport mechanism at low temperatures. The temperature dependence of the spin polarization is similar to that of (001)-oriented spin LEDs, and is dominated by the GaAs electron spin lifetime.

Convective Heat Transfer Near One-Dimensional and Two-Dimensional Wall Temperature Steps

Steady-state experiments with one-dimensional and two-dimensional calorimeters were used to study the convective heat transfer near sharp steps in wall temperature in a turbulent boundary layer. Data acquired under low and high freestream turbulence conditions indicated that spanwise turbulent diffusion is not a significant heat transport mechanism for a two-dimensional temperature step. The one-dimensional calorimeter heat transfer data were predicted within ±5 percent using the STAN7 boundary layer code for situations with an abrupt wall temperature step. The conventional correlation with an unheated starting length correction, in contrast, greatly under-predicts the heat transfer for the same experimental cases. A new correlation was developed that is in good agreement with near and far-field semi-analytical solutions and predicts the calorimeter heat transfer data to within ±2 percent for temperature step boundary condition cases.

Vertical dynamical transport of mesospheric constituents by dissipating gravity waves

Over 400 h of Na wind/temperature lidar observations, obtained at the Starfire Optical Range, NM, are used to study the vertical dynamical transport of Na in the mesopause region between 85 and 100 km . Dynamical transport occurs when dissipating, non-breaking gravity waves impart a net vertical displacement in atmospheric constituents as they propagate through a region. We show that the vertical constituent flux can be related in a simple way to the vertical heat flux. Breaking gravity waves also contribute to eddy transport by generating turbulence. Because eddy transport is a mixing process, it only occurs in the presence of a gradient in the concentration profile of the constituent, while dynamical transport can be sustained even in the absence of such a gradient. The dynamical Na flux is compared with the predicted eddy flux. The maximum downward dynamical flux of Na is −280 m/ s cm 3 at 88 km . The maximum downward eddy flux is −160 m/ s cm 3 at the same altitude assuming the diffusion coefficient is 200 m 2/ s . The observational results are consistent with theoretical predictions below 93 km and show that dynamical transport often exceeds the vertical transport associated with eddy diffusion. The theoretical models are used to predict the dynamical and eddy fluxes of atomic oxygen and show that for this constituent, dynamical transport is also a significant transport mechanism.

A theoretical analysis of transport phenomena in a hollow fiber membrane bioreactor with immobilized biocatalyst

The behavior of a hollow fiber (HF) membrane bioreactor with entrapped biocatalyst is analyzed and characterized from a theoretical point of view. The model is based on the numerical solution of the dimensionless balance equations governing mass transfer within the regions that can be defined for this reacting system, namely the fiber lumen, the membrane dense and spongy layers. The chemical reaction is supposed to take place solely in the last two regions (skin and sponge) where the biocatalyst is confined by entrapment and distributed according to the ratio E 3/ E 2. The evaluation of the significant transport mechanisms of the substrate consumption rates, and more generally of the reactor performance have been carried out by analyzing two representative parameters, i.e. the effectiveness factor and the performance index, as functions of the operating conditions, expressed in terms of a set of characteristic dimensionless groups. The enzyme distribution ratio, E 3/ E 2, its overall loading, the applied trans-membrane pressure (TMP) difference, the substrate feed concentration were investigated as the system key parameters. The reaction kinetics have been modeled with reference to Michaelis–Menten rate equation, modified to account for possible substrate and product inhibition. The results of the theoretical analysis enable to predict an optimal value of TMP and, therefore, of the permeate flux, as a trade-off between the necessity of reducing the transfer resistances in the substrate access to enzyme and that of allowing suitable residence time. The bioreactor performances are no longer affected by the distribution ratio E 3/ E 2, once that this parameter has reached almost unity value : this means that the enzyme can be effectively immobilized just in the membrane spongy region, with the advantage of a significant system simplification from both a technological and an economic standpoint. The model, however, is quite general and can be applied also to different reacting systems, in order to evaluate how the catalyst position within a specific support can affect their performances. A comparison between the theoretical model predictions and some literature experimental data has been also attempted, showing a good agreement for some typical operating conditions.

Large-Eddy Simulation of Sediment Transport: Currents over Ripples

A large-eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples. The code solves the Navier-Stokes equations and an advection-diffusion equation for the sediment, and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study. The main features of interest in this flow are seen because of the high resolution of the grid, especially near the ripple boundary. This yields a detailed bottom shear stress distribution, properly identified coherent structures, and resolved sharp gradients in the sediment concentration. Globally, the sediment is carried up into the flow from locations where the shear stress is high—on the upslopes of ripple crests—and is advected downstream by the current and upward by the vertical velocity component. Regions of positive vertical velocity are associated with Gortler vortices, which represent a significant sediment transport mechanism in this flow.

Transport of 137Cs And 239,240Pu with Ice-rafted Debris in the Arctic Ocean

Ice rafting is the dominant mechanism responsible for the transport of fine-grained sediments from coastal zones to the deep Arctic Basin. Therefore, the drift of ice-rafted debris (IRD) could be a significant transport mechanism from the shelf to the deep basin for radionuclides originating from nuclear fuel cycle activities and released to coastal Arctic regions of the former Soviet Union. In this study, 28 samples of IRD collected from the Arctic ice pack during expeditions in 1989-95 were analyzed for 137Cs by gamma spectrometry and for 239Pu and 240Pu by thermal ionization mass spectrometry. 137Cs concentrations in the IRD ranged from less than 0.2 to 78 Bq/kg (dry weight basis). The two samples with the highest 137Cs concentrations were collected in the vicinity of Franz Josef Land, and their backward trajectories suggest origins in the Kara Sea. Among the lowest 137Cs values are seven measured on sediments entrained on the North American shelf in 1989 and 1995, and sampled on the shelf less than six months later. Concentrations of 239Pu + 240Pu ranged from about 0.02 to 1.8 Bq/kg. The two highest values came from samples collected in the central Canada Basin and near Spitsbergen; calculated backward trajectories suggest at least 14 years of circulation in the Canada Basin in the former case, and an origin near Severnaya Zemlya (at the Kara Sea/Laptev Sea boundary) in the latter case. While most of the IRD samples showed 240Pu/239Pu ratios near the mean global fallout value of 0.185, five of the samples had lower ratios, in the 0.119 to 0.166 range, indicative of mixtures of Pu from fallout and from the reprocessing of weapons-grade Pu. The backward trajectories of these five samples suggest origins in the Kara Sea or near Severnaya Zemlya.

Lysimeter study of the fate of nutrients following subsurface injection and surface application of dairy pond sludge to pasture

The fate of nutrients in an organic waste (dairy pond sludge) was determined after surface application and subsurface injection into field plots on a free draining pasture soil. Soil monolith lysimeters were collected from the field plots and used to monitor plant uptake and leaching losses of nitrogen, phosphorus and potassium for 2 years following a single waste application, and for 1 year following two successive annual applications. Volatilisation losses of ammonia were low due to the low ammonium-nitrogen content of the waste and its near-neutral pH. Substantial nitrification occurred following waste injection. Nitrate leaching losses were low, even though nitrate accumulation following nitrification was high and macropore flow was a significant transport mechanism in this soil. The amount of nitrate leached was consistently higher following subsurface injection compared to surface application, although the concentration and amount lost in the drainage water was still relatively low. The low leaching losses were attributed to enhanced plant uptake of nutrients and possibly denitrification within the soil. Pasture plant production was increased by up to 40% following both methods of waste application.

Chernobyl fallout radionuclides in Lake Sniardwy, Poland

Following the introduction of Chernobyl radionuclides, in May 1986, to Lake Sniardwy, the largest lake in Poland, sediment cores and fish were periodically collected through 1989 and analysed for 144Ce, 134Cs and 137Cs. Within four months of the fallout, 134Cs had penetrated down to about 10 cm in one sediment core and, in cores collected from three sites, a year later, had penetrated from 14 to 24 cm. Laboratory measurement of the partitioning of a carrier-free spike of 137Cs between sediment and pore water yielded a K d of 4.3 × 10 3 ml g −1 ruling out molecular diffusion ( D eff = 0.06 cm 2 year −1) as a significant transport mechanism. Rapid vertical transport is ascribed to mixing through strong coupling of wind-driven currents to sediments in this shallow, polymictic lake (5.8 m mean depth) and to biological activity. Profiles of 134Cs at several sites and changes in profiles over three years at one site were described by eddy diffusive mixing (3–20 cm 2 year −1) of a layer of activity initially deposited on the sediment surface. The generally discontinuous nature of profiles of 144Ce is ascribed to its association with discreet ‘hot’ (nuclear fuel) particles. The average activity of 137Cs in the flesh of bream ( Abramis brama) increased up to 120 times that of pre-Chernobyl levels (measured in 1985) 1 year after the fallout event. The delayed maximum and subsequent decrease in 137Cs activity are described by a first order kinetic model with an uptake rate constant proportional to the time-dependent concentration of the isotope in surface sediments. Model calculations are substantially improved if the uptake rate constant includes a term, measuring the ‘availability’ of 137Cs to fish, calculated in terms of radial diffusion of radiocaesium into sediment particles. The inferred residence time of 137Cs in bream was about 1 year.