Determination of radionuclide exchangeability in freshwater systems

Two freshwater sediments were spiked with 57Co, 85Sr and 134Cs and left for adsorption times ranging from 1 day to over 60 days. Following adsorption, the 'exchangeable' pool of each radionuclide was measured using ammonium acetate extractions and a sequential leach procedure (Tessier et al., 1979), and the results were compared. Exchangeability was found to depend upon the sediment, radionuclide, sorption time and the identity of extracting agent. All three radionuclides showed a shift with increasing adsorption time from regular exchange sites to sites which are sterically hindered, but a fixation within the sediment was only observed for 57Co and 134Cs, with similar ammonium acetate extraction yields for both radionuclides. Misleading results were obtained during the sequential leach procedure due to redistribution and, therefore, the inclusion of a NH4+ leach after the MgCl2 extraction step was suggested. A mathematical model of element speciation was fairly successful at defining the 'exchangeable' fraction, suggesting that the chemically- and mathematically-defined fractions were similar.     

The competitive role of organic carbon and dissolved sulfide in controlling the distribution of mercury in freshwater lake sediments

The detailed distribution of mercury was studied in sediments and porewaters of two freshwater lakes, which were selected because of the contrasting conditions they present at their respective sediment-water interface (SWI). One lake is characterized by a SWI that remains oxic all year long whereas the other one shows a clear seasonal variation with the evolution of strongly anoxic conditions through the summer season. The results of the study clearly identify the importance of redox conditions on the geochemical behaviour of Hg at the SWI of both lakes but a very limited influence of an oxidized layer enriched in Fe and Mn oxyhydroxides at the top of the sediment of the oxic lake. In both lakes, a competitive effect on the cycling and mobility of the element was observed between natural organic matter and amorphous or organo-sulfide compounds. The proportion of Hg associated to natural organic matter in sediments showed a general increase with sediment depth. A fraction containing elemental Hg and Hg suspected to be bound to iron sulfides and organo-sulfides constituted the other major fraction of solid Hg in the sediments of both lakes. This second pool of Hg was generally larger at the top of the sediment where the production of dissolved sulfides is usually more detectable and it decreases with depth, suggesting that the metal is partially transferred from one pool being the sulfides including amorphous FeS and organo-sulfides to the organic matter pool. Methyl Hg represented less than 1% of the total Hg in sediments of both lakes. Our results obtained at different times of the summer season from two lakes contrasted by their SWI emphasize the competitive or alternating role played by dissolved and solid natural organic matter and sulfides on the fate of Hg in freshwater systems.     

Potential kinetic availability of metals in sulphidic freshwater sediments

The insolubility of metal sulphides is believed to limit the bioavailability of trace metals in sulphidic sediments. However, if non-equilibrium conditions are important, metals may be more available than simple thermodynamic calculations suggest. To investigate the possible dynamic supply of Cu, Ni and Zn in a sulphidic freshwater sediment, they were measured, along with iron, manganese and sulphide, by the technique of diffusive gradients in thin-films (DGT). DGT measures the supply of solute from sediment to solution in response to a local solute sink. Release of Mn, Cu, Zn and Ni was observed at the sediment surface and attributed to the supply from reductive dissolution of manganese oxides. The depth profile of simultaneously extractable metals (SEM) for Cu and Ni followed the shape of the Mn profile more closely than the profiles of either acid volatile sulphur (AVS) or Fe, again consistent with supply from Mn oxides. Solubility calculations for a mesocosm of homogenised sediment indicated supersaturation with respect to the sulphides of Fe, Cu, Ni and Zn, yet DGT measurements demonstrated a substantial supply of both trace metals and sulphide from the solid phase to the pore waters. Ratios of metals measured in pore waters by DGT were consistent with their release from iron and manganese oxides, indicating that supply, as much as removal processes, determines the pseudo-steady state concentrations in the pore waters. The observations suggest that trace metals are not immediately bound in an insoluble, inert form when they are in contact with sulphide. This has consequences for modelling metal processes in sediment, as well as for uptake by some biota.     

Characterization of bottom sediments in lakes using hydroacoustic methods and comparison with laboratory measurements

The acoustical properties of bottom sediments in two lakes were shown to be strongly correlated with clay content, organic C and total N concentrations, and other important sediment properties. The fractal dimension of the bottom echo was more strongly correlated with sediment physical and chemical properties than energy-based measures. The fractal dimension was also related to rates of PO₄-P and NH₄-N release from intact sediment cores and sediment oxygen demand. Measurements made at 430-kHz were more sensitive to differences in sediment properties than 201- or 38-kHz. Hydroacoustic measurements allow rapid assessment of properties important in lake restoration and water resource management.     

Sulfide formation in freshwater sediments, by sulfate-reducing microorganisms with diverse tolerance to salt

Understanding how sulfate-reducing microbes in freshwater systems respond to added salt, and therefore sulfate, is becoming increasingly important in inland systems where the threat from salinisation is increasing. To address this knowledge gap, we carried out mesocosm studies to determine how the sulfate-reducing microbial community in sediments from a freshwater wetland would respond to salinisation. The levels of inorganic mineral sulfides produced after 6 months incubation were measured to determine whether they were in sufficient quantity to be harmful if re-oxidized. Comparative sequence analysis of the dissimilatory sulfite reductase (DSR) gene was used to compare the sulfate-reducing community structure in mesocosms without salt and those incubated with moderate levels of salt. The amount of total S, acid volatile sulfide or chromium-reducible sulfide produced in sediments with 0, 1 or 5 g L^− 1 added salt were not significantly different. Sediments subjected to 15 g L^− 1 salt contained significantly higher total S and acid volatile sulfide, and levels were above trigger values for potential harm if re-oxidation occurred. The overall community structure of the sulfate-reducing microbiota (SRM) was explained by the level of salt added to sediments. However, a group of sulfate reducers were identified that occurred in both the high salt and freshwater treatments. These results demonstrate that freshwater sediments contain sulfate reducers with diverse abilities to respond to salt and can respond rapidly to increasing salinity, explaining the observation that harmful levels of acid volatile sulfides can form rapidly in sediments with no history of exposure to salt.     

Comparing zonal and CFD model predictions of isothermal indoor airflows to experimental data

It is inappropriate to use the assumption of instantaneously well-mixed zones to model airflows and pollutant transport in large indoor spaces. We investigate two approaches for describing the details of airflows in large indoor spaces, for accuracy and suitability for integration with multi-zone infiltration models. One approach, called the zonal method, was developed over the last 15 years to provide an improvement over the well-mixed assumption. The second approach is the use of a computational fluid dynamics simulation using a coarse grid model of the large indoor space. We compare velocity predictions from different formulations of zonal methods and coarse-grid k-epsilon computational fluid dynamics (CFD) models, to measurements, in a 2D mechanically ventilated isothermal room. Our results suggest that, when airflow details are required, coarse-grid CFD is a better-suited method to predict airflows in large indoor spaces coupled with complex multi-zone buildings, than are the zonal methods. Based on the comparison of pressure predictions from different models, we offer guidance regarding the coupling of a model of detailed airflow in large spaces to algebraic multi-zone infiltration models.     

Estimation of apparent rate coefficients for radionuclides interacting with marine sediments from Novaya Zemlya

To assess the impact of radionuclides entering the marine environment from dumped nuclear waste, information on the physico-chemical forms of radionuclides and their mobility in seawater-sediment systems is essential. Due to interactions with sediment components, sediments may act as a sink, reducing the mobility of radionuclides in seawater. Due to remobilisation, however, contaminated sediments may also act as a potential source of radionuclides to the water phase. In the present work, time-dependent interactions of low molecular mass (LMM, i.e. species < 10 kDa) radionuclides with sediments from the Stepovogo Fjord, Novaya Zemlya and their influence on the distribution coefficients (Kd values) have been studied in tracer experiments using 109Cd2+ and 60Co2+ as gamma tracers. Sorption of the LMM tracers occurred rapidly and the estimated equilibrium Kd(eq)-values for 109Cd and 60Co were 500 and 20000 ml/g, respectively. Remobilisation of 109Cd and 60Co from contaminated sediment fractions as a function of contact time was studied using sequential extraction procedures. Due to redistribution, the reversibly bound fraction of the gamma tracers decreased with time, while the irreversibly (or slowly reversibly) associated fraction of the gamma tracers increased. Two different three-compartment models, one consecutive and one parallel, were applied to describe the time-dependent interaction of the LMM tracers with operationally defined reversible and irreversible (or slowly reversible) sediment fractions. The interactions between these fractions were described using first order differential equations. By fitting the models to the experimental data, apparent rate constants were obtained using numerical optimisation software. The model optimisations showed that the interactions of LMM 60Co were well described by the consecutive model, while the parallel model was more suitable to describe the interactions of LMM 109Cd with the sediments, when the squared sum of residuals were compared. The rate of sorption of the irreversibly (or slowly reversibly) associated fraction was greater than the rate of desorption of the reversibly bound fractions (i.e. k3 > k2) for both radionuclides. Thus, the Novaya Zemlya sediment are supposed to act as a sink for the radionuclides under oxic conditions, and transport to the water phase should mainly be attributed to resuspended particles.     

Probability,exchangeability and extremes: with discussion of iceberg loading

The basic concepts of probability and utility, as well as their interrelationship, are reviewed. The duality embodied in the definitions of these quantities is emphasized. The assumptions of exchangeability as an alternative to independence, in assessing the implications of data sets and in problems of inference, is shown to offer the opportunity for considerable improvement in statistical analysis. Exchangeability implies that the order of a sequence of random events or quantities is not of significance. The use of a basic urn model in formulating problems involving exchangeability is demonstrated. The extension of the analysis to continuous random quantities as well as to problems of extremes is summarized. An example of the analysis of arrival of icebergs at discrete points in time at the location of an offshore structure is included and it is shown that the results include the uncertainty resulting from short data bases. The added risk associated with this uncertainty is acknowledged implicitly in the methodology.     

The distribution of selected organic molecules in freshwater sediment

The microbial degradation of organic material in lake sediment is dependent upon several parameters including the molecular nature of the organic material. In this study, the flux in the concentration of selected substrate molecules (cellulose, chitin and protein) was examined in sediment over a five-month period. In addition, the levels of metabolic products (free sugars, free amino acids and volatile acids) were also monitored. Results of the study indicate that while chitin and protein were actively metabolized, cellulose was less subject to degradation. The distribution of metabolic products over sample time indicated that aerobic metabolism was the primary mechanism of decomposition. Free sugars and dissolved amino acid levels always exceeded volatile acid concentrations.     

Measurement of sewer sediments with acoustic technology: from laboratory to field experiments

Monitoring sewer sediments is necessary to better understand sedimentation and erosion processes. Sonar is one of the available techniques to proceed to sewer sediment measurements. Extraction of numerical data, implementation of a new algorithm to identify the water-sediment interface, laboratory and field experiments have been done to evaluate the device, to quantify uncertainties and to test the sonar under various conditions. Results demonstrate that: 1) uncertainties in repeatability are less than 4%, 2) the sonar delivers accurate results under various conditions (small to large sewers and grit chambers), and 3) in situ measurements are affected by uncertainties, mainly due to the fact that the sensor is not in a fixed position but is floating on the free surface in the sewer. This device is useful and accurate for semi-automatic measurement but further research and improvements should be done to better know the position of the device in the section.     

Evaluation of cementation of Jeju coastal sediments using in situ tests

A series of SPT, CPT and SPS tests were performed for Jeju coastal sediments in order to investigate the possibility of detecting the cementation of granular sediments by in situ tests. Plotting the in situ test results into the G_max/q_c–q_c1 and (G_max/p_a)/N₆₀–(N₁)₆₀ relations suggested by Schnaid et al. (Proceedings of 1st international conference on site characterization, Balkema Pub, Rotterdam 2004) indicates that the Jeju calcareous sediments are cemented. It is also observed that the q_c/N ratio of Jeju sediments is much higher than the (q_c/p_a)/N₆₀–D₅₀ relation suggested by Kulhawy and Mayne (Manual on estimating soil properties for foundation design, EPRI, Ithaca 1990). It is suggested a relative density?>100?% obtained using the q_c–D_R–σ_v′ relation suggested for uncemented sands indicates cementation of the in situ sediments. The G_max/q_c value of Jeju sediments was found to be 2–6 times larger than that of uncemented sands at the same normalized cone resistance. It is concluded that in situ penetration resistances are relatively insensitive to the cementation of granular soil while the deformation moduli are significantly affected by the cementation, and that the relative density and G_max of cemented sediments can be seriously misjudged unless the effect of cementation on in situ penetration test results is considered.     

Composition of bacterial and archaeal communities in freshwater sediments with different contamination levels (Lake Geneva, Switzerland)

The aim of this study was to compare the composition of bacterial and archaeal communities in contaminated sediments (Vidy Bay) with uncontaminated sediments (Ouchy area) of Lake Geneva using 16S rRNA clone libraries. Sediments of both sites were analysed for physicochemical characteristics including porewater composition, organic carbon, and heavy metals. Results show high concentrations of contaminants in sediments from Vidy. Particularly, high contents of fresh organic matter and nutrients led to intense mineralisation, which was dominated by sulphate-reduction and methanogenesis. The bacterial diversity in Vidy sediments was significantly different from the communities in the uncontaminated sediments. Phylogenetic analysis revealed a large proportion of Betaproteobacteria clones in Vidy sediments related to Dechloromonas sp., a group of dechlorinating and contaminant degrading bacteria. Deltaproteobacteria, including clones related to sulphate-reducing bacteria and Fe(III)-reducing bacteria (Geobacter sp.) were also more abundant in the contaminated sediments. The archaeal communities consisted essentially of methanogenic Euryarchaeota, mainly found in the contaminated sediments rich in organic matter. Multiple factor analysis revealed that the microbial community composition and the environmental variables were correlated at the two sites, which suggests that in addition to environmental parameters, pollution may be one of the factors affecting microbial community structure.     

Spatial and vertical distribution of polycyclic aromatic hydrocarbons in mangrove sediments

The distribution and historical changes of polycyclic aromatic hydrocarbons (PAHs) contamination in mangrove sediments in Hong Kong SAR were investigated. Surface sediments (2-3 cm) collected from four mangrove swamps exhibited significant spatial variations in concentrations of total PAH (with SigmaPAHs ranging from 56 to 3758 ng g(-1) dry wt), as well as the composition of 16 USEPA priority PAH compounds. Within a small swamp with an area of 0.68 ha, the total PAH concentrations also differed from sampling site to site, indicating that the PAH contamination is localized and confined to a very small area within the same swamp. Discharges from municipal and industrial wastewater, urban runoff, oil leakage from boats and ships, and accidental oil spill are possible sources of the PAH contamination. The sediment depth profiles reveal that the surface sediment layer (0-5 cm) had lower total PAH concentrations than that in the bottom layer (15-20 cm), and PAH composition also changed with the sediment layers. Based on the estimated annual sediment deposition rate in Hong Kong SAR of around 0.4-0.5 cm, the present findings suggest that the PAH contamination was most serious between 1958 and 1979 but started to decline thereafter. Such decline was probably due to changes in petroleum usage in urban areas and a better control of wastewater discharges from 1980 onwards in this region.     

污染物离子在粘土介质中扩散系数和分配系数的测定(英文)

介绍了利用一种自制的特殊装置,测定污染物离子在粘土介质中迁移的扩散参数D和分配系数Kd的方法。通过一系列污染物在上海粘土中的纯扩散迁移试验,系统中污染物的质量不变,源浓度随试验的进行是下降的,运用一个简单的理论模型,通过拉普拉撕(Laplace)变换方法,对一个单独的试验,可同时推断出扩散系数和分配系数。通过拉普拉撕转换程序求出了阴离子(Cl-)和阳离子(K+,Ca2+,Zn2+,Cu2+)的扩散系数D和分配系数Kd。试验结果发现:阴离子在粘性土壤中的显扩散系数比阳离子的大;一价阳离子K+的显扩散系数比二价阳离子的大。     

Comparing different approaches to in situ measurement of building components thermal resistance

This paper introduces a comparison of different measuring methods of buildings fabric thermal resistance, including the test wall measuring points arrangements and measurement results, conducted in a test chamber in Cagliari (Italy) in summer 2009.Two methods and their measurement uncertainties are presented and compared by the compatibility of measurement study.The non-destructive method involves the heat-flow rate measurement through the test wall and its surface temperatures. The reliability of this method depends on the temperatures difference between the two environments separated by the building envelope. Thus two measurement series with different temperatures were carried out on the test wall.The destructive method instead foresees the acquisition of a sample by the use of a hollow drill, the building envelope layers thickness measurement and the thermal properties assignment to each different material. The wall R-value is the sum of each layer thermal resistance.The comparison shows that the R-values measured by the non-destructive method with a temperature difference of 10 °C and 7 °C and the one calculated from the destructive method obey the compatibility of measurement principle.     

Oxygen uptake of bottom sediments studied in situ and in the laboratory

Oxygen uptake by soft bottom sediments was measured in situ with an oxygen electrode in a bell jar. Values in the range 0·3-3·0 g O₂ m⁻² d⁻¹ were obtained at 19 localities in fresh and brackish water. Comparative measurements were made in the laboratory on sediment cores. These gave consistently lower values than the in situ measurements. Laboratory experiments showed that the oxygen uptake depended on the oxygen concentration and that the temperature coefficient decreased with increasing temperature. There was no simple correlation between oxygen uptake and content organic matter in sediments.     

Cadmium kinetics in freshwater clams (Unionidae) under field and laboratory conditions.

Cadmium dosing experiments were carried out with the freshwater clam Unio pictorum in flow-through micro-streams in order to obtain additional information on the kinetics of cadmium accumulation and elimination. Accumulation of cadmium in the kidney was rapid and within 3 weeks a concentration factor of 6000 was found. During elimination (29 weeks), the organs lose about one-third of their cadmium content rather rapidly, but no further elimination occurs. It can be concluded that the process of Cd accumulation is rapid and almost irreversible.     

Influence of organic carbon decomposition on calcite dissolution in surficial sediments of a freshwater lake

A multi-sensor approach was used to determine high-resolution porewater gradients of Ca(2+), CO(3)(2-), H(+) and O(2) in sediment cores along a depth transect from eutrophic Lake Sempach (Switzerland). We quantified the reproducibility of measurements and analyzed concentration profiles with a one-dimensional diffusion-reaction model to quantify benthic fluxes. Calculation of oxic respiration in the sediment showed that almost all settled organic carbon was degraded with O(2) at shallow depths while only 28% was decomposed aerobically at the deepest location. Fluxes were highest at 8 m depth (24.4 mM m(-2) d(-1)) and lowest at the deepest site of 85 m (7.1 mM m(-2) d(-1)). Dissolution of calcite from the sediment was also depth dependent. A total of 1.1 mM m(-2) d(-1) was found for the shallowest site, and values decreased with depth to 0.6 mM m(-2) d(-1) at the deepest site.     

Effects of ferric hydroxide on the anaerobic biodegradation kinetics and toxicity of vegetable oil in freshwater sediments

Biodegradation of vegetable oil in freshwater sediments exhibits self-inhibitory characteristics when it occurs under methanogenic conditions but not under iron-reducing conditions. The basis of the protective effect of iron was investigated by comparing its effects on oil biodegradation rate and the toxicity of oil-amended sediments to those of clay and calcium, which reduce the toxicity of oil-derived long-chain fatty acids by adsorption and precipitation, respectively. Kinetic parameters for an integrated mixed-second-order model were estimated by nonlinear regression using cumulative methane production as the response variable and used to compare the effects of the three treatment factors on the rate of oil biodegradation. Ferric hydroxide was the only factor that significantly (P<0.05) increased the rate of methane production from canola oil, whereas calcium significantly reduced the oil biodegradation rate. Measurement of sediment toxicity using the Microtox Solid-Phase Test showed that inhibitory products formed within 5 days of oil addition, but the sediment toxicity decreased over time as the extent of oil mineralization increased. None of the other amendments significantly reduced the toxicity of oil-containing sediments. Since ferric hydroxide stimulated the rate of oil biodegradation without affecting the toxicity of oiled sediments, it must operate through a mechanism that is different from those previously described for clay and calcium.