Sorption of chlorinated C1- and C2-hydrocarbons and monocyclic aromatic hydrocarbons on sea sediment

The sorption of chlorinated C₁- and C₂-hydrocarbons and monocyclic aromatic hydrocarbons on sea sediment was studied with a miscible displacement technique. Detection was done either by on-line UV-detection or off-line GC-analysis. Equilibrium partitioning coefficients between the salt water phase and the marine sediment were determined for 11 compounds by fitting their breakthrough curves to a local sorption equilibrium model. Based on the obtained partitioning coefficients and on the measurement of the organic carbon content of the sediment, the sorption into the organic carbon fraction was considered. Log K_oc data (K_oc = organic carbon-water partitioning coefficient) were calculated. A linear relationship between the log K_oc values and the log K_ow data (K_ow = octanol-water partitioning coefficient) was found (r = 0.94, n = 11). However, the sorption was lower than expected from the log K_ow data. Finally, the implications of the experimental results for the sorption behaviour of the compounds in the marine environment were evaluated. It was concluded that the sea sediment does not act as an important sink for these anthropogenic compounds.     

The stability of marine sediments at a tidal basin in San Francisco Bay amended with activated carbon for sequestration of organic contaminants

Recent laboratory studies show that adding activated carbon to marine sediments reduces the bioavailability of persistent organic contaminants, such as polychlorinated biphenyls, to benthic organisms. The present work investigates how mixing activated carbon into cohesive sediment affects the stability of sediment obtained from the intertidal zone at the Hunters Point Naval Shipyard Superfund site in South Basin, San Francisco Bay, CA. Our results show for these sediments that mixing activated carbon into sediment does not significantly affect stability of surface sediments, as measured by sediment erosion rate and critical shear stress for incipient motion, thus supporting the potential field application of this technique for in situ stabilization of persistent organic contaminants. Hydrodynamic modeling was used to estimate the maximum bottom shear stress encountered during high-wind storm events at the estuarine inlet from which the sediments were obtained. Comparison of estimated bottom shear stresses with measured critical shear stresses shows that surface sediments will not erode under normal, non-storm conditions. Bottom shear stresses caused by large waves under infrequent high-wind storm conditions may erode surface sediments for short periods of time. We conclude from sediment stability tests and hydrodynamic modeling that mixing activated carbon amendment with cohesive sediment at selected locations within South Basin will not reduce surface sediment stability nor result in significant erosion of treated sediments.     

Partitioning and desorption behavior of polycyclic aromatic hydrocarbons from disparate sources

Contaminated sediments pose a unique challenge for risk assessment or remediation because the overlying water column may transport contaminants offsite or to ecological receptors. This research compares the behavior of polycyclic aromatic hydrocarbons (PAHs) on marine sediments from two sites. The first site was affected by shipping activities and the second was impacted by a creosote seep. Organic carbon:water partitioning coefficients (Koc values) were measured with three solutions. Desorption was measured using Tenax beads. PAHs from the ship channel had lower Koc values than those from the creosote facility. For example, the average logKoc value of ship channel pyrene was significantly lower than that of creosote facility pyrene (4.39 +/- 0.35 and 5.29 +/- 0.09, respectively, when tested in 5 mM calcium chloride). These results were consistent with the greater desorption of pyrene, phenanthrene and benzo(a)pyrene from the ship channel than from the creosote facility sediments. Organic compound desorption from sediments can be considered to be a two-stage process, with a labile fraction that desorbs quickly and a refractory fraction that desorbs much more slowly. In both sediments, more than 75% of the benzo(a)pyrene was found to have partitioned into the refractory phase. The amounts of phenanthrene and pyrene that partitioned into the refractory phase were lower. Linear correlations of logKoc with log(CR/CL) (where CR and CL are the fractions of the compound in the refractory and labile phases, respectively, at time zero) showed that partitioning measurements made with the US EPA's Toxicity Characteristic Leaching Procedure fluid (US EPA, 1996) most closely matched predictions of desorption behavior. The data imply that with a larger data set, it may be possible to relate simple partitioning measurements to desorption behavior. Partitioning measurements were used to predict water concentrations. Despite having higher concentrations of carcinogenic PAHs [cPAHs, the seven PAHs categorized by the US EPA (2004) as class B2 carcinogens], creosote facility sediments were predicted to produce lower aqueous concentrations of cPAHs. These results indicate that both sediment and contaminant characteristics will impact contaminant release from sediments.     

The sequestration of PCBs in Lake Hartwell sediment with activated carbon

Recent laboratory studies with a marine sediment from a tidal mudflat in San Francisco Bay demonstrated that a low dose of activated carbon enhances the sequestration of PCBs and PAHs under well-mixed conditions. Here we compare activated carbon treatment for a freshwater sediment from a rural site, using PCB-contaminated sediment from Lake Hartwell, SC. An activated carbon dose of 2% of the dry sediment mass lowered total aqueous PCB concentrations by more than 95% after 1 month of treatment and more than 98% after 6 months. Aqueous PCB concentrations remained below detection limits following 18 months of sediment-carbon contact in slowly mixed systems. Uptake of PCBs into semipermeable membrane devices was reduced by 78%, 91% and 97% over 1, 6 and 18 months, respectively. These tests show that PCB sequestration with activated carbon improves with contact time and is not diminished by prolonged mixing with sediment. Desorption studies confirmed the sequestration, in which 74% of the total PCB mass could be desorbed from untreated Lake Hartwell sediment within 30 days as compared to only 7% after activated carbon treatment for 6 months. We compare these observations with marine sediment from San Francisco Bay and propose a conceptual model to rationalize how sediment properties relate to the reduction in aqueous PCB concentrations during activated carbon treatment.     

On the use of the partitioning approach to derive Environmental Quality Standards (EQS) for persistent organic pollutants (POPs) in sediments: a review of existing data

A review of experimental data has been performed to study the relationships between the concentration in water, pore water and sediments for different families of organic contaminants. The objective was to determine whether it is possible to set EQS for sediments from EQS defined for surface waters in the Daughter Directive of the European Parliament (COM (2006) 397). The analysis of experimental data showed that even though in some specific cases there is a coupling between water column and sediments, this coupling is rather the exception. Therefore it is not recommendable to use water column data to assess the chemical quality status of sediments and it is necessary to measure in both media. At the moment EQS have been defined for the water column and will assess only the compliance with good chemical status of surface waters. Since the sediment toxicity depends on the dissolved pore water concentration, the EQS developed for water could be applied to pore water (interstitial water); hence, there would be no need of developing another set of EQS. The partitioning approach has been proposed as a solution to calculate sediment EQS from water EQS, but the partitioning coefficient strongly depends on sediment characteristics and its use introduces an important uncertainty in the definition of sediment EQS. Therefore, the direct measurement of pore water concentration is regarded as a better option.     

Comparison of contaminants from different trophic levels and ecosystems

The present paper provides an overview of the priority contaminants and media from the Greenland part of the Arctic Monitoring and Assessment Program. Levels and accumulation patterns of heavy metals, POPs and a radionuclide (137Cs) are compared from the terrestrial, freshwater and marine ecosystems. Of the nine compounds presented, seven (Cd, Hg, Se, sigma PCB, sigma DDT, sigma HCH, HCB) increased in concentration towards higher trophic levels. For these contaminants the concentrations in soil and aquatic sediment were in the same order of magnitude, whereas the concentrations in marine biota were higher than found in the freshwater and terrestrial ecosystems probably due to the presence of longer food chains. Pb and 137Cs showed the reverse pattern compared with the other compounds. The concentrations in soil and aquatic sediments decreased in the order terrestrial, freshwater and marine ecosystems, which was reflected in the biota as well. Reindeer had similar or lower levels of Pb and 137Cs than lichens. Levels of Pb and 137Cs in marine biota did not show the same clear increase towards higher trophic as found for the other analysed compounds. Greenland Inuit contains considerably less mercury but higher levels of sigma PCB, sigma DDT and HCB than other Arctic marine top consumers.     

Natural rates of sediment containment of PAH, PCB and metal inventories in Sydney Harbour, Nova Scotia

Analyses of metal and organic contaminants were carried out on 41 sediment cores, dated using ²¹⁰Pb and ¹³⁷Cs, from the heavily industrialized region of Sydney Harbour, N.S. to evaluate the history of contamination and to predict the rates of natural containment of the harbour by sediment burial. Geochronologies for metals (eg. Pb, As) and polycyclic aromatic hydrocarbons (PAHs) are correlated with the development of the steel and coke industries in the Sydney region while polychlorinated biphenyl (PCB) geochronologies reflect the disposal of electrical equipment used in the steel mill/coking operations. Pb was derived mainly from atmospheric emissions and its concentration has declined exponentially with time in harbour sediments since the closure of the steel mill/coke ovens in the 1980s with a time constant of about 15 years. This represents the time scale for the circulation of this particle-associated contaminant in transient catchment basins prior to permanent deposition in the sediments. PAH and PCB sediment concentrations have also declined exponentially with time since the 1980s, but with a smaller time constant of 10 years owing to the fact that they enter the harbour directly with steel mill and coke oven effluent rather than through atmospheric pathways. Since the time dependence for the burial of metal and organic inventories can be modeled by first order processes, future contaminant levels can be predicted for surface sediments in Sydney Harbour. Mean sediment concentrations of metal and organic contaminants in the upper 5 cm throughout most of the harbour are predicted to decline to levels below the effects range-medium (above which organisms are very likely to be negatively affected by the presence of a contaminant) by 2030.     

Ecotoxicological evaluation of Mediterranean dredged sediment ports based on elutriates with oyster embryotoxicity tests after composting process

The ecotoxicological effect of dredged sediments was estimated by embryo-larval toxicity of the oyster Crassosstrea gigas in sediment elutriates (filtered and unfiltered). The study covers the main ports from the French Mediterranean coast. Composted sediments from a navy harbour (A), a commercial port (B) and two composite specimens (C and D) obtained after mixing various sediments were taken into consideration. Effective concentrations affecting 50% of larvae (EC50) were obtained from different elutriate concentrations (from 0 to 100%). Toxicity results obtained from filtered elutriates decreased according to the following gradient: sample A (5.68%), B (20.50%), C (37.60%) and D (47.17%). Chemical concentrations in whole sediments were in agreement with those in elutriates. Among the measured contaminants in elutriates, Cu and Zn resulted as the main contributors to toxicity. Dissolved organic carbon played an important role by exerting a protective effect against the toxicity of dissolved Cu. Toxicity results were interpreted on the basis of toxicity scores to give indication about sediment quality which provided more severe judgement than risk score based on chemical concentrations in sediments.     

Measurement of volatile organic compounds in sediments of the Scheldt estuary and the Southern North Sea

The concentrations and distribution of 13 priority volatile organic compounds (VOCs) were determined in sediments of the Scheldt estuary and the Belgian continental shelf, using a modified Tekmar LSC 2000 purge-and-trap system coupled to GC-MS. The method allows a sample intake of up to 50 g wet weight and detection limits are between 0.003 ng/g (tetrachloromethane) and 0.16 ng/g (m- and p-xylene). The repeatability (n = 5) varied between 4% (benzene) and 17% (toluene) and the recoveries ranged from 59% (1,1-dichloroethane) to 99% (tetrachloromethane). Because of the nature of the contaminants, special attention was paid to analyte losses and contamination of the samples during storage aboard the research vessel. Spiked sediment samples were prepared in the laboratory and stored aboard under the same conditions as the environmental samples. The recoveries for these samples varied between 94 and 130%, which suggests that storage had no adverse effect on the samples. No detectable VOC concentrations were found for most of the sampling stations. However, in the Antwerp harbour area, significant concentrations of VOCs were found. The sorption behaviour as predicted from laboratory equilibrium partitioning experiments gives an indication of the in situ partitioning behaviour of VOCs. Although VOCs in sediments should, in general, not be regarded as a major problem in the marine environment, high local concentrations may be a cause of concern.     

Binding of organic solutes to dissolved humic substances and its effects on adsorption and transport in the aquatic environment

Humic substances constitute a major fraction of dissolved organic matter in natural water and effluents. Their effect on the adsorption of organic contaminants to aquifer material was elucidated, and a model was proposed for the adsorption of organic solutes to aquifer solids in the presence of dissolved humic substances. The model is based on the assumption that organic solute binds to dissolved humic substances in a reversible manner to form a solute-humate complex. Following binding, both free and bound fractions of the organic solute are independently adsorbed onto the solid phase. In order to evaluate the validity of the model, the following parameters were determined: (1) the adsorption coefficient of the organic solute to clay; (2) the binding constant of the solute-humate complex; and (3) the adsorption of humic acid (HA) to clay, assuming that the solute-humate complex is adsorbed similarly to humic acid itself. Using these parameters in the model enabled the effect of dissolved humic substances on adsorption to be evaluated. Experimental results obtained for the adsorption of fluoranthene (a model compound of the PAH group) to clay in the presence of dissolved HA were compared with calculated values derived from the model described above. The sensitivity of the model to various parameters was evaluated and a prediction was made with respect to the effect of dissolved humic substances on the adsorption of a variety of organic solutes. It appears that dissolved humic substances solubilize organic solutes which have higher adsorption coefficients to clay than humic substances, but increase the adsorption of solutes having lower adsorption coefficients relative to humic substances.     

On the relationship between lake trophic level and lake sediments

The purpose of this work has been to study the relationship between lake type, as expressed by various trophic characteristics, and sediment type, as expressed by determinations of simple chemical data on nitrogen, phosphorus, carbon and loss on ignition. The study is based on two sets of data. The first one emanates from 71 lakes for which information is available from published sources on trophic level and N-content and organic content (loss on ignition) from surficial sediments. More detailed information has been obtained from 12 Swedish lakes/basins on the following water indicators of trophic level: transparency, chlorophyll-a, total-N, total-P, organic-N, inorganic-N. The sediment data from these 12 lakes include: mean values and characteristic values for organic content (loss on ignition, IG) C-, N- and P-contents from surficial sediments. A BPN-value (bioproduction number) is defined by the slope coefficient of the regression line between N-content and IG-content of surficial sediments (0–1 cm). It has been shown that the BPN-value provides most accurate information about lake trophic level on the scale from oligotrophy to eutrophy, provided that more than 50% of the lake area has lower loss on ignition than 20% (per dry substance). The BPN-value cannot be used as an indicator of trophic level if the IG-content is predominantly higher than 30% in a lake. In the latter case the ratio IG/N can be used as a means to differentiate lake humic level. The BPN-value, which is determined from sediment samples providing an even spread throughout the lake surface, yields lake typical information, whereas C/N ratios yield site specific information. A key diagrammatic interpretation between lake trophic type and lake sediments has been presented.     

Environmental stress and recovery: the geochemical record of human disturbance in New Bedford Harbor and Apponagansett Bay,Massachusetts (USA)

Sediments record the history of contamination to estuaries. Analysis of the concentrations of toxic organic compounds, contaminant and crustal metals, organic carbon content and isotopic composition in sediment cores from two estuarine systems in Buzzards Bay allowed reconstruction of human impacts over 350 years. Vertical distributions of the contaminants correlate with changes in the nature of watershed/estuarine activities. All contaminants were highly enriched (tens to hundreds times background) in modern New Bedford Harbor sediments. Enrichment began around the turn of the 20th century for all but PCBs, which were first synthesized in the 1930s. An increase in organic carbon content and a shift of carbon isotopes toward a more terrestrial signature illustrates increasing anthropogenic impact in New Bedford as population grew along with the industrial base. Institution of environmental protection measures in the late 20th century was reflected in decreased, although still substantially elevated, concentrations of contaminants. A lack of industrial development in Apponagansett Bay resulted in much lower concentrations of the same indicators, although specific contaminants related to the early whaling industry increased significantly above background as early as the late 18th century. The similarity of indicators in older portions of cores from NBH and unimpacted Apponagansett Bay demonstrates that cores can be used to establish reference conditions as successfully as using separate sites judged a priori to represent the reference state. The historical reconstruction approach provides the basis for establishing relationships between environmental stressors and factors that drive the stressors, as well as a framework for the assessment of ecological response(s) to environmental stressors over a range of time and/or exposure scales.     

Uncertainty and variability in risk from trophic transfer of contaminants in dredged sediments.

The risks associated with bioaccumulative contaminants must be considered when evaluating dredged material disposal alternatives. The bioaccumulation of organochlorines and other contaminants by higher trophic level organisms represents one of the most significant sources of uncertainty in risk assessment. Both population variability (e.g. true population heterogeneity in body weight, lipid content, etc.) and uncertainty (e.g. measurement error) in trophic transfer can lead to large errors in predicted risk values for ecological receptors. This paper describes and quantitatively evaluates sources of uncertainty and variability in estimating the risk to an ecological receptor (osprey) from the trophic transfer of polychlorinated biphenyls (PCBs) in sediments from the New York-New Jersey (NY-NJ) Harbor. The distribution of toxicity quotients is obtained using a food chain model for the osprey and specifying distributions for input parameters, which are disaggregated to represent either uncertainty or variability. PCB concentrations in sediment and water are treated as predominantly uncertain, whereas lipid content in fish, feeding preferences, and fish weight are assumed to contribute primarily to population variability in PCB accumulation. The analysis shows that point estimates of reasonable maximum exposure (RME) exceed the uncertainty bounds on the 95th percentile of variability. The analysis also shows that uncertainties in the sediment and water contaminant concentrations contribute more to the range of risk estimates than does the variability in the population exposure parameters. The separation of uncertainty and variability in food chain models can help to support management decisions regarding dredged material disposal by providing a quantitative expression of the confidence in ecological risk estimates. A rationale is provided for the distinction between uncertain and variable parameters based on management goals and data availability.     

Impacts of manipulated regime shifts in shallow lake model ecosystems on the fate of hydrophobic organic compounds

Regime shifts in shallow lakes may significantly affect partitioning of sediment-bound hydrophobic organic chemicals (HOCs) such as polychlorobiphenyls (PCB) and polycyclic aromatic hydrocarbons (PAH). In replicated experimental model ecosystems mimicking the alternative stable states ‘macrophyte-dominated’ and ‘suspended solid – phytoplankton dominated’, we tested the effects of macrophytes and benthivorous fish presence on mass distribution and bioaccumulation of hexachlorobenzene, PCBs and PAHs. HOC mass distributions and lipid-normalized concentrations in sediment (Soxhlet- and 6-h Tenax-extractable), suspended solids, macrophytes, periphyton, algae, zooplankton, invertebrates and carp revealed that mobile, i.e. less hydrophobic or less aged HOCs were more susceptible to ecological changes than their sequestered native counterparts. Macrophytes were capable of depleting considerable percentages of the bioavailable, fast desorbing HOC fractions in the sediment upper (bioactive) layer, but did not have a significant diluting effect on lipid-normalized HOC concentrations in carp. Carp structured invertebrate communities through predation and stimulated partitioning of HOCs to other system compartments by resuspending the sediment. These results show that shifts in ecosystem structure have clear effects on fate, risks and natural attenuation of sediment-bound organic contaminants.     

A multi-component statistic analysis for the influence of sediment/soil composition on the sorption of a nonionic surfactant (Triton X-100) onto natural sediments/soils

The contents of soil/sediment organic carbon and clay minerals (i.e. montmorillonite, kaolinite, illite, gibbsite and 1.4 nm minerals) for 21 natural soil/sediment samples and the sorption of Triton X-100 on these samples were determined. A multi-component statistic analysis was employed to investigate the importance of soil/sediment organic matters and clay minerals on their sorption of Triton X-100. The sorption power of soil/sediment composition for Triton X-100 conforms to an order of montmorillonite>organic carbon>illite>1.4 nm minerals (vermiculite+chlorite+1.4 nm intergrade mineral)>kaolinite. The sorption of Triton X-100 on a montmorillonite, a kaolinite and a humic acid were also investigated and consistent with the result of multi-component statistic analysis. It is clear that the sorption of Triton X-100 on soils or sediments is the combined contribution of soil/sediment organic matters and clay minerals, which depended on both the contents of soil/sediment organic matters and the types and contents of clay minerals. The important influence of illite on the sorption of nonionic surfactants onto soils/sediments is suggested and demonstrated in this paper. Surfactants for aquifer remediation application may be more efficient for the contaminated soils/sediments that contain little clay minerals with 2:1 structure because of the less sorption of nonionic surfactants on these soils/sediments.     

Contaminant levels in sediments and asteroids (Asterias rubens L., Echinodermata) from the Belgian coast and Scheldt estuary: polychlorinated biphenyls and heavy metals

The Southern Bight of the North Sea is particularly exposed to anthropogenic contamination, due to heavy urbanisation and industrialisation of its catchment area. The present work focuses on polychlorinated biphenyl (PCB) and metal contamination of the marine environment along and off the Belgian coast. Its objectives were to compare the concentrations of seven PCB congeners and four heavy metals in the sediments (a repository for anthropogenic contaminants) and in the asteroid Asterias rubens (a recognized bioindicator species). Nineteen sampling stations were considered between the mouth of the Scheldt Estuary and the southern limit of the Belgian coast (asteroids were found in 10 out of the 19 stations). PCB and metal concentrations measured in sediments and asteroids were in the range of values reported in previous studies. Stations under direct influence of the Scheldt were the most impacted by the considered contaminants. Metal concentrations varied according to the grain-size fraction considered. In asteroids, PCBs and metals were found to be selectively distributed among body compartments, and pyloric caeca were found to most efficiently discriminate between sampling stations contamination levels. PCB and metal analysis of sediments provided a physicochemical evaluation of the contamination, whereas analysis of asteroids introduced a biological dimension to the approach by taking into account bioavailability of the contaminants.     

Partitioning of marine antifoulants in the marine environment

The partitioning behaviour of the organic biocides, Irgarol 1051 and diuron and two inorganic biocides (copper and zinc) was investigated using six sediments of differing physico-chemical properties collected from unimpacted sites along the south coast of England. The kinetics of sorption and equilibrium partitioning between the sediments and seawater were investigated over a period of 20 days. Resulting organic carbon/water partition coefficients (log Koc) were related to suspended sediment concentration and ranged from 2.28 to 5.20 for diuron; and from 2.41 to 4.89 for Irgarol 1051. Sediment/water partition coefficients (log Kp) for copper and zinc varied from 2.46 to 5.08 l/kg and from 2.49 to 4.97 l/kg, respectively. Kinetic data indicated that there were significant interactions between the dissolved and particulate phases at the start of the experiments, just after mixing. This is thought to be a result of redistribution of organic carbon between the two phases.     

Sorption-desorption behavior of triazine and phenylurea herbicides in Kishon river sediments

Sorption and desorption hysteresis of widely applied triazine and phenylurea herbicides were studied for river sediments. Organic carbon normalized sorption coefficient (K_OC) values for all herbicides were significantly higher for the sediment from the downstream region of the river vs. the upstream sediment. On the basis of the measured K_OC values, the triazine herbicides can be arranged in the following order: terbutryn>terbuthylazine>ametryn>atrazine. Among the phenylurea herbicides, chlorotoluron exhibited higher sorption than isoproturon (K_OC values of 137 vs. 60 and 228 vs. 125 L/kg for the upstream and downstream sediments, respectively). Moreover, chlorotoluron exhibited lower desorption potential as compared with isoproturon (apparent hysteresis index values were 0.2–0.3 for chlorotoluron vs. 0.6–0.9 for isoproturon, measured with the upstream sediment). High sorption affinity of chlorotoluron to the sediments is probably due to stronger H-bonding interactions of the herbicide molecules with the sorbents. For both phenylurea herbicides, desorption hysteresis increased with decrease in sorbed amount. This behavior was opposite to the hysteresis trend observed for the triazines. The Cl-triazines (atrazine and terbuthylazine) exhibited higher desorption hysteresis than the S-triazines (ametryn and terbutryn). Therefore, the apparent hysteresis index values calculated for the Cl-triazines were lower than the values of S-triazines (about 0.4 and 0.7, respectively). Based on the relative strength of H-bonding interactions of Cl- and S-triazines with formate anion and on the desorption hysteresis data we suggest a gradient-derived hole-filling sorption mechanism for the triazine herbicides with the river sediments.     

Sediment nutrient characteristics and aquatic macrophytes in lowland English rivers

Aquatic macrophytes play an important role in the nutrient dynamics of streams. As a result, there is much interest in their use as trophic indicators. However, the relationship between aquatic macrophytes and the trophic status of rivers is a complex one, partly because of the effects of a wide range of environmental variables and partly because submerged, rooted macrophytes can absorb nutrients from the river sediments and/or the water column. Experiments which have tried to establish the relative importance of sediments or water as sources of nutrients are inconclusive and further work is needed to establish how sediment nutrient characteristics vary within and among rivers (spatially and temporally) and the inter-relationships between sediment nutrients, water column chemistry and macrophytes. This paper presents the initial findings from a study of 17 lowland rivers in southern England which is exploring the spatial variability of sediment characteristics (total and inorganic phosphorus, total nitrogen, organic carbon, silt-clay fraction and organic matter content) and the relationship with aquatic macrophytes. The preliminary analysis indicates that although sediment characteristics are highly variable within 100-m river reaches, the variability across the 17 rivers is even greater; this is despite the limited geographic and trophic range of the study sites. The results presented in this paper also give some indication of the sediment characteristics associated with five macrophyte species but it is too early to ascribe sediment preferences for particular species.     

Sediment-air equilibrium partitioning of semi-volatile hydrophobic organic compounds. Part 1. Method development and water vapor sorption isotherm

Contaminated sediments that become exposed to air as a result of dredging and disposal in confined disposal facilities are potential sources of air pollution. A critical parameter to develop emission estimation models is the equilibrium partition coefficient of contaminants, between sediment and air. In this first of two articles, we present a method, based on gas saturation in a flowing stream, to study both the adsorption of water and semi-volatile organic compounds on a sediment from the Campus Lake, Baton Rouge, LA, USA. The experimental set-up was used to determine the adsorption isotherm for water partitioning between sediment and pore-air. A detailed characterization of the sediment surface area and pore volume was used to develop an adsorption-condensation model for predicting water sorption on sediment. The model was used to estimate the importance of water adsorption on mineral surfaces and condensation in pores. This information serves, in the accompanying second article in the series, as the basis for the modeling of the partitioning of phenanthrene, and dibenzofuran.