Natural River Sediments Research Articles

Evaluating the effectiveness of the MicroPlastic Sediment Separator (MPSS)

Effective laboratory methods are a requirement to obtain accurate data on the contamination of the environment with microplastics. However, current methods often lack specification and validation of performance. The aim of this work was to provide the first evaluation of the effectiveness of the commercially available MicroPlastic Sediment Separator (MPSS) operated with sodium chloride. We performed density separation experiments with pristine plastic particles spiked into both commercial sand (experiment I) and natural river sediments (experiment II) and with environmental microplastics contained in natural river sediments (experiment III). The natural sediments were taken from the Lahn River in Germany, a medium-scale central European tributary. The pristine test particles were ground polypropylene fragments, sieved in three different size classes: larger than 1 mm, 0.5–1 mm, and 0.3–0.5 mm. In experiment I, the mean recovery rate was 97 % for particles of the largest size class (standard deviation s=6~%; n=30 per size class), but dropped to 75.33 % (s=21.29~%) in the medium and to 54 % (s=25~%) in the smallest size class. After density separation, 87 % of all unsuccessfully separated test particles were found at the inner walls of the MPSS. In experiment II, the recovery rate was not correlated with the particle size distribution of the natural river sediments. In experiment III, a subsequent second density separation step contributed on average 38 % (s=18~%; n=5) to the total number of extracted environmental microplastics. This study illuminates central aspects of a density separation and aims to contribute to quality improvements of recovery rate experiments and field studies for the generation of reliable data on microplastics in the environment.

Freshwater mussels promote functional redundancy in sediment microbial communities under different nutrient regimes

Abstract Animals are a critical component of biogeochemical cycles. While animal mediated fluxes of nutrients and energy have received considerable attention, the impacts of these fluxes on microbial community structure and function are comparatively understudied. Here, we investigated if freshwater mussel influences on biogeochemical cycling in stream sediment are accompanied by changes in sediment microbial community composition and ecoenzymatic activity, and if these relationships change under different nutrient regimes. We predicted that mussel effects on ecosystem function are reflected by modified sediment microbial communities. We hypothesized that if changes in either sediment ecoenzymatic function or microbial community composition are driven by mussel‐derived nutrient amendments, we should see muted changes in microbial community assemblages or function when a given nutrient is abundant. However, if microbial communities and function are influenced by other mussel functions, then we should see uniform changes regardless of nutrient availability. We transplanted freshwater mussels and natural river sediment to flow‐through mesocosms and monitored changes in microbial community composition over 1 week. Our results indicate that mussels always changed sediment microbial community composition, but the way communities changed was dependent on ambient nutrient concentrations. On the final day we measured the activity of ecoenzymes known to correlate to microbial function and nutrient availability. Mussels homogenized the stoichiometric ratios of ecoenzyme activities, indicating a consistent function of sediment microbes associated with freshwater mussels. Our results suggest that mussels may promote functional redundancy in sediment microbial communities and highlight the importance of animals in controlling biogeochemical transformations under changing nutrient conditions. Read the free Plain Language Summary for this article on the Journal blog.

Using Time-Lapse Resistivity Imaging Methods to Quantitatively Evaluate the Potential of Groundwater Reservoirs

In this study, we attempt to establish an alternative method for estimating the groundwater levels and the specific yields of an unconfined aquifer for the evaluation of potential groundwater reservoirs. We first converted the inverted resistivity into the normalized water content. Then, we inverted the parameters of the Brooks-Corey model from the vertical profiles of the water content by assuming that the suction head was in proportion to the elevation regarding a predefined base level. Lastly, we estimated the groundwater level, the theoretical specific yield, and the specific yield capacity from the Brooks-Corey parameters at every survey site in the study area. The contour maps of the time-lapse groundwater levels show that the groundwater flows downstream, with a higher hydraulic gradient near the river channel than in the area away from the main channel. We conclude that the estimated maximum specific yield capacities are consistent with that derived from the pumping tests in the nearby observation well. Additionally, the specific yield capacities are only three quarters to two thirds of the theoretical specific yields derived from the difference between the residual and saturated water contents in the Brooks-Corey model. We conclude that the distribution pattern of the specific yields had been subjected to the distribution of natural river sediments in the Minzu Basin, since the modern channel was artificially modified. Although we had to make some simple assumptions for the estimations, the results show that the surface resistivity surveys provide reasonable estimations of the hydraulic parameters for a preliminary assessment in an area with few available wells.

Chromium isotope fractionation during adsorption of chromium(III) by soils and river sediments

Chromium (Cr) isotope compositions of sedimentary rocks have been widely used to unravel fluctuations in atmospheric oxygen levels during geologic history. A fundamental framework of this application is that any Cr isotope fractionation in natural environments should be related to the redox transformation of Cr species [Cr(VI) and Cr(III)]. However, the behavior of Cr isotopes during non-redox Cr cycling is not yet well understood. Here, we present laboratory experimental results which show that redox-independent adsorption of Cr(III) by natural river sediments and soils can be accompanied by obvious Cr isotope fractionation. The observed Cr isotope fractionation factors (−0.06‰ – −0.95‰, expressed as 10<sup>3</sup>ln<i>α</i>) are much smaller than those caused by redox processes. Combined with previous studies on redox-independent Cr isotope fractionation induced by ligand-promoted dissolution, we suggest that the systematic shift to highly fractionated Cr isotope compositions of sedimentary rocks is likely to represent atmospheric oxygenation, but muted signals observed in some geologic periods may be attributed to non-redox Cr cycling and should be interpreted with caution.

Impacts of water temperature on phosphorus release of sediments under flowing overlying water

River sediment typically acts a ‘sink’ or ‘source’ of phosphorus for the overlying water owing to the adsorption of phosphorous by the sediment or release of phosphorous to the water, respectively. In this study, a method is proposed for the determination of the equilibrium concentration of the total phosphorus (ECTP) and total phosphorus release rate (Rr). Phosphorus release experiments were conducted on natural river sediments in flowing water with a low velocity under different water temperatures. An empirical equation was established for the correlation between ECTP and Rr at an arbitrary temperature and at 20 °C. The results indicate that an increase in the water temperature significantly enhances the phosphorus release. In comparison with total phosphorus, dissolved phosphorus and particulate phosphorus in the overlying water do not change with the water temperature. Thus, the results of this study aid in understanding the contaminant exchange between sediments and water in flowing rivers.

Greenhouse gas diffusive fluxes at the sediment–water interface of sewage-draining rivers

The purposes of this study were to analyse the spatiotemporal variations in greenhouse gas diffusive fluxes at the sediment–water interface of sewage-draining rivers and natural rivers, and investigate the factors responsible for the changes in greenhouse gas diffusive fluxes. Greenhouse gas diffusive fluxes at the sediment–water interface of rivers in Tianjin city (Haihe watershed) were investigated during July and October 2014, and January and April 2015 by laboratory incubation experiments. The influence of environmental variables on greenhouse gas diffusive fluxes was evaluated by Spearman’s correlation analysis and a multiple stepwise regression analysis. Sewage-draining rivers were more seriously polluted by human sewage discharge than natural rivers. The greenhouse gas diffusive fluxes at the sediment–water interface exhibited obvious spatiotemporal variations. The mean absolute value of the CO2 diffusive fluxes was seasonally variable with spring>winter>fall>summer, while the mean absolute values of the CH4 and N2O diffusive fluxes were both higher in summer and winter, and lower in fall and spring. The annual mean values of the CO2, CH4 and N2O diffusive fluxes at the sewage-draining river sediment–water interface were − 123.26 ± 233.78 μmol m−2 h−1, 1.88 ± 6.89 μmol m−2 h−1 and 1505.03 ± 2388.46 nmol m−2 h−1, respectively, which were 1.22, 4.37 and 134.50 times those at the natural river sediment–water interface, respectively. The spatial variation of the N2O diffusive fluxes in the sewage-draining rivers and the natural rivers was the most significant. As a general rule, the more serious the river pollution was, the greater the diffusive fluxes of the greenhouse gases were. On average for the whole year, the river sediment was the sink of CO2 and the source of CH4 and N2O. There were positive correlations among the CO2, CH4 and N2O diffusive fluxes. The main influencing factor for CO2 and N2O diffusive fluxes was the water temperature of the overlying water; however, the key factors for CH4 diffusive fluxes were the Eh of the sediment and the NH4+-N of the overlying water. River sediment can be either a sink or a source of greenhouse gases, which varies in different levels of pollution and different seasons. Human sewage discharge has greatly affected the carbon and nitrogen cycling of urban rivers.

Decay of elevated antibiotic resistance genes in natural river sediments after sedimentation of wastewater particles

Large-scale wastewater treatment plants (WWTPs) discharge hundreds of tons of total suspended solids (TSS) into surface waters every year. Additionally, a comparable amount is released by sewer overflows during heavy rain events in case of combined sewer systems. Along with sedimentation, particle-attached microorganisms and their antibiotic resistance genes (ARGs) are herewith transported to the riverbed of the receiving water. To better understand the dynamics of this process, a particulate wastewater fraction was added into batch reactors, which were previously filled with natural river sediments and tap water. In parallel, antibiotics (ABs) (erythromycin, tetracycline, ciprofloxacin, roxithromycin, penicillin V, and sulfamethoxazole) were spiked to investigate their capability to select for resistance. The abundance of six ARGs (ermB, tetM, blaTEM, sul1, CTX-M-32, and qnrS) as well as total bacteria (16S rDNA) was monitored in waters and in sediments for a duration of two months using quantitative PCR. Despite a continuous exposure to ABs (5 μg/L each), the abundance of ARGs remained unaffected. Addition of wastewater particles resulted in a sudden and strong increase of ARGs in waters (3–5 log units) and sediments (1–4 log units), however, elevated ARGs underwent a particular and complete decay. Our results indicate that the increased ARG abundances in receiving rivers are the result of a continuous import of ARGs from WWTP discharges or sewer overflow events. They further imply that elevated ARGs do not persist in receiving rivers, if this continuous import is removed. This seems to be the case merely for ARGs introduced by wastewater, given that a stable background concentration of ARGs was observed for the native population.

Fate and dynamics of metal precipitates arising from acid drainage discharges to a river system

Neutralisation of acid drainage creates metal-rich precipitates that may impact receiving water bodies. This study assessed the fate of over seven years of acid drainage discharges on the sediments of the lower River Murray (Australia), including the potential for periodic water anoxia to enhance risk via reductive dissolution of amorphous (Fe, Mn and co-precipitated and bound metal) oxide phases. With the exception of one site with restricted water exchange, elevated reducible/reactive metal(oid) (Fe, Ni, As, Co, Zn) concentrations were only observed in the localised wetland-riparian area within approximately 100 m of the discharges. Only a minor exceedance of national sediment quality guideline values occurred for Ni. In the main river channel, elevated reactive metal (Fe, Mn, Ni, Zn) concentrations were also only observed less than approximately 100 m from the drainage discharge point. This appears due to (a) rapid neutralisation of pH leading to metal precipitation and deposition in the localised discharge area, and/or (b) dilution of any metal precipitates entering the main channel with natural river sediments, and/or (c) flushing of precipitates downstream during higher flow conditions. The influence of deoxygenation on metal release was profound with large increases in the concentration of dissolved Fe, Mn, Zn, Ni, and As in the overlying water during laboratory experimental simulations. However, given in situ sediment metal contamination is very localised, it appears on a river reach scale that the acid drainage precipitates will not significantly contribute, over and above, the background release of these metals during these conditions.

Anthropogenic Reservoirs of Various Sizes Trap Most of the Sediment in the Mediterranean Maghreb Basin

The purpose of this study is to obtain a spatially explicit assessment of the impact of reservoirs on natural river sediment fluxes to the sea in the Mediterranean Maghreb Basin (MMB), a region where both mechanical erosion rates and the anthropogenic pressure on surface water resources are high. We combined modeling of riverine sediment yields (sediment fluxes divided by the drainage basin area) and water drainage intensities in a 5’ × 5’ grid point resolution (~10 km × 10 km) with a compilation of existing reservoirs in the area, and calculated sediment trapping based on average water residence time in these reservoirs. A total number of 670 reservoirs could be assembled from various sources (including digitization from Google maps), comprising large-scale, small-scale and hillside reservoirs. 450 of them could be implemented in our modeling approach. Our results confirm that natural sediment yields are clearly above the world average, with the greatest values for Morocco (506 t km−2 year−1), followed by Algeria (328 t km−2 year−1) and by Tunisia (250 t km−2 year−1). Including dams in the downstream routing of suspended sediments to the sea reduces the natural sediment flux in the entire MMB to the sea from 96 to 36 Mt km−1 year−1, which corresponds to an average sediment retention of 62%. Trapping rates are highest in the Tunisian basin part, with about 72%, followed by the Algerian (63%) and the Moroccan basin parts (55%). Small reservoirs and hillside reservoirs are quantitatively important in the interception of these sediments compared to large reservoirs. If we only considered the dams included in the widely used Global Reservoir and Dam (GRanD) database which comprises mainly large reservoirs sediment retention behind dams would account for 36% of the natural suspended particulate matter (SPM) flux to the Mediterranean Sea. Our data reveal negative correlation between sediment retention and natural erosion rates among the three Maghreb countries, which can be explained by the greater difficulties to build dams in steep terrains where natural sediment yields are high. Although the lowest sediment retention rates are found in the Moroccan part of the MMB, it is probably here where riverine sediment starvation has the greatest impacts on coastline dynamics. Understanding the impact of dams and related water infrastructures on riverine sediment dynamics is key in arid zones such as the MMB, where global warming is predicted to cause important changes in the climatic conditions and the water availability.

Sediment matrix characterization as a tool for evaluating the environmental impact of heavy metals in metal mining, smelting, and ore processing areas

In this work, the matrix characterization (mineralogy, total and local chemical composition, and total organic (TOC) and inorganic carbon (TIC) contents) of different types of sediments from mining- and metallurgy-influenced areas and the assessment of the impact of the matrix on the association of potentially hazardous metals with the mineral phases of these samples, which affect their mobility in the environment, are presented. For these purposes, sediment samples with different origins and from different locations in the environment were analyzed. Anthropogenic sediments from metal-rich post-flotation tailings (Lintich, Slovakia) represent waste from ore processing, natural river sediments from the Hornád River (Košice, Slovakia) represent areas influenced predominantly by the metallurgical industry, and lake sediments from a water reservoir Ružín (inflow from the Hornád and Hnilec Rivers, Slovakia) represent the impact of the metallurgical and/or mining industries. The total metal contents were determined by X-ray fluorescence (XRF) analysis, the local chemical and morphological microanalysis by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and the TOC and TIC contents by infrared (IR) spectrometry. The mobility/bioavailability of Cu, Pb, and Zn in/from sediments at the studied areas was assessed by ethylenediaminetetraacetic acid (EDTA) and acetic acid (AA) extraction and is discussed in the context of the matrix composition. The contents of selected potentially hazardous elements in the extracts were determined by the high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS).

Sorption and transformation of the reactive tracers resazurin and resorufin in natural river sediments

Abstract. Resazurin (Raz) and its reaction product resorufin (Rru) have increasingly been used as reactive tracers to quantify metabolic activity and hyporheic exchange in streams. Previous work has indicated that these compounds undergo sorption in stream sediments. We present laboratory experiments on Raz and Rru transport, sorption, and transformation, consisting of 4 column and 72 batch tests using 2 sediments with different physicochemical properties under neutral (pH = 7) and alkaline (pH = 9) conditions. The study aimed at identifying the key processes of reactive transport of Raz and Rru in streambed sediments and the experimental setup best suited for their determination. Data from column experiments were simulated by a travel-time-based model accounting for physical transport, equilibrium and kinetic sorption, and three first-order reactions. We derived the travel-time distributions directly from the breakthrough curve (BTC) of the conservative tracer, fluorescein, rather than from fitting an advective-dispersive transport model, and inferred from those distributions the transfer functions of Raz and Rru, which provided conclusive approximations of the measured BTCs. The most likely reactive transport parameters and their uncertainty were determined by a Markov chain–Monte Carlo approach. Sorption isotherms of both compounds were obtained from batch experiments. We found that kinetic sorption dominates sorption of both Raz and Rru, with characteristic timescales of sorption in the order of 12 to 298 min. Linear sorption models for both Raz and Rru appeared adequate for concentrations that are typically applied in field tracer tests. The proposed two-site sorption model helps to interpret transient tracer tests using the Raz–Rru system.

A New Method to Calculate Mixed-Layer Thickness Based on Sand Wave Movement

It is pointed out that the influence of the time step in the unsteady non-uniform sediment mathematical model must be considered when to calculate the mixed-layer thickness in this article. And then a new method to calculate the mixed-layer thickness is obtained based on the sand wave movement. This method can well reflect the effect of the time step on the mixed-layer thickness, and has a wide field of application in the numerical simulation of natural river sediment transport.

The role of black carbon in the sorption and desorption of phenanthrene on river sediments

Black carbon (BC) is ubiquitous in an aquatic environment. In the present study, the role of BC in the sorption and desorption of phenanthrene on the sediments sampled from the Yangtze River and the Yellow River of China has been studied. The BC content in total organic carbon ranged from 23 to 50% in sediment samples and BC accounted for 52.7–79.1% of the overall sorption, revealing the important role of BC in the sorption of phenanthrene on natural river sediments. The mean value of BC-water distribution coefficient (kBC) was about 50 times that of organic carbon (OC)-water distribution coefficient (kOC) for phenanthrene. The values of logkBC for the CTO375-combusted sediments ranged from 5.66 to 5.95, and the Freundlich exponent ranged from 0.40 to 0.67, suggesting that a difference exists among sorption capacities of BC from different river sediments. Therefore, the sources of BC should be taken into account when assessing its sorption capacity. For the desorption process, approximately 10.5–21.4 and 27.1–49.2% phenanthrene was desorbed from CTO375-combusted and original sediments, respectively. The results revealed that the OC-sorbed phenanthrene could not all desorb, and a fraction of BC-sorbed phenanthrene could desorb, although BC is considered a strong sorbent.

Exposure effect of fungicide kasugamycin on bacterial community in natural river sediment

The degradation of kasugamycin in water and the effects of kasugamycin on the bacterial communities in sediment and overlying water were analyzed over a 30-day period. Kasugamycin is generally regarded as nontoxic to microorganisms, but in this study we demonstrated that kasugamycin inhibited the growth of some aquatic bacteria but also stimulated the growth of other resistant bacteria. Microcosms were contaminated with kasugamycin at the 168.4 mg/L (700 times field application rate), and 1462.9 mg/L (6000 times field application rate). The percentages of kasugamycin degraded after 30 days of exposure were 34.1%, and 12.1% in the overlying water treated with 168.4 mg/L and 1462.9 mg/L, respectively. The degradation rates of kanamycin after 30 days of exposure were 1.92 mg/L/day for 164.8 mg/L treated water and 5.88 mg/L/day for 1462.9 mg/L treated water. Degradation rate of kasugamycin in overlying water was associated with an increase in the concentration of kasugamycin. Comparison of DGGE (denaturing gradient gel electrophoresis) profiles showed that the bacterial communities in treated microcosms were varied. Certain species were eliminated whereas some species were stimulated by the application of kasugamycin. The DGGE profiles and UPGMA (unweighted pair-group method using arithmetic averages) dendrograms revealed that the bacterial communities were more complex in treated sediment than in treated overlying water.

Fate of Beta Blockers in Aquatic-Sediment Systems: Sorption and Biotransformation

The fate of beta blockers (atenolol, acebutolol, bisoprolol, celiprolol, metoprolol, nadolol, pindolol, propranolol, and sotalol) was studied in surface water-sediment systems. A new analytical method was developed to determine the beta blockers in sediments by LC-ESI-tandem MS detection. The relative recoveries in sediments ranged from 89 +/- 7% (acebutolol) to 102 +/- 3% (nadolol) using deuterated surrogate standards. Beta blockers were present with concentrations up to 86 ng/g (bisoprolol) in the sediments of small German streams containing an elevated percentage of treated wastewater. Biotransformation studies and sorption isotherms of the beta blockers were performed with two natural river sediments ("Burgen", "Dausenau") differing in organic carbon content and particle size distribution. Biotransformation of beta blockers in the surface water-sediment systems exhibited a low to high persistence with 90% disappearance (DT(90)) ranging from 0.4-10 d (pindolol, atenolol) to >100 d (sotalol, propranolol or celiprolol). For sorption studies neither NaN(3) addition nor autoclavation led to a complete mass balance of the beta blockers, probably due to biotransformation. Isotherms at 6 h (apparent equilibrium, measuring aqueous and sediment phase) fitted by the Freundlich equation show that sorption of all beta blockers to the Burgen sediment were linear or close to it (i.e., n-values between 0.93 and 1.13), while in the Dausenau sediment the sorptions were slightly non linear (i.e., n-values 0.77-0.91). In river water the sorbed fraction is negligible in comparison to the dissolved fraction. Nevertheless, beta blockers can be detected with concentrations up to 86 ng/g (bisoprolol) in sediments of small streams containing more than 50% treated wastewater.

Analysis and Sorption of Psychoactive Drugs onto Sediment

An analytical method was developed to analyze eight psychoactive pharmaceuticals--including the antiepileptic carbamazepine, the opiates morphine, codeine, dihydrocodeine, the opiode tramadol, and the tranquilizers diazepam, oxazepam, temazepam--and the antibiotic sulfamethoxazole as well as three metabolites (10,11-dihydrocarbamazepine (DHC), 10,11-dihydroxy-10, 11-dihydrocarbamazepine, and N4-acetylsulfamethoxazole) in river sediments. Relative recoveries of all analytes exceeded 97% using either deuterated or 13C15N-labeled surrogate standards. Sorption isotherms of all analytes were constructed at pH 6.5-6.6 on two natural river sediments (Burgen and Dausenau) that differed in organic carbon contents and particle size distributions. Affinities of all analytes were up to an order of magnitude higher for the Dausenau sediment in comparison to the Burgen sediment. Isotherms were well described by the Freundlich model. Sorption of all analytes was linear on the Burgen sediment except for structurally similar carbamazepine (n = 0.90) and DHC (n = 0.88). Conversely, most analytes showed pronounced nonlinear sorption to the Dausenau sediment (n = 0.77-0.92) except for positively charged codeine, dihydrocodeine, and tramadol. Linear sorption of the latter was taken to arise from concentration-independent electrostatic interactions of the organocations with negatively charged surfaces on clay minerals or in the sediment organic matter. Desorption gave rise to hysteresis in 13 out of 16 investigated analyte-sorbent systems. Hysteresis was likely due to slow desorption kinetics beyond 24 h used in the experiment.

THE REMOBILIZATION OF METALS FROM IRON OXIDES AND SEDIMENTS BY METAL-EDTA COMPLEXES

Synthetic chelating agents such as EDTA form strong complexes with heavy metals and therefore have the potential to remobilize metals from sediments and aquifers. In natural waters EDTA is present almost exclusively in the form of metal-complexes. Therefore, remobilization of metals is always a metal-metal-EDTA exchange reaction. We have investigated, to our knowledge for the first time, the remobilization of metals from the surface of synthetic iron oxides and from a river sediment by different metal-EDTA complexes. The metals are exchanged as follows: MeEDTAdissolved C Me adsorbed ! Me EDTAdissolved C Meadsorbed The order of the remobilization rate of Zn 2C from goethite is CaEDTA > Fe(III)EDTA, reflecting the slow exchange reaction of Fe(III)EDTA. For the remobilization of Pb 2C from goethite, the rate was found to be Fe(III)EDTA > CaEDTA > ZnEDTA. Here, Fe(III)EDTA has surprisingly the fastest exchange rate. Only very limited remobilization of Pb 2C is possible from hydrous ferric oxide at pH 8 due to the very strong adsorption of Pb 2C. The order of remobilization of Zn 2C from a natural river sediment was found to be CaEDTA > CuEDTA > Fe(III)EDTA. The remobilization rate of Zn 2C with Fe(III)EDTA is only 12% of the rate with CaEDTA, illustrating the importance of EDTA speciation for assessing remobilization.

River sediment discharge to the oceans; present-day controls and global budgets

Empirical relationships between river sediment yields and a large number of hydroclimatic, biological, and geomorphological parameters are investigated at the global scale. For a set of 60 major world river basins, the environmental characteristics were extracted from various global computer data sets, and natural river sediment fluxes were taken from the literature. It is found that sediment yields can be best correlated by forming the products of hydroclimatic and geomorphological factors, that is, runoff intensity, basin slope, an index characterizing rock hardness, and an index characterizing rainfall variability over the year. The best parameter combination varies to some extent when the rivers are grouped according to their average climatic situation, but it is always a combination of the above mentioned parameters that yields the most significant models. In arid climates, however, regression coefficients are greater than in humid climates, indicating that erodibility is much greater than in arid climates. When extrapolated to the total continental area on the basis of the corresponding data sets, the empirical models result in a total sediment flux of about 16 Gigatons discharged to the oceans every year, which is in good agreement with previous estimates derived from compilations of world-wide river data. Consequently, two global maps are proposed showing the spatial distributions of river sediment yields on the continents as well as the inputs of these sediments to the oceans in a grid point scale. By far most of the sediments originate from the young orogenic belts of the continents, mainly due to a combination of steep morphologies and high runoff intensities.

The effect of phthalate ester plasticisers on the emergence of the midge ( Chironomus riparius) from treated sediments

Sediment toxicity tests with the midge, Chironomus riparius, were carried out in a natural river sediment spiked with either di-2-ethylhexyl phthalate (DEHP) or di-isodecyl phthalate (DIDP). The phthalates were each dosed at three levels in the sediment up to 10000 mg phthalate/ kg (dry weight basis) and the test organisms were introduced as larvae (< 24 hours post-hatch). No effects, in any of the treatments, were found on the development of the midge based on a comparison with the controls of the numbers and sex of the emerging adults. Sediment concentrations were measured at the start and finish of the study (28 day period) and good agreement with the nominal levels was found. Accumulation of the phthalates and any metabolites was determined on a sample of the emerging adults by measuring their radioactivity from the 14C-labelled test substances. A biota to sediment accumulation factor of 1.5 and 0.6 was found for DEHP and DIDP, respectively.

Size distribution and settling velocity of cohesive sediments during settling

Experiments on settling of cohesive sediments were carried out in turbulent flows in an annular flume using kaolinite clay as well as a natural river sediment. Results indicate that the finer fractions were able to deposit because they were settling as floes. Data on concentration and size distribution of dispersed samples were used to calculate the effective settling velocities for the different size fractions.