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[en] The use of a suction field aided by a jet system in a dredging process is one of the recent hydraulic methods in underwater dredging. The jet parameters have a great influence on the dredging efficiency. Jet induced impact energy that is required to initiate the bed motion and sediment pulsation is considered. Parametric sensitivity of various factors on dredging performance was carried out in an experimental set-up. Effects of changing the number of jets, jet geometry, suspension concentration, horizontal and vertical distances between jets and suction inlet on the dredging efficiency and the geometry of the excavation hole have been investigated. The results showed that several of these parameters have variable and significant effects on the dredging efficiency as well as the resulting geometry of the excavation. (author)
[en] In-situ characterization and assessment of arsenic (As) mobility in sediments was scarce. In this study, the distributions of labile As at a vertical resolution of 2 mm were obtained in the sediments of a large Lake Taihu through in-situ measurements using a Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique. The DGT-labile As, interpreted as DGT flux (FDGT), exhibited three different patterns in the lake, with all the patterns generally showing an increasing mobility followed by a decreasing mobility with sediment depth. The mobility of As could be characterized by the average FDGT (0.06–1.27 pg cm−2 s−1) in the top 10 mm surface sediments, the maximal FDGT (FDGT-M, 0.14–2.44 pg cm−2 s−1) in the end of the initial increasing phase of FDGT, and the diffusion length (ΔL, 28–66 mm) from the depth showing the FDGT-M to the sediment-water interface. The upward mobilization of labile As from the deep sediments to the surface sediments and overlying water became evident when FDGT-M > 1.7 pg cm−2 s−1 or ΔL < 41 mm. The results, for the first time, showed a prospect in in-situ risk assessment of the pollution of sediment As. It was suggested that the increasing mobility of As in the upper sediments was controlled by the reduction of As(V) and the reductive dissolution of Fe(III) (hydr)oxides, while the decreasing mobility in the deep sediments was attributed to immobilization of As(III) by secondary Fe(II)-bearing minerals. - Highlights: • The mobility of As in sediments was characterized in-situ with a high-resolution DGT. • The DGT-labile As in sediments exhibited three basic patterns in vertical distribution. • The mobility of labile As increased in the upper layers and decreased in the deep sediments. • A visible pollution of labile As appeared from the deep sediments when FDGT-M > 1.7 pg cm−2 s−1. • The DGT-labile As was a sensitive indicator in reflection of the pollution of sediment As. - In-situ DGT measurements showed a risk of As pollution from sediments when FDGT-M > 1.7 pg cm−2 s−1 or ΔL < 41 mm.
[en] The compaction effect may significantly flatten an excess 210Pb depth profile near the sediment-water interface, resulting in erroneous 210Pb dating. There are three mathematical approaches to correct for the compaction effect. In the present paper, some further deductions have been done to make the procedure of approach calculations more clearly and practically. Meanwhile, the three approaches are also compared using a case study selected from the literature
[en] A new method based on passive samplers has been developed to assess the diffusive flux of fluorene, fluoranthene and pyrene in the sediment bed and across the sediment-water interface. The dissolved compound concentration gradient in the sediment in the vertical direction was measured at the outlet of a storm water pond by using polyethylene strips as passive samplers. Simultaneously, the dissipation of a set of tracer compounds preloaded in the passive samplers was measured to estimate the effective diffusion coefficients of the pollutants in the sediment. Both measurements were used to evaluate the diffusive flux of the compounds according to Fick's first law. The diffusive fluxes of the 3 studied compounds have been estimated with a centimetre-scale resolution in the upper 44 cm of the sediment. According to the higher compound diffusion coefficient and the steeper concentration gradient in the surficial sediment layer, the results show that the net flux of compounds near the sediment interface (1 cm depth) is on average 500 times higher than in the deep sediment, with average fluxes at 1 cm depth on the order of 5, 0.1 and 0.1 ng/m2/y for fluorene, fluoranthene and pyrene, respectively. - Highlights: • Pore water concentration gradients of a set of PAHs were measured by passive samplers. • The diffusivities of PAHs in the sediment were measured using tracer compounds. • The investigated PAHs were fluorene, fluoranthene and pyrene. • The selected tracer compounds were 2H-labelled versions of the same PAHs. • The diffusive fluxes in the sediment and near the interface were calculated.
[en] Two physical in situ treatment techniques which are used to accelerate the natural processes that remove stranded oil on mixed coarse sediment beaches are discussed. These techniques involve sediment relocation from the upper parts of the beach to lower tidal zones (sediment relocation) and the mixing of surface oiled sediment using typical agricultural equipment (tilling). Sediment relocation was applied at two sites at different wave exposure and sediment conditions. Mechanical relocation of oiled sediments to lower sections of the intertidal zone significantly accelerated oil removal. Toxicity level was not increased in the near-shore environment by the sediment relocation treatments. In the tilling or mixing experiment the level of oil removal was accelerated in the short term, but after about ten days there was no evidence of increased effect over natural recovery. 3 figs
[en] In temperate zones, the addition of fertilizers is known to stimulate biodegradation in near-shore environments contaminated by oil spills. In this experiment the potential of stimulating biodegradation by the addition of fertilizers, under the conditions found at Svalbard, where beaches are frozen for about six months of each year, was of primary interest. Results showed that biodegradation does occur under Arctic conditions, and it can be stimulated by the addition of fertilizers without inducing adverse environmental effects. Monitoring methods and adjustment to the treatment strategies by using simple and inexpensive test kits were also described
[en] The importance of sample size in sampling natural sediments for contamination is discussed. It is shown that analytical results can vary widely if samples are too small to provide a realistic distribution of the grain sizes of the beach sediment with the normally high variability in natural sediment grain size. To guard against under or overestimation of contaminant concentration in the Svalbard field trials, samples greater than 1.5 litre were collected and extracted. In addition to large sample size, care was also taken to collect a vertical profile of equal proportions. Depth of oil penetration and accretion of surface sediments were also taken into account. Cross-shore differences in oil removal were avoided by collecting samples along a relatively narrow along-shore band within the inter-tidal zone, and edge effects were minimized by maintaining the sample exclusion zones within the oiled plot. The number of samples per plot is of obvious significance. The best indicator that the sample design was valid is the fact that, for some plots and treatments, differences in concentration could be defined at the 90 per cent confidence level