Impact of the Charleston Ocean Dredged Material Disposal Site on nearby hard bottom reef habitats

The deepening of shipping and entrance channels in Charleston Harbor (South Carolina, USA) was completed in April 2002 and placed an estimated 22 million cubic yards (mcy) of material in the offshore Charleston Ocean Dredged Material Disposal Site (ODMDS). To determine if sediments dispersed from the ODMDS were negatively affecting invertebrate and/or finfish communities at hard bottom reef areas around the disposal area, six study sites were established: three close to and downdrift of the ODMDS and three upcurrent and farther from the ODMDS. These sites were monitored biannually from 2000 to 2005 using diver surveys and annually using simultaneous underwater video tows and detailed sidescan-sonar. In general, the sediment characteristics of downdrift sites and reference sites changed similarly over time. Overall, the hard bottom reef areas and their associated communities showed little evidence of degradation resulting from the movement of sediments from the Charleston ODMDS during the study period.     

Trace metals, PAHs, and PCBs in sediments from the Jobos Bay area in Puerto Rico

This study provides baseline information on the extent of contamination in sediments of the Jobos Bay estuary and surrounding areas in Puerto Rico. Sediments from Jobos Bay area (n = 14) had higher overall average concentrations than those from La Parguera area (n = 5, used as reference site), in μg/g dw, for As (17 vs 9), Cu (29 vs 14), Pb (11 vs 4), and Zn (64 vs 28); and in %, for Fe (2.6 vs 0.6). Sediments (n = 8) screened for PAHs and PCBs exhibited total concentrations (ng/g dw) that ranged from 40.4 to 1912, and from not detected to 11.21, respectively. The quality of sediments of Jobos Bay could be classified as low to moderate pollution. The proximity to anthropogenic sources of contamination warrants a monitoring program for inorganic and organic pollutants in Jobos Bay area for an effective coastal management program of this tropical ecosystem.     

Anthropogenic influence on the degradation of an urban lake - The Pampulha reservoir in Belo Horizonte, Minas Gerais, Brazil

The artificial reservoir Lagoa da Pampulha in central Brazil has been increasingly affected by sediment deposition and pollution from urban and industrial sources. This study investigates water chemistry and heavy metal concentrations and their fractionation in the lake sediment using ICP-OES, ICP-MS, and XRD analyses. Fractionation analysis was done by sequential extraction under inert gas as well as after oxidation. The lake exhibits a permanent stratification with an oxygen-free hypolimnion below 2 m depth. Nutrient concentrations are enriched for phosphorous components (SRP, PO₄). In the sediment it was not possible to detect oxygen. Carbon, sulfur, and most of the analyzed heavy metals are enriched in the top sediment layer with a pronounced downward decrease, indicating the presence of an anthropogenic influence. Statistical analysis, including correlations and a Principal Component Analysis (PCA) of depth-related total concentration data, helps to distinguish presumably anthropogenic heavy metals from geogenic components. Some samples with high element concentrations in the sediment also show elevated concentrations in their pore water. Analyses of element distribution between sediment and pore water suggest a strong bonding of heavy metals to the anoxic sediment. The trend towards elevated solubility in the pore water of oxidized samples is clear for most of the analyzed elements. Fractionation analysis reveals characteristic associations of selected elements to specific mineral bonding forms. In addition, it indicates that the behavior of heavy metals in the sediment is strongly influenced by organic substances. These substances provide buffering against oxidation, acidification, and metal release. The high nutrient loading causes reducing conditions in the lake sediment. These conditions trigger the accumulation of sediments rich in S²⁻, which stabilizes the fixation of heavy elements. In the future, care must be taken to reduce the supply of contaminants and to prevent the release of heavy metals from sediments dredged for remediation purposes.     

Parameterization of near-bed processes under collinear wave and current flows from a two-phase sheet flow model

Sediment transport models require appropriate representation of near-bed processes. We aim here to explore the parameterizations of bed shear stress, bed load transport rate and near-bed sediment erosion rate under the sheet flow regime. To that end, we employ a one-dimensional two-phase sheet flow model which is able to resolve the intrawave boundary layer and sediment dynamics at a length scale on the order of the sediment grain. We have conducted 79 numerical simulations to cover a range of collinear wave and current conditions and sediment diameters in the range 210–460 μm$\mathrm{\mu }\mathrm{m}$. The numerical results confirm that the intrawave bed shear stress leads the free stream velocity, and we assess an explicit expression relating the phase lead to the maximum velocity, wave period and bed roughness. The numerical sheet flow model is also used to provide estimates for the bed load transport rate and to inspect the near-bed sediment erosion. A common bed load transport rate formulation and two typical reference concentration approaches are assessed. A dependence of the bed load transport rate on the sediment grain diameter is observed and parameterized. Finally, the intrawave near-bed vertical sediment flux is further investigated and related to the time derivative of the bed shear stress.     

Temporal variations of C,N, δC,and δN in organic matter collected by a sediment trap at Cuenca Alfonso,Bahía de La Paz,SW Gulf of California

To monitor the composition and the vertical flux of particulate matter from the sea surface, a sediment trap was moored in Cuenca Alfonso, Bahía de La Paz, a zone of high productivity in the southwestern Gulf of California. Carbonate-free samples from 2002 to 2005 were analyzed for C_org, N, δ¹³C, and δ¹⁵N. The results show seasonal and interannual variability, with the δ¹³C and δ¹⁵N values larger in spring and summer than in fall and winter. The C:N ratio and δ¹³C increased by 1.5 units from 2002 to 2003–2005, suggesting a change in the supply of organic matter and-or the use or preferential degradation of N_org. There was no interannual variation in δ¹⁵N. The occasional high δ¹⁵N values suggest that physical mechanisms, such as the shoaling and advection into the bay of ¹⁵N-rich subsurface equatorial water, occur over short time periods. The latter is presumed to be related to the periodic development of a significant cyclonic gyre in the southern Gulf.     

In situ record of sedimentary processes near the Rhône River mouth during winter events (Gulf of Lions,Mediterranean Sea)

The environment is impacted by natural and anthropogenic disturbances that occur at different spatial and temporal scales, and that lead to major changes and even disequilibria when exceeding the resiliency capacities of the ecosystem. With an annual mean flow of 1700 m³ s⁻¹, the Rhône River is the largest of the western Mediterranean basin. Its annual solid discharges vary between 2 and 20 Mt, with flood events responsible for more than 70% of these amounts.     

The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt

This study fits into a wider research program from RWS RIKZ concerning the exchange of sediment between the coast and the tidal basins, Western Scheldt and Wadden Sea, which are the largest basins along the Dutch Coast, over different time-scales. For both basins, questions about the evolution of the import/export at the mouth recently arose. In case of the Western Scheldt, which is the subject of this study, mainly the uncertainty about the future developments after the change from import to export at the mouth was noticed in the 1990s, which necessitated a more detailed study of this area.     

Conditions when anisotropy is negligible for solute transfer in sediment beds of lakes or streams

To reduce the complexity and save computation time, an isotropic and a scalar dispersion model are explored and compared to the anisotropic advection/dispersion model to study the interstitial flow in a stream and lake sediment induced by a periodic pressure wave. In these systems, the solute transport is controlled by the ratio (R = a/(LS)) of the pressure wave steepness (a/L) to the stream slope (S), and the dispersivity ratio (λ = α_L/L) that measures the longitudinal dispersivity (α_L) relative to the pressure wave length (L). Through a series of numerical experiments, the conclusion is reached that a scalar dispersion model can be applied with satisfactory results for advection-dominated transport, i.e. when R ?  0.1 and λ ? 0.01, or λ ? 0.0001, i.e. Peclet number (Pe) ? 10000; an isotropic dispersion model can be applied when R ? 10 or λ ? 0.001, and the full anisotropic advection/dispersion model has to be applied when R > 10 and λ > 0.001.     

Effects of Caragana Korshinskii Kom. Cover on runoff,sediment yield and nitrogen loss

Soil erosion is serious in the Loess Plateau of China. Deposition of the eroded sediment in lakes or rivers may lead to eutrophication, because the sediment carries a lot of nutrients. Field experiments were conducted to study soil erosion and loss of nitrogen (N) from a 15o hillslope with 30% (low) or 80% (high) coverage of the shrub Caragana korshinskii Kom. A bare soil plot was used for the comparison. The results showed that Caragana korshinskii cover significantly reduced runoff and soil erosion. In comparison to the bare soil plot, the vegetation covered plots had about 20% less runoff and 65% less sediment. In general, the concentration of N in both runoff water and the eroded sediment decreased with time and approached a steady value. However, the species of nitrate nitrogen (NO3) was exceptional which increased with time slightly. The soil erosion caused an N loss of about 250 mg/m2 for the bare soil plot, the low coverage of Caragana korshinskii reduced the N loss by 20% and the high coverage of Caragana korshinskii reduced the N loss by 40%. Moreover, the amount of total N in eroded sediment was 2 to 3 times higher than the value in runoff water. In the total N loss, the organic N was about 75-80%. Nevertheless, inorganic N in runoff water was 5 to 10 times higher than the value in eroded sediment. The species of NO3 was obviously higher than the species of ammonium nitrogen (NH4). NO3 was the main species of inorganic N loss and was about two thirds of total. The organic N was the main species of N in the eroded sediment.