Characterization and spatial distribution of particulate and soluble carbon and nitrogen from wildfire-impacted sediments

Purpose

The heavily forested Cache la Poudre (CLP) watershed in northern Colorado, USA, was impacted by the High Park wildfire in 2012. The wildfire burned land and vegetation immediately adjacent to the CLP River where blackened, ashy sediment samples were collected from five sites upstream of the City of Fort Collins drinking water intake to evaluate the spatial distribution and characteristics of burned sediments, along with quantifying and characterizing soluble compounds following a leaching experiment.

Materials and methods

At each site, samples were collected from three locations: (1) the edge of the bank adjacent to the water edge (downbank), (2) 1 m upslope of location 1 (midbank), and (3) 2 m upslope of location 1 (upperbank). All solid sediment samples were analyzed for elemental composition, and a subset of solid sediment samples were analyzed with ¹³C solid-state nuclear magnetic resonance spectroscopy. Sediments were mixed with the background CLP River water collected from upstream of the wildfire and allowed to leach for 6 and 24 h to determine the quantity and quality of water-soluble constituents. Filtered samples were analyzed for dissolved organic carbon (DOC), iron, manganese, and inorganic nutrient concentrations, by optical properties, and for disinfection byproduct (DBP) formation.

Results and discussion

Percent carbon and nitrogen content of the solid sediments were good predictors of leachate DOC concentration. The mean fluorescence index was higher for wildfire-impacted sediment leachates (1.50) compared to the background CLP River water (1.37), which may be due to changes in DOM molecular weight and oxidation of organic matter. All sediment leachates showed consistently higher haloacetonitrile and chloropicrin yields (DBP concentration/DOC concentration) compared to background CLP River water, whereas carbonaceous DBPs did not.

Conclusions

The collected sediments showed that burned material accumulated downstream near the river and was composed of inputs from burned soil and biomass along with the mobilization of unburned terrestrial material. The leachates of these sediments have different characteristics compared to the background CLP River water, indicating that DOM leached from sediments following a wildfire may increase aquatic DOC concentrations and N-DBP formation.

     

Use of batch leaching tests to quantify arsenic release from excavated urban soils with relatively low levels of arsenic

Purpose

In Japan, the excavated soils produced from constructions projects, which contain relatively low levels of arsenic (As), are considered as a potential concern if they could release significant amount of As to the environment. The aim of this study was to investigate the As leaching from excavated alkaline soils and, in particular, the influence of drying methods, pH of extracting solution, and consecutive washing on As leaching from these soils.

Materials and methods

Four excavated alkaline soils obtained from different construction sites in Tokyo, Japan, were used in this study. The soils were pretreated by three drying methods (air-dried, 40 °C-dried, and freeze-dried). Sequential extraction procedure was applied to partition As into five operationally defined chemical fractions. Batch leaching tests (initial pH-controlled leaching test and consecutive washing test) were conducted to investigate the As release under different leaching conditions.

Results and discussion

The As contents in the four soils were 9.22, 79.4, 6.75, and 11.7 mg kg^?1, respectively, and As was primarily associated with the residual phase. Arsenic leaching was strongly dependent on the extracting solution pH values. Strongly acidic extracting solution (pH 2) led to circumneutral leachates and limited As mobility, whereas the strong alkaline-extracting solution (pH 12) greatly enhanced the As release from these soils. The consecutive washing test results revealed a long-term release of As from these excavated soils. The pollution potential indices (PPIs) were successfully used to evaluate the pollution threat of As leaching from excavated soils. In addition, different drying methods resulted in variations in the short- and long-term release of As from these excavated soils.

Conclusions

The results revealed that the soil pretreatment and the leaching conditions should be considered if we want to use batch tests for the contamination assessment of excavated urban soils from construction projects. Different drying methods and single extraction may lead to misestimation of the As pollution level. High extraction efficiency with strong alkaline-extracting solution (pH 12) reveals that it could potentially be used to wash As from excavated alkaline soils.

     

Effect of hydrodynamics factors on sediment flocculation processes in estuaries

Purpose

Cohesive sediment is able to flocculate and create flocs, which are larger than individual particles and less dense. The phenomenon of flocculation has an important role in sediment transport processes such as settling, deposition and erosion. In this study, laboratory experiments were performed to investigate the effect of key hydrodynamic parameters such as suspended sediment concentration and salinity on floc size and settling velocity. Results were compared with previous laboratory and field studies at different estuaries.

Materials and methods

Experimental tests were conducted in a 1-L glass beaker of 11-cm diameter using suspended sediment samples from the Severn Estuary. A particle image velocimetry system and image processing routine were used to measure the floc size distribution and settling velocity.

Results and discussion

The settling velocity was found to range from 0.2 to 1.2 mm s^?1. Settling velocity changed in the case of increasing suspended sediment concentration and was controlled by the salinity. The faster settling velocity occurred when sediment concentration is higher or the salinity is lower than 2.5. On the other hand, at salinities higher than 20, in addition to increasing SSC, it was found that the situation was reversed, i.e. the lower the sediment concentration, the faster the settling velocity.

Conclusions

Sediment flocculation is enhanced with increasing sediment concentration but not with increasing salinity.

     

Bathymetric modeling of sediments and organic carbon of polluted rivers in southeastern China

Purpose

Rivers in low plains in the subtropical regions of China, where the population is dense and economies are active and well-developed, might be a large terrestrial carbon pool. This present study of the Sanyang wetlands in Wenzhou, southeastern China, aims to quantitatively estimate the volume of sediments in this region’s polluted river systems and their carbon storage.

Materials and methods

The bathymetry of river sections were surveyed using an echo sounder equipped with a differential GPS. An underwater digital elevation model (DEM) was then developed using the anisotropic ordinary Kriging method. Sediment samples were collected and analyzed for carbon content and sediment properties. Carbon storage in sediments was calculated using bathymetric and sediment analysis data.

Results and discussion

The studied rivers have been receiving organic pollutants from local residences and industries for decades. Results from a river network of 1.2 km² revealed a total carbon storage of 46.7 million kg in the sediments which had a volume of 1.4 million m³, with the upper 20 cm depth of sediments contributing about one third of this carbon storage.

Conclusions

The present work demonstrated that GIS technologies can be used to create digital river sediment surfaces and assess sediment amounts as well as determine the spatial distribution of sediments and their components. This could provide further insight into river restoration planning and other options from a carbon-balance perspective.

     

Hazard assessment of polycyclic aromatic hydrocarbons in water and sediment in the vicinity of coalmines

Purpose

Rivers feeding into the Loskop Dam, South Africa, pass through coal mining and heavily industrialised areas. Previous studies investigating mass mortalities of crocodile and fish in the river system, revealed the presence of organic compounds, including polycyclic aromatic hydrocarbons (PAHs), in their tissues.

Materials and methods

Samples were collected from nine sites within the dam in winter and summer. Liquid-liquid and microwave-assisted extraction was used for preconcentrating PAHs from water and sediment samples, respectively. Extracts were analysed by gas chromatography-mass spectrometry in the selected ion monitoring mode to determine the presence and levels of the 16 United States Environmental Protection Agency (US EPA)-priority PAHs.

Results and discussion

Significant levels of PAHs were found in both water and sediment samples. Concentrations were higher in sediments than in water, as well as in samples collected in winter. Levels of PAHs in sediments were generally higher than those reported by many researchers for other water bodies in industrialised areas. Zebrafish embryos were used to assess potential risks associated with the water and sediment, and to determine the effects of PAHs on aquatic life. Embryos, exposed to intact water and sediment samples, as well as to diluted sediment extracts, were monitored for 120 h post fertilisation. Sediment proved more toxic to zebrafish embryos than water, causing delayed embryo development and malformations.

Conclusions

These findings are alarming as they indicate that this water system is under stress. These findings can be typical of any water system situated in a coal mining and industrial region.

     

Sediment transport below a small alpine reservoir desilted by controlled flushing: field assessment and one-dimensional numerical simulation

Purpose

Sediment transport and riverbed sedimentation were investigated in an alpine stream below a small hydropower reservoir desilted by a controlled sediment flushing (CSF) operation. The term “controlled” refers to the operational tasks implemented to mitigate the downstream environmental impact of the operation. The experimental dataset acquired before, during, and after the CSF was also used to carry out and calibrate a one-dimensional sediment transport model of the monitored event.

Materials and methods

The investigated reservoir is located in the central Italian Alps, and its original storage was 160,000 m³, about 30% filled by a mixture of sand and silt/clay before the CSF. Downstream sediment concentration was controlled by releasing clear water from upstream reservoirs and regulating the work of earth-moving equipment in the emptied reservoir. A 3.6-km-long reach with average slope of 0.015 was monitored: concentration and grain size of suspended sediment were measured during the CSF and the riverbed alteration was evaluated by volumetric sampling and measurements of the deposits’ thickness. Sedimentation and River Hydraulics—One Dimensional (SRH-1D) was used to simulate sediment transport during the monitored CSF. Model parameters were calibrated by comparing the computed and the observed amount of sediment deposited along the study reach.

Results and discussion

Sediment flushing was carried out in October 2010 for 3 days. Ca. 16,000 m³ of sediment were evacuated, representing approximately 30% silt/clay and 70% sand. 2.4 Mm³ of clear water was released to reduce sediment concentration and increase transport capacity downstream. About 3000 m³ of sand was deposited in the study reach after the CSF, with maximum height up to 0.2 m. Although the riverbed before the CSF was simply set as mono-granular, after calibrating the parameters, good agreement was achieved between the depositional pattern computed by SRH-1D and the one observed, both in terms of deposit thickness and grain size of deposited sediment. The sensitivity analysis revealed a major role of the parameters controlling bed mixing processes in affecting the simulated deposition after the CSF.

Conclusions

Sediment below 0.1 mm in diameter was not detected in river deposits after the flushing: the effects on river biota associated with substrate clogging by very fine sediment were therefore minimized. After proper calibration, 1-D sediment transport modeling can effectively support the planning of CSF operations: to minimize the downstream environmental effects, concurrently achieving acceptable flushing efficiency, the analyzed scenarios as well as the model outputs need to be carefully evaluated from a multidisciplinary perspective.

     

Potential environmental impact of technosols composed of gossan and sulfide-rich wastes from São Domingos mine: assay of simulated leaching

Purpose

The aim of this study was to evaluate (i) the ability of two Technosols, prepared with gossan or sulfide-rich wastes plus mixtures of organic/inorganic amendments, to improve the characteristics of the wastes by the analysis of the variation of elemental concentrations in their simulated leachates, and (ii) the potential environmental risk of these Technosols evaluated through the concentrations of the elements leached from the tailings containing the two wastes, considering their mass in the São Domingos mine.

Materials and methods

Composite samples of two São Domingos mining wastes (gossan wastes—GW; sulfide-rich wastes—SW) were collected. Amendment mixtures, containing different organic/inorganic wastes (from green agriculture, distillation of Ceratonia siliqua and Arbutus unedo fruits, and limestone quarry), were applied at 12, 30, and 60 g/kg. Two sets of microcosm assays were performed under controlled conditions in greenhouse and monitored during 7 and 13 months for GW and SW, respectively. Materials from each pot/treatment (<5 cm of depth) were collected after 1, 4, 7, and 13 months of incubation and used to obtain simulated leachates (DIN extraction). The analytical parameters evaluated in the leachates were pH, electrical conductivity, and element concentrations using flame atomic absorption spectrometry for cations and graphite furnace atomic absorption spectrometry, ionic chromatography, and UV-VIS for elements that normally occur as anions in aqueous solution.

Results and discussion

Simulated leachates from SW had lower pH and higher concentrations of potentially hazardous elements than leachates from GW. The concentrations of As in leachates from GW-Technosols were higher than those in leachates from GW-control but <0.6 mg/kg. In GW-Technosols leachates, the pH and the concentrations of some nutrients (e.g., Ca and phosphates) also increased when compared to the control. In the SW-Technosols, the pH of the leachates increased only in the first month as long as limestone was present. In these leachates, a general decrease of the concentrations of some elements was observed (e.g., As, sulfate, Fe, Pb), especially in the first month. A clear influence of the dose and type of amendments was not observed during the experimental time span for both wastes.

Conclusions

The concentrations of elements like Al, Ca, Fe, and Pb in the leachates are controlled by both the pH of the solutions and the concentrations of phosphates and sulfates. The extensive mass of the studied mine wastes contribute to the release to the neighboring environment of considerable amounts of potentially hazardous elements. The rehabilitation of the mine wastes by the conception of Technosols, especially with sulfide-rich wastes, can reduce significantly their environmental impact.

     

Fine-sediment dynamics: towards an improved understanding of sediment erosion and transport

Purpose

Using Ells River, Alberta, Canada bed sediments, this study aims to determine (1) the erosion, transport, and deposition characteristics of cohesive bottom sediments, and (2) the influence of the microbial community in this regard.

Materials and methods

A 2-m annular flume was used to generate bed shear to assess cohesive sediment dynamics for eroded beds with consolidation/biostabilization periods of 1, 3, and 7 days. Additional optical particle sizing, image analysis, densitometry, and microbial analysis were employed to further the analysis with respect to bed erosion and eroded floc characteristics.

Results and discussion

Sediment dynamics can influence the benthic and planktonic community health within aquatic systems. The critical bed shear stress for erosion increased from 0.05 to 0.19 Pa (for 1- to 7-day runs). Consolidation (dry density) increased with time and depth and eroded biofilm biomass was observed to increase with time. The community structure of the eroded sediment did not change with time suggesting a stable well-established and highly selected community. Hydrocarbon-degrading bacteria were present within the microbial consortium. The sediment was highly hydrophobic (96 %) due to a high natural oil content which likely had a profound effect on sediment dynamics, flocculation, and sediment cohesion. Eroded sediment settled poorly, which will result in the long-range transport of associated contaminants.

Conclusions

The Ells River possesses some unique properties which should be considered when assessing contaminant source, fate, and effect. The most significant of these are small floc size, the hydrophobicity of the sediment, and the biological community as these were found to be influential in both the erosion and flocculation processes. It is important that any management strategies and operational assessments of reclamation strategies that may have implication on river health incorporate the sediment compartments (SS and bed sediment), biology, and the energy dynamics within the system in order to better predict the downstream flux of sediments.

     

Planktonic algal bloom significantly alters sediment bacterial community structure

Purpose

We determined the relationship of a planktonic algal bloom with spatial and temporal changes in sediment bacterial communities in a eutrophic urban river in the Taihu Basin, China.

Materials and methods

Surface sediments from different locations on the river, including with and without an algal bloom, were collected monthly for 1 year. Sediment bacterial communities were assessed by sequencing 16S rRNA gene amplicons using an Illumina MiSeq.

Results and discussion

There were distinct spatial and temporal changes in sediment bacterial community structure. Significant decrease in the Shannon diversity corresponded with the peak chlorophyll a (Chl a) concentration. Proteobacteria, Chloroflexi, and Bacteroidetes were the dominant phyla throughout the year; however, their relative abundances changed seasonally. At the time of peak Chl a concentrations, Proteobacteria and Bacteroidetes comprised a greater proportion of the bacterial community. Bacterial community structure also varied spatially at the operational taxonomic unit (OTU) level according to canonical correspondence analysis (CCA), especially in locations with an algal bloom compared with those without an algal bloom. There were positive correlations between multiple bacterial genera and Chl a content, suggesting the potential for facilitative relationships between phytoplankton and bacteria. Based on the CCA, water temperature, NH₄ ⁺-N, TN, NO₃ ^?-N, and TP in the sediment and Chl a contents in the water column were significantly correlated with sediment bacterial community structure (P < 0.05).

Conclusions

Planktonic algal blooms may influence sediment bacterial community structure at all taxonomic levels in urban rivers. This work emphasizes the need for more comprehensive studies regarding the impact of planktonic algal blooms on sediment bacterial communities.

     

Controls on microbial mercury transformations in contaminated sediments downstream of the Idrija mercury mine (West Slovenia) to the Gulf of Trieste (northern Adriatic)

Purpose

Concentrations and transformations of mercury were measured in river, estuarine, and marine sediments to determine factors affecting the fate of mercury entering the northern Adriatic Sea.

Materials and methods

Radiotracer methodology was used to compare rates of mercury methylation (²⁰³Hg), MeHg demethylation (¹⁴C), and sulfate reduction (³⁵S) in sediment depth profiles to concentrations of total and dissolved mercury species in the lower freshwater region of the Isonzo River, the coastal lagoons, and in the Gulf of Trieste, northern Adriatic Sea.

Results and discussion

Mercury was readily methylated and demethylated in all sediments, but the relative activity of these processes varied greatly with location. Methylation activity increased greatly from freshwater to the marine regions; however, demethylation was extremely high in the estuarine and lagoon sites. Ratios of methylation to demethylation were low in these coastal sites but increased further offshore in the gulf, which agreed with increased ratios of MeHg to total Hg (%MeHg) in gulf sediments. Comparisons of microbial activities indicated that sulfate reduction strongly controlled both methylation and demethylation. However, Hg methylation in coastal lagoon sediments was controlled by rapid demethylation and the bioavailability of Hg that was affected by Hg adsorption and precipitation. Methylation in offshore marine sites correlated with sulfate reduction but not the partitioning of Hg between pore water and solid phases. The decrease in sulfide production offshore exacerbated Hg methylation.

Conclusions

The freshwater to marine gradient in the Idrija/So?a/Isonzo/Adriatic region is dynamic, exhibiting horizontally variable rates of microbial activities and Hg transformations that create “hot spots” of MeHg accumulation that are controlled differently in each region.

     

Effects of biochar addition to estuarine sediments

Purpose

Biochar is a carbon-rich product, able to enhance soil fertility and mitigate CO₂ emissions. While biochar effects on agriculture are becoming known, its impact elsewhere, e.g., on estuarine ecosystems, has yet to be assessed. The main aim of the present study was to determine the effect of biochar on sediment–water retention, CO₂ emissions from sedimentary organic carbon decomposition, sediment pH and electrical conductivity, in aerobic conditions similar to those observed at low tide.

Materials and methods

Sediments from the Mondego Estuary (Portugal) were mixed with pine gasification biochar at different doses (5, 10, 14 %) and immersed in water with different salinity values (15, 25, 30) for 96 h. The influence of biochar on water retention, the residence time of water and CO₂ emissions between ?0.75 and ?1.5 MPa, total organic carbon, pH and electrical conductivity (EC) were determined. Carbon chemical composition and polycyclic aromatic hydrocarbon (PAH) concentrations were determined in sediments and biochar. Differences between biochar treatments after immersion in different water salinities were analysed using the Kruskal–Wallis test.

Results and discussion

Results showed that biochar was able to (a) increase sediment–water content in terms of quantity and residence time, (b) decrease CO₂ emissions, but only with a specific soil–water content and at the highest biochar dose, (c) increase sediment pH at all biochar doses and (d) increase sediment EC at the highest biochar dose. In contrast, the percentage of carbon mineralised was not modified. Biochar carbon was rich in PAHs and less decomposable than sedimentary carbon. The increments observed in sediment pH and EC were unable to change sediment alkaline or saline status according to standard classifications.

Conclusions

Our results suggest that the remarkable water adsorption capacity of biochar–sediment mixtures may be considered the main factor in regulating CO₂ emission rates from sediments, together with high PAH concentrations, which probably restrain the organic matter decomposition process.

     

Arsenic,lead, and uranium concentrations on sediments deposited in reservoirs in the Rio Grande Basin,USA–Mexico border

Purpose

The El Granero reservoir is the last reservoir of the Rio Conchos before it joins the Rio Grande at the Mexico–USA border. This reservoir, together with the San Marcos reservoir, is located in the arid region of Chihuahua, Mexico. High, naturally occurring radioactivity levels, as well as high arsenic (As) concentrations, have been found in both reservoirs. The main goal of this research was to establish the spatial and temporal distribution of trace and radioactive elements in surface sediments and cores collected from these reservoirs.

Materials and methods

Sediment cores were dated using ²¹⁰Pb and ¹³⁷Cs measurements and applying the constant rate of supply (CRS) model. Major, trace, and radioactive elements were determined in surface samples and three sediment cores. Radioactive elements were determined by both alpha and gamma spectrometry. Major and trace elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using the EPA 3051a method. Enrichment factors (EF), contamination factors (CF), and pollution load indexes (PLI) were calculated in order to identify the human impact in both reservoirs, whereas the chemical index weathering (CIW) was used to assess differences in the degree of weathering.

Results and discussion

High uranium (U) enrichment (EFs?=?24.9–54.7) was observed in core layers at the San Marcos reservoir, while in surface sediments, this enrichment was lower. The high variability of lead (Pb) and As in sediment cores from the Granero reservoir was attributed to human influence. Arsenic and Pb enrichment differences between entry and exit sediment cores were explained by the filtering capabilities of the elongated shape, the topography, and the presence of plants on the reservoir’s bed. The highest PLI was found at the entrance core of the Granero reservoir.

Conclusions

The natural element concentration levels of As, Pb, and U were established at the Granero reservoir. High EFs for As and Pb suggest an anthropogenic origin of these pollutants at specific time intervals. High U concentrations in the San Marcos area are explained as naturally occurring. The concentrations of As in most of the studied sediments could pose a risk to human health by As ingestion, since they are above the probable effect level (PEL).

     

Effect of aggregation on the adsorption of phosphorus onto air-dried sediment in contrasting shear flow conditions

Purpose

Fine sediments are usually collected in situ and air-dried for adsorption experiments, which may lead to particle aggregation and thus have a significant effect on phosphorus (P) adsorption under dynamic conditions. The main purpose of this study was to investigate the changes of aggregates due to drying with shear stress and the effects on the adsorption of P onto air-dried sediments under different shear rates after re-wetting.

Materials and methods

Sediment samples were collected from an alluvial river. Fine wet sediments (<31 μm) were wet-sieved and air-dried, and some air-dried sediments were further sonicated and served as the control. The grain size distribution of the three sediment samples (wet, dried, and sonicated) was measured to evaluate the particle aggregation level. The P sorption capacity of wet and dried sediments was determined by batch equilibrium experiments. The change of aggregate size with shear stress was investigated for dried and sonicated sediments. Sorption equilibrium experiments were performed to investigate the effect of shear stress on the P sorption with and without change of particle aggregation level, respectively.

Results and discussion

Fine particles agglomerated into larger aggregates during the drying process, resulting in a significant increase in the aggregate size. The sorption capacity was lower in aggregated sediment than in original wet sediment. Aggregate size in dried sediment decreased with the increase of shear rate, leading to an increase in the surface area and available adsorption sites, whereas the particle size of sonicated sediment was hardly affected. Accordingly, the P sorption amount of dried sediment increased with increasing shear rate, whereas that of sonicated sediment showed no significant change with shear rate after all sediments were suspended. There was a significant curvilinear correlation between aggregate size and P sorption amount for dried sediments, thus indicating that the P sorption amount increased significantly with decreasing aggregate size.

Conclusions

Sediment aggregation is an important factor affecting P adsorption besides the amount of suspended sediments and the exchange between suspended and bed sediments under dynamic conditions. The P equilibrium adsorption amount increases with shear stress for air-dried sediment. The effect of particle aggregation on the amount of P sorbed onto sediments should not be ignored, and thus, more attention should be paid to the pretreatment of sediment samples in the sorption experiments under dynamic conditions.

     

Sedimentary records of the Zrmanja River estuary,eastern Adriatic coast—natural vs. anthropogenic impacts

Purpose

Investigations of geochemical characteristics of sediments of the Zrmanja River estuary were done in order to determine the natural and anthropogenic factors influencing sediment composition in this area. For that purpose, spatial and temporal distribution of major and trace elements in the sediments and surrounding soils was studied.

Materials and methods

Sediment and soil samples, including one marl sample, were collected at 28 locations. All samples were subjected to total digestion and subsequently analysed by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) for total concentration of 20 elements (Ag, Al, As, Be, Ca, Cd, Co, Cr, Cu, Fe, Li, Mn, Ni, Pb, Rb, Sb, Sn, Sr, Ti and Y). Obtained concentrations and their normalized counterparts were used for assessment of factors influencing sedimentation in the study area.

Results and discussion

The results of the analysis showed that the composition of sediments of the Zrmanja River estuary is primarily determined by the composition of bedrock, existing hydrodynamic conditions and the relative isolation of the studied basin. Nevertheless, anthropogenic influences were observed as well. The composition of the Zrmanja River sediments reflects the impact of the ex-alumina factory “Jadral” and transfer by wind of the material from its immediate surroundings to the water system of the Zrmanja River. In addition, sedimentation in the Zrmanja River was found to be influenced by the construction of reservoirs and the HE “Velebit”, hydroelectric power plant located in the Zrmanja watershed.

Conclusions

The geochemical composition of recent sediments of the Zrmanja River estuary is controlled primarily by natural factors, although the influence of anthropogenic activities is also evident.

     

Factors controlling sediment trapping in two freshwater tidal wetlands in the Biesbosch area,The Netherlands

Purpose

A thorough understanding of mechanisms controlling sedimentation and erosion is vital for a proper assessment of the effectiveness of delta restoration. Only few field-based studies have been undertaken in freshwater tidal wetlands. Furthermore, studies that measured sediment deposition in newly created wetlands are also sparse. This paper aims to identify the factors controlling the sediment trapping of two newly created freshwater tidal wetlands.

Materials and methods

Two recently re-opened polder areas in the Biesbosch, The Netherlands are used as study area. Field measurements of water levels, flow velocities, and turbidity at both the in- and outlet of the areas were carried out to determine the sediment budgets and trapping efficiencies under varying conditions of river discharge, tide, and wind in the period 2014–2016.

Results and discussion

Short-term sediment fluxes of the two study areas varied due to river discharge, tide, and wind. A positive sediment budget and trapping efficiency was found for the first study area, which has a continuing supply of river water and sediment. Sediment was lost from the second study area which lies further from the river and had a lower sediment supply. The daily sediment budget is positively related to upstream river discharge, and in general, export takes place during ebb and import during flood. However, strong wind events overrule this pattern, and trapping efficiencies decrease for increasing wind strengths at mid-range river discharges and for the highest river discharges due to increased shear stress.

Conclusions

Delta restoration, based on sedimentation to compensate for sea-level rise and soil subsidence, could only be effective when there is a sufficient supply of water and sediment. Management to enhance the trapping efficiency of the incoming sediment should focus on directing sufficient river flow into the wetland, ensuring the supply of water and sediment within the system during a tidal cycle, creating sufficiently large residence time of water within the polder areas for sediment settling, and decreasing wave shear stress by the establishment of vegetation or topographic irregularities.

     

Carbon,iron, and aluminum responses to controlled water table fluctuations in sandy soil material

Purpose

Seasonal fluctuating water tables are common in sandy coastal plain soils, but their role in soil organic carbon dynamics is uncertain. Seasonal saturation influences biogeochemical processes that affect fate of organic matter and metals. A column study was conducted to test hypotheses that shallow water table (SWT) fluctuations reduce CO₂ loss (H1), increase leaching of dissolved organic C (DOC) and metals (Al and Fe) (H2), and result in greater net soil C storage (H3).

Materials and methods

The A- and E-horizon material of moderately well-drained sandy soils from five sites was collected for the study. Ten columns (two per site) of 90-cm height were packed to a thickness of 60 cm with E-horizon material, above which was packed 15 cm of A-horizon material from the same soil. Five columns were subjected to SWT treatments and five to deep water table (DWT) treatments. Upward CO₂ flux was measured using a NaOH (1 M) trap. Metal and DOC-C concentrations in leachates and in water sampled at the surface of columns were measured. Soil samples from the columns were collected by depth at 5–6-cm increments and tested for total and organic C, metals, and <50-μm material distribution by WT treatment.

Results and discussion

Upward flux of CO₂ was significantly less for SWT treatment. Higher DOC (for all events) and Fe concentrations (for first 18 days) were measured in SWT leachates as compared to DWT. Metal- and C concentrations were correlated (P?<?0.0001) for surface pore water samples of SWT but not for DWT. Net loss of C was significantly less under SWT condition. Results indicate significant water-table effects on magnitude and direction of C flux (solution or gaseous) and <50-μm particle distribution for sandy coastal plain soils materials.

Conclusions

Changes in water table depth can potentially alter not only net soil C storage but also the proportion of C converted to CO₂ versus DOC. Differences in the proportion would have consequences for C dynamics in ecosystems dominated by soils with fluctuating water tables such as occur extensively in the coastal plain of the SE USA and elsewhere.

     

Biochar improves sediment microbial fuel cell performance in low conductivity freshwater sediment

Purpose

The low conductivity of sediments for mass and electron transport is the most severe limiting factor in sediment microbial fuel cells (SMFCs), so that sediment ameliorations yielded more remarkable effects than electrode improvements. The objective of this research was to enhance the electricity generation of SMFCs with amendments of biochar to freshwater sediments for conductivity enhancement.

Materials and methods

Laboratory-scale SMFCs were constructed and biochars were produced from coconut shells at different temperatures. Variations in the power output, electrode potential, internal resistance, total organic carbon (TOC) content, and microbial communities were measured.

Results and discussion

Amending with biochar reduced the charge transfer resistances of SMFCs and enriched the Firmicutes (mainly Fusibacter sp.) in the sediment, which improved the SMFC power generation by two- to tenfold and enhanced the TOC removal rate by 1.7- to fourfold relative to those without the amendment.

Conclusions

The results suggested that biochar amendment is a promising strategy to enhance SMFC power production, and the electrical conductivity of biochar should be considered important when interpreting the impact biochar has on the electrical performance of soil or freshwater sediment MFCs.

     

Source of polynuclear aromatic hydrocarbons found in sediment in a region of expanding sugarcane cultivation of São Paulo State,Brazil

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are unintentional products that are classified as persistent toxic substances. The goal of the present study was to generate data on the presence of 15 priority PAHs that are found in surface sediment and core sediment in the region of the Turvo/Grande watershed, São Paulo State, Brazil, which is an area of expanding sugarcane cultivation, and to correlate these data with the sources of these PAHs and the guiding values for sediment quality analysis.

Materials and methods

Surface sediments and sediment cores were sampled during the rainy and dry seasons in February and July 2010. The extraction of PAHs from sediments was performed using a Soxhlet extractor, and then the extract was cleaned according to the methods of the US EPA 3630C (US EPA 1996) using a silica gel column. Quantification was performed using high performance liquid chromatography with fluorescence detection.

Results and discussion

The concentrations of all 15 PAHs decreased as the depth of the sediment core increased. Overall, the concentrations decreased along the sediment core; however, the RTURARG (region predominantly used for planting sugarcane and livestock) during the rainy season and the CAPRP sampling site (located at part of the Preto River dam) during the dry season showed increased concentrations in the first few sediment fractions, and then the concentrations decreased. Higher concentrations were observed in urban locations, and the concentration of naphthalene was higher than the probable effect level (PEL) determined by the Canadian environmental agency. The obtained diagnostic ratios indicate that the sediment from areas with an abundance of sugarcane was a pyrolytic source of PAHs, which indicates a contribution from burning straw to the PAH concentrations in those areas.

Conclusions

For all sampling sites and all PAHs, we found a decreasing trend in PAH concentrations with increasing sediment core depth, and the locations, such as CAPRP, that experienced a higher level of human activity had the highest total concentrations of PAHs. These locations were the only areas in which the PAH naphthalene was found in higher concentrations than the PEL. The diagnostic ratios reveal that regions with sugarcane plantations had predominantly pyrolytic sources of PAHs, indicating the contribution of PAHs from sugarcane straw burning.

     

Assessment of cytotoxicity and AhR-mediated toxicity of sediments from water level fluctuation zone in the Three Gorges Reservoir,China

Purpose

The Three Gorges Dam project is one of the biggest projects in the world. The water level fluctuation zone (WLFZ) was created with impoundment. The purpose of the current study is to investigate the ecotoxicological potential of the sediment extracts from the WLFZ and propose future WLFZ monitoring for early detection of environmental changes brought about by pollution.

Materials and methods

The investigation was performed by using cell-based in vitro bioassays to determine acute cytotoxicity (neutral red retention assay) and mechanism-specific aryl hydrocarbon receptor (AhR)-mediated activity (7-ethoxyresorufin-O-deethylase (EROD) induction assay) of sediment extracts with rainbow trout (Oncorhynchus mykiss) liver cells (RTL-W1).

Results and discussion

Results showed that the cytotoxicity and AhR-mediated toxicity potential of the sediment extracts from the WLFZ in the Three Gorges Reservoir (TGR) were moderate of level compared with the ecotoxicity of sediments from other river systems. However, according to a sediment classification system in Germany, sediments from some individual sites in the WLFZ showed strong toxicity. Compared to the results from the Yangtze River in our previous study, more attention should be paid to the aryl hydrocarbon receptor inducers in the WLFZ in TGR.

Conclusions

The in vitro bioassays used in this study may serve as a valuable tool to predict the potential ecological hazards of the organic pollutants in the WLFZ to the aquatic organisms in some extent.

     

Size characteristics of sediments eroded from three soils in China under natural rainfall

Purpose

The particle-size distribution of runoff sediment is important in understanding, characterizing and modeling the transport behavior of sediment and sediment-associated chemicals. The objective of this study was to investigate the particle-size distribution of sediments eroded from three soils in China under natural rainfall.

Materials and methods

Each of the three soils was packed to a depth of 30 cm in a 20?×?2.1 m runoff plot. Sediments yielded in nine natural rainfall events were analyzed for their particle-size distribution prior to and following dispersion.

Results and discussion

The sediment size measured in the undispersed condition was always larger than the one determined after chemical dispersion, indicating that part of the sediment was eroded in aggregated form. The degree of sediment aggregation depended on the clay content and the organic matter content of the sources. The mean sediment size quantified by mean weight diameter linearly increased with sediment yield for the two soils with relatively high clay content. The rate of increase was greater in the undispersed condition than that in the dispersed condition for these two soils. Comparing sediments to the corresponding source soil, the results of mean weight diameter and enrichment ratio both revealed that aggregate-size distribution was more sensitive to soil erosion than the primary particle-size distribution. Small aggregates, rather than the primary particles, were selectively eroded in the rainfall events.

Conclusions

These findings support the use of both dispersed and undispersed sediment-size distributions for the characterization of sediment transport and the associated sediment-bound nutrients and contaminants.