Sedimentation of metals in Sundarban mangrove ecosystem: Dominant drivers and environmental risks

Metal contamination from upstream river water is a threat to coastal and estuarine ecosystem. The present study was undertaken to unveil sedimentation processes and patterns of heavy metal deposition along the salinity gradient of a tropical estuary and its mangrove ecosystem. Sediment columns from three representative sites of differential salinity, anthropogenic interference, and sediment deposition pattern were sampled and analyzed for grain size distribution and metal concentrations as a function of depth. Sediments were dominantly of silty-medium sand texture. A suite of fluvial and alluvial processes, and marine depositional forcing control the sediment deposition and associated heavy metal loading in this estuary. The depth profile revealed a gradual increase in heavy metal accumulation in recent top layer sediments and smaller fractions (silt + clay), irrespective of tidal regimes. Alluvial processes and long tidal retention favor accumulation of heavy metals. Enrichment factor (0.52-15), geo-accumulation index (1.4-5.8), and average pollution load index (PLI = 2.0) indicated moderate to higher heavy metal contamination status of this estuary. This study showed that alluvial processes acted as dominant drivers for the accumulation of metals in sediments, which prevailed over the influence of marine processes. Longer tidal retention of the water column favored more accumulation of heavy metals. Metal accumulation in the sediments entails a potential risk of bioaccumulation and biomagnification through the food web, and may increasingly impact estuarine ecology, economy, and ultimately human health.

Distribution of Pb, Sr, and U isotopic signature and multielement composition of sediment in Lake Balaton (Hungary) at a sediment trap deep dredged 40 years ago near Balatongyörök-search for routes of recent pollution

We studied the Pb, Sr, and U isotopic composition and the concentration of toxic metal elements in sediment core samples collected in Lake Balaton at a sediment trap that was deep dredged in 1979, to analyze their changes in the last 40 years. Pb isotopic composition profiles of cores taken from the sediment trap showed different ²⁰⁶Pb/²⁰⁷Pb ratios ranging from 1.206 ± 0.002 at the bottom of the core (phase 1) compared to 1.185 ± 0.002 at the top of the core (phase 2). Phase 2 is the fraction reflecting isotopic signatures of the latest 40 years. At 80-100-cm depth, a transition zone was observed. Pb concentration together with Zn, Sb, Cu, Cd, and Fe showed elevated, 2-4 times higher values in the top phase of the sediment. The calculated Pb isotopic composition of pollutant Pb fraction was 1.177 ± 0.005 in the case of the ²⁰⁶Pb/²⁰⁷Pb and 2.456 ± 0.004 for ²⁰⁸Pb/²⁰⁷Pb, which shows good agreement with literature data for lead ores in Poland and Germany, but it is distinct from literature data for leaded fuel concerning Middle and Eastern Europe. The marked difference in the Pb signatures of phases enabled the construction of a sediment deposition rate map. U and Mo showed a characteristic concentration peak positioned exactly at the depth of the Pb signature transition. The isotopic signature of U based on ²³⁴U and ²³⁵U also showed a similar pattern. We suggest that the deposition of U and Mo can be related to cyanobacterial blooms in Lake Balaton in the late 1970s and early1980s.

Effect of sediment deposition on phosphate and hydrogen sulfide removal by granulated coal ash in coastal sediments

Granulated coal ash (GCA) is a strong in-situ capping material for removing PO₄-P and H₂S-S in contaminated coastal sediments. Although GCA performance is weakened by sediment deposition, related research is rare. To evaluate sediment deposition effects on PO₄-P and H₂S-S removal by GCA, GCA was placed on the top of sediment (C-GCA), was partially mixed with sediment (M-GCA), and was fully covered by sediment (N-GCA). Effective PO₄-P and H₂S-S removal from sediments occurred in the order of C-GCA > M-GCA > N-GCA. C-GCA and M-GCA significantly decreased PO₄-P and H₂S-S concentrations by 84- 90% and 100%, respectively, through calcium phosphate and iron sulfide precipitation. N-GCA was less effective in PO₄-P and H₂S-S removal than the control after 2.5 months, as fine sediment particles blocked the GCA pores, decreasing calcium and iron elution. The results provide a better understanding of how sediment deposition negatively impacted GCA performance.

Development of an automated sensor for in-situ continuous monitoring of streambed sediment height of a waterway

The sedimentary processes play a major role in every aquatic ecosystem, however, there are few automated options for in-situ monitoring of sediment displacement in the streambed of waterways. We present an automated optical instrument for in-situ continuous monitoring of sediment deposition and erosion of the streambed that requires no calibration. With a production cost of 32€, power consumption of 300 μA in sleep mode, and capacity to monitor the bedform of a waterway, the sensor was developed to evaluate the sediment dynamics of coastal areas with a wide spatial and temporal resolution. The novel device is intended to be buried in the sand and uses 32 infrared channels to monitor the streambed sediment height. For testing purposes, a maximum measuring length of 160 mm and 5 mm resolution was chosen, but these values are scalable. Sensors can be built with different ranges and precision according to the needs of the fieldwork. A laboratory experiment was conducted to demonstrate the working principle of the instrument and its behaviour regarding the turbidity originated by suspended sediment and the settling and deposition of the suspended particles. The device was deployed for 119 days in an estuarine area and was able to detect patterns in the sediment deposition and resuspension during the tidal cycles. Also, abnormal events occurred during the experiment as floods and algae blooms. During these events, the sensor was able to record exceptional erosion and sediment deposition rates. The reported automated instrument can be broadly used in sedimentary studies or management and planning of fluvial and maritime infrastructures to provide real-time information about the changes in the bedform of the watersheds.

Backshore nourishment of a beach degraded by off-road vehicles: Ecological impacts and benefits

Worldwide, spoil from maintenance dredging of navigation channels is increasingly used to opportunistically nourish beaches. This is often justified on the presumption that nourishment will improve public beach amenity and restore sandy beach habitat. However, this is not necessarily the case, especially for beaches that do not have an immediate threat of significant erosion. We addressed the ecological impacts and benefits of a backshore sand nourishment project conducted along an off-road vehicle (ORV) damaged section of Blacksmiths Beach, New South Wales, Australia. Sediment, sourced from dredging the inlet of nearby Lake Macquarie, was placed on the foredune, ORVs were excluded and low-density vegetation was planted. Sampling before and after the management interventions, at the Impact (nourished) site, two Control sites (with ORVs), and two Reference sites (without ORVs), assessed ecological impacts of nourishment and the efficacy of the interventions in rehabilitating vegetation and invertebrate communities degraded by ORVs. Nourishment initially had large negative impacts on vegetation cover, as well as on invertebrate abundance and richness. Recovery to a pre-nourished state was, however, observed for vegetation cover after 9 months and invertebrate communities after 21 months. Nevertheless, by the end of our study that extended 21 months post-nourishment and ORV exclusion, there was no evidence of change in the nourished site towards the state of Reference sites. Overall, our study suggests that small-scale backshore sand nourishments of ocean beaches may have only short-term negative impacts on foredune ecosystems when accompanied with some replanting. Nevertheless, where the frequency of sand disposals is greater than the required recovery time, or cumulative effects amass, longer-term or sustained impacts may occur. Our study does not support the efficacy of sand nourishment as a tool for ecological restoration, at least in the short term, without sustained replanting and weeding efforts aimed at reinstating the vegetation community.

Multiple stressors and stream macroinvertebrate community dynamics: Interactions between fine sediment grain size and flow velocity

Agricultural development has resulted in the degradation of freshwater ecosystems worldwide. Two key stressors impacting streams and rivers draining agricultural catchments are deposited fine sediment (e.g. due to erosion) and reduced flows (e.g. due to water abstraction, dams, or climate change). Past studies have identified fine sediment as a 'master stressor' in streams, but the effects of different sediment grain sizes in combination with reduced flow velocity are poorly understood. We manipulated deposited fine sediment (no added sediment; silt: 0-0.125 mm; fine sand: 0.125-0.250 mm; coarse sand: 1-2 mm) and flow velocity (fast: 26.5 cm/s; medium: 13.9 cm/s; slow: 0.0 cm/s) simultaneously in 60 outdoor stream mesocosms. We determined the individual and combined effects of these stressors on the benthic, drifting, and emerging stream macroinvertebrate communities. Both fine sediment and reduced flow velocity had pervasive detrimental impacts on stream invertebrate communities. Negative effects of sediment were worse at the smaller two grain sizes for some responses (abundance of Chironomidae, Copepoda, Psilochorema spp.); however, for several sediment-sensitive common taxa or community-level invertebrate metrics, effects were negative regardless of grain size. Although their combined effects were mainly additive, sediment impacts were worsened by reduced flow velocities in several cases. Our findings imply that (a) especially for sediment-sensitive species, all fine sediment <2 mm has profound negative effects, (b) sediment grain size matters for some invertebrate taxa, where severity of impacts increased as particle size decreased, and (c) negative effects of sedimentation can become worse when combined with reduced flow velocity.

Particle size segregation of turbidity current deposits in vegetated canopies

Interactions between ecology, hydrodynamics and sediments play central roles in the evolution of coastal and freshwater ecosystems. We set out to characterise interactions of a specific hydrodynamic phenomenon - turbidity currents - with vegetation and sediment dynamics. We measured hydrodynamics and sediment deposition rates when turbidity currents flowed into plant canopies in a lock-exchange flume experiment, using simulated vegetation and three real plant species, and varying the turbidity current's initial sediment concentration. The natural sediment used had an essentially bimodal size distribution, with coarse (6.2-104 μm) and fine (2.2-6.2 μm) fractions. In all cases, on entering the vegetation canopy, the turbidity current was initially inertially-dominated, but subsequently became drag-dominated. In the inertial regime, there was no size segregation in the deposited material. In the drag-dominated regime, the deposited material became increasingly dominated by fine sediment, at a rate dependent on the vegetation type. The transition between these two regimes occurred at a distance equivalent to 5.1-7.6 times the total water depth downstream of the lock gate. The size segregation of deposited sediment is posited to have consequences for substrate evolution, which in turn may affect vegetation growth. Thus, our findings point to a non-linear feedback mechanism between the spatial heterogeneity of vegetation canopies and that of the substrate they help to engineer.

Wind erosion on arable lands, associated with extreme blizzard conditions within the hilly area of Eastern Romania

The removal of wind-blown soils from cultivated fields is often expensive. It physically removes the most fertile portion of the soil that can lead to yield reduction in areas where wind erosion is a recurring problem. Soil nutrients and surface-applied chemicals can also be carried along with soil particles, contributing to off-site impacts. An extreme blizzard event has been investigated as case study within the Moldavian Plateau of eastern Romania. The results obtained, based on 45 snow core samples, show that the deposited sediment widely varied depending on land-use. The highest mean value of 5967 g m^-2 occurred on fallow land and it sharply decreased under winter wheat (445 g m^-2) and pasture (345 g m^-2). Regarding the major subunits of the Moldavian Plateau, the Jijia Rolling Plain (JRP) was the most affected, with mean value of 7547 g m^-2. The Barlad Plateau (BP) and the Falciu Rolling Plain (FRP) are depicted by smaller values, namely: 395 g m^-2 and 386 g m^-2. By using remote sensing it was estimated that 208,990 ha under fallow from JRP (34% of the total) were heavily subjected to wind erosion with a mean soil loss of 2.04 t ha^-1, while the depositional area comprised 175,440 ha (29% of the total). The deposited soil is loamy compared with the general clayey-loamy texture of local soils. The main source of the wind-blown material is represented by the close proximity fields under fallow of the depositional areas.

Particle deposition, resuspension and phosphorus accumulation in small constructed wetlands

To improve understanding of phosphorus (P) retention processes in small constructed wetlands (CWs), we analysed variations in sediment deposition and accumulation in four CWs on clay soils in east-central Sweden. Sediment deposition (in traps) generally exceeded the total suspended solids (TSS) load suggesting that resuspension and wetland base erosion were important. This was confirmed by quantification of particle accumulation (on plates) (1-23 kg m^-2 year^-1), which amounted to only 13-23% of trap deposition. Spatial mean P concentrations in accumulated sediment on plates (0.09-0.15%) were generally similar to temporal mean P concentrations of particles in water (0.11-0.15%). Deposition/accumulation was minor in one wetland with high hydraulic load (400 m year^-1), suggesting that such small wetlands are not efficient as particle sinks. Economic support for CWs are given, but design and landscape position are here demonstrated to be important for effective P retention.

Coral reef health response to chronic and acute changes in water quality in St. Thomas, United States Virgin Islands

It is suspected that land cover alteration on the southern coast of St. Thomas, USVI has increased runoff, degrading nearshore water quality and coral reef health. Chronic and acute changes in water quality, sediment deposition, and coral health metrics were assessed in three zones based upon perceived degree of human influence. Chlorophyll (p<0.0001) and turbidity (p=0.0113) were significantly higher in nearshore zones and in the high impact zone during heavy precipitation. Net sediment deposition and terrigenous content increased in nearshore zones during periods of greater precipitation and port activity. Macroalgae overgrowth significantly increased along a gradient of decreasing water quality (p<0.0001). Coral bleaching in all zones peaked in November with a regional thermal stress event (p<0.0001). However, mean bleaching prevalence was significantly greater in the most impacted zone compared to the offshore zone (p=0.0396), suggesting a link between declining water quality and bleaching severity.

Functional degradation of the water-sediment regulation scheme in the lower Yellow River: Spatial and temporal analyses

Heavy sedimentation has led to the phenomenon of a secondary perched river in the lower reaches of the Yellow River. The water-sediment regulation scheme (WSRS) using the Xiaolangdi Reservoir was first implemented in 2002 to try to solve this problem. In this study, we analyzed the spatial and temporal effects of the current WSRS (2005-2013) on the lower Yellow River. Our results suggest that the current WSRS is exhibiting a tendency towards functional degradation, meaning that the scheme is no longer as effective as it was initially for the lower Yellow River. Although the main river channel has been fully scoured in the lower reaches since the implementation of the WSRS, we found that the degree of erosion declined gradually in a top-down fashion from the braided reach, through the transitional reach, to the meandering reach. Of the total eroded sediment, 69.64% came from the braided reach and only 6.61% came from the meandering reach. In addition, the reduction in riverbed elevation-a key function of the WSRS-has clearly slowed since 2005. We discuss the mechanisms underlying this functional degradation of the current WSRS and future challenges for the management of the lower Yellow River. Insights gained from this study will likely be of use to those weighing up options for future implementations of the WSRS.

Effects of coastal managed retreat on mercury biogeochemistry

We investigated the impact of managed retreat on mercury (Hg) biogeochemistry at a site subject to diffuse contamination with Hg. We collected sediment cores from an area of land behind a dyke one year before and one year after it was intentionally breached. These sediments were compared to those of an adjacent mudflat and a salt marsh. The concentration of total mercury (THg) in the sediment doubled after the dyke was breached due to the deposition of fresh sediment that had a smaller particle size, and higher pH. The concentration of methylmercury (MeHg) was 27% lower in the sediments after the dyke was breached. We conclude that coastal flooding during managed retreat of coastal flood defences at this site has not increased the risk of Hg methylation or bioavailability during the first year. As the sediment becomes vegetated, increased activity of Hg-methylating bacteria may accelerate Hg-methylation rate.

The role of remediation, natural alkalinity sources and physical stream parameters in stream recovery

Acid mine drainage (AMD) negatively impacts not only stream chemistry, but also aquatic biology. The ultimate goal of AMD treatment is restoration of the biological community, but that goal is rarely explicit in treatment system design. Hewett Fork in Raccoon Creek Watershed, Ohio, has been impacted by historic coal mining and has been treated with a calcium oxide doser in the headwaters of the watershed since 2004. All of the acidic inputs are isolated to a 1.5 km stretch of stream in the headwaters of the Hewett Fork watershed. The macroinvertebrate and fish communities have begun to recover and it is possible to distinguish three zones downstream of the doser: an impaired zone, a transition zone and a recovered zone. Alkalinity from both the doser and natural sources and physical stream parameters play a role in stream restoration. In Hewett Fork, natural alkaline additions downstream are higher than those from the doser. Both, alkaline additions and stream velocity drive sediment and metal deposition. Metal deposition occurs in several patterns; aluminum tends to deposit in regions of low stream velocity, while iron tends to deposit once sufficient alkalinity is added to the system downstream of mining inputs. The majority of metal deposition occurs upstream of the recovered zone. Both the physical stream parameters and natural alkalinity sources influence biological recovery in treated AMD streams and should be considered in remediation plans.