Increased bioavailability of mercury in the lagoons of Lomb, Togo: the possible role of dredging

Evaluation of sediment dredging in remediating toxic metal contamination - a systematic review

Toxic metal pollution is a leading environmental concern for aquatic systems globally, and remedial dredging has been widely employed to mitigate its harmful impacts. In terms of the short-term impacts of remedial dredging, mixed results are reported in several studies. Despite its immediate negative impacts including saturation of water with toxic metals, increased turbidity, and sediment resuspension, positive impacts can be recorded over a stabilization period of 6-24 months after dredging. Nevertheless, the sustainability of these recorded positive effects cannot be ascertained as some studies have reported long-term regression in remediated sites' conditions. Evaluation of success determinants, site-measure compatibility, and determination of supplementary measures are keys to achieving and sustaining the projected benefits of remedial dredging and justifying its overall cost. This multicomponent study reviewed published literatures that documented the outcomes of short- and long-term dredging projects in toxic metal-polluted systems globally with a broad goal of examining how sediment removal impacts toxic metal dynamics in the aquatic system and understanding why the sustenance of positive impacts is controversial. In the meantime, this study also explored the preventative and remedial management strategies for attaining and sustaining positive dredging outcomes. The purpose of this study is to provide key recommendations for decision-making and policy development in aquatic toxic metal remediation.

Alkaline extraction: An optimal approach for extracting methylmercury from paddy soils

Accurately measuring the concentration of methylmercury (MeHg) is a critical part of Hg research. While analytical methods of MeHg have not been validated for paddy soils, which are one of the most important and active sites of MeHg production. Here we compared two methods most widely used to extract MeHg from paddy soils, i.e., CuSO₄/KBr/H₂SO₄-CH₂Cl₂ (referred to as acid extraction) and KOH-CH₃OH (referred to as alkaline extraction). By evaluating the formation of MeHg artifact using Hg isotope amendments and quantifying the extraction efficiency using the standard spike in 14 paddy soils, we propose that alkaline extraction is an optimal choice for paddy soils, with negligible MeHg artifact (accounting for 0.62-8.11 % of the background MeHg) and consistently high extraction efficiency (81.4-114.6 % for alkaline extraction compared with 21.3-70.8 % for acid extraction). Our finding highlights the importance of suitable pretreatment and appropriate quality controls during the measurement of MeHg concentrations.

Adaptive management of environmental challenges in West African coastal lagoons

Human activities in coastal lagoons over several decades have had a significant impact on their ecology and the valuable ecosystem services they provide. Although there are several management approaches to mitigate the problem, they are unable to link human needs and activities with changes in the state of the environment. This research provides this link via assessment of eleven lagoons in Ghana with a socio-ecological framework (Drivers (D), Activities (A), Pressure (P), State (S), Impact (I) on welfare (W), and Response (R) as a Measure (M); DAPSI(W)R(M)). Data were systematically obtained from relevant publications, previously conducted research, and national reports on the subject and were analyzed using this socio-ecological framework. Results show that basic biological and physiological needs such as food and shelter, social status and dominance, financial self-reliance, and self-actualization are the drivers of fishing, farming, settlements, salt mining, mangrove harvesting, industries, among others. These activities have contributed to pressures of selective extraction of fish and mangroves species, the introduction of heavy metals, organic materials, and smothering of substrates, consequently altering the environment by decreasing the oxygen rate and increasing the biochemical oxygen demand, organic matter, nutrients and pathogens, and reduction in lagoon areas and biodiversity. Thus, ultimately impacting human welfare, such as loss of revenue, employment, and seafood provision. Management options, including addressing the building and fuelwood material sources, afforestation and community ownership of lagoons, the prohibition of construction activities, and research-led management that can support decision-makers to improve the sustainability of these ecosystems, are highlighted. The findings have global implications for guiding local planners and state regulators in the applications of such integrated environmental management.

Geochemical Composition of the Lomé Lagoon Sediments, Togo: Seasonal and Spatial Variations of Major, Trace and Rare Earth Element Concentrations

The concentrations of major, trace (TE), and rare earth (REE) elements and their seasonal and spatial distribution were studied on the fine fraction (<63 µm) of the sediments of the Lomé lagoons (West Lake, East Lake, and Lake Bè). The sediments were collected on a total of nine sampling sites (three per Lake) during two campaigns (dry season and rainy season). The quality of the sediments was assessed on the basis of the enrichment factor (EF) and the labile or non-residual fraction (NRF) in relation to the values recommended for the quality of the sediments (Sediment Quality Guidelines, SQG). The distribution of rare earth elements shows enrichments in light rare earths superior to those of heavy rare earth elements during any season. Positive Ce anomalies are less noticeable and less variable between seasons than Eu anomalies. La/Yb ratios are positively correlated with the percentage of Al and Fe oxides and with the percentage of fine fractions. The main bearing phases of rare earth elements are, therefore, Al and Fe oxides and the finest fractions of the sediments. The concentrations of trace elements vary little, according to the seasons, but show strong variations from one element to another. The degrees of enrichment obtained are moderate for Bi, Cr, Ga, Mo, Pb, Sn, and Zn (1.5 < EF < 5) to significant for As, Cd, and Sb (5 < EF < 20) for all sites of Lake Bè. For the sites of West Lake, the degrees of enrichment obtained are moderate for As, Cd, Cu, Mo, and Pb (1.5 < EF < 5) to a significance for As, Bi, Cd, Pb, Sb, Sn, and Zn (5 < EF < 20). Only the East Lake sites show high degrees of enrichment for elements such as Sb and Sn (20 < EF < 40). Trace elements (TE) such as As, Cd, Cu, and Ni have total concentrations within the range of variation of the SQG concentrations (particularly Probable Effect Level (PEL) and Effect Range Median (ERM)), whereas Cr, Pb, and Zn total concentrations are higher. The ranking of priority sites with respect to the sediment contamination is determined according to ERM and PEL quotients in relation to the probability of toxicity for benthic organisms. For almost all the sites, the priority is lowest to medium-low with regard to As, Cd, and Cu and medium-high (Cr and Ni) to highest (Pb and Zn), particularly for the East and West Lakes. Moreover, the NRF can represent significant percentages of the total TE concentrations: 5% to 15% for As, Bi, Ni, V, Mo, and Sc, 15% to 25% for Co, Cu, and Sr, 25% to 40% for Pb and Zn and, lastly, 47% to 55% for Cd.

In situ simulation of thin-layer dredging effects on sediment metal release across the sediment-water interface

Dredging is widely applied to remediate contaminated sediments in aquatic ecosystems. However, the efficiency of thin-layer dredging for metal pollution control remains uncertain and even controversial. This study conducted an in-situ simulation experiment in Lake Taihu to investigate dredging effects on sediment metal release based on metal fractions, diffusion flux and kinetics parameters of metal resupply, using diffusive gradient in thin films (DGT), multi-microelectrode, and European Community Bureau of Reference (BCR) sequential extraction scheme. Results indicated that the exchange fluxes of metals did not necessarily correspond to total sediment metal concentrations or the contents of different sequentially-extracted metal fractions; there were appreciable decreases in Ni, Cd, Cu and Zn in terms of total sediment metal concentrations and metal fractions, whereas the bioavailability and release fluxes of labile Ni, Cu and Zn (but not Cd) were all notably promoted (by 136, 128 and 149%, respectively) in dredged area compared to those in un-dredged sediments. Further analysis on the kinetics of metal resupply by DGT technique and DGT-induced fluxes in sediments model (DIFS) showed higher concentrations of labile metals, with a larger resupply ability from sediments after dredging. Therefore, thin-layer dredging had the possibility to increase metal release from sediments to the water column. This was attributed to the remobilization of metal sulfides in anoxic deep sediments, as oxidation increased after dredging due to the introduction of oxygenated water, causing subsequent dissolution of sulfide-bound metals. In conclusion, dredging may not mitigate metal contamination, although it can reduce the total pollution load. Our findings indicated dual effects of dredging and provided new insights into the remobilization mechanism of metal release induced by dredging.

Major, minor, trace and rare earth elements in sediments of the Bijagós archipelago, Guinea-Bissau

Sixty sediment samples from four sites in the Bijagós archipelago were characterized for fine fraction, loss on ignition, major, minor and trace elemental composition (Al, Fe, Ca, Mg, Ti, P, Zr, Mn, Cr, Sr, Ba, B, V, Li, Zn, Ni, Pb, As, Co, U, Cu, Cs and Cd), and the elements of the La-Lu series. Element concentrations were largely explained by the Al content and the proportion of fine fraction content, with the exception of Ca and Sr. Sediments showed enhanced Ti, U, Cr, As and Cd concentrations with respect to estimated upper crust values, most likely mirroring a regional signature. Rare earth elements were in deficit relatively to the North American Shale Composite (NASC), mainly in coarser material. No pronounced Ce-anomaly was observed, while Eu-anomalies were positive in most analyzed sediments.

Estimation of mercury background values in sediment and biota of the Bijagós archipelago, Guinea-Bissau

This work evaluates the mercury (Hg) contamination status (sediments and biota) of the Bijagós archipelago, off the coast of Guinea-Bissau. Sediments exhibited very low concentrations (<1-12ngg(-1)), pointing to negligible sources of anthropogenic Hg in the region. Nevertheless, Hg is well correlated to the fine fraction, aluminium, and loss on ignition, indicating the effect of grain size and organic matter content on the presence of Hg in sediments. Mercury in the bivalves Tagelus adansoni and Senilia senilis did not vary considerably among sites, ranging within narrow intervals (0.09-0.12 and 0.12-0.14μgg(-1) (dry weight), respectively). Divergent substrate preferences/feeding tactics may justify slight differences between species. The value 11ngg(-1) is proposed as the sediment background concentration for this West-African coastal region, and concentrations within the interval 8-10ngg(-1) (wet weight) may be considered as reference range for S. senilis and T. adansoni in future monitoring studies.

Hospital Effluents Are One of Several Sources of Metal, Antibiotic Resistance Genes, and Bacterial Markers Disseminated in Sub-Saharan Urban Rivers

Data concerning the occurrence of emerging biological contaminants such as antibiotic resistance genes (ARGs) and fecal indicator bacteria (FIB) in aquatic environments in Sub-Saharan African countries is limited. On the other hand, antibiotic resistance remains a worldwide problem which may pose serious potential risks to human and animal health. Consequently, there is a growing number of reports concerning the prevalence and dissemination of these contaminants into various environmental compartments. Sediments provide the opportunity to reconstruct the pollution history and evaluate impacts so this study investigates the abundance and distribution of toxic metals, FIB, and ARGs released from hospital effluent wastewaters and their presence in river sediments receiving systems. ARGs (bla TEM, bla CTX-M, bla SHV, and aadA), total bacterial load, and selected bacterial species FIB [Escherichia coli, Enterococcus (ENT)] and species (Psd) were quantified by targeting species specific genes using quantitative PCR (qPCR) in total DNA extracted from the sediments recovered from 4 hospital outlet pipes (HOP) and their river receiving systems in the City of Kinshasa in the Democratic Republic of the Congo. The results highlight the great concentration of toxic metals in HOP, reaching the values (in mg kg(-1)) of 47.9 (Cr), 213.6 (Cu), 1434.4 (Zn), 2.6 (Cd), 281.5 (Pb), and 13.6 (Hg). The results also highlight the highest (P < 0.05) values of 16S rRNA, FIB, and ARGs copy numbers in all sampling sites including upstream (control site), discharge point, and downstream of receiving rivers, indicating that the hospital effluent water is not an exclusive source of the biological contaminants entering the urban rivers. Significant correlation were observed between (i) all analyzed ARGs and total bacterial load (16S rRNA) 0.51 to 0.72 (p < 0.001, n = 65); (ii) ARGs (except bla TEM) and FIB and Psd 0.57 < r < 0.82 (p < 0.001, n = 65); and (iii) ARGs (except bla TEM) and toxic metals (Cd, Cr, Cu, and Zn) 0.44 to 0.72, (p < 0.001, n = 65). These findings demonstrate that several sources including hospital and urban wastewaters contribute to the spread of toxic metals and biological emerging contaminants in aquatic ecosystems.

Spatial and seasonal variations of trace elements concentrations in liver and muscle of round Sardinelle (Sardinella aurita) and Senegalese sole (Solea senegalensis) along the Senegalese coast

Concentrations of 11 elements (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Se,V and Zn) were quantified in liver and muscle from two commercially important fish species from the Senegalese coast. The fish, a small pelagic species (the round sardinella) and a benthic species (the Senegalese sole) were collected from five sites during the dry and wet seasons in 2013. For both species, elements were more concentrated in liver than in muscle. There was no clear seasonal pattern in concentration of elements, however inter-site differences were observed. We found significant differences in element concentrations between the two studied species, likely associated with their behavior, feeding and habitat use. The concentrations of Cd, Fe, and Pb were significantly higher in sardinella whereas concentration of As, Cu, Cr, Mn and Se were highest in sole. The concentration of cadmium was particularly high in the liver of sardinella (from 0.9 to 56 mg kg(-1), with a mean ± sd of 17.2 ± 11.5 mg kg(-1)) and may be related to anthropogenic pressure such as the phosphate industry but also to the upwelling current which brings dissolved elements to the surface that are taken up by plankton. The results showed that concentrations of Cd and Pb were below the limit values established by the European Community and pose no threat to public health.

Zinc, copper, cadmium, and lead concentrations in water, sediment, and Anadara senilis in a tropical estuary

Spatial and seasonal contaminations of zinc, copper, cadmium, and lead were assessed simultaneously in water, sediment, and in the bivalve Arca senilis from the Milliardaires Bay (Cote d'Ivoire) between February and October 2008. The metal load in sediments doubled from the dry season to the rainy season. On the contrary, metal concentrations in waters decreased significantly from the dry season to the rainy season. Zn and Pb concentrations in A. senilis showed similar seasonal variation with sediments. On the other hand, A. senilis regulated Cu concentrations by eliminating about twelve times the concentration accumulated during the dry season. Apparent Zn, Cu, Cd, and Pb concentration gradients were observed, but no significant differences between stations for sediment, water, and A. senilis. Concentrations in sediment increased from stations close to Abidjan Harbor towards farther stations, while concentrations in A. senilis showed a reverse gradient. The distribution gradient of A. senilis indicates pollution from local sources, but a transplant experiment is needed to better understand the observed spatial trend. Zn and Cu concentrations may pose little risk to human health and the environment, but they are the highest on the regional scale. On the contrary, Cd and Pb concentrations in A. senilis exceeded the maximum allowable limits set by the European Commission. Complementary studies including chemical speciation should be considered to provide a more accurate assessment of the risk of heavy metals to the environment.

Trace metal distributions in the sediments from river-reservoir systems: case of the Congo River and Lake Ma Vallée, Kinshasa (Democratic Republic of Congo)

The contamination of drinking water resources by toxic metals is a major problem in many parts of the world, particularly in dense populated areas of developing countries that lack wastewater treatment facilities. The present study characterizes the recent evolution with time of some contaminants deposited in the Congo River and Lake Ma Vallée, both located in the vicinity of the large city of Kinshasa, capital of Democratic Republic of Congo (DRC). Physicochemical parameters including grain size distribution, organic matter and trace element concentrations were measured in sediment cores sampled from Congo River (n = 3) and Lake Ma Vallée (n = 2). The maximum concentration of trace elements in sediment profiles was found in the samples from the sites of Pool Malebo, with the values of 107.2, 111.7, 88.6, 39.3, 15.4, 6.1 and 4.7 mg kg(-1) for Cr, Ni, Zn, Cu, Pb, As and Hg, respectively. This site, which is characterized by intense human activities, is especially well known for the construction of numerous boats that are used for regular navigation on Congo River. Concerning Lake Ma Vallée, the concentration of all metals are generally low, with maximum values of 26.3, 53.6, 16.1, 15.3, 6.5 and 1.8 mg kg(-1) for Cr, Ni, Zn, Cu, Pb and As, respectively. However, the comparison of the metal profiles retrieved from the different sampled cores also reveals specific variations. The results of this study point out the sediment pollution by toxic metals in the Congo River Basin. This research presents useful tools for the evaluation of sediment contamination of river-reservoir systems.

Trace metals and persistent organic pollutants in sediments from river-reservoir systems in Democratic Republic of Congo (DRC): Spatial distribution and potential ecotoxicological effects

This paper discusses the occurrence and spatial distribution of metals and persistent organic pollutants (POPs: including organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), Polybrominated diphenyl ethers (PBDEs), and polycyclic aromatic hydrocarbons (PAHs) in sediments from a river-reservoir system. Surface sediments were sampled from thirteen sites of the Congo River Basin and Lake Ma Vallée, both situated in the vicinity of the capital city Kinshasa (Congo Democratic Republic). Sediment qualities were evaluated using toxicity test based on exposing Ostracods to the sediment samples. The highest metal concentrations were observed in sediments subjected to anthropogenic influences, urban runoff and domestic and industrial wastewaters, discharge into the Congo River basin. Ostracods exposed to the sediments resulted in 100% mortality rates after 6d of incubation, indicating the ultimate toxicity of these sediments as well as potential environmental risks. The POPs and PAHs levels in all sediment samples were low, with maximum concentration found in the sediments (area of pool Malebo): OCP value ranged from 0.02 to 2.50 with ∑OCPs: 3.3μgkg(-1); PCB ranged from 0.07 to 0.99 with Total PCBs (∑7×4.3): 15.31μgkg(-1); PAH value ranged from 0.12 to 9.39 with ∑PAHs: 63.89μgkg(-1). Our results indicate that the deterioration of urban river-reservoir water quality result mainly from urban stormwater runoff, untreated industrial effluents which discharge into the river-reservoirs, human activities and uncontrolled urbanization. This study represents useful tools incorporated to evaluate sediment quality in river-reservoir systems which can be applied to similar aquatic environments.