Distribution Of Metals In Sediments Research Articles

Impact of urban drainage on metal distribution in sediments of urban streams

The concentrations of total and extractable heavy metals (Cu, Zn, Pb, Cd, Cr and Ni) and other sediment properties were determined in sediments of three urban streams in Prague. The mean sediment concentrations (in mg kg(-1) (dry weight)) ranged within 0.2-3.2 (Cd), 20.2-61.7 (Cr), 16.3-135.2 (Cu), 17.8-42.5 (Ni), 20.2-114.8 (Pb) and 79.4-446.3 (Zn). The chemical distribution of metals, determined in four chemical sediment fractions of the sediment, indicates an increase of the distribution of Cu and Zn in more easily available fractions in sediments affected by combined sewer overflows. The highest percentages of Cd and Zn are in the most labile acid-soluble fraction (38-64% and 15-43% respectively), whereas Pb is bound mainly to reducible fraction.

Heavy metals in bottom sediments of Lake Umbozero in Murmansk Region, Russia

Sediment cores collected from different locations of Lake Umbozero were studied with respect to concentration and mobility of trace and heavy metals Co, Cu, Fe, Mn, Ni, Pb, U, and Zn. Lake Umbozero is the second largest lake in the Murmansk Region and subjected to contamination by air-borne emissions and river transportation from the nearby metallurgical and mining industries. Unlike its neighboring, more industry-prone Lake Imandra, Lake Umbozero is relatively unexplored with respect to its state of pollution. In our study, metal distribution in sediments was found to vary with respect to the cores, although in general the concentrations were at the same level throughout the lake indicating uniform horizontal distribution of metals. When compared to Lake Imandra, the concentrations of most of the metals studied were significantly lower and represented the levels in sediments measured in lakes of Kola Peninsula located further off from industrial pollutant sources. An exception was Pb the concentration of which was at the same level as in Lake Imandra, probably due to long-distance transport. Sediment layers were subjected to four-step sequential extraction procedure to reveal the metal distribution in soluble, exchangeable, acid-soluble, and residual fractions. Indicative of their potential higher lability, Mn, U, and Zn were generally found in exchangeable fraction; as also Mn and U extensively in the acid-soluble fraction.

Accumulations and risks of heavy metals in the sediments from 8 typical lakes in Wuhan, China

采集武汉市8个典型湖泊的表层沉积物,分析11种重金属的含量及其不同形态组成,研究了不同湖泊金属元素的富集与污染程度,探讨了沉积物中重金属的污染来源及其潜在生态风险.结果表明,沉积物中重金属Cd累积最严重,Zn和Hg也发生明显累积,龙阳湖污染较重,南太子湖和墨水湖污染中等,其它湖泊污染总体较轻.沉积物性质对重金属累积的影响不显著,城市工业活动强烈影响着重金属的分布.不同重金属的形态分布差异较大,Cd生物可利用态含量最高,其次为Mn、Zn、Co、Cu和Pb,而Sb和Hg以残留态占绝对优势,生态风险较小.相关分析和主成分分析表明,化石燃料燃烧、金属冶炼等是武汉市湖泊沉积物中重金属来源的主要贡献者,同时岩石风化等地球化学过程也影响着重金属的污染.;The content and species of 11 heavy metals in surface sediments from 8 typical lakes located in Wuhan, China, were investigated, and the accumulation, contamination, potential ecological risks and possible sources of metals were discussed in this paper. According to the metal geoaccumulation indexes, the results showed that the lake sediments had heavily polluted by Cd and evidently by Zn and Hg. On the whole, heavy pollutions of metals were observed in Lake Longyang and medium pollution in Lake Nantaizi and Lake Moshui, whereas the slight pollution in other lakes. Distribution of metals in sediments was observed to be not significant correlation with physicochemical properties of sediments, whereas distinctly influenced by urban industries. 11 metals distributed differently in lake sediments. The largest percentage of the form of available speciation was Cd in the lake sediments, with much higher potential ecological hazards, followed by Mn, Zn, Co, Cu and Pb. In contrary, As, Sb and Hg were dominantly associated with the residual fraction, indicating rare adverse biological effects. Correlation analysis and principal component analysis indicated that the heavy meal accumulation in the lake sediments were mainly attributed to the combustion of fossil fuel accompanied with metal refining, and followed with the effects of the biogeochemical processes of earth weathering.

Trace metal distribution in sediments of northern continental shelf of Crete Island, Eastern Mediterranean

The present study investigates the distribution of trace metals (Zn, Hg, Cd, Cu, and Pb), as indicators of pollution, in the surficial offshore shelf sediments along the northern coast of Heraklion Prefecture (Crete, Mediterranean Sea). The concentrations and the spatial distribution of the different trace metals, in relation to the sedimentological characteristics and the water circulation pattern of the entire continental shelf, are associated with human inshore sources of pollutants located along the coastline of the study area. Although the trace metal concentrations measured are higher than the background values, they are not considered to be dangerous to human health, as they are lower than the standard values given by the World Health Organisation, with only a few localised exceptions. Furthermore, results reveal the important role of local hydrodynamism that moves fine-grained material and associated trace metals offshore (seawards to wave breaking zone) and then transports them eastwards by entrapping them in the prevailing offshore shelf-water circulation.

The role of the smooth cordgrass Spartina alterniflora and associated sediments in the heavy metal biogeochemical cycle within Bahía Blanca estuary salt marshes

Background, aim, and scope Bahia Blanca estuary is characterized by the occurrence of large intertidal areas, including both naked tidal flats and salt marshes densely vegetated with Spartina alterniflora. The estuary is strongly affected by human activities, including industrial and municipal discharges, harbor maintenance, cargo vessels and boat navigation, oil storage and processing, etc. Even numerous studies have reported the occurrence and distribution of heavy metals in sediments and biota from this estuary, although the function of the halophyte vegetation on metals distribution was at present not studied. The main objective of the present study was to understand the potential role of the salt marshes as a sink or source of metals to the estuary, considering both the obtained data on metal levels within sediments and plants from the studied areas at naked tidal as well as vegetated flats.

Mechanisms of land–sea interactions – the distribution of metals and sedimentary organic matter in sediments of a river-dominated Mediterranean karstic estuary

This paper examines disposal of metals and the origin, characteristics, and distribution of sedimentary organic matter (SOM) in a Mediterranean karstic estuary in the north-eastern Adriatic. This environment offers a real-time, small model system for studies of geochemical processes in microtidal Mediterranean estuaries that are infilling with sediments and classified as river-dominated disequilibrium estuaries. The results have shown that the longitudinal distribution of heavy metals in sediments follows the sedimentation dynamics and deposition pattern of river-borne, clay mineral particles. The highest concentration of metals was found in the restricted upper part of the estuary, characterized by rapid deposition of clay particles and terrestrial sedimentary organic matter, and decreases toward the open sea. The vertical distribution of metals in sediment cores depends on the prevailing pH and Eh conditions. Significant increases of the concentrations of metals in the uppermost strata are the result of recent anthropogenic inputs. The share of the terrestrial component in SOM, estimated by N/C org atomic ratios and δ 13C values, decreases with distance from the river mouth. The small vertical variation in δ 13C values of SOM indicates that a fast sedimentation rate overrides the diagenetically determined decomposition. The results obtained indicate that river-borne inorganic particles, natural terrigenous organic material, and anthropogenic metal loads are trapped in sediments of the estuarine system. Under the prevailing conditions, there is negligible transport towards the open sea.

Combining multivariate analysis and geochemical approaches for assessing heavy metal level in sediments from Sudanese harbors along the Red Sea coast

Multivariate statistical analysis and geochemical approaches were exploited for the assessment of the level of some heavy metals (Mn, Fe, Ni, Cu, Zn and Pb) in sediments from Sudanese harbors along the Red Sea coast. Principal component analysis, as a multivariate statistical analysis approach, was applied to identify contribution sources by heavy metals in sediments. While a single source (crustal) was recorded in the bulk sediments and coarse sediment grains (grain-size 1000–500 µm), two sources (crustal and anthropogenic) were recorded in fine sediment grains (grain-size < 500 µm). Furthermore, enrichment factor (EF), as a geochemical approach, appointed polluted sites by heavy metals in the study area. Based upon a previous study addressed the interpretation of EF values, minor to moderate anthropogenic enrichment were recorded in sediments from some sites in the study area. The main anthropogenic activities that believed to be the major sources of pollution by heavy metals in the study area are discharges from oil refinery, industry, shipping activity and domestic waste. Hierarchical cluster analysis (HCA), as another multivariate statistical analysis approach, was applied for the concentrations of heavy metals in bulk sediments to group sediments according to their mineralogical composition. The output of HCA is that sediments from the Port-Sudan harbor can be divided mainly into three areas — east, west and south. For the Sawakin harbor, no apparent trend for the spatial distribution of heavy metals in sediments was recorded.

The spatial and temporal distribution of heavy metals in sediments of Victoria Harbour, Hong Kong

Victoria Harbour has received substantial loadings of pollutants from industrial and municipal wastewater discharged since the 1950s. Inputs of contaminants have declined dramatically during the last two decades as a result of better controls at the source and improved wastewater treatment facilities. To assess the spatial and temporal changes of metal contaminants in sediments in Victoria Harbour, core and grab sediments were collected. The central harbour areas were generally contaminated with heavy metals. The spatial distribution of trace metals can probably be attributed to the proximity of major urban and industrial discharge points, and to the effect of tidal flushing in the harbour. In the sediment cores, the highest concentrations of trace metals were observed to have accumulated during the 1950s–1980s, corresponding with the period of rapid urban and industrial development in Hong Kong. From the late 1980s, there has been a major decline in the concentrations of trace metals, due to a reduction in industrial activities and to the enactment of wastewater pollution controls in the territory. The Pb isotopic compositions of the sediments revealed the anthropogenic inputs of Pb to the harbour. The 206Pb/ 207Pb ratios varied from 1.154 to 1.190, which were lower than those of background geological materials in Hong Kong ( 206Pb/ 207Pb: 1.201–1.279). The data also indicated that the Pb in the harbour sediments most likely originated from mixed sources, including the leaded gasoline used in the past and other anthropogenic sources.

Sediment Quality and Resuspension Rate in Mangueira Bay, Brazil

Mangueira Bay is located in southern Brazil (between 30° and 32° south latitude). It is a significant regional resource that lends itself to activities such as tourism, fishing, and fish and shellfish breeding. Studies performed by Rio Grande University (FURG) have demonstrated that, due to its natural conditions, Mangueira Bay is an area of shelter and feeding for the local biota. As a result, the possibility of implementing intensive shrimp breeding has been analyzed. It is therefore important to study the hydrodynamics, water quality and sediments of this water body. This paper describes the spatial and temporal distribution of heavy metals in sediments during monitoring campaigns carried out in October 2002 and June 2003 as a part of the “Estuaries and Coastal Areas. Basis and Tools for a More Sustainable Development” (ECOSUD) project. The distribution of metals in the bed was estimated as well as the potential for resuspension due to the waves caused by typical storm conditions similar to those measured in the May 2003 campaign. This sediment resuspension in the seabed can repollute the water column. Classic expressions were used to estimate the wind-induced water waves, the resuspension rate and the concentration of suspended material. The results show differences in spatial distribution between the two campaigns, but in both cases the south of Mangueira Bay was found to be the proper area for aquaculture. Furthermore, in all of the sediment samples analyzed, the different metals were present in concentrations under the limits stipulated by the Canadian Environmental Quality Guidelines of the Canadian Environmental Protection Agency (CEPA). The calculation of sediment resuspension demonstrated that conditions favor shrimp breeding along the southwest coast under typical winds from the west, especially in the southern part of Mangueira Bay.

Regional geochemical baselines for Portuguese shelf sediments

Metal concentrations (Al, Cr, Cu, Ni, Pb and Zn) from the DGM-INETI archive data set have been examined for sediments collected during the 1970s from 267 sites on the Portuguese shelf. Due to the differences in the oceanographic and sedimentological settings between western and Algarve coasts, the archive data set is split in two segments. For both shelf segments, regional geochemical baselines (RGB) are defined using aluminium as a reference element. Seabed samples recovered in 2002 from four distinct areas of the Portuguese shelf are superimposed on these models to identify and compare possible metal enrichments relative to the natural distribution. Metal enrichments associated with anthropogenic influences are identified in three samples collected nearby the Tejo River and are characterised by the highest enrichment factors (EF; EF Pb < 3, EF Zn < 4). EF values close to 1 suggest a largely natural origin for metal distributions in sediments from the other areas included in the study.

Distribution of cadmium, chromium, copper, lead and zinc in marine sediments in Hong Kong waters

Partitioning of heavy metals (Cd, Cr, Cu, Pb, Zn) in marine sediments collected from various sites in Hong Kong waters were determined using sequential extraction method. Sediments from Kellette Bank, located in Victoria Harbour, had higher metal concentrations especially Cu and Zn than most other sites. Slightly over 20% of total Cu and Cr existed as readily available forms in Peng Chau and Kellette Bank. At most sampling sites, over 15% of the Cu existed as the exchangeable form indicating that Cu could be readily released into the aqueous phase from sediments. A significantly higher percentage of Pb and Zn was associated with the three non-residual fractions. Hence, there is a greater environmental concern for remobilization of Pb and Zn compared with Cr. The high amount of residual Cd (>50%) and the relatively lower Cd content indicate that little environmental concern is warranted for the remobilization of Cd. Distribution of metals in sediments collected from different depth at Kellette Bank shows that metal concentrations decreased with profile depth. The levels of Pb and Zn associated with the two readily available fractions increased sharply in the surface sediment. These metals represented the pollutants, which were introduced into the area in the mid-eighties through early nineties as a result of rapid economic and industrial development in the territory. As significant portions of these metals were bound to the readily available phases in the surface sediments, metal remobilization could be a concern.

Effects of ghost shrimp on zinc and cadmium in sediments from Tampa Bay, FL

This study investigated the effects that ghost shrimp have on the distribution of metals in sediment. We measured levels of HNO 3-extractable zinc and cadmium in surface sediment, in ghost shrimp burrow walls and in sediment ejected by the ghost shrimp from their burrows, at five sandy intertidal sites in Tampa Bay. Ghost shrimp densities and their rate of sediment ejection were also quantified, as were sediment organic content and silt + clay content. Densities of ghost shrimp ( Sergio trilobata and Lepidophthalmus louisianensis) averaged 33/m 2 at our sites, and they ejected sediment at an average rate of 28 g/burrow/day. Levels of both Zn and Cd were significantly higher in burrow walls than in surface sediments. Sediment ejected by the shrimp from their burrows had elevated levels of Zn (relative to surface sediments) at one of the sites. Sediment organic content and silt + clay content were higher in burrow-wall sediments than in ejected sediment, which in turn tended to have values above those of surface sediments. Differences in levels of HNO 3-extractable Zn and Cd among sediment types may be a consequence of these sediments differing in other physiochemical characteristics, though the differences in metal levels remained statistically significant for some sites after correcting for differences in organic content and silt + clay content. We conclude that the presence of ghost shrimp burrows contributes to spatial heterogeneity of sedimentary metal levels, while the ghost shrimp bioturbation results in a significant flux of metals to the sediment surface and is expected to decrease heterogeneity of metal levels in sedimentary depth profiles.

Monitoring the Effect of Nearshore Development and Production in the Beaufort Sea

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98500, "Monitoring the Impact of Nearshore Development and Production in the Arctic Beaufort Sea, Alaska," by G. Durell and J. Neff, Battelle; J. Brown, Exponent; and J. Trefry, Florida Inst. of Technology, prepared for the 2006 SPE International Conference on Health, Safety, and Environment in Oil and Gas Exploration and Production, Abu Dhabi, UAE, 2–4 April. The Arctic Nearshore Impact Monitoring in Development Area (ANIMIDA) project was initiated to monitor effects of nearshore development and production in the Beaufort Sea, Alaska. This program was expanded in 2004 with the 5-year Continuation of ANIMIDA (CANIMIDA). Contaminant levels are assessed in sediment, water, and biota, and geophysical measurements are being made to determine the sources and fate of natural and anthropogenic hydrocarbons and metals. Introduction Development and production of oil and gas on the North Slope of Alaska has until recently occurred only on land. However, there is an increasing interest in developing the unique offshore environment of northern Alaska. The ANIMIDA project was a 5-year study started in 1999 to assess potential spatial and temporal changes related to oil development and production in the Beaufort Sea. ANIMIDA provided baseline data and monitoring results for potential chemical contaminants from development and natural sources, ecology, and subsistence whaling in the vicinity of the Northstar and Liberty development sites. Monitoring for Northstar included preconstruction, construction, and early production periods; development of Liberty is on hold, and monitoring there has generated baseline preconstruction data. The 5-year CANIMIDA began in 2004 to continue and expand the nearshore monitoring in this area. The CANIMIDA technical-task objectives include the following.Characterize hydrocarbon and metal distributions in sediments in the study area.Determine sources, concentrations, and dispersion pathways for suspended sediment and associated chemicals.Characterize potential anthropogenic chemicals in amphipods, bivalves, and fish, and determine bioaccumulation and biological effects of anthropogenic chemicals.Monitor the unique ecosystem and local subsistence whaling. Methods Field surveys are conducted in the spring and summer. Bottom and suspended sediment, water, and biota samples are analyzed for more than 40 polycyclic aromatic hydrocarbons (PAHs), a suite of individual saturated and total petroleum hydrocarbons, chemical biomarkers and tracers, 13 trace and major metals, and supporting geophysical parameters. Deployed blue mussels and semi-permeable-membrane devices (SPMDs) also are used as sampling devices to concentrate contaminants and assess potential for contaminant bioaccumulation. In addition, suspended-sediment transport and loading to the Beaufort Sea (primarily from rivers) are being determined, along with natural sources of contaminants that may also have anthropogenic sources (e.g., from oil and gas activities). Field work is being conducted during the relatively brief open-water season (July and August) and also during the spring high-flow and ice-breakup periods when there is a large influx of fresh water and suspended solids; the spring sampling is conducted both under ice offshore and in the major source rivers.

Distribution of heavy metals in sediments of Igbede, Ojo and Ojora rivers of Lagos, Nigeria

The distribution of some heavy metals, namely Cd, Pb, Zn, Fe, Cu, Cr and Mn in epipellic sediments of Igbede, Ojo and Ojora rivers of Lagos was studied weekly in the early summer (November) of 2003. The levels of selected trace metals were determined using Atomic Absorption Spectrophotometer (UNICAM 969 AAS SOLAR). Trends in heavy metal burdens in the sediments revealed weekly variations in all the rivers assessed. Statistical analyses also showed different mean levels of trace metals in the aquatic environments, the distribution of which followed the sequence Fe > Zn > Mn > Pb > Cu > Cr > Cd, Fe > Zn > Cu > Mn > Pb > Cr > Cd and Fe > Zn > Mn > Cu > Cr > Pb > Cd in Igbede, Ojo and Ojora rivers respectively. Fe recorded the highest concentration levels (1,582.95 ± 96.57 μ g/g–1,910.34 ± 723.19 μ g/g) in all the sediments investigated while the Cd levels (0.06 ± 0.10 μ g/g–0.47 ± 0.36 μ g/g) were the lowest. Expectedly, trace metal concentrations in fine grain muddy sediments of the Igbede and Ojo coastline were much higher than those of Ojora which consist of coarse and sandy deposits covering the near shore area. Generally, the results obtained fell within tolerable limits stipulated by World Health Organization (WHO).

Trace metal distribution in sediments of the Pearl River Estuary and the surrounding coastal area, South China

Surface sediments and sediment cores collected at the Pearl River Estuary (PRE) and its surrounding coastal area were analysed for total metal concentrations, chemical partitioning, and Pb isotopic compositions. The distribution of Cu, Cr, Pb, and Zn demonstrated a typical diffusion pattern from the land to the direction of the sea. Two hotspots of trace metal contamination were located at the mixed zone between freshwater and marine waters. The enrichment of metals in the sediments could be attributed to the deposition of the dissolved and particulate trace metals in the water column at the estuarine area. The similar Pb isotopic signatures of the sediments at the PRE and its surrounding coastal area offered strong evidence that the PRE was a major source of trace metals to the adjacent coastal area. Slightly lower 206Pb/ 207Pb ratios in the coastal sediments may indicate other inputs of Pb in addition to the PRE sources, including the inputs from Hong Kong and other parts of the region.

Distribution of Heavy Metals in Sediments of the Detroit River

The spatial distribution of 17 metals in the sediments of the Detroit River was established using metal concentrations from a river-wide survey. The survey (1999) was based on a stratified random sampling design that divided the river into upper, middle, and lower reaches and subsequently into U.S. and Canadian sides of the river. Results based on strong extraction, using concentrated acids, revealed that the Lowest Effect Level (LEL) for As, Cd, Cu, and Hg was exceeded at more than 75% of sampling sites and the Severe Effect Level (SEL) for As at 16.2% of sites. Most of the metals were homogenously distributed throughout reaches of the river, although sites with elevated concentrations were localized mainly along the middle and lower reaches as a result of a pattern of contamination sources and geographic complexity of the river, especially a spatial/temporal variability in water flow. A comparison of the results of a strong to a weak extraction (cold 5% acetic acid, to assess metal bioavailability) revealed two groups of sediment type. The first group with a “high” weak/strong ratio (bioavailable metals; about 1 for Ca, Mg, Na and from 0.6 to 0.4 for the rest of metals) was observed at sites with low flow velocities below 0.4 m s −1. The “low” ratio (non-bioavailable metals; 0.25 for Ca, Mg, Na and from 0.15 to 0.05 for other metals) was observed at sites with flow velocities greater than 0.6 m s −1. The data indicate that the sediment conditions, dependent on flow distribution, regulate not only the distribution of heavy metals but also can regulate metal bioavailability.

The effects of mining in Northern Romania on the heavy metal distribution in sediments of the rivers Szamos and Tisza (Hungary)

AbstractThe breach in the dam of a tailing pond in the processing plant of a gold mine in Baia Mare (NW Romania) in January 2000 lead to an ecological disaster in the rivers Szamos and Tisza. It was mainly caused by the released slurries containing cyanides and heavy metals. The investigation of surface sediment samples and depth profiles of the years 2000 and 2001 documented the pollution of the rivers Szamos and Tisza on Hungarian territory. In the Szamos and large areas of the Tisza floodplains a significant enrichment with the heavy metals Pb, Cd, Cu, and Zn is existent. The identification of the binding forms with sequential extraction showed that Cd and Zn exist in an easily available form. Therefore the remobilization is possible and an environmental hazard in the downstream areas must be considered. The anthropogenically caused portions of heavy metal concentrations decrease considerably with increasing distance to the mining region. The comparison of the concentrations of heavy metals in both years shows that the pollution is not only caused by the accident in January 2000. The increase of concentrations is based on the permanent input of pollutants into the rivers. The pollution is mainly caused by the erosion of exposed tailing dumps and the poor maintenance of the treatment plants.

Comparison of components and distribution of heavy metals in sediments, suspended particulate matter and surface coatings in natural waters

Comparison of components and distribution of heavy metals in sediments, suspended particulate matter and surface coatings in natural waters

Spatio-Temporal Distribution of Heavy Metals in Sediments and Surface Water in Stubbs Creek, Nigeria

Spatio-Temporal Distribution of Heavy Metals in Sediments and Surface Water in Stubbs Creek, Nigeria

Spatial distribution of heavy metals in sediments from the Gulf of Paria,Trinidad

The Gulf of Paria receives heavy metal input from urban runoff, industrial and agricultural activity, sewage and domestic wastes: both from the west coast and from inland areas of Trinidad. Non-residual concentrations of nine metals, as well as total mercury concentrations, were used to determine spatial distributions of heavy metals in sediments in the Gulf of Paria. Surficial sediment samples were collected at 37 stations, which included the mouths of 11 major rivers that flow into the Gulf of Paria. Stations were sampled twice during the wet season (July 1998 and November/ December 1998) and twice during the dry season (March 1999 and April 1999). Sediments were analyzed for aluminium (Al), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), manganese (Mn), nickel (Ni), zinc (Zn) and mercury (Hg). Total Organic Carbon (TOC) and grain size analyses were also performed on the sediments. Principal component analysis showed that sediments from river mouths subject to greatest land use and anthropogenic input, were distinct from other sediments in the Gulf of Paria. This was due to higher Pb, Zn, Cu and Hg concentrations (3.53-73.30 microg g(-1), 45.8-313.9 microg g(-1), 8.43-39.71 microg g(-1) and 0.03-0.10 microg g(-1), respectively). Sediments further from the coast were also distinct due to their higher Al, Fe, Cr and Mn concentrations (1.37-3.16 mg g(-1), 9.51-18.91 mg g(-1) , 17.22-28.41 microg g(-1) and 323.6-1,564.2 microg g(-1), respectively). Cd and Pb were higher in the wet season while Ni was higher in the dry season. Pb, Zn, Cu and Hg were correlated with each other and with TOC. Correlation was also observed between Al, Fe, Cr, Mn and Ni. Al, Fe, Cr and Mn were correlated with percentage clay in sediments. The results suggest that Pb, Zn, Cu and Hg are preferentially removed by organic matter, which settles at the river-mouths, while Al, Fe, Cr, Mn, and Ni become associated with clay minerals and are transported away from the coast.