Tributyltin in environmental samples from the Former Derecktor Shipyard, Coddington Cove, Newport RI

Tributyltin (TBT) was detected in all 24 surface sediment (top 2 cm) samples collected from Coddington Cove, Newport, RI. TBT surface sediment concentrations ranged from 32 to 372 ng Sn/g with a mean concentration of 146 ng Sn/g. Analyses of selected core sections detected TBT in at least the top 18 cm at all 7 stations where cores were collected. No consistent TBT concentration trends with depth for these cores suggest mixing is an important process in the sediment column. In one core (station 28), TBT was found in the 76-86 cm section at a concentration of 141 ng Sn/g; thus sediments are a significant sink for TBT. However, sediment mixing processes can enhance releases of bioavailable TBT. Mussels, clams and fish from Coddington Cove contain TBT at concentrations ranging from 9.2 to 977 ng Sn/g. TBT concentrations in lobsters were below the detection limit (<6 ng Sn/g). Based on available screening criteria, TBT concentrations in Coddington Cove sediment is likely to be having an adverse effect on the biota at some locations.     

Accumulation of organotin in Littorina littorea and Mya arenaria from Danish coastal waters

Organotin concentrations were determined in seawater, sediment and selected molluscs collected from the vicinity of the island of Fyn, Denmark. The extent of organotin contamination varied with the proximity of sampling sites to marinas and commercial shipping activities. Seasonal reductions in environmental TBT concentrations coincided with removal of pleasure craft from marinas during the winter season. At marina sites, concentration factors for TBT were 150-1500 for sediment and 500-10,000 for Littorina littorea (compared with seawater concentrations). Away from marina sites, Mya arenaria concentrated TBT by a factor of 57,000 to 220,000. The concentrations of TBT in M. arenaria from relatively clean sites were more than 10 times higher than found in L. littorea from heavily polluted marinas. M. arenaria were absent from marina sites and their immediate surroundings. Of the total organotin found in M. arenaria throughout the year, 80-90% was in the form of TBT. In contrast, in L. littorea approximately 60-70% of the total organotin was present as TBT in spring, but this proportion fell to 20-40% during autumn and winter. M. arenaria shows potential as a bioindicator organism of TBT pollution as it appears to have a very limited ability to metabolise and eliminate TBT, yet can tolerate quite high levels of TBT exposure. L. littorea might be used as a bioindicator of short-term fluctuations in TBT concentrations at heavily polluted sites. It is concluded that continued presence of TBT in the Danish coastal ecosystem may still pose an ecotoxicological threat.     

Butyltins in water, biofilm, animals and sediments of the west coast of India

Biofilm, fish, oyster, mussel, clam, surface seawater, suspended particulate matter (SPM), and sediment samples were collected from marine and/or estuarine waters of the west coast of India. These samples were analysed for butyltin derivatives such as dibutyltin (DBT) and tributyltin (TBT). The concentrations DBT plus TBT varied between 2.4 and 8.3, 163 and 363 ng/l, 5 and 2853 ng/g dry wt in the SPM, seawater and sediment samples, respectively, of the Marmugao harbour. The values of DBT plus TBT ranged between 0.60 and 29, 123 and 242 ng/l and 1.4 and 65 ng/g dry wt in SPM, water and sediment samples, respectively, collected from the Mandovi estuary. In the Dona Paula Bay the DBT plus TBT varied from 10 to 89 ng/l in surface seawater, and TBT from 10 to 513 ng/g in biofilm samples. For the coastal sediment samples the concentration of DBT plus TBT ranged between 36 and 133 ng/g dry wt of sediment. For the animal samples the DBT plus TBT ranged between 58 and 825 ng/g dry wt of the tissue. Mussel tissues contained the highest amount of DBT plus TBT (825 ng/g dry wt tissue), whereas highest TBT concentration was recorded in the oyster (732 ng/g dry wt). TBT was generally the most abundant butyltin compound in most of the samples suggesting fresh inputs and/or less degradation of TBT. A wide range of the observed butyltin concentrations suggests the presence of localized areas of contamination. Leaching of tributyltin-containing antifouling paints from the ocean going ships, fishing and recreational boats, barges, and the inputs of TBT from the Goa shipyard and dry dock facility situated in the harbour are the probable sources of the DBT and TBT in the samples of the west coast of India. Higher levels of TBT were observed in biofilm relative to that in the surrounding seawater. When fed on TBT contaminated biofilm of the diatom Navicula subinflata, butyltin concentrations in the clam Paphia malabarica increased over the period of feeding suggesting the importance of biofilm in the transfer of butyltins to higher group of organisms.     

Butyltin compounds in sediment and fish from the Polish Coast of the Baltic Sea

Concentrations of mono- (MBT), di- (DBT), and tri-(TBT) butyltin compounds were measured in eggs, liver, and muscle of nine species of fish from four regions of the Baltic Sea - the Firth of Vistula, the Gulf of Gdańsk, Puck Bay, and the mouth of the Vistula River. The overall concentration ranges among all the fish sampled from the four sites were: < 7 to 79 ng/g for MBT, 6 to 1100 ng/g for DBT, 7 to 3600 ng/g for TBT, and 16 to 4800 ng/g for total BTs, on a wet wt basis. The highest concentration of total BTs was found in herring liver from the Firth of Vistula (4800 ng/g, wet wt) and in roach muscle from Puck Bay (3300 ng/g, wet wt), while the least concentration was found in burbot eggs and liver from the Vistula River (39 and 32 ng/g, wet wt, respectively). TBT was the major form of BTs present in most samples analyzed. Sediment samples collected from shipyards in the Gulf of Gdańsk contained butyltin concentrations ranging from 1.2 to 46 μg/g (dry wt) for MBT, 2.0 to 42 μg/g for DBT, and 2.6 to 40 μg/g for TBT. As with the fish, the majority of the BTs in sediment were present as TBT, which suggested recent exposure of the aquatic environment of the region to TBT.     

Levels and distributions of polybrominated diphenyl ethers,hexabromocyclododecane, and tetrabromobisphenol A in sediments from Taihu Lake,China

The concentrations and distribution of polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD), and tetrabromobisphenol A (TBBPA) were investigated in 28 sediment samples collected from Taihu Lake, Eastern China. The results showed that all three classes of compounds were detected in 28 sediment samples but that PBDEs were the main contaminants in the study area. The total PBDE concentrations ranged from 3.77 to 347 ng/g dry weight (dw) with a mean value of 72.8 ng/g dw, whereas the concentrations ranged from 0.168 to 2.66 and from 0.012 to 1.30 ng/g dw for HBCD and TBBPA, respectively. In all of the sediment samples, BDE-209 was the predominant congener, accounting for 95.9 to 98.6 % of the total PBDEs, which indicated that commercial mixtures of deca-BDEs were the main sources of PBDE contamination. Higher concentrations of PBDEs were found in samples collected from Meiliang Bay and Gonghu Bay near the inflow river, which suggested that inflow runoff might play an important role in the PBDE levels in Taihu Lake sediments.     

Butyltins in shellfish, finfish, water and sediment from the Coos Bay estuary (Oregon, USA)

Butyltin was measured in tissue, water and sediment samples collected throughout Coos Bay estuary between 1986 and 1997. Butyltin-induced shell deformation in commercial oysters prompted this assessment. Butyltins are released from tributyltin oxide (TBTO)-containing boat paint. Clams from three sites contained tributyltin (TBT) levels ranging from 168 to 457 microg/kg in 1992-1994. Consequently, Oregon Health Division (OHD) issued a shellfish consumption advisory in 1995; it was rescinded in 1997 after US Environmental Protection Agency (EPA) revised the TBTO oral reference dose (RfD). The revised RfD changed the tissue screening level from 112 to 1120 microg TBT/kg. Fifty percent of sediment samples collected during 1991-1997 and analyzed for total butyltins (TotBT) exceeded the Puget Sound Dredge Disposal Analysis Screening Level (PSDDA SL) of 73 microg TBT/kg dw. TotBT at three shipyards selected for site assessment by EPA ranged from 24 to 68,613 microg/kg dw. Sediment and tissue butyltins have been decreasing in all but one area of the estuary.     

Concentration of butyltin species in sediments associated with shipyard activity

Tributyltin (TBT) and dibutyltin (DBT) were analyzed in sediment samples collected from intertidal locations in Portland and Boothbay Harbor, Maine (USA) in 1990 and 1992. Surface sediment TBT concentrations ranged from 24 to 12 400 ng gm(-1) (dry wt basis). Sediments with the highest TBT concentrations were associated with shipyard hull washing/refinishing activities. Analysis of different layers in core samples found that butyltin concentrations decreased with depth at the Boothbay site and remained relatively constant with depth at the Portland site. Elutriate analysis showed that soluble TBT was released from a heavily contaminated sediment. The resulting TBT seawater concentration 1400 ng liter(-1) was < 0.03% of reported seawater solubilities of TBT and was only 0.14% of the total TBT in the sediment sample. This suggests that the TBT in the sediments analyzed is in a bound matrix form, such as paint particles, that releases the biocide slowly. The results indicate that there is a potential for future release of TBT from the resuspension of fine sediments at certain locations in Maine.     

Sources and fate of butyltins in the St. Lawrence Estuary ecosystem

Butyltins (BTs) were determined in sediment, zooplankton, benthic fish and invertebrates in the St. Lawrence Estuary and its mixing zone with the Gulf of St. Lawrence (Canada) in an attempt to assess sources and fate of these compounds in a large ecosystem before the enforcement of the world-wide ban of TBT-based antifouling paints. All BTs (MBT, DBT and TBT) were found along the studied area (450 km) where the traffic of large vessels occurs around the year. Concentrations of total butyltins (BTs) in surface sediment were below 6 ng Sn g(-1) d.w. Total BTs concentrations found in zooplankton samples at the mouth of the Saguenay Fjord were the highest (793 ng Sn g(-1) d.w.), indicating the influence of the Fjord on the St. Lawrence contamination. Although a relatively low contamination level was measured in sediment, total BTs concentrations ranged from 9 to 489 ng Sn g(-1) d.w. for benthic organisms. Biota-sediment accumulation factors (BSAFs), calculated on the basis of the organic carbon content in the sediment (concentrations normalized to 1% Corg), ranged from 0.9 to 98.3, and are an indicator of an important source of BTs from the Saguenay Fjord particulate matter. This may be explained by the fact that when TBT is released in a large and deep well stratified coastal environment, it could bind to the suspended particulate matter and then be taken in charge by water column organisms and may be mostly metabolised before it reaches bottom sediment. Sediment is not considered as the main contributor to the contamination of fish and invertebrates. It is expected that any reduction of direct inputs of TBT from ship hulls in a near future should result in a rapid reduction of butyltins in the St. Lawrence ecosystem.     

Contamination of butyltin compounds in Malaysian marine environments

Concentration of butyltin compounds (BTs), including tributyltin (TBT), dibutyltin (DBT) and monobutyltin (MBT) and total tin (SigmaSn) were determined in green mussel (Perna viridis), 10 species of muscle fish and sediment from coastal waters of Malaysia. BTs were detected in all these samples ranging from 3.6 to 900 ng/g wet wt., 3.6 to 210 ng/g wet wt., and 18 to 1400 ng/g dry wt. for mussels, fish and sediments, respectively. The concentrations of BTs in several locations of this study were comparable with the reported values from some developed countries and highest among Asian developing nations. Considerable concentration of BTs in several locations might have ecotoxicological consequences and may cause concern to human health. The parent compound TBT was found to be highest than those of its degradation compounds, DBT and MBT, suggesting recent input of TBT to the Malaysian marine environment. Significant positive correlation (Spearman rank correlation: r2=0.82, P<0.0001) was found between BTs and SigmaSn, implying considerable anthropogenic input of butyltin compounds to total tin contamination levels. Enormous boating activities may be a major source of BTs in this country, although aquaculture activities may not be ignored.     

Flame retardants, surfactants and organotins in sediment and mysid shrimp of the Scheldt estuary (The Netherlands)

Sediment and mysids from the Scheldt estuary, one of the largest and most polluted estuaries in Western Europe, were analyzed for a number of contaminants that have been shown to possess endocrine-disrupting activity, i.e. organotins, polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD), tetrabromobisphenol A (TBBPA), nonylphenol ethoxylates (NPE) and transformation products, nonylphenol (NP) and nonylphenol ether carboxylates (NPEC). In addition, in vitro estrogenic and androgenic potencies of water and sediment extracts were determined. Total organotin concentrations ranged from 84 to 348 ng/g dw in sediment and 1110 to 1370 ng/g dw in mysid. Total PBDE (excluding BDE-209) concentrations ranged from 14 to 22 ng/g dw in sediment and from 1765 to 2962 ng/g lipid in mysid. High concentrations of BDE-209 (240-1650 ng/g dw) were detected in sediment and mysid (269-600 ng/g lipid). Total HBCD concentrations in sediment and mysid were 14-71 ng/g dw and 562-727 ng/g lipid, respectively. Total NPE concentrations in sediment were 1422 ng/g dw, 1222 ng/g dw for NP and 80 ng/g dw for NPEC and ranged from 430 to 1119 ng/g dw for total NPE and from 206 to 435 ng/g dw for NP in mysid. Significant estrogenic potency, as analyzed using the yeast estrogen assay, was detected in sediment and water samples from the Scheldt estuary, but no androgenic activity was found. This study is the first to report high levels of endocrine disruptors in estuarine mysids.     

Organotin contamination in the Atlantic Ocean off the Iberian Peninsula in relation to shipping

Imposex in female snails, a bioindicator of TBT contamination, and the presence of organotins in snails' tissue and sediments were studied at nine sites off the western Iberian Peninsula. The study was part of a European project (acronym HIC-TBT) co-financed by the EU-LIFE programme, intending to investigate and communicate the impact of organotins from ships in marine ecosystems. Snails and sediments were sampled during two cruises in May/June 1999 and in January 2000 in areas of high, intermediate and low-shipping density. Imposex was found in female snails from several sampling sites, some of which had an imposex incidence of 100%. Differences in sensitivity were found between species; hence comparison of imposex levels between locations where different species were collected was not straightforward. Total organotin concentrations in sediments (sum of butyl and phenyltin compounds) ranged from 21 to 185 ng Sn g(-1) with higher values for most sites sampled in the vicinity of shipping lanes. Organotin concentration in snails' tissue ranged from <5 to 196 ng Sn g(-1), which are similar to those found in snails from other offshore areas contaminated by TBT. Overall, these results give further support to the recent ban on the use of organotin based antifouling paints to all ship size.     

Concentrations of organotin compounds in sediment and clams collected from coastal areas in Vietnam

Levels of butyltin (BT) and phenyltin (PT) compounds were determined in sediments and clam Meretrix spp. collected from north and central coastal areas in Vietnam. Concentrations of TBT in sediments ranged from 0.89 to 34 ng g(-1) dry wt and those in clams ranged from 1.4 to 56 ng g(-1) wet wt. The levels of TBT in sediments and clams from Vietnam were within limits reported from other countries. Further, the TBT level in clams was lower than the tolerable average residue level (TARL) estimated based on tolerable daily intake (TDI). Trace amounts of PTs were also found in both sediment and clam samples. In sediments from north and central Vietnam, the concentrations of TBT were highest in the order of Hue (28 ng g(-1) dry wt), Cua Luc (15 ng g(-1) dry wt), Sam Son (6.3 ng g(-1) dry wt), and Tra Co (5.5 ng g(-1) dry wt). Among the clams from north and central Vietnam, the levels of TBT in clams from Cua Luc were dramatically high at 47 ng g(-1) wet wt. TBT formed the principal butyltin species in sediment at all sites studied. The ratios of TBT in sediment were higher among BT compounds at all study sites. Of total BTs, TBT was the dominant species in clams from almost all sites studied. In spatial distribution, TPT showed a pattern similar to TBT, suggesting the use of TPT as an antifouling paint. The partition coefficient between sediment and calms was calculated. The partition coefficients of TBT and TPT were 2.01 (0.56-5.5) and 9.23 (3.1-20), respectively. These results show that sediment-bound TBT is a source of contamination to clams in addition to dissolved TBT.     

Horizontal distribution of butyltins in surface sediments from an enclosed bay system, Korea

Tributyltin (TBT), dibutyltin (DBT), and monobutyltin (MBT) compounds were quantitatively determined from surface sediment samples (total 59 stations) covering a whole basin where harbors, shipyards, and aquaculture farms were located. Butyltin compounds were detected from all the stations covering 640 km(2) of an enclosed bay system. TBT concentrations ranged from 4 to 382 ng/g as tin on a dry weight basis, and total butyltin concentrations, from 27 to 1763 ng/g. Horizontal distribution of TBT concentration showed apparent negative gradients from harbors and shipyards, indicating that its contamination was closely related to boating and dry-docking activities. However, TBT concentrations were decreased steeply from source areas. Elevated DBT and MBT levels in creeks imply the possible input of DBT from industrial wastewater. Total butyltin concentrations in sediments are significantly correlated with particulate organic carbon concentration for the subset of stations that are distant from source areas.     

Factors influencing organotin distribution in different marine environmental compartments, and their potential health risk

Tributyltin (TBT) and triphenyltin (TPT) in different marine environmental compartments such as seawater, sediments, and inshore fishes were investigated in 21 Taiwanese harbors between 2001 and 2004 in order to determine the major factors influencing their distribution. The existence of major input sources and the limited water exchange rate inside the harbors were indicated by higher TBT concentrations in seawater from inner harbor than from outer harbor areas. The levels of TBT in sediments were found to be mainly affected by their geographic distribution, water exchange rates and shipping activity. No significant correlations in TBT concentrations between water, sediment and fish suggested TBT accumulation by fish might not result from water and sediment, but from their food. TPT were detected in most fish samples, but found in few sediment samples and none in seawater, indicating fish could be as a target element for monitoring contaminated levels of TPT in the aquatic environment. Mean concentrations of TBT in fish muscle higher than tolerable average residue levels (TARLs), and mean hazard indices of TBT and TPT higher than 1 suggested consumption of fishes from Taiwanese harbor areas might have potential high risk to human health.     

Tributyltin distribution and producing androgenic activity in water,sediment, and fish muscle

This study investigated the concentrations of Tributyltin (TBT) in water, sediment, and fish muscle samples taken from Kaohsiung Harbor and Kaoping River estuary, Taiwan. TBT concentrations in water and sediment samples ranged from less than 18.5 to 34.1 ng Sn L^?1 and from 2.44 to 29.7 ng Sn g^?1 weight per weight (w/w), respectively. Concentrations in the TBT‐contaminated fish muscle samples ranged from 10.8 to 79.6 ng Sn g^?1 w/w. The TBT concentrations in fish muscle were higher than those in water and sediment samples. The fish muscle/water TBT bioconcentration factor (BCF) ranged from 590 to 3363 L kg^?1. Additionally, the water samples were assessed for androgenic activity with an MCF7‐AR1 human breast cancer cell line. The androgenic activity ranged from 0.94 to 3.1 ng‐dihydrotestosterone per litre water (ng‐DHT L^?1). Higher concentrations of TBT in water and sediment samples occurred in the dry season, but the androgenic activity had higher values in the rainy season.     

Airborne PCB as a contamination risk on freeze-drying of sediment

In a study of polychlorinated biphenyls (PCB) in freeze-dried sediments from Swedish lakes, the samples were found to contain unexpectedly high concentrations of PCBs. In one of the lakes (Bj?rken), e.g. the concentration of PCB-28 was found to be 189 ng/g dw after freeze-drying compared to 7 ng/g dw after air-drying of the sediment and therefore our hypothesis was that the contamination had occurred during the freeze-drying procedure. Hence, a dry reference sediment (RS) was used in order to identify potential sources of PCB contamination. The investigation included freeze-drying of the dry RS in five different laboratories, exposure to the air in the freeze-drying room, storage at room temperature and air-drying in a certain fume hood and in a laminar flow clean bench. The pattern and concentrations of PCBs detected in the exposed RS were strongly influenced by low chlorinated PCB congeners under all of these conditions with exception for air-drying in the fume hood. Storage in the laboratory air resulted in a slight elevation in the concentrations of low chlorinated PCB congeners, whereas higher concentrations of all PCB congeners were observed after freeze-drying. In all experiments the contamination appeared to be due to adsorption of PCB from the laboratory air. On the basis of these findings we recommend that reference material be subjected to the entire procedure, including sample preparation, in order to monitor contamination and that a procedure involving solvent extraction of wet sediment samples is used, whenever possible, instead of procedures involving dry samples.     

Occurrence of polychlorinated naphthalenes, polychlorinated biphenyls and short-chain chlorinated paraffins in marine sediments from Barcelona (Spain)

Polychlorinated naphthalenes (PCNs), short-chain chlorinated paraffins (SCCPs) and polychlorinated biphenyls (PCBs) were analysed in marine sediment samples collected from the coastal area of Barcelona (Spain) and near of a submarine emissary coming from a waste water treatment plant located at the mouth of the Besòs River (Barcelona). An integrated sample treatment based on Soxhlet extraction followed by a simple clean-up with Florisil and graphitized carbon cartridge was employed. Gas chromatography coupled to ion-trap tandem mass spectrometry (GC-MS/MS) and gas chromatography-mass spectrometry in electron capture negative ionization mode, were used for PCN and SCCP determinations, respectively, while for PCB analysis gas chromatography with electron capture detection (GC-ECD) was used. The method developed provided low limits of detection (0.001-0.003 ng g(-1) dry weight (dw) for PCNs, 1.8 ng g(-1) for SCCPs and 0.006-0.014 ng g(-1)dw for PCBs) and good run-to-run precisions (lower than RSD 8%) for the analysis of sediment samples. Concentration levels ranging from 0.17 to 3.27 ng g(-1)dw for PCNs, between 0.21 and 1.17 microg g(-1)dw for SCCPs, and from 2.33 to 44.00 ng g(-1) (dw) for PCBs, were found in the coastal sediments, while for samples collected near to the submarine emissary higher levels (from 2.02 to 6.56 ng g(-1)dw for PCNs, between 1.25 and 2.09 microg g(-1)dw for SCCPs and from 22.34 to 37.74 ng g(-1)dw for PCBs) were obtained. The results obtained provide new data about the occurrence of PCN and SCCP in the coastal area of Barcelona.     

Tributyltin contamination around an oil terminal in Sullom Voe (Shetland)

This report provides clear evidence of tributyltin (TBT) contamination of coastal waters close to an oil terminal handling large tankers. Dogwhelk populations from Sullom Voe, Shetland, have been examined for the development by females of certain male sexual characteristics (e.g. a penis and vas deferens). This condition, termed imposex, is a response to exposure to tributylin compounds, and can be quantified by use of the relative penis size index (RPSI). Dogwhelk populations throughout Sullom Voe were strongly affected by TBT (RPSI 34 to 81%), and up to 90% of the females in the oil terminal area were sterile. Queen scallops collected in the Voe contained enhanced concentrations of tin in adductor muscle and gonad tissue (0.02 to 0.23 mg kg(-1) wet weight), but it is considered unlikely that their growth and development is adversely affected. Tin was rarely detected in scallops, queens, or dogwhelks outside the Voe, although the development of a vas deferens in many of the female dogwhelks indicated a degree of contamination of the open waters of Yell Sound. TBT was detectable (2 ng litre(-1) as tin) in only two sea water samples taken from the terminal area.     

Tributyltin distribution in the coastal environment of Peninsular Malaysia

The occurrence of tributyltin (TBT) is reported in the coastal waters of a few selected sites in Peninsular Malaysia. Water, bivalves and sediment samples collected were analysed specifically for TBT using sensitive analytical methods which involved a solvent extraction procedure with appropriate clean-up followed by graphite furnace atomic absorption spectrometric measurements. The levels of TBT in the seawater in unexposed areas were found in the range from <3.4 to 20 ng litre(-1) as compared to coastal areas with high boat and ship activities where TBT levels in seawater were generally above 30 ng litre(-1), with the highest level found at 281.8 ng litre(-1). TBT levels in the tissues of random cockle and soft-shell clam samples from local markets were found in the range from <0.5 to 3.7 ng g(-1) wet weight. The levels of TBT found in green mussel samples both from the market (23.5 ng g(-1) wet weight) and those from a mussel farm (14.2 ng g(-1) wet weight) indicate slight accumulation of TBT. In sediments, TBT levels were found ranging from <0.7 ng g(-1) dry weight in unexposed coastal sites to as high as 216.5 ng g(-1) dry weight for a site within a port area.     

Occurrence of tributyltin (TBT)-resistant bacteria is not related to TBT pollution in Mekong River and coastal sediment: with a hypothesis of selective pressure from suspended solid

Tributyltin (TBT) is organotin compound that is toxic to aquatic life ranging from bacteria to mammals. This study examined the concentration of TBT in sediment from and near the Mekong River and the distribution of TBT-resistant bacteria. TBT concentrations ranged from <2.4 to 2.4 ng/g (dry wt) in river sediment and <2.4-15 ng g(-1) (dry wt) in harbor sediment. Viable count of total bacteria ranged from 2.0 x 10(4) to 1.4 x 10(7)cfu/g, and counts of TBT-resistant bacteria ranged <1.0 x 10(2) to 2.5 x 10(4)cfu/g. The estimated occurrence rate of TBT-resistant bacteria ranged from <0.01 to 34% and was highest in upstream sites in Cambodia. The occurrences of TBT in the sediment and of TBT-resistant bacteria were unrelated, and chemicals other than TBT might induce TBT resistance. TBT-resistant bacteria were more abundant in the dry season than in the rainy season. Differences in the selection process of TBT-resistant bacteria between dry and rainy seasons were examined using an advection-diffusion model of a suspended solid (SS) that conveys chemicals. The estimated dilution-diffusion time over a distance of 120 km downstream from a release site was 20 days during dry season and 5 days during rainy season, suggesting that bacteria at the sediment surface could be exposed to SS for longer periods during dry season.