Biosensor applications in contaminated estuaries: Implications for disaster research response.

BACKGROUND: Given the time and monetary costs associated with traditional analytical chemistry, there remains a need to rapidly characterize environmental samples for priority analysis, especially within disaster research response (DR2). As PAHs are both ubiquitous and occur as complex mixtures at many National Priority List sites, these compounds are of interest for post-disaster exposures. OBJECTIVE: This study tests the field application of the KinExA Inline Biosensor in Galveston Bay and the Houston Ship Channel (GB/HSC) and in the Elizabeth River, characterizing the PAH profiles of these region's soils and sediments. To our knowledge, this is the first application of the biosensor to include soils. METHODS: The biosensor enables calculation of total free PAHs in porewater (C free), which is confirmed through gas chromatography-mass spectrometry (GC-MS) analysis. To determine potential risk of the collected soils the United States Environmental Protection (USEPA) Agency's Regional Screening Level (RSL) Calculator is used along with the USEPA Region 4 Ecological Screening Values (R4-ESV) and Refined Screening Values (R4-RSV). RESULTS: Based on GC-MS results, all samples had PAH-related hazard indices below 1, indicating low noncarcinogenic risks, but some samples exceeded screening levels for PAH-associated cancer risks. Combining biosensor-based C free with Total Organic Carbon yields predictions highly correlated (r > 0.5) both with total PAH concentrations as well as with hazard indices and cancer risks. Additionally, several individual parent PAH concentrations in both the GB/HSC and Elizabeth River sediments exceeded the R4- ESV and R4-RSV values, indicating a need for follow-up sediment studies. CONCLUSIONS: The resulting data support the utility of the biosensor for future DR2 efforts to characterize PAH contamination, enabling preliminary PAH exposure risk screening to aid in prioritization of environmental sample analysis.

Seasonal Differences in Contaminant Accumulation in Neotropical Migrant and Resident Songbirds.

For many years, it has been hypothesized that Neotropical migrants breeding in the United States and Canada accumulate organochlorine pesticides (OCPs) while on their wintering grounds in Latin America. We investigated the seasonal accumulation of persistent organic pollutant (POPs) in migrant and resident passerines in Texas, Yucatán, and Costa Rica collected during the fall, winter, and spring from 2011 to 2013. A total of 153 birds were collected, and all contained detectable levels of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and OCPs with dichlorodiphenyldichloroethylene (DDE) being the most predominant pesticide. OCPs and PCBs were the predominant contaminants, accounting for ≥80 % of the total POPs burden, whereas PBDEs accounted for ≤16 %. Only spring migrants from Texas had significantly greater DDE concentrations (64.6 ng/g dry weight [dw]) than migrants collected in Costa Rica (23.2 ng/g dw). Resident birds in Texas had significantly greater levels of DDE (121 ng/g dw) and ΣPBDEs (34.8 ng/g dw) compared with residents in Yucatán and Costa Rica. For ΣPCBs, resident birds from Costa Rica had significantly lower concentrations (9.60 ng/g dw) compared with their migrant counterparts (43.7 ng/g dw) and residents from Texas (48.3 ng/g dw) and the Yucatán (32.1 ng/g dw). Migrant and resident passerines had similar congener profiles for PCBs and PBDEs suggesting similar exposure and retention of these contaminants. No significant accumulation of DDE was observed in migrants while on their wintering grounds. Relatively high concentrations of PBDEs in resident birds from Costa Rica warrant future studies of PBDE contamination in Latin America.

Seasonal and body size-dependent variability in the bioaccumulation of PAHs and their alkyl homologues in pearl oysters in the central Arabian Gulf.

Spatiotemporal concentration patterns for 19 parents and their alkyl homologues were measured in Pinctada radiata from 7 locations in the central Arabian Gulf around Qatar in the winter, spring and summer (2014-2015). The concentrations of PAHs ranged from 20 to 2240 (262 ± 38.0 ng·g-1 dw) with the highest occurrence in the Doha harbor (738.4 ± 197.3 ng·g-1 dw) and the lowest in the west coast of Qatar (48.3 ± 5.8 ng·g-1 dw). Residual PAHs in the oysters were about two times higher in winter than in spring and summer (P < 0.05). PAHs in oysters are dominated by 2 and 3 rings PAHs and their alkyls. Alkylated PAHs (APAHs) comprised >55 % of the ΣPAHs. Statistically significant differences in PAHs profiles among oysters were due in part to differences in lipid contents and shell biometrics. Principal component analysis (PCA) and diagnostic ratios for sources identifications suggested that PAHs accumulations in oysters were due to petrogenic and fuel combustion.

Comparative toxicity of hydrocarbons for evaluation of Lysmata boggessi as an experimental proxy for deep-water column micronekton.

The potential impacts of sub-surface hydrocarbon plumes to deep-water column micronekton are an important consideration in a more complete understanding of ecosystem effects resulting from deep-sea oil spills. However, evaluating toxicity in these organisms presents multiple challenges, and the use of a shallow-water proxy species allows comparison and validation of experimental results. This study thus examined the suitability of the peppermint shrimp, Lysmata boggessi, as an experimental proxy for ecologically important deep-sea zooplankton/micronekton in hydrocarbon toxicity assays. This crustacean species occurs in shallow coastal marine environments throughout the western Atlantic, Caribbean and Gulf of Mexico, is similar in size to the mesopelagic organisms previously tested and is readily available via commercial aquaculture. The effects of 1-methylnaphthalene and fresh Macondo oil (MC252) on L. boggessi were assessed in 48-h constant-exposure toxicity tests, and acute thresholds were compared to previously determined LC50s for oceanic mid water Euphausiidae, Janicella spinacauda, Systellaspis debilis, Sergestes sp., Sergia sp. and the mysid shrimp Americamysis bahia. Acute thresholds and the calculated critical target lipid body burden (CTLBB) for the shallow-water L. boggessi were comparable to the deep-water species tested, suggesting that L. boggessi may be a suitable proxy for some mesopelagic micronekton species in acute hydrocarbon exposures.

Persistent organic pollutants in eggs from south Texas Aplomado falcons.

A program to reintroduce the Northern Aplomado falcon (Falco femoralis septentrionalis) in south Texas and the southwestern United States was initiated in the late 1970s. Fledgling Aplomado falcons were first released in the Laguna Atascosa National Wildlife Refuge in 1993 and the first nesting pair in the area was recorded by 1995. During 2004-2017 we collected addled eggs from nesting pairs in the Laguna Atascosa National Wildlife Refuge and Matagorda Island in south Texas, to determine if environmental contaminants in Aplomado falcon eggs had decreased over time and if eggshell thickness values were similar to those in the pre-DDT era. We analyzed organochlorine pesticides, PCBs, and PBDEs in 60 egg homogenates by gas chromatography-mass spectrometry. Eggshells were measured to determine thickness and to correlate with contaminant concentrations. Mean concentration in eggs were 244 ng/g ww for p,p'- DDE, 270 ng/g ww for PCBs and 10 ng/g ww for PBDEs. These values were lower than those reported in a previous study for eggs collected between 1999 and 2003, with a mean of 821 ng/g ww for p,p'-DDE and 1228 ng/g ww for total PCBs. Eggshell thickness ranged from 0.206 mm to 0.320 mm (n = 156). Overall, contaminant concentrations in eggs of Aplomado falcons were low, at levels not likely to impact the recovery of the species. Data from this and previous studies indicate that DDE has decreased significantly in eggs of Aplomado falcons over the last 25 years in south Texas. Breeding populations have been steady at over 30 breeding pairs in south Texas since 2011, although they decreased to 24 pairs in 2018 following Hurricane Harvey.

Polycyclic aromatic hydrocarbon status in post-hurricane Harvey sediments: Considerations for environmental sampling in the Galveston Bay/Houston Ship Channel region.

Hurricane Harvey led to a broad redistribution of sediment throughout Galveston Bay and the Houston Ship Channel (GB/HSC), but the resulting changes in chemical contaminant distributions have yet to be characterized. To address this question, we collected and analyzed post-Harvey sediment for concentrations of the EPA 16 Priority Pollutant polycyclic aromatic hydrocarbon (PAHs), determining the extent to which the spatial distribution and sourcing of contaminants may have changed in contrast to historical surface sediment data (<5 cm) from the National Oceanic Atmospheric Administration (NOAA) available for the years 1996-2011. We found a small, but detectable increase from pre- to post-Harvey in PAH concentrations, with PAH diagnostic sourcing indicating combustion origins. Of the detected PAHs, none exceeded Sediment Quality Guideline values. Overall, we have added to the understanding of PAH spatial trends within the GB/HSC region, and developed a reference PAH baseline to inform future studies.

Polycyclic aromatic hydrocarbons in soil of the Canadian river floodplain in Oklahoma.

The accumulation of polycyclic aromatic hydrocarbons (PAH) in soil, plants, and water may impart negative effects on ecosystem and human health. We quantified the concentration and distribution of 41 PAH (n = 32), organic C, total N, and S (n = 140) and investigated PAH sources using a chronosequence of floodplain soils under a natural vegetation succession. Soil samples were collected between 0- and 260-cm depth in bare land (the control), wetland, forest, and grassland areas near a closed municipal landfill and an active asphalt plant (the contaminant sources) in the north bank of the Canadian River near Norman, OK. Principal component, cluster, and correlation analyses were used to investigate the spatial distribution of PAH, in combination with diagnostic ratios to distinguish pyrogenic vs. petrogenic PAH suites. Total PAH concentration (SigmaPAH) had a mean of 1300 ng g(-1), minimum of 16 ng g(-1), and maximum of 12,000 ng g(-1). At 0- to 20-cm depth, SigmaPAH was 3500 +/- 1600 ng g(-1) (mean +/- 1 SE) near the contaminant sources. The most common compounds were nonalkylated, high molecular weight PAH of pyrogenic origin, i.e., fluoranthene (17%), pyrene (14%), phenanthrene (9%), benzo(b)fluoranthene (7%), chrysene (6%), and benzo(a)anthracene (5%). SigmaPAH in the control (130 +/- 23 ng g(-1)) was comparable to reported concentrations for the rural Great Plains. Perylene had a unique distribution pattern suggesting biological inputs. The main PAH contamination mechanisms were likely atmospheric deposition due to asphalt production at the 0- to 20-cm depth and past landfill operations at deeper depths.

Mesocosm experiments to better understand hydrocarbon half-lives for oil and oil dispersant mixtures.

Concerns on the timing and processes associated with petroleum degradation were raised after the use of Corexit during the Deepwater Horizon oil spill. There is a lack of understanding of the removal of oil associated with flocculate materials to the sediment. Mesocosm studies employing coastal and open-ocean seawater from the Gulf of Mexico were undertaken to examine changes in oil concentration and composition with time. The water accommodated fractions (WAF) and chemically enhanced WAF (CEWAF) produced using Macondo surrogate oil and Corexit were followed over 3-4 days in controlled environmental conditions. Environmental half-lives of estimated oil equivalents (EOE), polycyclic aromatic hydrocarbons (PAH), n-alkanes (C10-C35), isoprenoids pristane and phytane, and total petroleum hydrocarbons (TPH) were determined. EOE and PAH concentrations decreased exponentially following first-order decay rate kinetics. WAF, CEWAF and DCEWAF (a 10X CEWAF dilution) treatments half-lives ranged from 0.9 to 3.2 days for EOE and 0.5 to 3.3 days for PAH, agreeing with estimates from previous mesocosm and field studies. The aliphatic half-lives for CEWAF and DECWAF treatments ranged from 0.8 to 2.0 days, but no half-life for WAF could be calculated as concentrations were below the detection limits. Biodegradation occurred in all treatments based on the temporal decrease of the nC17/pristane and nC18/phytane ratios. The heterogeneity observed in all treatments was likely due to the hydrophobicity of oil and weathering processes occurring at different rates and times. The presence of dispersant did not dramatically change the half-lives of oil. Comparing degradation of oil alone as well as with dispersant present is critical to determine the fate and transport of these materials in the ocean.

Measured and predicted acute toxicity of phenanthrene and MC252 crude oil to vertically migrating deep-sea crustaceans.

Deep-water column micronekton play a key role in oceanic food webs and represent an important trophic link between deep- and shallow-water ecosystems. Thus, the potential impacts of sub-surface hydrocarbon plumes on these organisms are critical to developing a more complete understanding of ocean-wide effects resulting from deep-sea oil spills. This work was designed to advance the understanding of hydrocarbon toxicity in several ecologically important deep-sea micronekton species using controlled laboratory exposures aimed at determining lethal threshold exposure levels. The current study confirmed the results previously determined for five deep-sea micronekton by measuring lethal threshold levels for phenanthrene between 81.2 and 277.5 µg/L. These results were used to calibrate the target lipid model and to calculate a critical target lipid body burden for each species. In addition, an oil solubility model was used to predict the acute toxicity of MC252 crude oil to vertically migrating crustaceans, Janicella spinacauda and Euphausiidae spp., and to compare the predictions with results of a 48-h constant exposure toxicity test with passive-dosing. Results confirmed that the tested deep-sea micronekton appear more sensitive than many other organisms when exposed to dissolved oil, but baseline stress complicated interpretation of results.

Biological response to dissolved versus dispersed oil.

The water-soluble compounds of oil (e.g. low molecular weight PAHs) dissolve as a function of their physicochemical properties and environmental conditions, while the non-soluble compounds exist as dispersed droplets. Both the chemical and physical form of oil will affect the biological response. We present data from a mesocosm study comparing the microbial response to the water-soluble fraction (WSF), versus a water-accommodated fraction of oil (WAF), which contains both dispersed and dissolved oil components. WAF and WSF contained similar concentrations of low molecular weight PAHs, but concentrations of 4- and 5-ring PAHs were higher in WAF compared to WSF. Microbial communities were significantly different between WSF and WAF treatments, primary productivity was reduced more in WSF than in WAF, and concentrations of transparent exopolymeric particles were highest in WSF and lowest in the controls. These differences highlight the importance of dosing strategy for mesocosm and toxicity tests.

Spatio-temporal distribution and sources identifications of polycyclic aromatic hydrocarbons and their alkyl homolog in surface sediments in the central Arabian Gulf.

The quantitative analysis of 18 parents and their alkyl homologs was performed in sediment samples from the central Arabian Gulf (Gulf) around Qatar Peninsula in six sequential seasons, winter 2014 to spring 2015, at 21 locations with a water depth range of 1.5-60 m. PAHs distribution was patchy with higher concentrations found inside semi-enclosed coastal areas like harbors and bays. The mean PAHs concentration was 112 ng·g-1 dry weight with a range of 0.6 to 1560 ng·g-1 and a variability coefficient of 2.4. The PAHs mean concentration was highest in the winter by a factor of 5 compared to mean summer concentration. A significant seasonal variability in the concentrations of ∑PAHs is mainly attributed to variability in the concentrations of the low molecular weight PAHs fraction and the less alkylated PAHs. Alkylated-PAHs were the most dominant PAHs comprising about 50% of the ∑PAHs, and with about 6 times higher than the mean concentrations in the winter compared to the mean summer concentration. The LPAHs concentrations correlated negatively with temperature and ∑PAHs correlated positively with % clay. Principal component analysis was used to identify sources of PAHs. PAHs in the Gulf have mixed sources with an estimated 57% from petroleum and 43% from pyrogenic sources. Coastal water hydrodynamics and lateral transport processes affect the distribution and composition of PAHs in the central Gulf.

Passive dosing yields dissolved aqueous exposures of crude oil comparable to the CROSERF (Chemical Response to Oil Spill: Ecological Effects Research Forum) water accommodated fraction method.

The Chemical Response to Oil Spill: Ecological Effects Research Forum's water accommodated fraction procedure was compared with 2 alternative techniques in which crude oil was passively dosed from silicone tubing or O-rings. Fresh Macondo oil (MC252) was dosed at 30 mg/L using each approach to investigate oil dissolution kinetics, which was monitored by fluorometry as estimated oil equivalents (EOEs). Subsequent experiments with each dosing method were then conducted at multiple oil loadings. Following equilibration, test media were analytically characterized for polyaromatic hydrocarbons (PAHs) using gas chromatography (GC)-mass spectrometry and dissolved oil using biomimetic solid-phase microextraction (SPME). The results showed that equilibrium was achieved within 72 h for all methods. Measured PAH concentrations were compared with oil solubility model predictions of dissolved exposures. The concentration and composition of measured and predicted dissolved PAHs varied with oil loading and were consistent between dosing methods. Two-dimensional GC compositional data for this oil were then used to calculate dissolved toxic units for predicting MC252 oil acute toxicity across the expected range of species sensitivities. Predicted toxic units were nonlinear with loading and correlated to both EOE and biomimetic SPME. Passive dosing methods provide a practical strategy to deliver and maintain dissolved oil concentrations while avoiding the complicating role that droplets can introduce in exposure characterization and test interpretation. Environ Toxicol Chem 2018;37:2810-2819. © 2018 SETAC.

Relationships between pesticides and organic carbon fractions in sediments of the Danshui River estuary and adjacent coastal areas of Taiwan.

In order to understand the fate of pesticides in marine environments, concentrations of pesticides and different carbonaceous fractions were determined for surface sediments in the Danshui River and nearby coastal areas of Taiwan. The major compounds detected were tetrachlorobenzene, HCHs, chlordane, aldrin, DDDs, DDEs and DDTs. Total concentrations of pesticides in the sediments ranged from not detectable to 23 ng g(-1), with the maximum value detected near the discharge point of the marine outfall from the Pali sewage treatment plant. These results confirm that pesticides persist in estuarine and nearby coastal environments of the Danshui River well after their ban. Concentrations of total pesticides significantly correlate with concentrations of total organic carbon and black carbon in these sediments, suggesting that total organic carbon and black carbon regulate the distribution of trace organic pollutants in fluvial and coastal marine sediments.

Distribution and residue profile of organochlorine pesticides and polychlorinated biphenyls in sediment and fish of Lake Manzala, Egypt.

Sediment and fish (Oreochromis niloticus) samples collected from Lake Manzala were analyzed to assess the spatial distribution of OCPs and 96 PCBs. Relatively higher concentrations of chlorpyrifos, ∑DDT, and HCB were found, particularly at the Bahr Al-Baqar drain station, which has uncontrolled inputs of untreated domestic, agricultural, and industrial wastes. The concentrations of ∑PCBs ranged from 19 to 69 ng/g dw and from 7.4 to 29 ng/g dw for sediment and fish samples, respectively. Ratios of DDT to its metabolites suggest that the source of ∑DDT is from past usage of technical DDT in the regions surrounding the lake. Sediment quality guidelines were exceeded in 88, 75, and 42% of sediments for the Effects Range Low (ERL) for ∑PCBs, ∑DDT, and 4,4'-DDE, respectively. Sediment from the Bahr Al-Baqar drain exceeded the Probable Effects Level (PEL) for DDT isomers 2,4' and 4,4'. All fish samples from Lake Manzala were well below action and tolerance levels set by US EPA for ∑DDT, chlordane, dieldrin, heptachlor, mirex, and PCBs. HIGHLIGHTS: Distributions of OCPs and PCBs in sediment and tilapia from Lake Manzala were investigated. Chlorpyrifos, ∑DDT, and HCB sediment concentrations were spatially variable and relatively elevated. ∑96PCBs, ∑DDT, and 4,4'-DDE exceeded the Effects Range Low in 88, 75, and 42% of the sediments, respectively. The major input from Cairo, the Bahr Al-Baqar drain, exceeded the Probable Effects Level for DDT isomers in sediments. OCP and PCB concentrations in tilapia were below action and tolerance levels set by the US-EPA.

Relationship between carbonaceous materials and polychlorinated biphenyls (PCBs) in the sediments of the Danshui River and adjacent coastal areas, Taiwan.

Persistent organic pollutants, POPs (e.g., polychlorinated biphenyls) can seriously and deleteriously affect environmental quality and human health. These organic pollutants are exhibiting high affinities to solid phases and thus, quickly end up in sediments. To better understand the role of carbonaceous materials in the transport and distributions of POPs in terrestrial and near-shore environments, concentrations of PCBs and carbonaceous materials (including total organic carbon, black carbon and total carbohydrates), were determined in surface sediments of the Danshui River and nearby coastal areas, Taiwan. Total concentrations of PCBs in the sediments ranged from non-detectable to 83.9 ngg(-1), dry weight, with the maximum value detected near the discharge point of the marine outfall from the Pali Sewage Treatment Plant. These results suggest that the sewage treatment plant has discharged PCBs in the past and the concentrations are still high due to their persistence; alternatively, PCBs are still being discharged in the estuarine and near-shore environment of the Danshui River. Organic carbon and black carbon concentrations correlated well with those of total PCBs in the sediments, suggesting that both organic carbon and black carbon significantly affect the distribution of trace organic pollutants through either post-depositional adsorption, or by co-transport of similar source materials. The field results demonstrate that black carbon and plays an important role in the general distribution of PCBs, while concentrations of some specific PCBs are affected by both black carbon and organic carbon concentrations.

Composition and depth distribution of hydrocarbons in Barataria Bay marsh sediments after the Deepwater Horizon oil spill.

In 2010, an estimate 4.1 million barrels of oil were accidentally released into the Gulf of Mexico (GoM) during the Deepwater Horizon (DWH) Oil Spill. One and a half years after this incident, a set of subtidal and intertidal marsh sediment cores were collected from five stations in Barataria Bay, Louisiana, USA, and analyzed to determine the spatial and vertical distributions and source of hydrocarbon residues based on their chemical composition. An archived core, collected before the DWH oil spill from the same area, was also analyzed to assess the pre-spill hydrocarbon distribution in the area. Analyses of aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and stable carbon isotope showed that the distribution of petroleum hydrocarbons in Barataria Bay was patchy and limited in areal extent. Significant TPH and ΣPAH concentrations (77,399 µg/g and 219,065 ng/g, respectively) were detected in the surface sediments of one core (i.e., core A) to a depth of 9 cm. Based on a sedimentation rate of 0.39 cm yr(-1), determined using (137)Cs, the presence of anthropogenic hydrocarbons in these sediment core deposited ca. 50 to 60 years ago. The historical background hydrocarbon concentrations increased significantly at the sediment surface and can be attributed to recent inputs. Although the oil present in the bay's sediments has undergone moderate weathering, biomarker analyses performed on core A samples likely indicated the presence of hydrocarbons from the DWH oil spill. The effects of oiling events on Barataria Bay and other marsh ecosystems in this region remain uncertain, as oil undergoes weathering changes over time.

Spatial and temporal distribution of water column total polycyclic aromatic hydrocarbons (PAH) and total petroleum hydrocarbons (TPH) from the Deepwater Horizon (Macondo) incident.

Pre-spill background concentrations of TPH and PAH in water samples from the Gulf of Mexico are compared with samples (over 20,000) collected during and after the Deepwater Horizon incident (13,000 stations). Samples were collected by multiple response agencies, trustees and BP and reported in the Gulf Science Data. The samples were collected from a few m to over 800 km in all directions from the wellhead. During the incident, samples with the highest concentrations of hydrocarbons were collected proximal to the wellhead or in samples collected from surface slicks and dispersant use. Of the 13,172 water sample TPH concentrations reported, 84% were below 1 µg/L (background). Of the 16,557 water sample PAH concentrations reported, 79% were below 0.056 µg/L (the median field blank, background). The percentage of samples below background increased rapidly after the well was capped. The spatial and temporal distributions of these hydrocarbon data are presented.

Edward D. Goldberg's proposal of "the Mussel Watch": Reflections after 40years.

We chronicle the extensive influence over the past forty years of Professor Edward D. Goldberg and his call in 1975 for a "Mussel Watch" or bivalve sentinel organism approach to assess geographic status and temporal trends of several chemicals of environmental concern in the coastal ocean. Examples of local, regional, national and international programs are discussed briefly as are examples of interesting useful findings and limitations to the Mussel Watch concept. Mussel Watch continues to provide useful data about status and trends of chemical contamination in coastal ecosystems.

Uptake and depuration of toxic halogenated aromatic hydrocarbons by the American oyster (Crassostrea virginica): a field study.

Uptake and depuration of toxic chlorinated compounds such as planar polychlorinated biphenyls (PCBs 77, 126, 169), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) were monitored during a 50-day field study where American oysters (Crassostrea virginica) were transplanted back and forth between a heavily polluted area (Houston Ship Channel, SC) and a relatively unimpacted area (Hanna's Reef, HR) within Galveston Bay, TX. In general, low molecular weight, less lipophilic compounds accumulated in the oysters to a larger extent than high molecular weight ones. Estimated half-lives for planar PCB congeners 77 and 126 were 28 and 51 days, respectively for depuration of newly contaminated oysters (HR-SC-HR) while longer half-lives (42 and 60 days, respectively) were observed for the same compounds as they were eliminated from chronically contaminated individuals (SC-HR). Estimated half-lives for 2,3,7,8-TCDD and 2,3,7,8-TCDF were 35 and 36 days, respectively and were similar to the tetrachlorinated biphenyls (PCBs 77 and 81). Compared with ortho-substituted PCB congeners of the same chlorination level, the more toxic PCBs take longer to depurate from the oysters. With few exceptions, elimination of all toxic compounds investigated proceeded at a slower rate from the chronically exposed population that from the newly contaminated one.