Dynamic exposure of organisms and passive samplers to hydrophobic chemicals

Passive sampling of herbicides above sediments at sites with losses of submerged macrophytes in a mesotrophic lake

Declines of submerged macrophytes (SUM) were monitored in littoral zones of the deep, mesotrophic lake Suhrer See (Northern Germany) since 2017. Drastic losses coincided with intense agriculture in sandy sub-catchments and precipitation. All lines of evidence pointed to a causal connection with subsurface discharge indicating that herbicide application might have caused the effects. Passive sampling was applied in 2022 to elucidate, whether herbicides were really present at sites of losses and if so, in ecotoxicological relevant concentrations. Samplers were exposed on top of lake sediments in 2 m depth and under worst case conditions, i.e., at sites, known for losses of the whole functional group of SUM and at the beginning of the vegetation period. At this time, SUM diaspores were most vulnerable to repression of development and the subsurface discharge was high in the same instance. The potential ecotoxicological relevance of detected herbicide concentrations was assessed with a toxic units (TU) approach, with reference to acute effect concentrations (EC50 of green algae, 72 h, growth). The TU ranged from 0.001 to 0.03. Most concentrations exceeded the threshold of relevance set by an assessment factor of 1000, i.e., TU > 0.001. Locally applied herbicides acted by suppressing developmental stages, and the sum of TU exceeded 0.02 at all sites, mainly due to diflufenican. Not applied locally, terbuthylazine and its relevant metabolites, including terbutryn, acted by inhibiting photosynthesis, and the sum of TU reached 0.005. On this base, diflufenican was assessed to be likely a main stressor, all other detected herbicides to be potentially relevant. Uncertainties and knowledge gaps were specified. The result of the chemical risk assessment was counterchecked for consistence with biological monitoring data within a whole lake perspective. Concepts of empirical and advanced causal attribution methodology were applied to get a grip to the ecological causal field and to protection.

Effect of Polymer Aging on Uptake/Release Kinetics of Metal Ions and Organic Molecules by Micro- and Nanoplastics: Implications for the Bioavailability of the Associated Compounds

The main driver of the potential toxicity of micro- and nanoplastics toward biota is often the release of compounds initially present in the plastic, i.e., polymer additives, as well as environmentally acquired metals and/or organic contaminants. Plastic particles degrade in the environment via various mechanisms and at different rates depending on the particle size/geometry, polymer type, and the prevailing physical and chemical conditions. The rate and extent of polymer degradation have obvious consequences for the uptake/release kinetics and, thus, the bioavailability of compounds associated with plastic particles. Herein, we develop a theoretical framework to describe the uptake and release kinetics of metal ions and organic compounds by plastic particles and apply it to the analysis of experimental data for pristine and aged micro- and nanoplastics. In particular, we elucidate the contribution of transient processes to the overall kinetics of plastic reactivity toward aquatic contaminants and demonstrate the paramount importance of intraparticulate contaminant diffusion.

Calibration of an acute toxicity model for the marine crustacean, Artemia franciscana, nauplii to support oil spill effect assessments

Oil spill risk and impact assessments rely on time-dependent toxicity models to predict the hazard of the constituents that comprise crude oils and petroleum substances. Dissolved aromatic compounds (ACs) are recognized as a primary driver of aquatic toxicity in surface spill exposure scenarios. However, limited time-dependent toxicity data are available for different classes of ACs to calibrate such models. This study examined the acute toxicity of 14 ACs and 3 binary AC mixtures on Artemia franciscana nauplii at 25 °C. Toxicity tests for 3 ACs were also conducted at 15 °C to evaluate the role of temperature on toxicity. The ACs investigated represented parent and alkylated homocyclic and nitrogen-, sulfur- and oxygen-containing heterocyclic structures with octanol-water partition coefficients (log K_ow) ranging from 3.2 to 6.6. Passive dosing was used to expose and maintain concentrations in toxicity tests which were confirmed using fluorometry, and independently validated for 6 ACs using GC-MS analysis. Mortality was assessed at 6, 24, and 48 h to characterize the time course of toxicity. No mortality was observed for the most hydrophobic AC tested, 7,12-dimethylbenz[a]anthracene, due to apparent water solubility constraints. Empirical log LC₅₀ s for the remaining ACs were fit to a linear regression with log K_ow to derive a critical target lipid body burden (CTLBB) based on the target lipid model. The calculated 48 h CTLBB of 47.1 ± 8.1 μmol/g octanol indicates that Artemia nauplii exhibited comparable sensitivity to other crustaceans. A steep concentration-response was found across all compounds as evidenced by a narrow range (1.0-3.1) in the observed LC₅₀ /LC₁₀ ratio. Differences in toxicokinetics were noted, and no impacts of temperature-dependence of AC toxicity were found. Toxicity data obtained for individual ACs yielded acceptable predictions of observed binary AC mixture toxicity. Results from this study advance toxicity models used in oil spill assessments.

Calibration and field application of the Atlantic HLB Disk containing Chemcatcher® passive sampler - Quantitative monitoring of herbicides, other pesticides, and transformation products in German streams

The Chemcatcher® (CC) passive sampler containing an Atlantic HLB-L Disk (AD) was calibrated in a laboratory-based flow-through tank over 21 days under stirring for 38 polar organic pesticides with log K_ow ranging from -1.7 to 3.8. The resultant sampling rates R_s range from 0.025 to 0.068 L/d. In 2018, field trials were conducted in the German rivers Mulde and Havel, as well as in 7 agricultural streams in Lower Saxony and Saxony-Anhalt. For 36 detected pesticides, the overall low concentrations were 0.2 to 49.4 ng/L. The determined pesticide profiles reflect agricultural use and were dominated by triazine herbicides including transformation products, by neonicotinoid insecticides, and by the herbicide mecoprop. Additional single hot spots were provided by the herbicides metamitron, isoproturon, and MCPA (showing the overall largest value of 49.4 ng/L). Notably, the detected waterborne pesticides include banned herbicides and associated transformation products in concentration ratios suggesting also recent input. This concerns in particular atrazine and its transformation products 2-OH-atrazine, deethylatrazine and deisopropylatrazine. An extended target screening of AD-CC extracts from the river Havel revealed the additional presence of other organic micropollutants including biocides, surfactants and industrial chemicals, and demonstrated the AD-CC applicability up to log K_ow of 4.5.

Groundwater sampling in karst terranes: passive sampling in comparison to event-driven sampling strategy

Karst aquifers are very easily contaminated because of the surficial features that commonly exist in karst terranes. Pollutant releases into sinkholes, sinking streams, and/or losing streams commonly result in concentrated solutes rapidly infiltrating and migrating through the subsurface to eventually discharge at downgradient springs unless intercepted by production wells, but slow percolation through soils also may result in serious contamination of karst aquifers. The unique features of karst terranes tend to cause significant problems in the interpretation of results obtained from water-quality grab samples of karst groundwater. To obtain more representative samples, event-driven sampling was proposed some decades ago, but event-driven sampling can be difficult and expensive to implement. In this paper, application of passive-sampling strategies is advocated as a means for effectively obtaining representative water-quality samples from karst aquifers. A passive-sampling methodology may be particularly useful for karst aquifers that may be found in complexly folded and faulted terranes. For example, a groundwater tracing investigation of a contaminated site in a karst terrane confirmed that several offsite springs and wells are connected to the contaminated site. Tracer recoveries suggested transport rates that were relatively slow for flow in a karstic aquifer (~0.02 m/s). Breakthrough curves were erratic and spiky. To obtain representative groundwater samples, a passive-sampling methodology is recommended.

Uptake and Release Kinetics of Organic Contaminants Associated with Micro- and Nanoplastic Particles

A generic theoretical framework is presented for describing the kinetics of uptake and release of organic compounds that associate with plastic particles. The underlying concepts account for the physicochemical features of the target organic compounds and the plastic particles. The developed framework builds on concepts established for dynamic speciation analysis by solid-phase microextraction and the size-dependent reactivity features of particulate complexants. The theoretical framework is applied to interpretation of literature data, thereby providing more rigorous insights into previous observations. The presented concepts enable predictions of the sink/source functioning of plastic particles and their impact on the dynamic chemical speciation of organic compounds in aqueous environmental media and within biota. Our results highlight the fundamental influence of particle size on the uptake and release kinetics. The findings call for a comprehensive description of the physicochemical features of plastic particles to be provided in experimental studies on micro- and nanoplastics in different types of aquatic environmental media.

Characterization of the accumulation of metals and organic contaminants on a novel active-passive sampling device under controlled water flow conditions

Recently, a new sampling device combining active and passive sampling (APS) was developed for the measurement of time-averaged concentrations of metal species and both polar and non-polar organic contaminants in water. By coupling a diffusion cell (loaded with a set of sorbents selective for different substances) with a small pump and a flow meter, the APS device is able to perform in situ measurements that are independent of the hydrodynamic conditions in the exposure medium. In the present study, the diffusion layer thickness (δ) at the sorbent/solution interface within the diffusion cell was characterised under controlled flow conditions. Laboratory tests indicated that, in the range of flow rates investigated, the average diffusion layer thickness (δ¯) varied from ∼60 to ∼110 μm, depending on the type of substance measured and the position of the sorbent with respect to the flow direction. Due to its ability to maintain an approximately constant δ¯, good to excellent agreement was found between measurements performed with the APS device in non-complexing media and concentrations measured in discrete water samples for all the substances investigated. These results suggest that the APS device could overcome issues affecting the quantitative interpretation of measurements by conventional passive sampling devices and serve as a useful tool for simultaneously monitoring a wide range of contaminants in water.

Headspace Passive Dosing of Volatile Hydrophobic Organic Chemicals from a Lipid Donor-Linking Their Toxicity to Well-Defined Exposure for an Improved Risk Assessment

High hydrophobicity and volatility of chemicals often lead to substantial experimental challenges but were here utilized in headspace passive dosing (HS-PD) to establish and maintain exposure: the pure chemical served as a passive dosing donor for controlling exposure at saturation, whereas triglyceride oil containing the chemical was used to control lower exposure levels. These donor solutions were added to glass inserts placed in the closed test systems. Mass balance calculations confirmed a dominant donor capacity for all chemicals except isooctane. This HS-PD method was applied to algal growth inhibition and springtail lethality tests with terpenes, alkanes, and cyclic siloxanes. Headspace concentrations above the lipid donors were measured for three chemicals to determine their chemical activity, using saturated vapor as the analytical standard and thermodynamic reference. Toxicity was related to chemical activity and calculated concentrations in membranes at equilibrium with the lipid donor. For both tests and all chemicals, toxic effects were observed within or above the reported range for baseline toxicity, meaning that no excess toxicity was observed. The toxicity of siloxanes was markedly higher to the terrestrial springtail than the aquatic algae, which is consistent with a more efficient mass transfer of these volatile hydrophobic chemicals in air compared to water.

Theory and modelling approaches to passive sampling

Designs and applications of passive samplers for various environmental compartments have been broadened significantly since their introduction. Understanding the theory behind passive sampling is essential for proper development of sampling methods and for accurate interpretation of the results. Theoretical underpinnings of passive sampling have been explored using different approaches. The aim of this review is to describe passive sampling theory and modelling approaches presented in the literature in a manner that allows researchers to obtain comprehensive understanding of them and to recognize the assumptions behind each approach together with their applicability to a given passive sampling technique. A common approach originates from Whitman's two-film theory and produces an exponential model that describes the entire passive sampling process. This approach, however, is based on several assumptions including linear exchange kinetics between the sampled medium and the passive sampler. Two-phase air passive samplers with a well-defined barrier are commonly modeled based on the zero-sink assumption, which assumes efficient trapping of analytes in the receiving phase. This assumption may become invalid under various scenarios; consequently, other approaches to modelling have been introduced including simulation of the sampling process by approximate temporal-steady states in hypothetical segments in the adsorption phase. Another approach uses dynamic models to determine accumulation of analytes in passive samplers. Dynamic models are capable of describing mass accumulation in the passive sampler, its transient response, and its response to fluctuations in environmental concentrations. Finally, empirically calibrated models, attempting to simplify the process of passive sampling rate determination, are also presented. In general, dynamic models are used to establish a profound understanding of the sampling process and analyse the applicability of the simpler models and their assumptions, while the simplified models are desirable and practical for most users. Nonetheless, due to the advancement in the computational tools, application of the dynamic models could be made simple and user-friendly.

A passive sampling model to predict PAHs in butter clams (Saxidomus giganteus), a traditional food source for Native American tribes of the Salish Sea Region

Native Americans face disproportionate exposures to environmental pollution through traditional subsistence practices including shellfish harvesting. In this study, the collection of butter clams (Saxidomus giganteus) was spatially and temporally paired with deployment of sediment pore water passive samplers at 20 locations in the Puget Sound region of the Salish Sea in the Pacific Northwest, USA, within adjudicated usual and accustomed tribal fishing grounds and stations. Clams and passive samplers were analyzed for 62 individual PAHs. A linear regression model was constructed to predict PAH concentrations in the edible fraction of butter clams from the freely dissolved fraction (C_free) in porewater. PAH concentrations can be predicted within a factor of 1.9 ± 0.2 on average from the freely dissolved PAH concentration in porewater using the following equation: PAHClam=4.1±0.1×PAHporewater This model offers a simplified, cost effective, and low impact approach to assess contaminant levels in butter clams which are an important traditional food.

Partitioning and Bioaccumulation of Legacy and Emerging Hydrophobic Organic Chemicals in Mangrove Ecosystems

Knowledge regarding partitioning behavior and bioaccumulation potential of environmental contaminants is important for ecological and human health risk assessment. While a range of models are available to describe bioaccumulation potential of hydrophobic organic chemicals (HOCs) in temperate aquatic food webs, their applicability to tropical systems still needs to be validated. The present study involved field investigations to assess the occurrence, partitioning, and bioaccumulation behavior of several legacy and emerging HOCs in mangrove ecosystems in Singapore. Concentrations of synthetic musk fragrance compounds, methyl triclosan (MTCS), polychlorinated biphenyls, organochlorine pesticides, and polycyclic aromatic hydrocarbons were measured in mangrove sediments, clams, and caged mussels. Freely dissolved concentrations of the HOCs in water were determined using silicone rubber passive samplers. Results showed that polycyclic musks and MTCS are present in mangrove ecosystems and can accumulate in the tissues of mollusks. The generated HOC concentration data for mangrove water, sediments, and biota samples was further utilized to evaluate water-sediment partitioning (e.g., K_oc values) and bioaccumulation behavior (e.g., BAF and BSAF values). Overall, the empirical models fit reasonably well with the data obtained for this ecosystem, supporting the concept that general models are applicable to predict the behavior of legacy and emerging HOCs in mangrove ecosystems.

Mechanistic Model Describing the Uptake of Chemicals by Aquatic Integrative Samplers: Comparison to Data and Implications for Improved Sampler Configurations

Aquatic integrative passive samplers are used to determine aqueous concentrations of polar organic pollutants, yet their uptake mechanisms are poorly understood. We introduce a one-dimensional model to simulate uptake by a passive sampler, Chemcatcher. The model considers the uptake as molecular diffusion through a series consisting of the aqueous boundary layer (ABL), the membrane filter (MF), and the sorbent disk with concurrent sorption by matrix of the MF and the disk. Uptake profiles of ∼20 polar chemicals measured over a week and a month were accurately modeled. Characteristic behaviors such as lag phases, linear and curved uptake, and equilibrating behavior were explained well by the model. As the model is mechanistically based, it was able to show the combined influences of the MF/water ( K_MF/w) and disk/water ( K_disk/w) partition coefficients, diffusion coefficients, and the ABL thickness on the sampling rates. On the basis of the model results, we offer three concrete recommendations for achieving the linear uptake needed for measuring time-weighted average concentrations: (i) use a MF that does not significantly sorb chemicals (e.g., log K_MF/w < 3) to avoid lag phases, (ii) use a sorbent with strong sorption properties (e.g., log K_disk/w > 6) for effective trapping of chemicals on the disk top layer, and (iii) make the ABL and/or the MF thicker so that the diffusion toward the disk slows.

An exponential model based new approach for correcting aqueous concentrations of hydrophobic organic chemicals measured by polyethylene passive samplers

Although low density polyethylene (PE) passive samplers show promise for the measurement of aqueous phase hydrophobic organic chemicals (HOCs), the lack of a practical and unsophisticated approach to account for non-equilibrium exposure conditions has impeded widespread acceptance and thus application in situ. The goal of this study was to develop a streamlined approach based on an exponential model and a convection mass transfer principle for correcting aqueous concentrations for HOCs deduced by PE samplers under non-equilibrium conditions. First, uptake rate constants (k₁), elimination rate constants (k₂), and seawater-PE equilibrium partition coefficients (K_PEWs) were determined in laboratory experiments for a diverse suite of HOCs with logK_ow range of 3.4-8.3. Linear relationships between log k₂ and logK_ow, and between log K_PEW and logK_ow were established. Second, PE samplers pre-loaded with ¹³C-labeled performance reference compounds (PRCs) were deployed in the ocean to determine their k₂in situ. By applying boundary layer and convection mass transfer theories, ratio (C) of k₂ values in field and laboratory exposures was estimated. This C value was demonstrated a constant that was only determined by water velocities and widths of PE strips. A generic equation with C and logK_ow as parameters was eventually established for extrapolation of non-equilibrium correction factors for the water boundary layer-controlled HOCs. Characterizing the hydrodynamic conditions indicated the sampler configuration and mooring mode should aim at sustaining laminar flow on the PE surface for optimal mass transfer. The PE estimates corrected using this novel approach possessed high accuracy and acceptable precision, and can be suited for a broad spectrum of HOCs. The presented method should facilitate routine utilization of the PE samplers.

Predicting Partitioning and Diffusion Properties of Nonpolar Chemicals in Biotic Media and Passive Sampler Phases by GC × GC

The chemical parameters needed to explain and predict bioavailability, biodynamics, and baseline toxicity are not readily available for most nonpolar chemicals detected in the environment. Here, we demonstrate that comprehensive two-dimensional gas chromatography (GC × GC) retention times can be used to predict 26 relevant properties for nonpolar chemicals, specifically: partition coefficients for diverse biotic media and passive sampler phases; aquatic baseline toxicity; and relevant diffusion coefficients. The considered biotic and passive sampler phases include membrane and storage lipids, serum and muscle proteins, carbohydrates, algae, mussels, polydimethylsiloxane, polyethylene, polyoxymethylene, polyacrylate, polyurethane, and semipermeable membrane devices. GC × GC-based chemical property predictions are validated with a compilation of 1038 experimental property data collected from the literature. As an example application, we overlay a map of baseline toxicity to fathead minnows onto the separated analyte signal of a polychlorinated alkanes (chlorinated paraffins) technical mixture that contains 7820 congeners. In a second application, GC × GC-estimated properties are used to parametrize multiphase partitioning models for mammalian tissues and organs. In a third example, we estimate chemical depuration kinetics for mussels. Finally, we illustrate an approach to screen the GC × GC chromatogram for nonpolar chemicals of potentially high concern, defined based on their GC × GC-estimated biopartitioning properties, diffusion properties, and baseline toxicity.

Chemodynamics of Soft Nanoparticulate Metal Complexes: From the Local Particle/Medium Interface to a Macroscopic Sensor Surface

The lability of a complex species between a metal ion M and a binding site S, MS, is conventionally defined with respect to an ongoing process at a reactive interface, for example, the conversion or accumulation of the free metal ion M by a sensor. In the case of soft charged multisite nanoparticulate complexes, the chemodynamic features that are operative within the micro environment of the particle body generally differ substantially from those for dissolved similar single-site complexes in the same medium. Here we develop a conceptual framework for the chemodynamics and the ensuing lability of soft (3D) nanoparticulate metal complexes. The approach considers the dynamic features of MS at the intraparticulate level and their impact on the overall reactivity of free metal ions at the surface of a macroscopic sensing interface. Chemodynamics at the intraparticulate level is shown to involve a local reaction layer at the particle/medium interface, while at the macroscopic sensor level an operational reaction layer is invoked. Under a certain window of conditions, volume exclusion of the nanoparticle body near the medium/sensor interface is substantial and affects the properties of the reaction layer and the overall lability of the nanoparticulate MS complex toward the reactive surface.

Enhanced adsorption of benzene vapor on granular activated carbon under humid conditions due to shifts in hydrophobicity and total micropore volume

A series of hydrophobic-modified (polydimethylsiloxane (PDMS) coating) activated carbons (ACs) were developed to answer a fundamental question: what are the determinants that dominate the adsorption on ACs under humid conditions? Using column experiments, an inter-comparison among bare-AC and PDMS-coated ACs was conducted regarding the association of surface characteristics and adsorption capacity. Primary outcomes occurred in two dominating markers, hydrophobicity and total micropore volume, which played a key role in water adsorption on ACs. However, their contributions to water adsorption on ACs substantially differed under different Pwater/Pair conditions. Hydrophobicity was the only contributor in Pwater/Pair=0.1-0.6, while the two markers contributed equally in Pwater/Pair=0.7-1.0. Furthermore, PDMS-coated AC had a significant increase in benzene adsorption capacities compared to bare-AC at 0-90% relative humidity, while these differences were not significant among PDMS-coated ACs. It is thus presumed that the balance between the two markers can be shifted to favor almost unchanged benzene adsorption capacities among PDMS-coated ACs over a large range of relative humidity. These findings suggest potential benefits of PDMS coating onto ACs in enhancing selective adsorption of hydrophobic volatile organic compounds under high humid conditions. To develop new porous materials with both high total micropore volume and hydrophobicity should thus be considered.

Internal Concentration and Time Are Important Modifiers of Toxicity: The Case of Chlorpyrifos on Caenorhabditis elegans

The internal concentration of chemicals in exposed organisms changes over time due to absorption, distribution, metabolism, and excretion processes since chemicals are taken up from the environment. Internal concentration and time are very important modifiers of toxicity when biomarkers are used to evaluate the potential hazards and risks of environmental pollutants. In this study, the responses of molecular biomarkers, and the fate of chemicals in the body, were comprehensively investigated to determine cause-and-effect relationships over time. Chlorpyrifos (CP) was selected as a model chemical, and Caenorhabditis elegans was exposed to CP for 4 h using the passive dosing method. Worms were then monitored in fresh medium during a 48-h recovery regime. The mRNA expression of genes related to CYP metabolism (cyp35a2 and cyp35a3) increased during the constant exposure phase. The body residue of CP decreased once it reached a peak level during the early stage of exposure, indicating that the initial uptake of CP rapidly induced biotransformation with the synthesis of new CYP metabolic proteins. The residual chlorpyrifos-oxon concentration, an acetylcholinesterase (AChE) inhibitor, continuously increased even after the recovery regime started. These delayed toxicokinetics seem to be important for the extension of AChE inhibition for up to 9 h after the start of the recovery regime. Comprehensive investigation into the molecular initiation events and changes in the internal concentrations of chemical species provide insight into response causality within the framework of an adverse outcome pathway.

Isotopic exchange on solid-phase micro extraction fiber in sediment under stagnant conditions: Implications for field application of performance reference compound calibration

An overlooked issue for field application of in situ performance reference compound (PRC) calibration methods is the validity of the assumption that both the sorption of a target compound and desorption of its corresponding PRC follow the first-order kinetics with the same rate constants under stagnant conditions. In the present study, disposable polydimethylsiloxane fibers of 2 sizes (7 and 35 µm) impregnated with 8 (13) C-labeled or deuterated PRCs were statically deployed into different marine sediments, from which the kinetics for sorption of the target compounds and desorption of the PRCs were characterized. Nonsymmetrical profiles were observed for exchange of the target analytes and their corresponding PRCs in sediment under stagnant conditions. The hysteretic desorption of PRCs in the kinetic regime may be ascribed to the low chemical potential between the fiber and sediment porewater, which reflects the inability of water molecules to rapidly diffuse through sediment to solvate the PRCs in the aqueous layer around the fiber surface. A moderate correlation (r = 0.77 and r = 0.57, p < 0.05 for both regressions) between the PRC-calibrated equilibrium concentrations of 1,1-dichloro-2,2-bis-(chlorophenyl) ethylene (p,p'-DDE) and polychlorinated biphenyl (PCB)-153 and the lipid normalized levels in worms (Neanthes arenaceodentata) was obtained in co-exposure tests under simulating field conditions, probably resulting from slightly overestimated bioavailability because of the hysteretic desorption of PRCs and toxic effects. Environ Toxicol Chem 2016;35:1978-1985. © 2015 SETAC.

Metabolism, bioaccumulation, and toxicity of pesticides in aquatic insect larvae

Aquatic insects having a high diversity are good biotic indicators for freshwater quality. Their larvae living in freshwater are sensitive to pesticides, and its impacts has been examined not only through laboratory toxicity studies using water and sediment exposure but also through higher-tier micro-/mesocosm studies and field monitoring. Many sophisticated statistical methods have been applied to assess the impacts of pesticides at levels from species to community, but their body burden has been studied much less, especially in relation to toxicity. We review the uptake, metabolism with relevant detoxifying enzymes, and depuration of pesticides in aquatic insect larvae, which determine their body burden and help to understand the toxicity profiles specific to each chemical class. We also discuss experimental conditions, environmental factors, and species sensitivity in relation to the bioconcentration/-accumulation and toxicity of pesticides.

Passive samplers accurately predict PAH levels in resident crayfish

Contamination of resident aquatic organisms is a major concern for environmental risk assessors. However, collecting organisms to estimate risk is often prohibitively time and resource-intensive. Passive sampling accurately estimates resident organism contamination, and it saves time and resources. This study used low density polyethylene (LDPE) passive water samplers to predict polycyclic aromatic hydrocarbon (PAH) levels in signal crayfish, Pacifastacus leniusculus. Resident crayfish were collected at 5 sites within and outside of the Portland Harbor Superfund Megasite (PHSM) in the Willamette River in Portland, Oregon. LDPE deployment was spatially and temporally paired with crayfish collection. Crayfish visceral and tail tissue, as well as water-deployed LDPE, were extracted and analyzed for 62 PAHs using GC-MS/MS. Freely-dissolved concentrations (Cfree) of PAHs in water were calculated from concentrations in LDPE. Carcinogenic risks were estimated for all crayfish tissues, using benzo[a]pyrene equivalent concentrations (BaPeq). ∑PAH were 5-20 times higher in viscera than in tails, and ∑BaPeq were 6-70 times higher in viscera than in tails. Eating only tail tissue of crayfish would therefore significantly reduce carcinogenic risk compared to also eating viscera. Additionally, PAH levels in crayfish were compared to levels in crayfish collected 10 years earlier. PAH levels in crayfish were higher upriver of the PHSM and unchanged within the PHSM after the 10-year period. Finally, a linear regression model predicted levels of 34 PAHs in crayfish viscera with an associated R-squared value of 0.52 (and a correlation coefficient of 0.72), using only the Cfree PAHs in water. On average, the model predicted PAH concentrations in crayfish tissue within a factor of 2.4 ± 1.8 of measured concentrations. This affirms that passive water sampling accurately estimates PAH contamination in crayfish. Furthermore, the strong predictive ability of this simple model suggests that it could be easily adapted to predict contamination in other shellfish of concern.

Modeling the transport of organic chemicals between polyethylene passive samplers and water in finite and infinite bath conditions

Understanding the transfer of chemicals between passive samplers and water is essential for their use as monitoring devices of organic contaminants in surface waters. By applying Fick's second law to diffusion through the polymer and an aqueous boundary layer, the authors derived a mathematical model for the uptake of chemicals into a passive sampler from water, in finite and infinite bath conditions. The finite bath model performed well when applied to laboratory observations of sorption into polyethylene (PE) sheets for various chemicals (polycyclic aromatic hydrocarbons, polychlorinated biphenyls [PCBs], and dichlorodiphenyltrichloroethane [DDT]) and at varying turbulence levels. The authors used the infinite bath model to infer fractional equilibration of PCB and DDT analytes in field-deployed PE, and the results were nearly identical to those obtained using the sampling rate model. However, further comparison of the model and the sampling rate model revealed that the exchange of chemicals was inconsistent with the sampling rate model for partially or fully membrane-controlled transfer, which would be expected in turbulent conditions or when targeting compounds with small polymer diffusivities and small partition coefficients (e.g., phenols, some pesticides, and others). The model can be applied to other polymers besides PE as well as other chemicals and in any transfer regime (membrane, mixed, or water boundary layer-controlled). Lastly, the authors illustrate practical applications of this model such as improving passive sampler design and understanding the kinetics of passive dosing experiments.

Guidance for improving comparability and relevance of oil toxicity tests

The complex nature and limited aqueous solubility of petroleum substances pose challenges for consistently characterizing exposures in aquatic life hazard assessments. This paper reviews important considerations for the design, conduct and interpretation of laboratory toxicity tests with physically and chemically dispersed oils based on an understanding of the behavior and toxicity of the hydrocarbons that comprise these substances. Guiding principles are provided that emphasize the critical need to understand and, when possible, characterize dissolved hydrocarbon exposures that dictate observed toxicity in these tests. These principles provide a consistent framework for interpreting toxicity studies performed using different substances and test methods by allowing varying dissolved exposures to be expressed in terms of a common metric based on toxic units (TUs). The use of passive sampling methods is also advocated since such analyses provide an analytical surrogate for TUs. The proposed guidance is translated into a series of questions that can be used in evaluating existing data and in guiding design of future studies. Application of these questions to a number of recent publications indicates such considerations are often ignored, thus perpetuating the difficulty of interpreting and comparing results between studies and limiting data use in objective hazard assessment. Greater attention to these principles will increase the comparability and utility of oil toxicity data in decision-making.

Biotic ligand model does not predict the bioavailability of rare Earth elements in the presence of organic ligands

Due to their distinct physicochemical properties, rare earth elements (REEs) are critical to high-tech and clean-energy industries; however, their bioavailability is still largely unexplored. In this paper, the bioavailability of several REEs has been carefully examined for the freshwater alga, Chlamydomonas reinhardtii. In the presence of organic ligands (L), the biouptake of REEs was much higher than that predicted by the biotic ligand model (BLM). Enhancement of the biouptake flux was observed for six ligands (metal = thulium) and six REEs (ligand = citric acid), indicating that this could be a common feature for these metals. In order to explore the mechanism for the enhanced uptake, Tm internalization was carefully evaluated. The Tm internalization flux (Jint) followed first-order (Michaelis-Menten) kinetics with a calculated maximum internalization flux (Jmax) of (1.1 ± 0.08) × 10(-14) mol · cm(-2) · s(-1) and an affinity constant for the reaction of the metal with the transport sites (KTm-R) of 10(7.1) M(-1). In the presence of citric acid, malic acid, or NTA, the Jint for Tm was more than 1 order of magnitude higher than that predicted by the BLM when algae were exposed to a constant 10(-9) M Tm(3+). The bioavailability of the metal complexes could not be explained by a piggyback internalization (through an anion channel) or the contribution of labile complexes. The enhanced biouptake was attributed to the formation of a ternary Tm complex {L-Tm-R} at the metal transport site. In the natural environment where organic ligands are ubiquitous, classic models are unlikely to predict the bioavailability of REEs to aquatic organisms.

Partitioning of humic acids between aqueous solution and hydrogel. 2. Impact of physicochemical conditions

The effects of the physicochemical features of aqueous medium on the mode of partitioning of humic acids (HAs) into a model biomimetic gel (alginate) and a synthetic polyacrylamide gel (PAAm) were explored. Experiments were performed under conditions of different pH and ionic strength as well as in the presence or absence of complexing divalent metal ions. The amount of HA penetrating the gel phase was determined by measuring its natural fluorescence by confocal laser scanning microscopy. In both gel types, the accumulation of HA was spatially heterogeneous, with a much higher concentration located within a thin film at the gel surface. The thickness of the surface film (ca. 15 μm) was similar for both types of gel and practically independent of pH, ionic strength, and the presence of complexing divalent metal ions. The extent of HA accumulation was found to be dependent on the composition of the medium and on the type of gel. Significantly more HA was accumulated in PAAm gel as compared to that in alginate gel. In general, more HA was accumulated at lower background salt concentration levels. The distribution of different types of HA species in the gel body was linked to their behavior in the medium and the differences in physicochemical conditions inside the two phases.

Use of passive samplers for improving oil toxicity and spill effects assessment

Methods that quantify dissolved hydrocarbons are needed to link oil exposures to toxicity. Solid phase microextraction (SPME) fibers can serve this purpose. If fibers are equilibrated with oiled water, dissolved hydrocarbons partition to and are concentrated on the fiber. The absorbed concentration (Cpolymer) can be quantified by thermal desorption using GC/FID. Further, given that the site of toxic action is hypothesized as biota lipid and partitioning of hydrocarbons to lipid and fibers is well correlated, Cpolymer is hypothesized to be a surrogate for toxicity prediction. To test this method, toxicity data for physically and chemically dispersed oils were generated for shrimp, Americamysis bahia, and compared to test exposures characterized by Cpolymer. Results indicated that Cpolymer reliably predicted toxicity across oils and dispersions. To illustrate field application, SPME results are reported for oil spills at the Ohmsett facility. SPME fibers provide a practical tool to improve characterization of oil exposures and predict effects in future lab and field studies.

Passive sampling methods for contaminated sediments: state of the science for organic contaminants

This manuscript surveys the literature on passive sampler methods (PSMs) used in contaminated sediments to assess the chemical activity of organic contaminants. The chemical activity in turn dictates the reactivity and bioavailability of contaminants in sediment. Approaches to measure specific binding of compounds to sediment components, for example, amorphous carbon or specific types of reduced carbon, and the associated partition coefficients are difficult to determine, particularly for native sediment. Thus, the development of PSMs that represent the chemical activity of complex compound-sediment interactions, expressed as the freely dissolved contaminant concentration in porewater (Cfree ), offer a better proxy for endpoints of concern, such as reactivity, bioaccumulation, and toxicity. Passive sampling methods have estimated Cfree using both kinetic and equilibrium operating modes and used various polymers as the sorbing phase, for example, polydimethylsiloxane, polyethylene, and polyoxymethylene in various configurations, such as sheets, coated fibers, or vials containing thin films. These PSMs have been applied in laboratory exposures and field deployments covering a variety of spatial and temporal scales. A wide range of calibration conditions exist in the literature to estimate Cfree , but consensus values have not been established. The most critical criteria are the partition coefficient between water and the polymer phase and the equilibrium status of the sampler. In addition, the PSM must not appreciably deplete Cfree in the porewater. Some of the future challenges include establishing a standard approach for PSM measurements, correcting for nonequilibrium conditions, establishing guidance for selection and implementation of PSMs, and translating and applying data collected by PSMs.

Velocity dependent passive sampling for monitoring of micropollutants in dynamic stormwater discharges

Micropollutant monitoring in stormwater discharges is challenging because of the diversity of sources and thus large number of pollutants found in stormwater. This is further complicated by the dynamics in runoff flows and the large number of discharge points. Most passive samplers are nonideal for sampling such systems because they sample in a time-integrative manner. This paper reports test of a flow-through passive sampler, deployed in stormwater runoff at the outlet of a residential-industrial catchment. Momentum from the water velocity during runoff events created flow through the sampler resulting in velocity dependent sampling. This approach enables the integrative sampling of stormwater runoff during periods of weeks to months while weighting actual runoff events higher than no flow periods. Results were comparable to results from volume-proportional samples and results obtained from using a dynamic stormwater quality model (DSQM). The paper illustrates how velocity-dependent flow-through passive sampling may revolutionize the way stormwater discharges are monitored. It also opens the possibility to monitor a larger range of discharge sites over longer time periods instead of focusing on single sites and single events, and it shows how this may be combined with DSQMs to interpret results and estimate loads over extended time periods.

Polyoxymethylene passive samplers to monitor changes in bioavailability and flux of PCBs after activated carbon amendment to sediment in the field

Field and laboratory exposures of polyoxymethylene passive samplers to sediments and the water column were applied to monitor changes in bioavailability and flux of polychlorinated biphenyls (PCBs) following a pilot-scale amendment of activated carbon in Grasse River. Following amendment, reductions in passive sampler uptake tracked reductions in bioaccumulation in a freshwater invertebrate, which supports a biological basis for utilizing passive samplers for in situ site investigations following a remediation. Freely dissolved concentrations of PCBs were reduced in sediment pore waters compared to untreated sediments indicating reduced bioavailability of PCBs after activated carbon amendment. Freely dissolved PCB concentrations in sediment pore water in treated sites were also lower than overlying water concentrations indicating a reversal of the sediment from being a source to a sink of PCBs from the water column. These observations indicate that activated carbon amendment to sediment limits contaminant exposure to both the benthic and pelagic food webs through reductions in bioavailability and flux of PCBs into the water column.

Monitoring PAH contamination in water: comparison of biological and physico-chemical tools

The suitability of biological methods and chemical-based passive samplers to determine exposure to PAHs was tested by deploying zebra mussels and SPMDs along the Seine River over 11 months. The concentration of 13 PAHs was analyzed every month in both water and mussels. The sum of the PAH concentrations in mussels, initially at 299 ng gdry wt(-1), reached 2654, 3972 and 3727 ng g(-1) at the end of exposure in the three sampling points taken through the river. The respective SPMD-available concentrations of TPAHs reached 9, 52 and 34 ng L(-1). Results showed seasonal variations of total PAH concentrations in the mussels, characterized by a decrease during spawning. The non-achievement of steady state concentration that was observed in mussels may be accounted for by the temporal variation of environmental concentrations. Thus, a bioaccumulation model based on kinetic rather than simple equilibrium partitioning was found to be more appropriate to describe PAH content in mussels. Moreover, biodynamic kinetic modeling proved useful to better understand the uptake and loss processes of pyrene. It clearly shows that these processes are markedly influenced by the biological state of the zebra mussels. The most realistic hypothesis is that the temporal variation of the biodynamic parameters may originate from a decrease of the mussels' metabolization of PAHs during spawning. Since SPMD passive samplers cannot integrate such biological factors, they are poor predictors of PAH bioavailability in mussels.

Speciation analysis of aqueous nanoparticulate diclofenac complexes by solid-phase microextraction

The dynamic sorption of an organic compound by nanoparticles (NPs) is analyzed by solid-phase microextraction (SPME) for the example case of the pharmaceutical diclofenac in dispersions of impermeable (silica, SiO(2)) and permeable (bovine serum albumin, BSA) NPs. It is shown that only the protonated neutral form of diclofenac is accumulated in the solid phase, and hence this species governs the eventual partition equilibrium. On the other hand, the rate of the solid/water partition equilibration is enhanced in the presence of the sorbing nanoparticles of SiO(2) and BSA. This feature demonstrates that the NPs themselves do not enter the solid phase to any appreciable extent. The enhanced rate of attainment of equilibrium is due to a shuttle-type of contribution from the NP-species to the diffusive supply of diclofenac to the water/solid interface. For both types of nanoparticulate complexes, the rate constant for desorption (k(des)) of bound diclofenac was derived from the measured thermodynamic affinity constant and a diffusion-limited rate of adsorption. The computed k(des) values were found to be sufficiently high to render the NP-bound species labile on the effective time scale of SPME. In agreement with theoretical prediction, the experimental results are quantitatively described by fully labile behavior of the diclofenac/nanoparticle system and an ensuing accumulation rate controlled by the coupled diffusion of neutral, deprotonated, and NP-bound diclofenac species.

Development of a new time-integrative sampler using in situ solvent extraction

Despite the great success of time-weighted average passive sampling of hydrophobic contaminants, such as PCBs and PAHs, the sampling of polar organic compounds still presents a challenge because the equilibrium between water and most sampling phases is attained in a relatively short time. In this study, we proposed a new time-integrative sampler using in situ solvent extraction (TISIS) for polar organic chemicals. The sampler was composed of a 15 cm poly(dimethylsiloxane) (PDMS) tubing, with an internal diameter of 0.5 mm and wall thickness of 0.5 mm, through which an extraction solvent (acetonitrile) was passed. Four polar organic contaminants, caffeine, atrazine, diuron and 17α-ethynylestradiol, were chosen for the evaluation of the performance of the sampler. Without the use of in situ solvent extraction, the PDMS tubing when exposed to a constant aqueous concentration of the four model compounds was able to linearly accumulate those compounds for less than 12 h and equilibrium between the PDMS tubing and water was attained in 2 d under our laboratory conditions. However, TISIS when exposed to a constant aqueous concentration was able to linearly accumulate all the model compounds without any exposure time limitation. The measured sampling rates at three different extraction flow rates (0.2, 0.5, 1.5 mL min(-1)) were similar, regardless of the chemicals, indicating that the overall mass transfer from aqueous solution to the extraction solvent was most likely dominated by partitioning to the PDMS tubing and the internal diffusion within PDMS. In addition, a pulsed exposure experiment confirmed that TISIS operated in a time-integrative mode when the environmental concentration was highly fluctuated.

Dissolved organic carbon enhances the mass transfer of hydrophobic organic compounds from nonaqueous phase liquids (NAPLs) into the aqueous phase

The hypothesis that dissolved organic carbon (DOC) enhances the mass transfer of hydrophobic organic compounds from nonaqueous phase liquids (NAPLs) into the aqueous phase above that attributable to dissolved molecular diffusion alone was tested. In controlled experiments, mass transfer rates of five NAPL-phase PAHs (log K(OW) 4.15-5.39) into the aqueous phase containing different concentrations of DOC were measured. Mass transfer rates were increased by up to a factor of 4 in the presence of DOC, with the greatest enhancement being observed for more hydrophobic compounds and highest DOC concentrations. These increases could not be explained by dissolved molecular diffusion alone, and point to a parallel DOC-mediated diffusive pathway. The nature of the DOC-mediated diffusion pathway as a function of the DOC concentration and PAH sorption behavior to the DOC was investigated using diffusion-based models. The DOC-enhanced mass transfer of NAPL-phase hydrophobic compounds into the aqueous phase has important implications for their bioremediation as well as bioconcentration and toxicity.

Metal speciation and toxicity of Tamar Estuary water to larvae of the Pacific oyster, Crassostrea gigas

As part of the PREDICT Tamar Workshop, the toxicity of estuarine waters in the Tamar Estuary (southwest England) was assessed by integration of metal speciation determination with bioassays. High temporal resolution metal speciation analysis was undertaken in situ by deployment of a Voltammetric In situ Profiling (VIP) system. The VIP detects Cd (cadmium), Pb (lead) and Cu (copper) species smaller than 4 nm in size and this fraction is termed 'dynamic' and considered biologically available. Cadmium was mainly present in the dynamic form and constituted between 56% and 100% of the total dissolved concentration, which was determined subsequently in the laboratory in filtered discrete samples. In contrast, the dynamic Pb and Cu fractions were less important, with a much larger proportion of these metals associated with organic ligands and/or colloids (45-90% Pb and 46-85% Cu), which probably reduced the toxicological impact of these elements in this system. Static toxicity tests, based on the response of Crassostrea gigas larva exposed to discrete water samples showed a high level of toxicity (up to 100% abnormal development) at two stations in the Tamar, particularly during periods of the tidal cycle when the influence of more pristine coastal water was at its lowest. Competitive ligand-exchange Cu titrations showed that natural organic ligands reduced the free cupric ion concentration to levels that were unlikely to have been the sole cause of the observed toxicity. Nonetheless, it is probable that the combined effect of the metals determined in this work contributed significantly to the bioassay response.

Quantitative evaluation of laboratory uptake rates for pesticides, pharmaceuticals, and steroid hormones using POCIS

Polar organic chemical integrative samplers (POCIS) are useful in monitoring for a wide range of chemicals in aquatic systems; however, a lack of available uptake rate data for compounds of environmental interest is one limitation in the application of these samplers to environmental studies. In this study, laboratory calibration experiments were conducted with POCIS for 65 compounds at 25°C under flowing conditions to determine chemical-specific uptake rates (R(s)). Experimental uptake rates measured in this study ranged from 0.034 to 1.33 L/d, and uptake rates were determined for 36 compounds with no previously reported values. Experimentally determined uptake rates were applied to data obtained from POCIS samplers deployed downstream of three wastewater treatment plant (WWTP) effluent discharges and in four surface waters influenced by agricultural runoff. Time-weighted average concentrations for atrazine and metolachlor determined using uptake rates generated in this study compare well with results from composited grab sampling previously conducted in agricultural watersheds in Nebraska, USA.

Chemical techniques for assessing bioavailability of sediment-associated contaminants: SPME versus Tenax extraction

The traditional approach for predicting the risk of hydrophobic organic contaminants (HOCs) in sediment is to relate organic carbon normalized sediment concentrations to body residues or toxic effects to organisms. However, due to the multiple variables controlling bioavailability, this method has limitations. A matrix independent method of predicting bioavailability needs to be used in order to be universally applicable. Both chemical activity (freely dissolved chemical concentrations) measured by solid-phase microextraction (SPME) and bioaccessibility (rapidly desorbing fraction) estimated by Tenax extraction have been developed to predict bioavailability of sediment-associated HOCs. The objectives of this review are to summarize a number of studies using matrix-SPME or Tenax extraction to estimate bioavailability and/or toxicity of different classes of HOCs and evaluate the strengths and weakness of these two techniques. Although the two chemical techniques assess different components of the matrix, estimates obtained from both techniques have been correlated to organism body residues. The advantages of SPME fibers are their applicability for use in situ and their potential usage for a wide array of contaminants by selection of appropriate coatings. Single time-point Tenax extraction, however, is more time- and labor-effective. Tenax extraction also has lower detection limits, making it more applicable for highly toxic contaminants. This review also calls for additional research to evaluate the role of sequestrated contaminants and ingestion of sediment particles by organisms on HOC bioavailability. The use of performance reference compounds to reduce SPME sampling time and linking chemical based bioavailability estimates to toxicological endpoints are essential to expand the applications of these methods.

Bioremediation of marine sediments contaminated by hydrocarbons: experimental analysis and kinetic modeling

This work deals with bioremediation experiments on harbor sediments contaminated by aliphatic and polycyclic aromatic hydrocarbons (PAHs), investigating the effects of a continuous supply of inorganic nutrients and sand amendments on the kinetics of microbial growth and hydrocarbon degradation. Inorganic nutrients stimulated microbial growth and enhanced the biodegradation of low and high molecular weight hydrocarbons, whereas sand amendment increased only the removal of high molecular weight compounds. The simultaneous addition of inorganic nutrients and sand provided the highest biodegradation (>70% for aliphatic hydrocarbons and 40% for PAHs). A semi-empirical kinetic model was successfully fitted to experimental temporal changes of hydrocarbon residual concentrations and microbial abundances. The estimated values for parameters allowed to calculate a doubling time of 2.9 d and a yield coefficient biomass/hydrocarbons 0.39 g C biomass g-1C hydrocarbons, for the treatment with the highest hydrocarbon biodegradation yield. A comparison between the organic carbon demand and temporal profiles of hydrocarbons residual concentration allowed also to calculate the relative contribution of contaminants to carbon supply, in the range 5-32%. This suggests that C availability in the sediments, influencing prokaryotic metabolism, may have cascade effects on biodegradation rates of hydrocarbons. Even if these findings do not represent a general rule and site-specific studies are needed, the approach used here can be a relevant support tool when designing bioremediation strategies on site.

Field measurement of diffusional mass transfer of HOCs at the sediment-water interface

The sediment to water diffusive flux of PAHs and PCBs was measured under field conditions with a novel infinite-sink benthic flux chamber that deployed semipermeable membrane devices (SPMD) as a sorbing material. Fluxes were measured before and after in situ capping of sediments in Oslo Harbour with clean clay. The fluxes of native pyrene and PCB 52 from uncapped contaminated sediment measured with the flux chamber were 0.3-1.6 microg m(-2) d(-1) and 2-8 ng m(-2) d(-1), respectively. Fluxes from the capped sediment were reduced by 93-97%. The in situ measured fluxes were compared to fluxes independently calculated from freely dissolved concentrations in pore water and overlying water, measured using equilibrium passive samplers, diffusive boundary layer (DBL) thickness, measured by an alabaster dissolution method and literature values of diffusion coefficients. Measured fluxes from the uncapped sediment agreed well with calculated fluxes, the median of the ratio of the measured flux over the calculated flux was 0.9 with an inter quartile range of 0.5-1.6.

Controlling and maintaining exposure of hydrophobic organic compounds in aquatic toxicity tests by passive dosing

The risk assessment of hydrophobic organic compounds (HOCs) in aquatic toxicity or bioconcentration tests is a challenge due to their low aqueous solubilities, sorption and losses leading to poorly defined exposure and reduced test sensitivity. Passive dosing overcomes these problems via the continual partitioning of HOCs from a dominating reservoir loaded in a biocompatible polymer such as silicone, providing defined and constant freely dissolved concentrations and eliminating spiking with co-solvents. This study characterised the performance of a passive dosing format for aquatic tests with small organism such as invertebrates and algae, consisting of PDMS silicone cast into the base of the glass test vessel. The PDMS silicone was loaded by partitioning from a methanol solution containing PAHs (logK(OW) 3.56-6.63) as model compounds, followed by removal of the methanol with water. This resulted in highly reproducible PDMS silicone HOC concentrations. When shaking, release of PAHs into aqueous solution was rapid and reproducible, and equilibrium partitioning was reached within 5h for all compounds. The buffering capacity was sufficient to maintain stable concentrations over more than 10 weeks. This format was applied in a 48h Daphnia magna immobilisation assay to test the toxicity of a range of PAHs at their aqueous solubility. D. magna immobilisation did not show a trend with aqueous solubility or hydophobicity (K(OW)) of the PAHs. However, the immobilisation data for all compounds could be fitted with one maximum chemical activity response curve. Those PAHs with the lowest maximum chemical activities resulted in no immobilisation. Naphthalene and phenanthrene showed full toxicity at aqueous solubility, and passive dosing was also used for the concentration-response testing of these compounds. The freely dissolved aqueous concentrations causing 50% immobilisation (EC-50) were 1.96 mg L(-1) for naphthalene and 0.48 mg L(-1) for phenanthrene. Therefore, passive dosing is a practical and economical means of improving the exposure of HOCs in aquatic toxicity or bioconcentration tests.