Joy A. McGrath

Predicting sediment metal toxicity using a sediment biotic ligand model: methodology and initial application

An extension of the simultaneously extracted metals/acid-volatile sulfide (SEM/AVS) procedure is presented that predicts the acute and chronic sediment metals effects concentrations. A biotic ligand model (BLM) and a pore water-sediment partitioning model are used to predict the sediment concentration that is in equilibrium with the biotic ligand effects concentration. This initial application considers only partitioning to sediment particulate organic carbon. This procedure bypasses the need to compute the details of the pore-water chemistry. Remarkably, the median lethal concentration on a sediment organic carbon (OC)-normalized basis, SEM*(x,OC), is essentially unchanged over a wide range of concentrations of pore-water hardness, salinity, dissolved organic carbon, and any other complexing or competing ligands. Only the pore-water pH is important. Both acute and chronic exposures in fresh- and saltwater sediments are compared to predictions for cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) based on the Daphnia magna BLM. The SEM*(x,OC) concentrations are similar for all the metals except cadmium. For pH = 8, the approximate values (micromol/gOC) are Cd-SEM*(xOC) approximately equal to 100, Cu-SEM*(x,OC) approximately equal to 900, Ni-SEMoc approximately equal to 1,100, Zn-SEM*(x,OC) approximately equal to 1,400, and Pb-SEM*(x,OC) approximately equal to 2,700. This similarity is the explanation for an empirically observed dose-response relationship between SEM and acute and chronic effects concentrations that had been observed previously. This initial application clearly demonstrates that BLMs can be used to predict toxic sediment concentrations without modeling the pore-water chemistry.

Predicting the toxicity of neat and weathered crude oil: toxic potential and the toxicity of saturated mixtures.

The toxicity of oils can be understood using the concept of toxic potential, or the toxicity of each individual component of the oil at the water solubility of that component. Using the target lipid model to describe the toxicity and the observed relationship of the solubility of oil components to log (Kow), it is demonstrated that components with lower log (Kow) have greater toxic potential than those with higher log (Kow). Weathering removes the lower-log (Kow) chemicals with greater toxic potential, leaving the higher-log (Kow) chemicals with lower toxic potential. The replacement of more toxically potent compounds with less toxically potent compounds lowers the toxicity of the aqueous phase in equilibrium with the oil. Observations confirm that weathering lowers the toxicity of oil. The idea that weathering increases toxicity is based on the erroneous use of the total petroleum hydrocarbons or the total polycyclic aromatic hydrocarbons (PAHs) concentration as if either were a single chemical that can be used to gauge the toxicity of a mixture, regardless of its makeup. The toxicity of the individual PAHs that comprise the mixture varies. Converting the concentrations to toxic units (TUs) normalizes the differences in toxicity. A concentration of one TU resulting from the PAHs in the mixture implies toxicity regardless of the specific PAHs that are present. However, it is impossible to judge whether 1 microg/L of total PAHs is toxic without knowing the PAHs in the mixture. The use of toxic potential and TUs eliminates this confusion, puts the chemicals on the same footing, and allows an intuitive understanding of the effects of weathering.

Validation of the target lipid model for toxicity assessment of residual petroleum constituents: Monocyclic and polycyclic aromatic hydrocarbons

A method is presented for developing scientifically defensible, numeric guidelines for residual petroleum-related constituents, specifically monocyclic aromatic hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs), in the water column. The guidelines are equivalent to a HC5 (i.e., hazard concentration to 5% of the tested species, or the concentration that protects 95% of the tested species). The model of toxicity used in this evaluation is the target lipid model (TLM) that was developed for assessing the toxicity of type I narcotic chemicals. An acute to chronic ratio is used for chronic expression and sublethal effects. The TLM is evaluated by comparing predicted and observed toxicity of these petroleum components. The methodology is capable of predicting both the acute and chronic toxicity of MAHs and PAHs in single exposures and in mixtures. For acute exposures, the TLM was able to predict the toxicity to within a factor of three to five. The use of toxic units was an effective metric for expressing the toxicity of mixtures. Within the uncertainty bounds, the TLM correctly predicted where sublethal effects of edemas, hemorrhaging, and other abnormalities were observed to occur in early life-stage exposure to PAHs. The computed HC5s were lower than no-observed-effect concentrations based on growth, reproduction, and mortality endpoints and sublethal effects. The methodology presented can be used by the oil spill community to compare residual concentrations of PAHs against defensible, numeric guidelines to assess potential ecological impacts.

Validation of the narcosis target lipid model for petroleum products: Gasoline as a case study

The narcosis target lipid model (NTLM) was used to predict the toxicity of water-accommodated fractions (WAFs) of six gasoline blending streams to algae (Pseudokirchnereilla subcapitata, formerly Selenastrum capricornutum), juvenile rainbow trout (Oncorhynchus mykiss), and water flea (Daphnia magna). Gasolines are comprised of hydrocarbons that on dissolution into the aqueous phase are expected to act via narcosis. Aquatic toxicity data were obtained using a lethal-loading test in which WAFs were prepared using different gasoline loadings. The compositions of the gasolines were determined by analysis of C3 to C13 hydrocarbons grouped in classes of n-alkanes, iso-alkanes, aromatics, cyclic alkanes, and olefins. A model was developed to compute the concentrations of hydrocarbon blocks in WAFs based on gasoline composition and loading. The model accounts for the volume change of the gasoline, which varies depending on loading and volatilization loss. The predicted aqueous composition of WAFs compared favorably to measurements, and the predicted aqueous concentrations of WAFs were used in the NTLM to predict the aquatic toxicity of the gasolines. For each gasoline loading and species, total toxic units (TUs) were computed with an assumption of additivity. The acute toxicity of gasolines was predicted to within a factor of two for algae and daphnids. Predicted TUs overestimated toxicity to trout because of experimental factors that were not considered in the model. This analysis demonstrates the importance of aliphatic hydrocarbon loss to headspace during WAF preparation and the contribution of both aromatic and aliphatic hydrocarbons test to the toxicity of gasolines in closed systems and loss of aliphatics to headspace during WAF preparation. Model calculations indicate that satisfactory toxicity predictions can be achieved by describing gasoline composition using a limited number of aromatic and aliphatic hydrocarbon blocks with different octanol-water partition coefficients.

PETROTOX: An aquatic toxicity model for petroleum substances

A spreadsheet model (PETROTOX) is described that predicts the aquatic toxicity of complex petroleum substances from petroleum substance composition. Substance composition is characterized by specifying mass fractions in constituent hydrocarbon blocks (HBs) based on available analytical information. The HBs are defined by their mass fractions within a defined carbon number range or boiling point interval. Physicochemical properties of the HBs are approximated by assigning representative hydrocarbons from a database of individual hydrocarbons with associated physicochemical properties. A three-phase fate model is used to simulate the distribution of each structure among the water-, air-, and oil-phase liquid in the laboratory test system. Toxicity is then computed based on the predicted aqueous concentrations and aquatic toxicity of each structure and the target lipid model. The toxicity of the complex substance is computed assuming additivity of the contribution of the individual assigned hydrocarbons. Model performance was evaluated by using direct comparisons with measured toxicity data for petroleum substances with sufficient analytical characterization to run the model. Indirect evaluations were made by comparing predicted toxicity distributions using analytical data on petroleum substances from different product categories with independent, empirical distributions of toxicity data available for the same categories. Predictions compared favorably with measured aquatic toxicity data across different petroleum substance categories. These findings demonstrate the utility of PETROTOX for assessing environmental hazards of petroleum substances given knowledge of substance composition.

Application of the narcosis target lipid model to algal toxicity and deriving predicted-no-effect concentrations

The narcosis target lipid model (TLM) was developed to predict the toxicity of chemicals to aquatic organisms that act via narcosis. It is based on the hypothesis that target lipid is the site of toxic action within the organism, that octanol is the appropriate surrogate, and that target lipid has the same physical-chemical properties in all organisms. Here the TLM is extended to available freshwater green algal toxicity data to support a narcosis toxic mode-of-action (TMoA) effect assessment. For each species, significant linear relationships were observed between log(median effective concentration [EC50]) and log(Kow) of the test chemicals. The slope of the log-log relationship statistically was similar to the universal narcosis slope of -0.945 that was derived from an earlier analysis of the TLM to nonalgal species. Critical target lipid body burdens (CTLBB), C(L)* were estimated for each algal species from the intercepts of the regressions and found to be within the range (43-398 micromol/g octanol) reported previously, indicating that algae exhibit a similar sensitivity distribution relative to other aquatic species. The TLM is used to derive the predicted-no-effect concentrations (PNECs) using the hazardous concentration to 5% species (HC5) statistical extrapolation procedure. This calculation requires an analysis of the variability of the universal slope, the C(L)*, and the acute-to-chronic ratio. The PNECs derived using this procedure were consistent with chronic-no-effect concentrations reported for narcotic chemicals. This is in contrast to PNECs derived from limited chemical-specific toxicity data and default application factors. It is concluded that coupling the TLM to the HC5 extrapolation procedure allows optimal use of available toxicity data for deriving environmental quality criteria with a narcotic TMoA.

Modeling polycyclic aromatic hydrocarbon bioaccumulation and metabolism in time-variable early life-stage exposures.

Recent laboratory investigations into the bioaccumulation and toxicity of polycyclic aromatic hydrocarbons (PAH) have focused on low-level, time-variable exposures to early life-stage fish. Polycyclic aromatic hydrocarbon body-burden residues reported in these studies were lower than critical body-burden residues predicted by the target lipid model (TLM). To understand this discrepancy, a time-variable uptake and depuration model of PAH bioaccumulation was developed. Kinetic constants were fit using measured exposure and tissue concentrations. The resulting lipid-water partition coefficients (K(LW)) were uncorrelated with the octanol-water partition coefficient (K(OW))--a qualitatively unrealistic finding considering that numerous studies have reported a positive correlation between the two. Because PAHs are known to be metabolized, the comparison of K(LW) with K(OW) suggests that metabolism may be occurring in early life-stage fish. Therefore, the uptake and depuration model was modified to include metabolism while assuming linearity of K(LW) with K(OW). Calculated metabolism rates were positively correlated with K(OW)--a finding qualitatively similar to those of other studies. The present study provides a reasonable explanation for the discrepancy between the TLM predictions and the measured toxic effect levels. Given the time-variable exposure concentrations, the maximum measured body burdens used to relate to toxic effects may be underestimated. In addition, the maximum body burden of parent PAH plus metabolites may be a better measure in relating tissue concentrations to toxic effects. Incorporating these refinements in relating body burdens to toxic effects may result in a better comparison between TLM predictions and measured effect levels.

Tissue‐based risk assessment of cyclic volatile methyl siloxanes

Cyclic volatile methyl siloxanes (cVMS) are important consumer materials that are used in personal care products and industrial applications. These compounds have gained increased attention in recent years following the implementation of chemical legislation programs worldwide. Industry-wide research programs are being conducted to characterize the persistence, bioaccumulation, and toxicity (PBT) properties of cVMS materials. As part of this larger effort, a tissue-based risk assessment was performed to further inform the regulatory decision-making process. Measured tissue concentrations of cVMS compounds in fish and benthic invertebrates are compared with critical target lipid body burdens (CTLBBs) as estimated with the target lipid model (TLM) to evaluate risk. Acute and chronic toxicity data for cVMS compounds are compared with data for nonpolar organic chemicals to validate application of the TLM in this effort. The analysis was extended to estimate the contribution from metabolites to the overall cVMS-derived tissue residues using a food chain model calibrated to laboratory and field data. Concentrations of cVMS materials in biota from several trophic levels (e.g., invertebrates, fish) are well below the estimated CTLBBs associated with acute and chronic effects. This analysis, when combined with the limited biomagnification potential for cVMS compounds that was observed in the field, suggests that there is little risk of adverse effects from cVMS materials under present-day emission levels.

EXTENSION AND VALIDATION OF THE TARGET LIPID MODEL FOR DERIVING PREDICTED NO-EFFECT CONCENTRATIONS FOR SOILS AND SEDIMENTS

Substance risk assessments require estimation of predicted no-effect concentrations (PNECs) in soil and sediment. The present study applies the target lipid model (TLM) and equilibrium partitioning (EqP) model to toxicity data to evaluate the extrapolation of the TLM-derived aquatic PNECs to these compartments. This extrapolation assumes that the sensitivity of aquatic species is similar to that of terrestrial and benthic species. The acute species sensitivity distribution, expressed in terms of species-specific critical target lipid body burdens, was computed using the TLM-EqP framework and found to span a similar range as the aquatic organism species sensitivity distribution but with a slightly lower median value (less than 2 times). The species sensitivity distribution for acute-to-chronic ratios also exhibited a similar range and distribution across species, suggesting similar mechanisms of action. This hypothesis was further tested by comparing empirical soil/sediment chronic effect levels to the calculated PNEC derived using TLM-EqP. The results showed that 95% of the compiled chronic effects data fell above the PNEC, confirming an adequate protection level. These findings support the conclusion that TLM-derived aquatic PNECs can be successfully extrapolated to derive credible PNECs for soil and sediment compartments.

Application of the target lipid model for deriving predicted no‐effect concentrations for wastewater organisms

The target lipid model (TLM) was applied to literature data from 10 microbial toxicity assays to provide a quantitative effects assessment framework for wastewater treatment plant organisms. For the nonpolar organic chemicals considered, linear relationships between the logarithm of the median effect concentrations (EC50) and log(K(OW)) conformed to the TLM for all endpoints with the exception of nitrification inhibition. Additional experimental data for the nitrification inhibition endpoint were generated for 16 narcotic chemicals using a procedure that allowed testing of volatile substances. Results obtained from the present study demonstrated that the nitrification inhibition endpoint was not adequately described by the TLM consistent with previous literature data. Acute to chronic ratios (ACRs) defined as the ratio of the EC50 to the 10% effect concentration (EC10) were available for two of the endpoints investigated and ranged from 1.1 to 2.3 for the Tetrahymena growth assay and from 2.4 to 24.1 for the nitrification inhibition endpoint. No inhibitory effects for any of the microbial endpoints investigated were observed for compounds with log(K(OW)) >5. The critical target lipid body burdens (C(L)(*)) were calculated for the nine microbial toxicity endpoints conforming to the TLM and ranged from 252 to 2,250 micromol/g octanol. The Microtox light inhibition (C(L)(*) = 252 micromol/g octanol) and Tetrahymena pyriformis growth (C(L)(*) = 254 micromol/g octanol) assays were found to be the most sensitive endpoints. The predicted no-effect concentration (PNEC) derived using the HC5 (hazardous concentration to 5% of test organisms) statistical extrapolation procedure was calculated using TLM parameters for substances with log(K(OW)) from 0 to 5. Results from this analysis demonstrate PNECs for narcotic compounds are protective of wastewater organisms excluding nitrifying bacteria. Further model improvement is needed if protection of nitrifying bacteria in wastewater treatment systems is required.