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.

Environmental assessment of 5-ethylidene-2-norbornene using modeled and measured fate and effects results.

Several predictive models were used to assess aquatic exposure, persistence (P) and potential for long-range transport (LRT) of 5-ethylidene-2-norbornene (ENB). Such estimations are components of the assessment process for persistent, bioaccumulative and toxic (PBT) substances, which are also referred to as persistent organic pollutants (POPs). An ecological exposure assessment for ENB from manufacturing activities was conducted based on physical/chemical properties, monitoring data, and degradation, transport and distribution estimates. Based on the results of several model predictions, chronic exposure of aquatic organisms is not expected, due to the anticipated residence time of ENB in aquatic ecosystems. These modeled results consistently predict ENB does not present the potential to persist in the environment. Volatilization from water to the air is calculated to occur at a relatively rapid rate for ENB based on its Henry's Law constant. Once in the air, ENB is expected to degrade rapidly due to oxidation by hydroxyl radicals and ozone based on calculated atmospheric half-lives of 57 and 27 min, respectively. Additionally, ENB is not predicted to undergo long-range transport based on the short atmospheric half-life due to oxidation by hydroxyl radicals and ozone. Additionally, based on predicted exposure from site-specific emission using the EPA model EFAST, ENB is not expected to reach concentrations of concern for chronic aquatic toxicity endpoints.