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1 Ecological Risk Characterization

1Ecological Risk CharacterizationIntroduction:Risk characterization integrates the available data on effects and exposureassessments to evaluate the risk of toxicological impacts on organisms exposedto the chemical of interest. In this exercise, we will evaluate risks using the“quotient method”. The quotient method involves comparing Predicted ExposureConcentrations (PECs) to Predicted No Effect Concentrations (PNEC). We willalso evaluate whether there is potential for bioaccumulation of triclosan.Risk Characterization:In the Exposure Assessment exercise, you calculated PECs (µg/L) fortriclosan in surface waters at a distance of 1,000 metres from sources of municipalwastewater using dilution scenarios with 10th, 50th and 90th percentile dilutionfactors. The 10th percentile dilution factors are considered to be conservativeestimates of the exposure of aquatic organisms to down-the-drain chemicals.Consider all three dilution scenarios as PEC values for the quotient method forquantitative risk characterization.Risks in the Aquatic Environment:From the Effects Assessment exercise, evaluate the data for the acutetoxicity of aquatic organisms exposed to triclosan. From the previous EffectsAssessment tutorial, utilize the various endpoints of acute toxicity in aquaticorganisms, including your calculations for mortalities of daphnia and medaka, andthe acute toxicity data from the literature. Also, review the endpoints for chronictoxicity, which includes the data on reproduction in Daphnia magna andCeriodaphnia dubia, and in post-embryonic amphibian development (i.e. fromMarlatt et al., 2015). Use NOEC data for your estimates of the PNEC. If there wereno NOECs determined in the individual toxicity tests, estimate the NOEC bydividing the LC50 (or EC50) by a factor of 100, or by dividing the LOEC by a factorof 10.2Using an Excel spreadsheet, log-transform all of the NOECs for acutetoxicity and determine the mean NOEC and the 95% confidence limits around themean. Take the lower 95% confidence limit as a “conservative” estimate of thePNEC for aquatic organisms exposed to triclosan. Using this PNEC for acutetoxicity, determine the ratio of PEC to PNEC (PEC/PNEC) for aquatic organismsexposed to triclosan in wastewater effluents at a distance of 1,000 metersdownstream of the source under 10th, 50th and 90th percentile dilution scenarios. Ifthe ratio is =1, then there is a risk of adverse effects to aquatic organisms exposedto triclosan in surface waters.Determine the risk quotient using NOEC data for chronic toxicity inCeriodaphnia dubia (i.e. reproduction) and in post-embryonic frogs (i.e.development) exposed to triclosan at a distance of 1,000 meters downstream fromthe source under 10th, 50th and 90th percentile dilution scenarios.Risks in the Terrestrial Environment:From the Effects Assessment exercise, evaluate the data for the acutetoxicity of terrestrial organisms (i.e. cucumber, red wiggler worm) exposed totriclosan in soil. Use the NOEC data for your estimates of the PNEC. For the mostsensitive endpoint (i.e. toxicity to cucumber), apply an assessment (i.e. “safety”)factor of 10 to the NOEC to account for the fact that there are very few toxicologicaldata for terrestrial organisms exposed to triclosan. Determine the ratio of PEC toPNEC (PEC/PNEC) for terrestrial organisms exposed to triclosan in agriculturalsoils where biosolids were applied. If the ratio is =1, then there is a risk of adverseeffects to terrestrial organisms exposed to triclosan in soil.Bioaccumulation Potential:The log Kow value for triclosan is 4.8, so this compound may bioaccumulatein exposed organisms; especially through bioconcentration from water in aquaticorganisms. There are various empirical relationships between log Kow andbioconcentrations factors (BCF) for aquatic organisms that have been proposed inthe literature:3log BCF = 0.542 log Kow + 0.124 Neely et al. (1974)log BCF = 0.85 log Kow – 0.70 Veith et al. (1979)log BCF = log Kow – 1.32 Mackay (1982)Using each of these relationships, estimate the BCF for aquatic organismsexposed to triclosan. Compare these values to the BCF values that have beendetermined experimentally for organisms exposed to triclosan (Table 1).Table 1: Data on bioconcentration factors estimated for triclosan in aquaticorganisms.Test Organism Exposure BCF ReferenceZebra fish Lab exposure 2,532 – 4,157 Orvos et al. 2002Common carp Lab exposure 15-90 NITE 2005White fish Field samples 2,000 – 5,200 Balmer et al 2004Algae Field samples 700 – 1,500 Coogan et al 2007Snail Field samples 1,200 Coogan et al 2008References:Balmer ME et al. 2004. Occurrence of methyl triclosan, a transformation productof the bacteriocide triclosan, in fish from various lakes in Switzerland. EnvironSci Technol 38:390-395.Coogan MA et al. 2008 Snail bioaccumulation of triclocarban and triclosan andmethyl triclosan in a North Texas, USA stream affected by wastewatertreatment plant runoff. Environ Toxicol Chem 27:1788-1793.Coogan MA et al 2007. Algal bioaccumulation of triclocarban, triclosan and methyltriclosanin a North Texas wastewater treatment plant receiving stream.Chemosphere 67:1911-1918.Mackay D. 982. Correlation of bioconcentration factors. Environ Sci Technol16:274-278.Neely BW et al. 1974. Partition coefficient to measure bioconcentration potentialof organic chemicals in fish. Environ Sci. Technol 8:1113-1115.4NITE(National Institute of Technology and Evaluation of Japan) 2005.Biodegradation and bioconcentration of existing chemical substances underthe Chemical Substances Control Law. DR et al. 2002. Aquatic toxicity of triclosan. Environ. Toxicol Chem 21:1338-1349.Veith GD et al. 1979. Measuring and estimating the bioconcentration factor ofchemicals in fish. J Fish Res Bd Can 36:1040-1048.Assignment:Risk Characterization and Management1. Present your ecological risk assessments for aquatic and terrestrialorganisms exposed to triclosan by completing all of the steps described inthe Exposure Assessment, Effects Assessment and Risk Characterizationtutorials. Describe any of the steps in this procedure that are “conservative”approaches for calculating a risk quotient (i.e. based on worst casescenarios). Based on these calculations, if you were to recommend furtherstudies to more fully assess the risks of triclosan in the environment (i.e. aPhase II, Tier B assessment), what would they be?2. Estimate the Bioconcentration Factors (BCFs) for triclosan in aquaticorganisms using the three empirical relationships provided above. Comparethese data to the BCF values that have been determined experimentally(Table 1) and comment on any differences or similarities. The log Kow valueof 4.8 for triclosan was determined at a pH = 7. However, for chemicalsthat can ionize, such as triclosan, the Kow will vary with pH. Triclosan is aphenolic compound with a pKa value of 7.9. In aquatic environments at theupper range of the pH for natural waters (i.e. apprioximately pH=8), wouldyou expect that triclosan would show greater or lesser potential forbioaccumulation than at a lower pH of 7?3. “Risk Management” involves developing actions or policies that will reduceor eliminate the risk of adverse effects to organisms. Risk management5could involve banning or phasing out a chemical. Is there justification forusing this approach to prevent adverse effects to aquatic and terrestrialorganisms exposed to triclosan? An alternative approach is to take stepsto reduce the amount of a chemical released into the environment. Discussways in which this approach could be used to reduce the risks to aquaticand terrestrial organisms exposed to triclosan.

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