1 Ecological Risk Characterization

1 Ecological Risk Characterization Introduction: Risk characterization integrates the available data on effects and exposure assessments to evaluate the risk of toxicological impacts on organisms exposed to the chemical of interest. In this exercise, we will evaluate risks using the “quotient method”. The quotient method involves comparing Predicted Exposure Concentrations (PECs) to Predicted No Effect Concentrations (PNEC). We will also evaluate whether there is potential for bioaccumulation of triclosan. Risk Characterization: In the Exposure Assessment exercise, you calculated PECs (µg/L) for triclosan in surface waters at a distance of 1,000 metres from sources of municipal wastewater using dilution scenarios with 10th, 50th and 90th percentile dilution factors. The 10th percentile dilution factors are considered to be conservative estimates of the exposure of aquatic organisms to down-the-drain chemicals. Consider all three dilution scenarios as PEC values for the quotient method for quantitative risk characterization. Risks in the Aquatic Environment: From the Effects Assessment exercise, evaluate the data for the acute toxicity of aquatic organisms exposed to triclosan. From the previous Effects Assessment tutorial, utilize the various endpoints of acute toxicity in aquatic organisms, including your calculations for mortalities of daphnia and medaka, and the acute toxicity data from the literature. Also, review the endpoints for chronic toxicity, which includes the data on reproduction in Daphnia magna and Ceriodaphnia dubia, and in post-embryonic amphibian development (i.e. from Marlatt et al., 2015). Use NOEC data for your estimates of the PNEC. If there were no NOECs determined in the individual toxicity tests, estimate the NOEC by dividing the LC50 (or EC50) by a factor of 100, or by dividing the LOEC by a factor of 10. 2 Using an Excel spreadsheet, log-transform all of the NOECs for acute toxicity and determine the mean NOEC and the 95% confidence limits around the mean. Take the lower 95% confidence limit as a “conservative” estimate of the PNEC for aquatic organisms exposed to triclosan. Using this PNEC for acute toxicity, determine the ratio of PEC to PNEC (PEC/PNEC) for aquatic organisms exposed to triclosan in wastewater effluents at a distance of 1,000 meters downstream of the source under 10th, 50th and 90th percentile dilution scenarios. If the ratio is =1, then there is a risk of adverse effects to aquatic organisms exposed to triclosan in surface waters. Determine the risk quotient using NOEC data for chronic toxicity in Ceriodaphnia dubia (i.e. reproduction) and in post-embryonic frogs (i.e. development) exposed to triclosan at a distance of 1,000 meters downstream from the source under 10th, 50th and 90th percentile dilution scenarios. Risks in the Terrestrial Environment: From the Effects Assessment exercise, evaluate the data for the acute toxicity of terrestrial organisms (i.e. cucumber, red wiggler worm) exposed to triclosan in soil. Use the NOEC data for your estimates of the PNEC. For the most sensitive 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 toxicological data for terrestrial organisms exposed to triclosan. Determine the ratio of PEC to PNEC (PEC/PNEC) for terrestrial organisms exposed to triclosan in agricultural soils where biosolids were applied. If the ratio is =1, then there is a risk of adverse effects to terrestrial organisms exposed to triclosan in soil. Bioaccumulation Potential: The log Kow value for triclosan is 4.8, so this compound may bioaccumulate in exposed organisms; especially through bioconcentration from water in aquatic organisms. There are various empirical relationships between log Kow and bioconcentrations factors (BCF) for aquatic organisms that have been proposed in the literature: 3 log 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 organisms exposed to triclosan. Compare these values to the BCF values that have been determined experimentally for organisms exposed to triclosan (Table 1). Table 1: Data on bioconcentration factors estimated for triclosan in aquatic organisms. Test Organism Exposure BCF Reference Zebra fish Lab exposure 2,532 – 4,157 Orvos et al. 2002 Common carp Lab exposure 15-90 NITE 2005 White fish Field samples 2,000 - 5,200 Balmer et al 2004 Algae Field samples 700 – 1,500 Coogan et al 2007 Snail Field samples 1,200 Coogan et al 2008 References: Balmer ME et al. 2004. Occurrence of methyl triclosan, a transformation product of the bacteriocide triclosan, in fish from various lakes in Switzerland. Environ Sci Technol 38:390-395. Coogan MA et al. 2008 Snail bioaccumulation of triclocarban and triclosan and methyl triclosan in a North Texas, USA stream affected by wastewater treatment plant runoff. Environ Toxicol Chem 27:1788-1793. Coogan MA et al 2007. Algal bioaccumulation of triclocarban, triclosan and methyltriclosan in a North Texas wastewater treatment plant receiving stream. Chemosphere 67:1911-1918. Mackay D. 982. Correlation of bioconcentration factors. Environ Sci Technol 16:274-278. Neely BW et al. 1974. Partition coefficient to measure bioconcentration potential of organic chemicals in fish. Environ Sci. Technol 8:1113-1115. 4 NITE(National Institute of Technology and Evaluation of Japan) 2005. Biodegradation and bioconcentration of existing chemical substances under the Chemical Substances Control Law. http://www.safe.nite.go.jp/denglish/kizon/KIZON_start-hazkizon.html. Orvos 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 of chemicals in fish. J Fish Res Bd Can 36:1040-1048. Assignment: Risk Characterization and Management 1. Present your ecological risk assessments for aquatic and terrestrial organisms exposed to triclosan by completing all of the steps described in the Exposure Assessment, Effects Assessment and Risk Characterization tutorials. Describe any of the steps in this procedure that are “conservative” approaches for calculating a risk quotient (i.e. based on worst case scenarios). Based on these calculations, if you were to recommend further studies to more fully assess the risks of triclosan in the environment (i.e. a Phase II, Tier B assessment), what would they be? 2. Estimate the Bioconcentration Factors (BCFs) for triclosan in aquatic organisms using the three empirical relationships provided above. Compare these data to the BCF values that have been determined experimentally (Table 1) and comment on any differences or similarities. The log Kow value of 4.8 for triclosan was determined at a pH = 7. However, for chemicals that can ionize, such as triclosan, the Kow will vary with pH. Triclosan is a phenolic compound with a pKa value of 7.9. In aquatic environments at the upper range of the pH for natural waters (i.e. apprioximately pH=8), would you expect that triclosan would show greater or lesser potential for bioaccumulation than at a lower pH of 7? 3. “Risk Management” involves developing actions or policies that will reduce or eliminate the risk of adverse effects to organisms. Risk management 5 could involve banning or phasing out a chemical. Is there justification for using this approach to prevent adverse effects to aquatic and terrestrial organisms exposed to triclosan? An alternative approach is to take steps to reduce the amount of a chemical released into the environment. Discuss ways in which this approach could be used to reduce the risks to aquatic and terrestrial organisms exposed to triclosan.