Screening assessment on MBMBP phenols: chapter 10


Potential to Cause Ecological Harm

Data concerning measured levels of MBMBP in air, water, soil and sediment in Canada were not found. The Screening Information Data Set initial assessment report (OECD 2001) reports that no quantitative monitoring data are available globally, including Canada. No data could be found for concentrations of MBMBP in wildlife in Canada or worldwide.

Environmental concentrations were calculated based on potential losses during plastic processing. Processing plant effluents were assumed to be treated by municipal sewage treatment plants (STPs) before discharge to the environment. The local concentration in STP effluent is calculated using ChemSim, a modelling program developed for Environment Canada that predicts aquatic concentrations downstream from point sources of a substance’s release (Canadian Hydraulics Centre 2003). In order to run ChemSim, inputs, including the loading rate, are required. In this case, the loading rate is the mass of MBMBP in STP effluent released in one day. To calculate the loading rate, the following conservative assumptions were made:

  • The upper range of the quantity of MBMBP imported in 2000 is 100 tonnes (100 000 kg). For the conservative exposure scenario, we have assumed that this quantity is imported by one distributor who sells to one customer who
  • uses the total quantity of MBMBP as a plastic additive;
  • uses the total quantity at one processing facility in one calendar year; and
  • discharges processing plant effluent into a municipal sewage treatment plant.
  • For calculating the amount of MBMBP released from the processing facility into the municipal sewage plant, the Organisation for Economic Co-operation and Development (OECD) emission release scenario for plastic additives was used, with a release percentage of 0.65% (OECD 2003c). For determining the number of operating days (300 per year), the European Union Technical Guidance Document was used (European Chemicals Bureau 2003).
  • The 92.2% removal rate for MBMBP at the STP was calculated using the STP fugacity model within the EPI Suite of models, version 3.10 (EPIsuite 2001).

The ChemSim model run assumed a standard river in Southern Ontario with a flow rate of 5 m3/s and predicted a maximum concentration of MBMBP of 7.46 × 10−3 mg/L, 50 m from the point of impingement (release). Further assumptions required to run ChemSim (pertaining to river flow and channel geometry) are listed in the ChemSim report for this substance (Environment Canada 2004). The predicted environmental concentration for the aquatic medium (PECaq) is therefore 7.46 × 10−3 mg/L.

Most of the MBMBP in STP influent is removed (92.2%) and ends up in the sewage sludge. Since application of sewage sludge to agricultural land is a possibility, we have considered an exposure scenario involving sewage sludge-amended soil. No data on MBMBP concentrations in Canadian sewage sludge or soil were found. For the conservative soil exposure scenario, we calculate a concentration of MBMBP in sewage sludge to be 257 mg/kg dry weight, based on standard calculations, adapted to MBMBP, for estimating the concentration of a substance in sewage sludge (Droste 1997). Using this MBMBP concentration in sewage sludge and assuming that MBMBP-containing sludge is applied to the land for 10 years (OMOE and OMAFRA 1996) and that no or little biodegradation of the MBMBP occurs, this would result in a soil concentration of 1.64 mg/kg dry weight. The predicted environmental concentration for soil (PECsoil) is therefore 1.64 mg/kg dry weight.

Experimental toxicity data exist for effects in aquatic organisms (green algae, water flea and fish; OECD 2001). There is some uncertainty connected to these data, because the toxicity values are all above the water solubility limit. The critical toxicity value (CTV) selected is the lowest acceptable chronic value of 0.89 mg/L, the lowest-observed-effect concentration (LOEC) for immobility in Daphnia magna (water flea). No toxicity data were found for effects on soil- or sediment-dwelling organisms, terrestrial plants, or wildlife.

A - In the Aquatic Compartment

The conservative exposure scenario considered the release of MBMBP to the aquatic medium following industrial processing of the imported substance entirely at one site and the subsequent treatment and release from an STP. The PEC for the aquatic medium is 0.00746 mg/L.

The CTV for this assessment is the lowest acceptable chronic value (21-day LOEC) of 0.89 mg/L for immobility in Daphnia magna. The CTV is then divided by an application factor of 10 to account for uncertainty in extrapolating from laboratory to field conditions and for intraspecies and interspecies variations in sensitivity, giving a predicted no-effect concentration (PNEC) of 0.089 mg/L.

Therefore, the quotient for risk to aquatic species is calculated as follows:

Risk quotient =

Risk quotient

As this calculated risk quotient is much less than 1, it is predicted that MBMBP is unlikely to have harmful effects on pelagic organisms.

B - In Other Environmental Compartments

The soil CTV for MBMBP is 2670 mg/kg dry weight, which is the LOEC calculated using an equilibrium partitioning approach. The PNEC is determined by dividing the CTV by an application factor of 10 to account for extrapolation from laboratory to field conditions and intraspecies and interspecies variations in sensitivity. Therefore, the PNEC for soil-dwelling organisms is 267 mg/kg dry weight.

A risk quotient can thus be calculated as follows:

Risk quotient =

Risk quotient

Since this quotient is significantly less than 1, it is predicted that MBMBP is unlikely to have harmful effects on soil-dwelling invertebrate organisms exposed to sewage sludge--amended agricultural land.

Uncertainties in Evaluation of Ecological Risk

Exposure Characterization

There is some uncertainty associated with the exposure characterization. There is a lack of monitoring data for MBMBP; environmental concentrations had to therefore be estimated using models. The use of models to predict actual concentrations based on current releases introduces uncertainties that are not easily quantifiable. Model selection, model inputs, release scenarios, site-specific information and meteorology are all factors that will affect the predicted exposure values.

Because of a lack of current information, conservative scenarios had to be developed. These scenarios also included some assumptions, such as the percentage of MBMBP released in STP effluent. Additionally, some sources, such as small companies that did not meet the reporting threshold of the section 71 survey, may not be included in the assessment and may account for a certain volume of MBMBP released to the environment. Current monitoring data from sites where MBMBP could be released, as well as from sites located far from point sources, would be very useful to support the assumptions in this assessment. However, given the available information, the scenarios developed are considered conservative (e.g., high release amounts were assumed for these sources). Even if the uncertainty about each actual exposure value is high, the confidence in the resulting conclusion is good.

Effects Characterization

Some uncertainty is associated with the PNEC determination. Due to MBMBP’s low water solubility, homogeneous solutions could be reached in the toxicity studies only by using the maximum allowable dispersant (castor oil) concentration. Although effects were observed, concentrations of MBMBP in the environment are unlikely to reach the toxicity thresholds reported in the studies. Nevertheless, the studies were robustly summarized, and toxicity data can be used to select a conservative toxicity value for the assessment of risk to aquatic organisms.

There are no experimental toxicity data for soil-dwelling organisms. Equilibrium partitioning was used to estimate the toxicity of MBMBP to soil-dwelling organisms based on data for aquatic species. Also, the soil exposure concentration was estimated using conservative assumptions, such as lack of biodegradation.

 

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