Uncertainty

Uncertainties associated with the ecological screening assessment of MBMBP are discussed below.

Exposure Characterization

There is some uncertainty associated with the exposure characterization. There is a lack of monitoring data for MBMBP, so environmental concentrations had to 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 might not be included in the assessment and may account for a certain volume of MBMBP released to the environment, such as small companies not meeting the reporting threshold of the Section 71 survey. 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 on each actual exposure value is high, the confidence in our resulting conclusion is good.

Effects Characterization

Some uncertainty is associated with the ENEV 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.