TR 061 – Environmental Exposure Assessment

TR 061 : Environmental Exposure Assessment | September 1994

In ECETOC Technical Report No. 51 (ECETOC, 1993) a process for the Environmental Risk Assessment of Substances was described which is applicable to all substances, whether new or existing, and to all environmental compartments. The scheme generally follows a stepwise approach in which, if necessary, increasingly refined estimates of the Predicted Environmental Concentrations (PECs) and the Predicted No-Effect Concentrations (PNECs) can be compared in stages.

A number of steps in the overall process could at the time not be described in detail. The main objective of the work presented in this report was to provide more detailed information on how to perform an assessment of environmental exposure on a regional and on a local scale.

Mathematical distribution and fate models are required in the screening and confirmatory phases of environmental exposure assessment. Sensitivity analyses and a critical review of parameters have shown that regional generic fugacity models of the "Mackay level III" type are, in principle, a suitable tool for performing risk characterisation on a regional scale. The model used and partly described in this report is HAZCHEM; the full description can be found in ECETOC (1994b). Such models can be used to point at environmental compartments of concern in a qualitative manner. For chemicals mainly released via diffuse sources, they may also be used quantitatively, provided that the amounts emitted into the environment can be estimated with a sufficient degree of accuracy. The model published by Mackay et al (1992) was used as a basis and was adapted to represent a generic European geographic scenario.

To calculate environmental concentrations in local scenarios, approaches for the water, soil and air compartments are proposed. For the water compartment the discharge (direct or via a WWTP) into a river can adequately be modelled with RIVMODEL which is included in the HAZCHEM package. Two scenarios have been proposed, a lowland river scenario with low flow rate and a mountain area river scenario with higher flow rate. A scenario for local air and soil modelling is presented which includes the most relevant exposure routes. Details on indirect human exposure are given in ECETOC (1994a).

Reliable data on release and emission of a substance are the key elements for the calculation of realistic PECs for the different environmental compartments. In the majority of cases release of a substance is determined by the process involved including dedicated treatment and not by the physico-chemical data of the substance. Emission estimation requires scenarios which cover release, elimination and dilution processes. As could be demonstrated for e.g. chemical intermediates, it is important to use process-oriented release data, to consider all dilution processes (internal and external) and to use reliable elimination data e.g. in a biological waste water treatment plant. For emission scenarios default values for process data, elimination and dilution should be given which cover an average (generic) situation and which can be overwritten when substance-specific data are available. Using only worst case data would lead to unrealistically high PECs.

Furthermore, for the calculation of PECs information on the kinetics of primary biodegradation (degradation of the parent compound) is needed. It is difficult or almost impossible to derive kinetic biodegradation data directly from simple screening tests on ready biodegradation as they are normally available at base set level of the notification of new chemicals. Since ready biodegradation tests are based on the measurement of ultimate biodegradation, no direct correlation between the results from these tests and primary biodegradation exists. Therefore - as a first step - default half-life times/rate constants have to be assigned to substances according to the results obtained in screening tests.

For the prediction of biodegradation rates in the waste water treatment plant a tiered approach including three steps is proposed, whereby steps 2 and 3 are required only if the PEC needs to be refined. At the screening phase a default rate constant of 3 h^-1 can be assigned to ready biodegradable compounds and to those which reached the corresponding pass level after acclimatisation. At the confirmatory phase rate constants should be directly determined from measurement of primary biodegradation or based on respirometric methods whereas at the investigative phase comparative measurements of influent and effluent concentrations replace the use of default values or calculated rate constants.

An attempt was made to derive biodegradation half-life times for surface waters and soil by evaluating a biodegradation database and industry biodegradation data but due to the limitations of the available data no recommendations on scientifically based default values to be used at screening level could be given although the data suggest that the default values used within existing risk assessment schemes may be overly conservative. At the confirmatory phase the biodegradation rates in soil and surface waters will have to be derived from primary biodegradation testing requiring specific analytical methods or radiolabelled materials. For soil, standard simulation tests are available.