HAZARD, EXPOSURE AND ECOLOGICAL RISK ASSESSMENT For the purposes of this discussion, hazard assessment and ecological risk assessment may be regarded as two branches of environmental assessment. Other branches include resource management and environmental impact assessment. Taken together, these constitute perhaps the most diverse assessment dealing with all the environmental aspects of a project at the planning stage. Although these broad fields are interrelated, there is a considerable diversity in the way that environmental problems are defined and analysed

under each discipline. Hazard assessment may be considered broadly as a methodology for analysing and investigating the effects of chemicals on the natural environment (Suter, 1993). It has primarily been developed as a discipline by aquatic toxicologists, and it is in this field of environmental protection that the techniques are most highly developed. The expected concentration of a given chemical in the environment is compared with the estimated toxic threshold using a tiered toxicity testing system. This comparison is then used to determine whether a given release or discharge is safe, hazardous or whether the implications are unknown or uncertain. Hazard assessment does not by

SUMMARY The regulation of chemicals entering aquatic environments is based largely on the level of hazard they pose, as assessed using an iterative framework. Hazard assessment is carried out using toxicity tests to gauge the magnitude of any likely problem together with a determination of the key physico-chemical parameters. This information can be used in two ways. First, it may be used to set an environmentally ‘safe’ level. Second, it may be used in conjunction with an exposure assessment to prepare a probabilistic risk assessment. In theory this process leads to a probability value indicating the likelihood of unacceptable impacts in the receiving environment.