Advances in Water Treatment and Enviromental Management

CONTROL OF DANGEROUS SUBSTANCES IN UK SURFACE WATERS 47Figure 1. Schematic flow chart for the derivation of environmental quality standards.between the effects of the particular compound and those of other potentiallytoxic compounds present. It is important to compare the tentative EQSwith the highest observed concentration which has no effect on theecosystem, rather than with the lowest concentration which does have aneffect. Any discrepancies between the field data and the tentative EQS requirethe reassessment of the available field and laboratory data and, if necessary,the application of a different safety factor.The setting of EQS values relies heavily on knowledge of bioaccumulationand toxicity effects. This information is often incomplete and depends oninterpretation. The field data are often inadequate, especially for the lessfrequently occurring organic compounds, to verify the tentative EQS derivedfrom the laboratory data. The application of safety factors must thereforetake account of the different toxicities and actions of the chemicals on thetarget organisms and the quality of the toxicity data available.When deriving EQS values for substances which are designed for a specificpurpose, such as pesticides, it is particularly important that toxicity testsare available for the target species as the sensitivities of different taxonomicgroups to these compounds vary widely.