Advances in Water Treatment and Enviromental Management

CONTROL OF DANGEROUS SUBSTANCES IN UK SURFACE WATERS 49an arbitrary safety factor of 10 to laboratory toxicity test results. It wasthus possible, using bioaccumulation, to verify the tentative EQS.When setting EQSs, it is important to establish whether the standard shouldbe set as total or dissolved concentration or, as in the case of ammonia, theun-ionised species. This approach requires a knowledge of the speciationof the compound in the environment, and is of particular importance forinorganic compounds. For organic compounds, the EQS tends to beexpressed as total concentration.A decision has also to be made on the degree of compliance required for thestandard to protect the use. The options include:– 100 per cent compliance (maximum allowable concentrations)– 95 per cent compliance (5 per cent exceedance can be tolerated)– 50 per cent compliance (annual mean concentration)The decision depends on the source of the substance and on the differencebetween acute and chronic toxicities.Table 2 shows the List II substances for which EQS values have been adoptedin the UK and another 11 compounds which are currently under discussion.Table 2. Substances for which EQS values have been published