Advances in Water Treatment and Enviromental Management

46 WATER TREATMENTcan be used to control multiple point sources. It also makes proper use ofthe receiving water capacity. The common uses of both fresh and salinewaters have been defined (Mance 1984), taking into account the use-relateddirectives adopted by the EC (eg the Abstraction to Drinking Water Directive(CEC 1975) and the Freshwater Fish Directive (CEC 1978), Table 1.Table 1. Designated uses of surface waters for the derivation of EQS values- Not applicableIn practice, however, the setting of a reliable EQS for all water uses isfrequently not possible as the information on harmful effects is often limited.4. DERIVATION OF ENVIRONMENTAL QUALITY STANDARDS (EQSs)Figure 1 illustrates the approach adopted for the derivation of EQSs in the UK.Available data on the toxic effects, both from laboratory studies and fieldobservations, are critically assessed. Particular attention is paid to theexperimental test method used.The lowest credible adverse effect concentration is derived from the availabletoxicity data. Particular emphasis is placed on chronic effects noted afterlong-term exposure, or exposure effects at specific sensitive life stages ofthe target species. A safety factor is then applied to obtain a tentative EQS,which depends on the quality and extent of the available toxicity data. Ifonly acute (short term) LC 50 values are available, an arbitrary safety factorof 100 is usually applied. This factor tends to be reduced to 10 if chronicLC 50 data are available for sensitive species. For some substances, ‘no effectsconcentrations’ (NOEC) are reported for long-term chronic studies. Thesevalues may be used as tentative standards without applying a safety factor,provided the test design is acceptable, and that they refer to relevant speciesand are supported by the other data available.The tentative EQS derived from the laboratory toxicity data is verified bycomparison with the available field toxicity data, taking care to distinguish