Advances in Water Treatment and Enviromental Management

CONTROL OF DANGEROUS SUBSTANCES IN UK SURFACE WATERS 45the responsible authorities at the time (the Water Authorities in Englandand Wales) of the content of the Dangerous Substances Directive and the‘daughter’ directives then published. The circular also implemented thestandards laid down in the directives and those agreed nationally for theList II substances (arsenic, chromium, copper, inorganic lead, nickel andzinc). A similar circular was issued by the Scottish Development Departmentto the River Purification Boards in Scotland (SDD 1985). This DoE circularhas recently been superseded by the updated Circular 7/89 (DoE 1989)implementing all directives adopted so far for List I substances and EQSsfor an additional six List II substances or groups of substances (vanadium,boron, organotins, mothproofing agents, pH and iron).3. THE ENVIRONMENTAL QUALITY OBJECTIVE (EQO) APPROACHThe EC Dangerous Substances Directive (CEC 1976) provides two alternativeapproaches for control, the uniform emission standard (UES) or limit valueapproach and the environmental quality standard (EQO) approach.The EQO approach has been generally applied in the UK for the managementof discharges to surface waters. The EQO approach is based on the premisethat a minimum acceptable concentration of a pollutant can be definedwhich does not interfere with the use of the water. This premise does nothold for certain particularly dangerous compounds, which require differentcontrol strategies. For instance, DDT control demands prohibition of use ortotal ban on discharges (complete recycling or destruction).A distinction has to be made between EQOs and EQSs. The EQO defines theuse for which the water is intended (eg abstraction for drinking water) whereasthe EQS specifies the concentration of the substance which should not beexceeded to protect the particular use of the water. For example, in theAbstraction for Drinking Water Directive the EQS for nitrate is 50 mg/l.In the EC Dangerous Substances Directive environmental quality standards(EQSs) are described as environmental quality objectives (EQOs). This hasled to confusion as quality objectives are viewed by those countries whichopted for the limit value approach as long-term goals demanding very lowstandards and not as in the UK for setting consents for discharges.To apply the UK EQO approach, the different uses of water need to bedefined and standards, in terms of the maximum acceptable concentrationof the particular contaminant, must be derived. These concentrations mustbe low enough to protect the water’s uses by taking into account thevulnerable targets requiring protection: man, his food sources or other livingorganisms. The standards can then be used to derive consents for individualdischarges, taking into account the dilution in the receiving water, but alsothe concentration of the substance already present in the receiving waterfrom other sources. The EQO approach can deal with diffuse inputs and