Tham khảo tài liệu 'results for water polution act india ushotsearchs - great document_2', khoa học tự nhiên, công nghệ môi trường phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | basis of newly elaborated water quality objectives too often or too soon after establishing practices designed to comply with earlier water quality objectives. In the UK for example the 1991 Water Act allows for the revision of water quality objectives although such a review can only take place at intervals of at least five years or if the NRA requests such a review following consultation with water users and other appropriate bodies. Adaptation of monitoring programmes surveillance systems and laboratory practices are necessary in the implementation of water quality objectives. Two problems deserve special mention in this respect the detection limit of laboratory equipment and agreement on a criterion for the attainment of water quality objectives. Experience in many countries shows that laboratory techniques should have a detection limit that is preferably one order of magnitude lower than the water quality objective for the substance in question. In the case of hazardous substances this may require sophisticated laboratory equipment and specially trained personnel and may lead to high costs for laboratory analyses. Usually water quality criteria used as a basis for elaborating water quality objectives already have a built-in margin of safety so that for the most part a certain number of monitoring data may exceed the established water quality objective and forewarn of a certain risk without requiring immediate action. In most cases this advance warning ensures that action can be taken before real damage occurs. For hazardous substances some countries consider that the water quality objective has been attained if at least 90 per cent of all measurements within a period of three years comply with the water quality objective or if the mean value of the concentration of the substance is less than or equal to half the concentration value of the water quality objective. Another approach requires the use of the mean concentration of a substance as an evaluation .
