Article 6
1. For the purpose of designating and revising the designation of vulnerable zones, Member States shall: (a) within two years of notification of the Directive, monitor the nitrate concentration in freshwaters over a period of one year: (i) at surface water sampling stations, laid down in Article 5 (4) of Directive 75/440/EEC and/or at other sampling stations which are representative of surface waters of Member States, at least monthly and more frequently during flood periods; (ii) at sampling stations which are representative of the groundwater aquifers of Member States, at regular intervals and taking into account the provisions of Directive 80/778/EEC; (b) repeat the monitoring programme outlined in (a) at least every four years, except for those sampling stations where the nitrate concentration in all previous samples has been below 25 mg/l and no new factor likely to increase the nitrage content has appeared, in which case the monitoring programme need be repeated only every eight years; (c) review the eutrophic state of their fresh surface waters, estuarial and coastal waters every four years. 2. The reference methods of measurement set out in Annex IV shall be used.