Regulations relative to measures relating to the prevention, reduction and elimination of pollution of water. For the purpose of the designation and revision of designations of nitrate vulnerable zones, the Department of the Environment shall monitor at least every 4 years the nitrate concentration in freshwaters over a period of 1 year: (i) at surface water sampling stations laid down in article 5(4) of Directive 75/440/EEC or at other sampling stations which are representative of surface waters, or at both, at least monthly and more frequently during flood periods; and (ii) at sampling stations which are representative of groundwater aquifers, at regular intervals and taking into account the provisions of Directive 80/778/EEC. the Department shall furthermore review every 4 years the eutrophic state of fresh surface waters, estuarial and coastal waters. Where the nitrate concentration in all previous samples taken in pursuance of the Directive at any sampling station has been below 25 mg/1 and no new factor likely to increase the nitrate content has appeared, the requirements of monitoring of nitrate concentrations shall have effect in relation to that sampling station as if the reference to 1997 and the reference to 4 years were references to 2001 and 8 years respectively. Monitoring under this regulation shall be carried out using the reference methods of measurement set out in Schedule 2.