The European Commission proposed legislation in 1975 to secure clean drinking water for the people of Europe (well, the member States of the EU, anyway).
This proposal resulted in the passing of Directive 80/778 EEC. The member States were given five years to bring their systems into compliance.
None saw any reason to seek a derogation or extension of time. Events showed that they were not very pressed to comply with the Directive, although they were legally obliged to comply; they simply transposed the Directive incorrectly or partially or did not monitor their drinking water sufficiently frequently to permit valid judgements on compliance or non compliance to be made. Alternatively the people were not informed of the results of testing.
Even so, pressure was applied to reduce the burden of Directive 80/778 EEC and the Commission replaced it with Directive 98/83 EC. Now the consumer’s entitlement is to the delivery of clean wholesome water from the tap: previously it was in respect of the water entering the system, not leaving it.
Directive 75/440/EEC prescribes requirements for the quality of surface waters from which drinking water is to be abstracted. As far back as 1991 the European Court of Justice made it clear in Commission v Germany  ECR I-4983 that the member States were obliged to draw up a plan to improve such waters and to follow the plan in stages. Window dressing will be insufficient; the plan must be real and must be followed.
In Commission v UK Case –340/96  ECR I-2023 the European Court of Justice judged that the UK had not properly transposed Directive 98/83 EC because the domestic authorities had too much discretion in enforcement.
The European Court of Justice has ruled that the obligations to deliver clean wholesome water are strict.