3.—(1) Subject to paragraphs 5 and 6, the enforcing authority must monitor the water for tritium where—
(a)an anthropogenic source of tritium or other artificial radionuclides is present within the catchment area for the supply; and
(b)it cannot be shown on the basis of other surveillance programmes or investigations that the level of tritium is below the parametric value for tritium.
(2) Where monitoring for tritium is required by sub-paragraph (1), samples must be taken in accordance with paragraph 6.
(3) If the concentration of tritium in any such sample exceeds the parametric value for tritium, the enforcing authority must carry out an investigation of the presence of other artificial radionuclides.