13.—(1) Subject to paragraphs (2), (4) and (6), in each year every water undertaker or combined licensee must take or cause to be taken, from the point at which water leaves each treatment works which it uses to supply water to water supply zones, the standard number of samples for analysis—
(a)for determining the concentration of residual disinfectant,
(b)for determining whether, in relation to the colony counts and turbidity parameters (items 5 and 13 in Schedule 2), water leaving treatment works meets the specifications for those parameters set out in Schedule 2, and
(c)for testing for compliance with the prescribed concentrations or values in respect of the coliform bacteria, E. coli, and nitrite parameters (in Part II of Table A in Schedule 1 and item 19 in Table B of Schedule 1 respectively) for leaving water treatment works.
(2) Paragraph (3) applies where in each of the two preceding years the results of the analysis of the samples taken, subject to paragraph (4), have established that—
(a)in respect of coliform bacteria, E. coli or nitrite parameters, the maximum concentration has not been exceeded;
(b)in respect of the turbidity parameter, the specified value of 1 NTU has been met;
(c)in respect of the colony counts parameter, there has been no significant increase.
(3) The number of samples taken in respect of the particular parameter in the following year from the point at which water leaves that treatment works may be the reduced number.
(4) In respect of the coliform bacteria and the E. coli parameters (items 1 and 2 in Part II of Table A in Schedule 1), the reduced number of samples may only be taken in accordance with paragraph (3) if the water undertaker or combined licensee is of the opinion that—
(a)there is no foreseeable risk that the supply will exceed the maximum concentration for that parameter, and
(b)the treatment works is designed to secure that, in the event of a failure of the disinfection process, water that has not been disinfected cannot enter the supply.
(5) Samples required to be taken by this regulation must be taken at regular intervals.
(6) Where a particular treatment works is in use for part only of a year, the minimum number of samples to be taken from that works in that year must bear to the standard number or, as the case may be, the reduced number, the same proportion as the number of days in that year in which the treatment works has been in use bears to 365.