3. –
(1) Subject to sub-paragraph (2), where in the case of particular waters to which the classification BW1 applies –
(a)any requirement relating to the parameter for pH, colour or transparency cannot be complied with as a result of exceptional weather or geographical conditions; or
(b)any requirement relating to any parameter cannot be complied with as a result of the waters having undergone a process of natural enrichment from the soil without human intervention,
the Secretary of State may by notice served on the National Rivers Authority modify or disapply that requirement in relation to those waters.
(2) The Secretary of State shall not exercise his powers under this paragraph so as to permit a risk to public health.