7 Del. Admin. Code § 7401-7.0

Current through Reigster Vol. 28, No. 6, December 1, 2024

Section 7401-7.0 - Critical Flows

7.1 For all waters of the State, all water quality standards and criteria, except those for toxic substances, shall not apply at those times when the freshwater flow or net advective flow falls below that value that is equal to the flow of 7-day duration with recurrence interval of 10 years (generally known as the 7Q10 or the Q7-10). However, at all times all waters shall be free of materials and substances as listed in subsection 6.5 .2.

7.2 For all waters of the state, water quality criteria for toxic substances as specified in subsection 4.6 .3 shall not apply at those times when the freshwater or net advective flow falls below the following values:

7.2.1 The harmonic mean flow, for human carcinogens;

7.2.2 The flow of 30-day duration with recurrence interval of 5 years (generally known as the 30Q5 or Q30-5), for human systemic toxicants and for ammonia fresh water aquatic life chronic toxicity criteria;

7.2.3 The flow of 7-day duration with recurrence interval of 10 years (generally known as the 7Q10 or the Q7-10), for compounds having a chronic toxicity criterion; and

7.2.4 The flow of 1-day duration with recurrence interval of 10 years (generally known as the 1Q10 or the Q1-10), for compounds having an acute toxicity criterion.

7.3 These critical flows shall also be used as design flows for developing water quality-based discharge limitations for the referenced groups of parameters. The Department shall consider scientifically reasonable requests for seasonally adjusted flows or the use of dynamic modeling techniques for this purpose on a case-by-case basis.

7.4 Nothing in Section 7.0 shall be construed as allowing any reduction in efficiency of, or suspension of, required pollution control practices, whether applied to point or nonpoint sources, during periods when flows are less than those specified for suspension of standards applicability in subsections 7.1 -7.3.

7 Del. Admin. Code § 7401-7.0

14 DE Reg. 1392 (06/01/11)
21 DE Reg. 241 (9/1/2017) (Final)