R 323.1090 Applicability of water quality standards.

Rule 90. (1) The requirements prescribed by these rules shall not apply within mixing zones, except for the requirements prescribed in R 323.1050, or as otherwise specified by these rules. (2) Water quality standards prescribed by these rules are minimally acceptable water quality conditions and shall apply at all flows equal to or exceeding the design flows, except where the department determines that a more restrictive design flow is necessary. The design flows in lotic systems shall be as follows: (a) Unless otherwise stated in this rule, the design flow is equal to the lowest of the 12 monthly 95% exceedance flows. The 95% exceedance flow is the flow equal to or exceeded 95% of the time for the specified month. (b) For human health values, the design flow is equal to the harmonic mean flow. (c) For wildlife values, the design flow is equal to the 90-day, 10-year low flow (90Q10). (3) A maximum of 4 seasonal design flows may be granted when determining surface water effluent limitations for ammonia or substances not addressed by R 323.1057 if it is determined by the department that the use of such design flows will protect water quality and be consistent with the protection of the public health, safety, and welfare. The seasonal design flows shall be the lowest of the monthly 95% exceedance flow for the months in each season. (4) Alternate design flows may be used for intermittent wet weather discharges as necessary to protect the designated uses of the receiving water.