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Kentucky’s New Legislation- Modifying Water Discharge KPDES Permits Allowing for Rapid and Complete Mixing

Posted: August 7th, 2021

Authors: Karen T.  William S. 

On March 24, 2021, Kentucky House Bill (HB) 386 was enacted into law. The purpose of the legislation was to amend Kentucky’s regulations on water quality standards to recognize the effect of rapid and complete mixing (RCM) on industrial discharges and clarify the reinstatement of mixing zones for bioaccumulative chemicals of concern (BCC) that existed prior to September 8, 2004.  This legislation provides another tool for compliance with water quality standards for facilities using a submerged high-rate multi-port diffuser at the point of discharge into the receiving stream.

Federal regulations at 40 CFR 122.44(d)(ii) (incorporated by reference at 401 Kentucky Administrative Regulation (KAR) 5:065 Section 1(4)) allow permitting authorities, when determining reasonable potential (for a pollutant to be present above the water quality standard) , to account for “dilution of the effluent in the receiving water”. RCM and mixing zones are two distinct tools a permitting authority can use to consider dilution of the receiving water and where water quality criteria are met. RCM is applied in situations where complete mixing occurs, whereas mixing zones are applied in situations where incomplete mixing occurs. Therefore, RCM and mixing zones are independent of each other, but both can be used to determine the appropriate water quality-based effluent limitations.

The codification of HB 386 serves to provide Kentucky dischargers who invest in certain technologies with an additional tool for meeting water quality criteria without any need for additional rulemaking from the Kentucky Energy and Environment Cabinet (EEC). Instead, dischargers who have installed submerged high-rate multi-port diffusers or outfall structures and want to benefit from this provision request EEC to modify their Kentucky Pollutant Discharge Elimination System (KPDES) permit limits and conditions to reflect the effect of RCM when they apply for a permit or have their permit renewed or modified. Kentucky Division of Water (KDOW) is currently reviewing the options for evaluating RCM.

The RCM provisions in Section 2 of HB 386 serve to define RCM and to require that EEC recognize and apply the effects of RCM when establishing the discharge limits and conditions for Kentucky Pollutant Discharge Elimination System (KPDES) discharges whenever the discharge occurs through a submerged high-rate multi-port diffuser or outfall structure.  The legislation recognizes that when discharges occur through a submerged high-rate multi-port diffuser or outfall structure, the resulting effluent is mixed so rapidly and completely upon discharge that water quality criteria are met almost immediately in the limited area surrounding or downstream from the point of discharge.

Because the point of compliance set in permits is end-of-pipe, the effluent limits in permits that use this technology should reflect this effect that occurs immediately after leaving the pipe. HB 386 recognizes this need by mandating that EEC consider the effect of RCM when setting permit limits (i.e., in issuing, renewing or modifying KPDES permits).

RCM is a distinct concept from a mixing zone.  401 KAR 10:001 Section 1(49) state that a mixing zone is an area where the “discharge is in transit and progressively diluted from the source to the receiving system”. Therefore, a mixing zone allows water quality criteria to be exceeded within a larger zone authorized by EEC.

KDOW is currently reviewing permits with diffusers to determine their next course of action based on the new act.  The United States Environmental Protection Agency (U.S. EPA) Region 4 will also be reviewing KPDES permits with changes based on the new act during the public comment period.

ALL4 will continue to track HB 386 and its impact on KPDES dischargers.  ALL4 will be co-presenting on this topic at the Kentucky Chamber 24th Annual Kentucky Environmental Permitting & Reporting Conference. If you have any questions, please contact Karen Thompson (kthompson@all4inc.com) at 859-281-1664 or William Shane (wshane@all4inc.com) at 859-223-0123.


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