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Water Quality | Nondegradation Rule Update | Comment Period Ends April 23, 2024 | Public Meeting April 23, 2024

  • March 08 2024

The proposed notice of rulemaking for the Nondegradation of Water Quality and Mixing Zones in Surface and Ground Water Rule Updates was published in Issue 5 of the Montana Administrative Register today (March 8, 2024) as Notice No. 17-439.

These rule updates incorporate changes adopted by the 2023 Legislative Session under Senate Bill 285. SB 285 directed the department to, among other things, adopt criteria for crediting nitrogen attenuation at the drainfield and riparian zone based on soil type in making nonsignificance determinations for non-permitted septic system discharges.

SB 285 required the department to adopt new criteria to determine when septic system discharges that are not subject to permit requirements result in nonsignificant changes in water quality.

SB 285 also required the department to consider elevation and the horizontal distance between the discharge and the surface water in the direction of ground water flow and that nondegradation analysis for surface water impacts must be limited to a maximum of 1/2 mile.

The rule update also includes changes to replace inconsistent standard groundwater mixing zone lengths that were based on wastewater treatment system size, lot size, subdivision size, and type of water system, which results in widely different standard mixing zones for wastewater treatment systems designed for the same use and the same design wastewater flow. The proposed changes are necessary to provide flexible and consistent standard mixing zone lengths that are based on the most important factors, design flow and the nature of the wastewater discharge.

Finally, the department proposes to remove references to level 1a and level 1b throughout the rule because they are outdated and unnecessary. Technological improvements in nitrogen removal have made it necessary to address those systems that provide additional nitrogen reductions. The department proposes to add level 3 and level 4 approvals to provide distinct designations for different amounts of nitrogen treatment instead of the single level 2 designation that does not specifically account for different nitrogen reductions and effluent concentrations in rule. In conjunction with those rules the department has also updated rules regarding the requirements for classifying wastewater treatment systems as level 2, 3 or 4 treatment and the long-term operation and maintenance requirements for those treatment systems.

Public comment on the rule updates will be accepted through April 23, 2024, and a public hearing will be held on April 23, 2024 at 1:00 PM in Room 111 of the Metcalf Building and online.

If you would like to provide comments regarding the Nondegradation of Water Quality and Mixing Zones in Surface and Ground Water Rule Updates, please email them to

Thank you for your interest in this process. We are interested in your feedback!

Associated Materials and Links:

How to submit public comment:

Mail comments to:
Department of Environmental Quality
P.O. Box 200901
Helena, Montana 59620-0901

Or email comments to:

Comment period ends April 23, 2024 at 5:00 PM

Public meeting information:

April 23, 2024 at 1:00 PM
Room 111, Metcalf Building
1520 E 6th Ave.
Helena, MT 59601

Or, join online via Zoom
Webinar ID: 851 5190 8418
Passcode: 221161
Telephone: (206) 337-9723

Tags: Public Comment, Water, Public Meeting and Rule Making