Nondegradation Determinations & Mixing Zones

The Water Quality Act (Montana Code Annotated (MCA)) requires the Department of Environmental Quality to protect high quality state water from degradation. The nondegradation rules (17.30.701 et seq. Administrative Rules of Montana (ARM)) were adopted to implement the Act. The nondegradation rules apply to any activity that may affect the quality of surface or ground water.

Nondegradation determinations are typically associated with a mixing zone. A mixing zone is a limited area, within a surface water or ground water, where dilution of a discharge may occur. All applicable water quality standards and nondegradation limits must be met at the end of a mixing zone. There are numerous restrictions on the size, the location, the changes that are allowed within a mixing zone, and if the Department may grant a mixing zone. Please read the mixing zone rules (ARM 17.30.501 et seq.) for more details.

Application:  Simply put, the purpose of the nondegradation rules are to protect high quality state ground and surface waters. High quality waters are those waters whose quality is higher than the established standards (high quality state waters are defined in 75-5-103(10), MCA). Some nondegradation limits are set at definite concentrations called trigger levels (listed in Department Circular DEQ-7) or at a percentage of the lowest applicable water quality standard. Other nondegradation limits are qualitative, such as those for nitrogen and phosphorus in surface water. Whenever a person conducts an activity that may impact water quality, they must comply with the nondegradation requirements (this applies whether the activity is or is not regulated by the Department). If the activity is regulated by the Department, the Department will ensure compliance with the nondegradation requirements prior to issuing its permit, license or other authorizations. A person may also request a nondegradation significance determination and submit information to the Department to demonstrate the activity will cause nonsignificant degradation of state waters. The proposed activity may not begin until the Department has determined the activity will cause nonsignificant degradation or has issued an authorization to degrade (authorizations to degrade are explained later).

The Water Quality Act (75-5-317, MCA) exempts certain activities from the nondegradation requirements (i.e., automatically classifies them as "nonsignificant"). Exemptions are based on the activities low potential for harm to human health and the environment, and must meet the criteria listed in 75-5-301(5)(c), MCA. The nondegradation rules only apply to "new or increased sources" as of April 29, 1993 (ARM 17.30.702(16)). This clause exempts discharges that were existing or permitted, authorized or approved by the Department (or its predecessor) prior to April 29, 1993. These exemptions only apply to the nondegradation requirements, they do not apply to the state water quality standards which include the human health and aquatic life standards listed in the surface and ground water standards and in WQB-7.

A new community wastewater treatment system for a small town or subdivision would be an example of an activity requiring a nondegradation significance determination. One of the constituents of concern in domestic wastewater is nitrate. In this example, the system consists of a lagoon and an infiltration/percolation (IP) cell. The IP cell allows the treated wastewater to infiltrate into the ground water at a certain rate. A mixing zone would be specified starting at the IP cell and extend in the direction of ground-water flow for a specified distance.

The entity proposing the activity must demonstrate that the nitrate (as nitrogen) concentration at the end of the mixing zone will remain below the nondegradation trigger level, 5 milligrams per liter (mg/L), when the wastewater system is operating.

Many dischargers are required to monitor their discharge quality or the water in the mixing zone to ensure long-term compliance with the nondegradation requirements.  This is a simple example, many mixing zones include multiple parameters with more complex water quality limits.

Significance determinations in surface water can be similar to the example described above. However, different limits often apply to the same constituent depending on whether the constituent is discharged to ground water or surface water. For example, the trigger level for nitrate (as nitrogen) is 5 mg/L in ground water, but is 0.01 mg/L in surface water.

If the Department determines a proposed activity will cause significant degradation of state waters, the applicant has the options of submitting more information to change the determination, appealing the decision to the Department Director or the Board of Environmental Review, or submitting an application to degrade state waters. An application to degrade state waters recognizes that the activity will degrade state waters, but must demonstrate the reasons why the Department should allow the activity. The information required to request an authorization to degrade is in 75-5-303, MCA and ARM 17.30.701 et seq. Anyone may submit comments or information to the Department regarding a significance determination or a request to authorize degradation.

Fees:  The Subdivision Section charges a fee for the nondegradation review pursuant to ARM 17.36.802(1); the rest of the Department includes the review as part of other fees. The fee schedule for reviews of authorizations to degrade are listed in ARM 17.30.201(3)(b)&(c).

Other Information: Various programs within the Department’s Water Quality Division support practices that help ensure the nondegradation of state waters. The Source Water Protection Program is directly responsible for identifying the susceptibility of public water supply systems to potential contaminant sources and provides technical assistance in the development of local water resource protection plans. The program also provides reviews of certain proposed activities in watersheds of surface water-based public water supply systems.

The Department’s Engineering Bureau works in conjunction with Permitting at DEQ to assure  nondegradation standards will be achievable in the Montana Pollutant Discharge Elimination System (MPDES) permit or the Montana Ground Water Pollution Control System (MGWPCS) permit.