Montana Pollutant Discharge Elimination System (MPDES)

The goal of the Montana Pollutant Discharge Elimination system (MPDES) program is to control point source discharges of wastewater such that water quality is the receiving streams is protected. Levels of water quality that are required to maintain the various beneficial uses of the receiving streams are set forth in the Water Quality Standards (WQS).

All point sources of wastewater discharge are required to obtain and comply with MPDES permits. The effluent limitations and other conditions contained in MPDES permits are based upon preservation of the WQS, with certain categories of wastewaters being required to be treated to a federally-specified minimum level (technology-based treatment) in addition to WQS requirements. WQS requirements for pollutant levels in the discharge are calculated at the average design wastewater flow and the seven-day, ten-year low stream flow (7Q10) in the receiving stream. At streamflows below the 7Q10, the WQS and MPDES do not give further protection from pollutant discharges.

The Nondegradation Rules are a part of the WQS that apply to new or increased sources of pollution. These rules prohibit increases in discharge of toxic and deleterious materials to state waters, unless it is affirmatively demonstrated to the DEQ that a change is justifiable as a result of necessary economic or social development and will not preclude present and anticipated use of these waters.

Some common pollutants that are limited under Nondegradation are nutrients, heavy metals, and toxic organic pollutants. These same pollutants could also be limited under the WQS in existing dischargers’ permits. The difference would be that the WQS levels would be calculated to achieve less than chronic toxicity levels instream at the 7Q10, whereas nondegradation limits in new or enlarged point source discharges would be set at baseline instream concentrations plus a "trigger level" amount which would define the "significance" threshold.

Each MPDES permit issued is designed to protect the receiving stream quality at the point of discharge. In addition, recognizing the dynamic nature of streams and the potential additive or cumulative effects of pollutants, MPDES permits also address stream reach or basin wide pollution problems. A calculation process called total maximum daily load (TMDL) is used to apportion allowable pollutant discharge levels among the various dischargers. If reductions of a given pollutant in a stream reach or basin are found necessary to meet WQS, the TMDL process is used to apportion the reductions among the dischargers in that reach or basin.

The program is funded by application and annual permit fees. Activities of program staff include public education, reviewing applications, determining effluent limits and best management practices, environmental assessments, public participation and information retrieval, effluent and instream data review and management, field inspections, enforcement, regulation and guidance preparation, program planning and administration.