Administrative Rules of Montana
Department 17 - ENVIRONMENTAL QUALITY
Chapter 17.36 - SUBDIVISIONS/ON-SITE SUBSURFACE WASTEWATER TREATMENT
Subchapter 17.36.3 - Subdivision Requirements
Rule 17.36.322 - SEWAGE SYSTEMS: SITING

(1) Gravity-fed subsurface sewage treatment systems may not be used if natural slopes are greater than 15 percent. A pressure-dosed sewage treatment system with a design flow of 5000 gallons per day or less may be used on slopes greater than 15 percent and up to 25 percent, if a professional engineer or a person qualified to evaluate and identify soil in accordance with Department Circular DEQ-4 submits adequate evidence that there will be no visible outflow of liquid downslope from the subsurface sewage treatment system.

(2) The department may grant a waiver, pursuant to ARM 17.36.601 and after consultation with the local health department, to allow pressure-dosed subsurface sewage treatment systems on slopes greater than 25 percent and up to 35 percent if a professional engineer or a person qualified to evaluate and identify soil in accordance with Department Circular DEQ-4 submits adequate evidence that there will be no visible outflow of liquid downslope from the subsurface sewage treatment system.

(3) Subsurface sewage treatment systems may not be installed on unstable landforms, as defined in ARM 17.36.101.

(4) No component of any sewage treatment system may be located under structures or driveways, parking areas or other areas subjected to vehicular traffic, or other areas subject to compaction, except for those components of the system designed to accommodate such conditions. Drainfields must not be located in swales or depressions where runoff may flow or accumulate.

(5) Pursuant to 76-4-104(6)(i), MCA, a proposed drainfield mixing zone must meet the requirements of ARM 17.36.122(6).

(6) For lots two acres in size or less, the applicant shall physically identify the drainfield location by staking or other acceptable means of identification. For lots greater than two acres in size, the department may require the applicant to physically identify the drainfield location.

(7) The department may require the applicant to show detailed lot layouts on a contour map if the department determines that there is a question about suitability of the drainfield location.