(1) Subsurface sewage treatment systems may not be used if natural slopes are greater than 15%; however, the department may, by waiver granted pursuant to ARM 17.36.601, allow a sewage treatment system with a design flow of 5000 gallons per day or less on slopes between 15% and 25%, if a registered professional engineer or a person qualified to evaluate and identify soil in accordance with ASTM standard D5921-96el (Standard Practice for Subsurface Site Characterization of Test Pits for On-Site Septic Systems) submits adequate evidence that there will be no visible outflow of liquid downslope from the subsurface sewage treatment system.

(2) Subsurface sewage systems may not be installed on unstable landforms, as defined in ARM 17.36.320.

(3) No component of any sewage treatment system may be located under structures or driveways, parking areas or other areas subjected to vehicular traffic, 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.

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

(5) 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.