(1) Except as provided in ARM 17.36.916, all wastewater treatment systems must be designed and constructed in accordance with the applicable requirements in ARM 17.36.913 and in Department Circular DEQ-4.

(2) Department Circular DEQ-4, 2023 edition, which sets forth standards for subsurface sewage treatment systems, and Department Circular DEQ-2, 2016 edition, which sets forth design standards for public sewage systems, are adopted and incorporated by reference for purposes of this subchapter. All references to these documents in this subchapter refer to the editions set out above. Copies are available from the Department of Environmental Quality, P.O. Box 200901, Helena, MT 59620-0901.

(3) Wastewater treatment systems must be located to maximize the vertical separation distance from the bottom of the absorption trench to the seasonally high ground water level, bedrock, or other limiting layer, but under no circumstances may this vertical separation be less than four feet of natural soil.

(4) A replacement area or replacement plan must be provided for each new or expanded wastewater treatment system. Replacement areas and plans must comply with the requirements of this subchapter.

(5) A site evaluation must be performed for each wastewater treatment system. As determined by the reviewing authority, the site evaluation may include the following:

(a) soil descriptions for proposed wastewater treatment systems. Soil descriptions must be based on data obtained from test holes within 25 feet of each wastewater treatment location. Test holes must be at least eight feet in depth unless a limiting layer precludes digging to eight feet;

(b) percolation test results within the boundaries of the proposed wastewater treatment system; and

(c) if the applicant or the reviewing authority has reason to believe that ground water will be within seven feet of the surface at any time of the year within the boundaries of the system, the applicant must provide data to demonstrate that the minimum separation distance required by (3) between the absorption trench bottom and the seasonally high ground water level can be maintained. The reviewing authority may require the applicant to install ground water observation wells to a depth of at least eight feet to determine the seasonally high ground water level. The applicant shall monitor the observation wells through the seasonally high ground water period. Measurement must occur for a long enough period of time to detect a peak and a sustained decline in the ground water level.

(6) If a department-approved public collection and treatment system is readily available within a distance of 200 feet of the property line for connection to a new source of wastewater, or as a replacement for a failed system, and the owner or managing entity of the public collection and treatment system approves the connection, wastewater must be discharged to the public system. For purposes of this rule:

(a) a public system is not "readily available" if there is evidence demonstrating that connection to the system is physically or economically impractical, or that easements cannot be obtained; and

(b) a connection is "economically impractical" if the cost of connection to the public system equals or exceeds three times the cost of installation of an approvable system on the site.

(7) Wastewater treatment systems, except for sealed components that are designed, constructed, and tested as set out in ARM 17.36.918, may not be located in drainage ways.