(1) The applicant shall submit a storm drainage plan in accordance with department Circular DEQ-8 to the reviewing authority.

(2) Storm drainage plans must be prepared by a professional engineer if the subdivision application proposes either of the following:

(a) six or more lots; or

(b) a lot proposed with greater than 25 percent impervious area.

(3) Storm drainage plans that are required to be designed by a professional engineer under (2) must meet the requirements in ARM 17.36.121 except for the following:

(a) a three-year expiring approval in ARM 17.36.121 does not apply;

(b) certification and as-built drawing requirements listed in ARM 17.36.121(2) through (4) do not apply for retention ponds for residential lots, designed solely for, and located on, the lot where the additional runoff is generated; and

(c) certification and as-built drawing requirements listed in ARM 17.36.121(2) through (4) are required for storm water facilities that treat additional runoff generated on 2 or more lots.

(4) A storm drainage plan must include a maintenance plan for all drainage structures. The maintenance plan must describe the drainage structures, provide a maintenance schedule, and designate the entity responsible for performing maintenance. The reviewing authority may require the applicant to create a homeowner's association or other legal entity that will be responsible for maintenance of storm drainage structures and that will have authority to charge appropriate fees. The maintenance plan must include easements and agreements as necessary for operation and maintenance of all proposed storm drainage structures or facilities.

(5) The applicant shall obtain an easement if the reviewing authority determines the easement is needed to allow adequate operation and maintenance of the facilities. The easement must be filed with the county clerk and recorder at the time the certificate of subdivision approval issued under this chapter is filed. The easement must be in one of the following forms:

(a) in writing signed by the grantor of the easement; or

(b) if the same person owns both parcels, shown on the plat or certificate of survey for the proposed subdivision.

(6) The requirements of (1), (2), (3), and (4) are not required under the following circumstances:

(a) subdivisions located entirely within a first-class or second-class municipality, as described in 7-1-4111, MCA, or within a Municipal Separate Storm Sewer System (MS4) general permit area, as defined in ARM 17.30.1102, if:

(i) the applicant submits to the reviewing authority a letter of consent from the municipal or MS4 entity on a form provided by the department; and

(ii) the municipal or MS4 entity either accepts the stormwater into a municipal storm water system or requires the applicant to comply with municipal or MS4 storm water drainage design standards;

(b) lots 5 acres or larger in size if the applicant provides information demonstrating that the total impervious area on the lot will be less than 5% of the lot area, including easements and right-of-ways; or

(c) lots that are exempt from review under 76-3-207(1)(a), (d), (e), or (f), MCA, if the parcels are used for one living unit and no more than 25% of each qualifying parcel is impervious.  To qualify for this exemption, the applicant must provide an estimate of each parcel's total impervious area and verification of the intended use. A change in the percentage of impervious area beyond 25% or changing from more than one living unit will require review.

(7) If material will be displaced or added within a delineated floodplain, the applicant shall provide evidence that the floodplain permit coordinator has been notified and that appropriate approvals can be  obtained. Adequate evidence must be in the form of a floodplain permit or certification from the floodplain coordinator.

(8) If applicable, the applicant shall obtain an MPDES permit for storm water discharges, pursuant to ARM Title 17, chapter 30.

(9) Storm water that reaches state surface waters must be treated prior to discharge if the reviewing authority determines that untreated storm water is likely to degrade the receiving waters.

(a) minimum treatment of storm water consists of removal of settleable solids and floatable material. The reviewing authority may require more extensive treatment if deemed necessary to protect state waters from degradation;

(b) plans for the treatment facility must be approved by the reviewing authority.

(10) The department may grant a waiver from any of the requirements in this rule pursuant to the provisions of ARM 17.36.601.