2020 MS4 General Permit Section 20 MCM 5: Post-Construction Stormwater Management 2020 MS4 General Permit Section 20 MCM 5: Post-Construction Stormwater Management

Section 20 MCM 5: Post-Construction Stormwater Management

20.1

MCM 5: Post-Construction Stormwater Management. [Minn. R. 7090]

20.2

New permittees must develop, implement, and enforce, and existing permittees must revise their current program, as necessary, and continue to implement and enforce, a Post-Construction Stormwater Management program that prevents or reduces water pollution after construction activity is completed. The program must address construction activity with land disturbance of greater than or equal to one acre, including projects less than one acre that are part of a larger common plan of development or sale, within the permittee's jurisdiction and that discharge to the permittee's MS4. The permittee must incorporate Section 20 requirements into their program. [Minn. R. 7090]

20.3

To the extent allowable under state or local law, the permittee must develop, implement, and enforce a regulatory mechanism(s) that incorporates items 20.4 through 20.15. A regulatory mechanism(s) for the purposes of the General Permit may consist of contract language, an ordinance, permits, standards, written policies, operational plans, legal agreements, or any other mechanism, that will be enforced by the permittee. [Minn. R. 7090]

20.4

The permittee's regulatory mechanism(s) must require owners of construction activity to submit site plans with postconstruction stormwater management BMPs designed with accepted engineering practices to the permittee for review and confirmation that regulatory mechanism(s) requirements have been met, prior to start of construction activity. [Minn. R. 7090]

20.5

The permittee's regulatory mechanism(s) must require owners of construction activity to treat the water quality volume on any project where the sum of the new impervious surface and the fully reconstructed impervious surface equals one or more acres. [Minn. R. 7090]

20.6

For construction activity (excluding linear projects), the water quality volume must be calculated as one (1) inch times the sum of the new and the fully reconstructed impervious surface. [Minn. R. 7090]

20.7

For linear projects, the water quality volume must be calculated as the larger of one (1) inch times the new impervious surface or one-half (0.5) inch times the sum of the new and the fully reconstructed impervious surface. Where the entire water quality volume cannot be treated within the existing right-of-way, a reasonable attempt to obtain additional right-ofway, easement, or other permission to treat the stormwater during the project planning process must be made. Volume reduction practices must be considered first, as described in item 20.8. Volume reduction practices are not required if the practices cannot be provided cost effectively. If additional right-of-way, easements, or other permission cannot be obtained, owners of construction activity must maximize the treatment of the water quality volume prior to discharge from the MS4. [Minn. R. 7090]

20.8

Volume reduction practices (e.g., infiltration or other) to retain the water quality volume on-site must be considered first when designing the permanent stormwater treatment system. The General Permit does not consider wet sedimentation basins and filtration systems to be volume reduction practices. If the General Permit prohibits infiltration as described in item 20.9, other volume reduction practices, a wet sedimentation basin, or filtration basin may be considered. [Minn. R. 7090]

20.9

Infiltration systems must be prohibited when the system would be constructed in areas:

a. that receive discharges from vehicle fueling and maintenance areas, regardless of the amount of new and fully reconstructed impervious surface;

b. where high levels of contaminants in soil or groundwater may be mobilized by the infiltrating stormwater. To make this determination, the owners and/or operators of construction activity must complete the Agency's site screening assessment checklist, which is available in the Minnesota Stormwater Manual, or conduct their own assessment. The assessment must be retained with the site plans;

c. where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour;

d. with less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock;

e. of predominately Hydrologic Soil Group D (clay) soils;

f. in an Emergency Response Area (ERA) within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, Subp. 13, classified as high or very high vulnerability as defined by the Minnesota Department of Health;

g. in an ERA within a DWSMA classified as moderate vulnerability unless the permittee performs or approves a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater;

h. outside of an ERA within a DWSMA classified as high or very high vulnerability unless the permittee performs or approves a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater;

i. within 1,000 feet up-gradient or 100 feet down gradient of active karst features; or

j. that receive stormwater runoff from these types of entities regulated under NPDES for industrial stormwater: automobile salvage yards; scrap recycling and waste recycling facilities; hazardous waste treatment, storage, or disposal facilities; or air transportation facilities that conduct deicing activities.

See "higher level of engineering review" in the Minnesota Stormwater Manual for more information. [Minn. R. 7090]

20.10

For non-linear projects, where the water quality volume cannot cost effectively be treated on the site of the original construction activity, the permittee must identify, or may require owners of the construction activity to identify, locations where off-site treatment projects can be completed. If the entire water quality volume is not addressed on the site of the original construction activity, the remaining water quality volume must be addressed through off-site treatment and, at a minimum, ensure the requirements of items 20.11 through 20.14 are met. [Minn. R. 7090]

20.11

The permittee must ensure off-site treatment project areas are selected in the following order of preference:

a. locations that yield benefits to the same receiving water that receives runoff from the original construction activity;

b. locations within the same Department of Natural Resource (DNR) catchment area as the original construction activity;

c. locations in the next adjacent DNR catchment area up-stream; or

d. locations anywhere within the permittee's jurisdiction. [Minn. R. 7090]

20.12

Off-site treatment projects must involve the creation of new structural stormwater BMPs or the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP. Routine maintenance of structural stormwater BMPs already required by the General Permit cannot be used to meet this requirement. [Minn. R. 7090]

20.13

Off-site treatment projects must be completed no later than 24 months after the start of the original construction activity. If the permittee determines more time is needed to complete the treatment project, the permittee must provide the reason(s) and schedule(s) for completing the project in the annual report. [Minn. R. 7090]

20.14

If the permittee receives payment from the owner of a construction activity for off-site treatment, the permittee must apply any such payment received to a public stormwater project, and all projects must comply with the requirements in items 20.11 through 20.13. [Minn. R. 7090]

20.15

The permittee's regulatory mechanism(s) must include the establishment of legal mechanism(s) between the permittee and owners of structural stormwater BMPs not owned or operated by the permittee, that have been constructed to meet the requirements in Section 20. The legal mechanism(s) must include provisions that, at a minimum:

a. allow the permittee to conduct inspections of structural stormwater BMPs not owned or operated by the permittee, perform necessary maintenance, and assess costs for those structural stormwater BMPs when the permittee determines the owner of that structural stormwater BMP has not ensured proper function;

b. are designed to preserve the permittee's right to ensure maintenance responsibility, for structural stormwater BMPs not owned or operated by the permittee, when those responsibilities are legally transferred to another party; and

c. are designed to protect/preserve structural stormwater BMPs. If structural stormwater BMPs change, causing decreased effectiveness, new, repaired, or improved structural stormwater BMPs must be implemented to provide equivalent treatment to the original BMP. [Minn. R. 7090]

20.16

The permittee must maintain a written or mapped inventory of structural stormwater BMPs not owned or operated by the permittee that meet all of the following criteria:

a. the structural stormwater BMP includes an executed legal mechanism(s) between the permittee and owners responsible for the long-term maintenance, as required in item 20.15; and

b. the structural stormwater BMP was implemented on or after August 1, 2013. [Minn. R. 7090]

20.17

The permittee must implement written procedures for site plan reviews conducted by the permittee prior to the start of construction activity, to ensure compliance with requirements of the permittee's regulatory mechanism(s). [Minn. R. 7090]

20.18

The permittee must ensure that individuals receive training commensurate with their responsibilities as they relate to the permittee's Post-Construction Stormwater Management program. Individuals includes, but is not limited to, individuals responsible for conducting site plan reviews and/or enforcement. The permittee must ensure that previously trained individuals attend a refresher-training every three (3) calendar years following the initial training. [Minn. R. 7090]

20.19

The permittee must maintain written enforcement response procedures (ERPs) to compel compliance with the permittee's regulatory mechanism(s) required in Section 20. At a minimum, the written ERPs must include:

a. a description of enforcement tools available to the permittee and guidelines for the use of each tool; and

b. name or position title of responsible person(s) for conducting enforcement. [Minn. R. 7090]

20.20

For each site plan review conducted by the permittee, the permittee must document the following:

a. supporting documentation used to determine compliance with Section 20 of the General Permit, including any calculations for the permanent stormwater treatment system;

b. the water quality volume that will be treated through volume reduction practices (e.g., infiltration or other) compared to the total water quality volume required to be treated;

c. documentation associated with off-site treatment projects authorized by the permittee, including rationale to support the location of permanent stormwater treatment projects in accordance with items 20.10 and 20.11;

d. payments received and used in accordance with item 20.14; and

e. all legal mechanisms drafted in accordance with [item 20.15, including date(s) of the agreement(s) and name(s) of all responsible parties involved. Minn. R. 7090]

20.21

For each training in item 20.18, the permittee must document:

a. general subject matter covered;

b. names and departments of individuals in attendance; and

c. date of each event. [Minn. R. 7090]

20.22

The permittee must document any enforcement conducted pursuant to the ERPs in item 20.19, including verbal warnings. At a minimum, the permittee must document the following:

a. name of the person responsible for violating the terms and conditions of the permittee's regulatory mechanism(s);

b. date(s) and location(s) of the observed violation(s);

c. description of the violation(s);

d. corrective action(s) (including completion schedule) issued by the permittee;

e. referrals to other regulatory organizations (if any); and

f. date(s) violation(s) resolved. [Minn. R. 7090]

20.23

The permittee must conduct an annual assessment of the Post-Construction Stormwater Management program to evaluate program compliance, the status of achieving the measurable requirements in Section 20, and determine how the program might be improved. Measurable requirements are activities that must be documented or tracked as applicable to the MCM (e.g., inventory, trainings, site plan reviews, inspections, enforcement, etc.). The permittee must perform the annual assessment prior to completion of each annual report and document any modifications made to the program as a result of the annual assessment. [Minn. R. 7090]