APPENDIX C. BENCHMARK MONITORING WAIVER FOR INDUSTRIAL STORMWATER INFILTRATION AND PONDING

This Appendix addresses requirements for the Benchmark Monitoring Waiver for industrial stormwater infiltration devices, and industrial stormwater ponds as defined in Appendix E. (Note that effluent limit monitoring is not exempt from monitoring.) The Benchmark Monitoring Waiver is not granted unless all applicable requirements of the permit, and specifically this Appendix, are complied with. Note that Parts III and VII of the permit have specific additional sector or subsector requirements regarding stormwater infiltration. Permittees must be in compliance with Part III and VII requirements in order to utilize infiltration or ponding as part of a stormwater treatment system. Violation of any specific requirements that may affect the operation of the industrial stormwater pond or infiltration device may result in revocation of the monitoring waiver.

Part I. Requirements for a Benchmark Monitoring Waiver for Infiltration Devices

Unless specifically prohibited from obtaining a Benchmark Monitoring Waiver under the sector or subsector requirements of Part VII, a Benchmark Monitoring Waiver is authorized for infiltration devices that are operated in accordance with the applicable requirements of the permit, including this Appendix. For infiltration devices that meet the requirements of a Benchmark Monitoring Waiver, bypasses or overflow of stormwater from storm events that exceed the Benchmark Monitoring Waiver design capacity (specified in this Appendix), are not required to be monitored for benchmark parameters. (Note that effluent limit monitoring is not exempt from monitoring.) To obtain a Benchmark Monitoring Waiver, the Permittee must comply with the following terms and conditions which shall be considered the least stringent acceptable values for obtaining a Benchmark Monitoring Waiver:

A. Design Requirements for a Benchmark Monitoring Waiver for Infiltration Devices

Infiltration devices shall be designed consistent with accepted engineering practices. Designs shall be approved by a professional engineer or other licensed professional. Accepted practices are generally described in the applicable portions of the Minnesota Stormwater Manual (Chapter 12-8 and other parts). Other applicable technical sources can be used as appropriate. The design shall meet the following minimum requirements for a Benchmark Monitoring Waiver:

1. Infiltration devices shall be designed and operated to infiltrate at a long term expected rate of no less than 0.2 inches per hour to no greater than 1.63 inches per hour. (See the Minnesota Stormwater Manual for detailed information on soils and long term versus perk test or infiltrometer measurements).

2. The infiltration device shall provide, at minimum, a storage volume that will contain the entire volume of runoff to the infiltration device, up to and including the 2-year, 24-hour storm event.

3. Infiltration devices shall infiltrate the design storage volume (which may be equal to or greater than the runoff from the 2 year event) within 48 hours at the long term infiltration rate. The calculated design volume of runoff shall be based on United States Weather Bureau Technical Paper 40 (USWB TP 40) and the runoff characteristics of the watershed to the infiltration device.

4. Infiltration devices shall have suitable soils to provide treatment at the design long term infiltration rate. Testing shall be conducted to ensure that the infiltration device stormwater storage area has at least three feet of suitable soils between the stormwater storage area and either groundwater, the soil elevation leaving evidence of seasonally saturated soils, or fractured bedrock, whichever is least. Soil testing shall be based on the appropriate testing recommendations of the Minnesota Stormwater Manual or equivalent professional sources.

B. Operation and Maintenance

1. The infiltration device shall be maintained and operated to meet the design criteria. In addition, outlets, overflows or bypasses shall be designed, maintained and modified as needed to expedite maintenance including periodic cleaning and repair. For example, pre-settling of solids, removal of floatable material, and other maintenance actions which allow the Permittee to provide effective long term operation of the infiltration device.

2. Newly constructed or up-graded infiltration devices shall be visually inspected after all precipitation events for 30 days after initiation of operation, and thereafter in accordance with inspection requirements outlined in Part III.F of the permit or, if applicable, Part VII of the permit, to ensure that infiltration is occurring at the appropriate rate and the device is operating correctly.

3. Appropriate access, equipment, and training for staff must be provided for operation and maintenance of the infiltration devices.

4. Permit violations regarding the design, operation, and maintenance of an infiltration device, may be grounds for the Agency to revoke the Benchmark Monitoring Waiver.

C. Documentation

The design basis for meeting the criteria for a Benchmark Monitoring Waiver under this part must be kept with the SWPPP. All design assumptions, operational and maintenance methods, tests, calculations and monitoring shall be kept with the SWPPP. Portions of the SWPPP that are essential to operations may be summarized with specific references, while design and reference documents must be made available within 72 hours of request.

Part II. Requirements for a Benchmark Monitoring Waiver for Industrial Stormwater Ponds

Unless specifically prohibited from obtaining a Benchmark Monitoring Waiver under the sector or subsector requirements of Part VII, a Benchmark Monitoring Waiver is authorized for industrial stormwater ponds that are operated in accordance with the applicable requirements of the permit, including this Appendix. For industrial stormwater ponds that meet the requirements of a Benchmark Monitoring Waiver, bypasses or overflow of stormwater from storm events that exceed the Benchmark Monitoring Waiver design capacity (specified in this Appendix), are not required to be monitored for benchmark parameters. (Note that effluent limit monitoring is not exempt from monitoring, under this part.) To obtain a Benchmark Monitoring Waiver, the Permittee must comply with the following terms and conditions which shall be considered the least stringent acceptable values for obtaining a Benchmark Monitoring Waiver.

A. Design Requirements for an Industrial Stormwater Pond Benchmark Monitoring Waiver

Industrial stormwater ponds qualifying for a Benchmark Monitoring Waiver shall be designed consistent with accepted engineering practices and approved by a professional engineer or other licensed professional. Generally accepted practices are described in the applicable portions of the Minnesota Stormwater Manual (Chapter 12-9 and other parts). Other applicable technical sources can be used as appropriate. The design shall meet the following minimum requirements:

1. The industrial stormwater pond must be designed to have a permanent dead storage volume, which is the volume below the normal outlet, that is equal to or exceeds the entire runoff volume to the pond which would result from the 5-year, 24-hour rainfall event (based on USWB TP 40).

2. The industrial stormwater pond shall be designed and operated to eliminate scour and re-suspending of sediment at high flows, so that benchmark values will be expected to be met up to the 10-year, 24-hour storm event (event based on USWB TP 40).

3. Industrial stormwater pond permanent storage (dead storage below the outlet) volume must be designed to eliminate scour and re-suspension of settled solids for the expected flow velocities. The maximum permanent storage (or dead storage) depth must be adjusted for the site conditions to provide enough sediment storage, and to prevent scour. The depth shall be limited to prevent anaerobic conditions from developing in the pool. The optimum depth in the permanent pool usually varies between 3 feet to 10 feet deep, depending on the site-specific conditions of flow and the nature of the pollutants.

4. Skimmers, screens, or equivalent collection devices shall be incorporated into the outlets so that floatable materials in the industrial stormwater pond will not be discharged. Such devices shall be inspected and maintained to prevent clogging or discharge of collected material. Collected materials shall be disposed of properly.

B. Operation and Maintenance

1. The industrial stormwater pond shall be maintained and operated to meet design criteria. In addition, outlets, overflows or bypasses shall be designed, maintained and modified, as needed to expedite maintenance including periodic cleaning and repair. For example, pre-settling of solids, removal of floatable material, or other maintenance actions which allow the Permittee to provide effective long term operation of the industrial stormwater pond.

2. Newly constructed or up-graded industrial stormwater ponds shall be visually inspected after all precipitation events for 30 days after initiation of operation, and thereafter in accordance with inspection requirements outlined in Part III.F of the permit or, if applicable, Part VII of the permit, to ensure that the industrial stormwater pond is operating correctly.

3. Access, equipment, and training for appropriate staff must be provided for operation and maintenance of the industrial stormwater pond.

4. The Permittee shall operate and maintain all industrial stormwater ponds as required by this permit, including all applicable specific requirements of this Appendix and any restrictions in the sector or subsector specific requirements of Part VII. Permit violations regarding the design, operation, and maintenance of an industrial stormwater pond, may be grounds for the Agency to revoke the Benchmark Monitoring Waiver.

C. Documentation

All design assumptions, operational and maintenance methods, tests, calculations and monitoring shall be kept with the SWPPP. Portions of the SWPPP that are essential to operations may be summarized with specific references, while design and reference documents must be made available within 72 hours of request.

This page was last edited on 2 April 2013, at 21:16.