MS4 Staff Site Plan Review Guidance

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This document provides guidance on how to comply with the site plan review process in the Minnesota Pollution Control Agency’s (MPCA) Authorization to Discharge Stormwater Associated with Small Municipal Separate Storm Sewer Systems (MS4) under the National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) Permit Program (MS4 Permit). The MS4 Permit requires you, the permittee, to perform and document site plan reviews for construction projects that disturb one acre or more of land to ensure active and post-construction requirements of your regulatory mechanism (e.g. code, ordinance, law) are met. The MS4 Permit requires your regulatory mechanism to be as stringent as the Authorization to Discharge Stormwater Associated with Construction Activity under the NPDES/SDS Program general permit (MNR100001/Construction Stormwater Permit).

The items identified below are the minimum that must be evaluated during your stormwater site plan review under the MS4 and Construction Stormwater Permits. For each site plan review performed, you must document the project name, location, total acreage to be disturbed, owner and operator of the proposed construction activity, and any stormwater related comments used to approve or deny the project. If you’d like to conduct a more comprehensive review, please see the MPCA’s SWPPP Checklist. During the site plan review, you must verify that site plans include (an) acceptable:

Best Management Practices (BMPs) to minimize erosion

Stabilization schedule must be no less than:
14 days for all exposed soils
7 days if a discharge point is within one mile of a special or impaired water
24 hours for areas within 200 ft of a public water during fish spawning times
24 hours for areas of ditches and swales within 200 ft of the property edge or surface water discharge point and 14 days for remainder

BMPs to minimize erosion are the first line of defense for protecting stormwater from sediment running off your site. Soil without proper erosion control, such as blanket, mulch, or vegetation is prone to erosion.

• Erosion control BMPs
• Schedule & effective methods to immediately stabilize soil
• Energy dissipation at pipe outlets
  
• 100 foot or more buffer from a special or impaired water
  
• Schedule to stabilize drainage ditches or swales
  
• Plan to route water around unstabilized areas
  
• Plan to discharge water from BMPs to vegetated areas
  

BMPs to minimize the discharge of sediment and other pollutants

Properly installed and maintained sediment control BMPs, such as silt fence or bioroll, prevent sediment from discharging from your site. They allow stormwater to pass through, but filter out sediment. When reviewing site plans, ensure that sediment controls to be used will be effective for the scale of the site.

• Downgradient perimeter control
  
• Storm drain inlet protection
  
• Sediment control for temporary soil stockpiles
  
• Vehicle tracking BMPs
  
• Temporary sedimentation basins, if applicable
  
• 50 foot buffer maintained adjacent to surface waters or redundant sediment controls otherwise
  
• Plan for using polymers or flocculants if conventional erosion and sediment controls cannot be used:
  
  • Chemicals must be appropriate for the soil type
    
  • Chemicals must be used and dosed correctly
    

Site inspections and rainfall records

Insepction schedule must be no less than:
Every 7 days during active construction
Within 24 hours after a 0.5 inch rain event
If work is suspended due to frozen ground, inspections must begin within 24 hours after runoff occurs or construction starts
For parts of the site with permanent cover but work is ongoing elsewhere, inspections can be once per month
If the entire site has permanaent cover and there is no active construction, inspections can be once per month for twelve months

• Schedule for inspecting & recording inspections
  
• Documentation of inspections, including:
  
  • date and time of inspection
    
  • inspector name
    
  • inspection findings and corrective actions
    
  • recent rainfall events of 0.5 inches in 24 hours
    
  • observed discharge and discharge points
    
  • proposed amendments to the site plan
    

BMPs for dewatering activities

• Plan to discharge sediment-laden water to a sedimentation basin
  
• Plan for dewatering to prevent:
  
  • discharge of sediment laden water,
    
  • erosion and downstream impacts
    

BMP maintenance

BMP maintenance schedule must be no less than:
End of next business day for nonfunctioning BMPs
End of next business day or as soon as conditions allow for nonfunctioning perimeter control
72 hours or as soon as conditions allow for draining sedimentation basins
24 hours for tracked sediment
7 days for removing deltas and sediment from surface water and restabiliztion

• Schedule and procedure to repair and replace adjusted and non-functioning BMPs or perimeter control devices when sediment reaches ½ the device height
  
• Schedule to inspect surface waters and streets
  
• Schedule and procedure to maintain sediment basins
  
• Procedure to remove tracked sediment
  
• Schedule to inspect infiltration areas
  

Management of solid and hazardous waste

• Cover on building products, pesticides, herbicides, insecticides, fertilizers, chemicals, and landscape materials
  
• Plan for properly storing and disposing of waste
  

Washout operations specifications:
Washout wastes must not contact the ground
Washout operations must not result in runoff

• Securing portable toilets
  
• Process for containing washout wastes
  
• Defined limited area for vehicle washing, fueling, and maintenance and a plan to contain washing runoff
  
• Prohibition of engine degreasing onsite
  

Final stabilization when construction is complete

• Process for stabilizing soils with 70% perennial cover and ditches with permanent cover
  
• Process for cleaning sediment basins and conveyance systems of accumulated sediment
  
• Removal of temporary erosion & sediment control BMPs
  
• For residential construction: plan to complete temporary erosion protection and downgradient perimeter control before the property is sold
  
• For construction on agricultural land: the land must be returned to its preconstruction use
  

Use of temporary sediment basins, if applicable

Temporary sediment basin live storage:
If the live storage volume is calculated, it must accommodate a 2-year, 24 hour flood event, but cannot be less than 1,800 cubic ft
If live storage is not calculated, then the basin must accommodate 3,600 cubic ft of live storage

• Use of temporary sediment basin if 10 or more acres of disturbed soil drain to one place, or 5 acres if the discharge point is within one mile of a special or impaired water
  
• Documented live storage volume from each acre drained
  
• Design that:
  • prevents short-circuiting & floating debris discharge
    
  • allows for complete basin drawdown
    
  • has a stabilized emergency overflow
    
  • withdraws water from the surface
    
  • includes energy dissipation for the basin outlet
    
  • is outside of surface waters
    
  • avoids draining to wetlands
    
  • is operational before construction starts
    

Post-construction stormwater management

Infiltration prohibitions and limitations:
Infiltration is prohibited where: industrial facilities cannot under an NPDES Industrial Stormwater permit, vehicle fueling/maintenance occurs, there is less than 3 ft between the infiltration device and bedrock/seasonably saturated soils, and high levels of contaminants will be mobilized
Infiltration is limited within: clay soils, 1,000 ft upgradient or 100 ft downgradient of active karsts, a Drinking Water Supply Management Area, or areas with infiltration rates of >8.3 inches/hour

• Preference for Green Infrastructure techniques
  
• No net increase (new development) or net decrease (redevelopment) in:
  
  • stormwater discharge volume,
    
  • total suspended solids, and
    
  • total phosphorus
    
• Reason infiltration is not allowed, if applicable, and alternative treatment system