Liners for stormwater management

Liners are designed to limit infiltration of water from a stormwater Best Management Practice (BMP) into underlying and adjacent soil.

Conditions where liners are required

Under the Construction Stormwater General Permit:

• The Permittee(s) must ensure filtration systems with less than three (3) feet of separation from seasonally saturated soils or from bedrock are constructed with an impermeable liner.
• The SWPPP must identify additional or different measures necessary (e.g. impervious liner in pond bottom) to assure compliance with surface and groundwater standards in Minn. R. chs. 7050 and 7060 in karst areas and to ensure protection of drinking water supply management areas (see Minn. R. 4720.5100, subp. 13).

Although this second bullet does not specifically require a liner, a liner represents on method of complying with the permit requirement.

Certain sectors covered under an Industrial Stormwater Permit are required to have liners for ponds constructed after April 5, 2010.

• Automobile salvage yards (Sector M)
• Air transportation facilities that conduct deicing activities (Sector S)
• Hazardous waste treatment, storage, or disposal facilities operating as a Solid Waste Management Unit (Sector K)
• Timber product facilities operating under an SIC code of 2491 (Sector A - wood preserving)
• Scrap recycling and waste recycling facilities (Sector N)

Conditions where liners are recommended

Liners are recommended for the following conditions.

• Circumstances where a permanent pool is needed but difficult to maintain due to site conditions. Examples include constructed wet detention ponds or constructed wetlands requiring a permanent pool and underlain by areas with Hydrologic Soil Group A soils, gravel, or fractured bedrock
• Permeable pavement designs in compacted fill soils.
• Areas where seepage from a BMP into the groundwater would otherwise occur but should be avoided due to risk of groundwater contamination. These include
  • Confirmed stormwater hotspots in areas where the potential for groundwater pollution is high. The potential for groundwater pollution can be determined using existing pollution sensitivity maps developed by the Minnesota Department of Natural Resources. Note these maps cover large geographic areas and should be used for initial screening at a site.
  • Active karst terrain
• Locations where recommended horizontal or vertical separation distances cannot be achieved
• Use impermeable liner as needed to separate tree BMPs from road, parking lot, sidewalk or adjacent walls or building foundation

Liner specifications

Specifications are provided for different levels of liners. Although specifications for clay liners are included with Levels 1 and 2, clay liners are generally not recommended.

• Level 1 liner - for sites with high potential for groundwater contamination
  • HDPE (high density polyethylene) liner criteria:
    • Minimum thickness shall be 60 mils.
    • Design according to the criteria in Table 3 of the NRCS 313, Waste Storage Facility technical standard.
    • Install according to NRCS Wisconsin Construction Specification 202, Polyethylene Geomembrane Lining.
  • GCL (geosynthetic clay liner) liner criteria:
    • Design according to the criteria in Table 4 of NRCS 313, Waste Storage Facility technical standard.
    • Install according to NRCS Wisconsin Construction Specification 203, Geosynthetic Clay Liner.
  • Clay liner criteria (essentially the same as the clay below landfills but not as thick):
    • 50 percent fines (200 sieve) or more
    • An in-place hydraulic conductivity of 1 x 10^-7 centimeters per second (cm/s) or less
    • Average liquid limit of 25 or greater, with no value less than 20
    • Average PI of 12 or more, with no values less than 10
    • Clay installed wet of optimum if using standard Proctor, and 2 percent wet of optimum if using modified Proctor
    • Clay compaction and documentation as specified in NRCS Wisconsin Construction Specification 300, Clay Liners
    • Minimum thickness of two feet
    • Specify method for keeping the pool full or use of composite soils below liner
• Level 2 liner - for sites with low groundwater pollution potential
  • HDPE liner criteria:
    • Minimum thickness shall be less than 40 mils
    • Design according to the criteria in Table 3 of the NRCS 313, Waste Storage Facility technical standard
    • Install according to NRCS Wisconsin Construction Specification 202, Polyethylene Geomembrane Lining
  • Silt/Clay liner criteria:
    • 50 percent fines (200 sieve), or 20 percent fines and a PI of 7
    • An in-place hydraulic conductivity of 1 x 10^-5 centimeters per second or less
    • Soil compaction and documentation as specified in NRCS Wisconsin Construction Specification 204, Earthfill for Waste Storage Facilities
    • Minimum thickness of two feet
    • Specify method for keeping the pool full or use of composite soils below liner
• Liners for facilities covered by an Industrial Stormwater Permit and where ponds are allowed with a constructed liner
  • The industrial stormwater pond must be lined with a synthetic liner that is chemically compatible with materials expected to enter the pond, must be Ultra Violet (UV) stable, and must be designed to restrict infiltration to less than 500 gallons per acre per day (5.41 x 10^-7 centimeters per second).
  • The industrial stormwater pond must be designed in accordance with accepted engineering practices. (See MPCA Recommended Pond Design Criteria, Polyvinyl Chloride Liner guidelines, and High Density Polyethylene Liner Guidance)

Decision process for selecting liner level

The following table summarizes the process for selecting a liner.