Infiltration Exemptions

The following activities are exempt from the 90% infiltration standard:

• Sites (or areas) that meet the definition of redevelopment
• New development with less than 10% connected impervious and a cumulative impervious area less than one acre
• Agricultural development related to the production of food and fiber
• Expansion of municipal or county roads
• Areas with a measured native soil infiltration rate less than 0.6 inches/hour, or textured as clay, not easily removed or manipulated

Sites or areas that are exempt from the infiltration standard should still attempt to maximize their site's “stay-on”. Three of the most common types of exemptions requested are discussed below.

Even on sites with clay soils, designing practices to increase stay-on is possible. Traditional infiltration practices that rely on ponding water, like bioretention, may need an underdrain and a shallower depth to meet drawdown requirements. Permeable pavement works best by infiltrating a distributed depth of water, which can work well on soils with lower infiltration rates. Other practices, such as green roofs or water re-use, do not depend on in-situ soils at all to keep runoff on the site.

It's more difficult to meet the infiltration standard on sites with clay soils, so an alternative approach may be used, detailed below.

• If entire site has 5+ feet of clay-textured soils (or with measured infiltration rate <0.6 in/hr) that can't be removed with conventional excavation equipment, the requirements below may be used:
  • Dedicate 2% of the site to infiltration practices that maximize infiltration and conform to applicable standards, e.g. maximum drawdown times;
  • Provide infiltration calculations and associated summary table.

• If partial areas of site have 5+ feet of clay-textured soils (or with measured infiltration rate <0.6 in/hr) that can't be removed with conventional excavation equipment, the requirements below may be used:
  • Provide a map delineating exempt and non-exempt areas;
  • Infiltrate 90% of the predevelopment volume from non-exempt areas;
  • Dedicate 2% of the site to infiltration practices that maximize infiltration and conform to applicable standards;
  • Provide infiltration calculations and summary table for both non-exempt areas and full site.

It's recommended that designers wanting to request an exemption first discuss with staff for an official determination. Exemption requests must provide a preliminary site plan and soils testing results in accordance with the DNR's Site Evaluation for Stormwater Infiltration. The evaluation will need to cover all of the site to be considered for exemption.

There are no exemptions for areas with shallow groundwater, so these sites will typically need to meet the infiltration standard. Using shallow practices to infiltrate roof water is recommended, as there is no separation requirement. Bioretention can also be used, as the engineered soil qualifies as a “filtering layer” and satisfies the required separation for any runoff source. For small sites and practices infiltrating relatively small volumes of water, groundwater can be up to one foot beneath the bottom of a practice while still functioning. Underdrains are highly recommended in these situations, as groundwater can fluctuate with the season and over time.

Presence of redoximorphic (redox) features may not reflect the current hydrologic conditions and are not definitive indicators of groundwater elevation. These features can occur in tight soils above the groundwater table and in recently drained soils, which may retain the presence of reduced features. It's important to remember that historically wet soils have been systematically drained for many years across Dane County. The soils investigation should indicate standing water, if encountered, which will provide the true groundwater elevation. For the most accurate reading, especially in tight soils, boreholes should be left open and measured after 24 hours.

Sites with shallow bedrock should take an approach similar to those with shallow groundwater, detailed above. Bedrock infiltration rates are highly variable and will need to be determined by a geotechnical engineer and/or through discussion with WRE staff. Because bedrock may have a more direct path to groundwater, an additional 1 foot of sand (finer than coarse) below the engineered soil layer is necessary to meet separation requirements.

Wisconsin Chapter 151 has infiltration prohibitions for certain land uses, but these are not exemptions. Sites with prohibited areas are still expected to meet the infiltration standard.