4.1 Abstraction/Filtration

Goal 1 - Abstraction/Filtration

Promote abstraction and filtration of surface water where feasible for the purposes of improving water quality and increasing groundwater recharge throughout the watershed.

Discussion

Development and the associated creation of new impervious surface increase the volume of stormwater runoff.  The new runoff volume can convey more pollutants to receiving waters and may increase erosion and sediment transport, negatively affecting water quality.  Development also decreases the amount of stormwater that naturally percolates into the soil to recharge groundwater, thus reducing baseflow in streams, changing hydrology in groundwater-fed wetlands, and decreasing water availability in drinking water aquifers.  The Minnehaha Creek subwatershed is almost entirely developed, and those hydrologic impacts have already occurred.

Abstraction of stormwater (retained on site through infiltration, evapotranspiration, or capture and reuse) reduces the amount of runoff from site conveying pollutants.  The most common type of abstraction, infiltration, reduces runoff, which helps recharge groundwater.  Filtration offers an opportunity to use soil to naturally cleanse stormwater prior to discharge.  Increased aabstraction and filtration in the Minnehaha Creek subwatershed is desirable for several reasons:

  • A reduction in the amount of pollutant loading into Minnehaha Creek and the lakes conveyed by runoff;
  • An increase in shallow groundwater recharge to help increase baseflow in the Creek;
  • A reduction in runoff peak flows and overall volumes that now erode streambanks;
  • Protection of the hydrology of the groundwater-fed wetlands in the subwatershed; and
  • The lake TMDLs will rely in part on increased infiltration in the lakesheds to help achieve lake water quality goals.

A key strategy to achieve this goal is the adoption of a volume management standard for new development and redevelopment that requires the abstraction of one inch of rainfall.   Much of the subwatershed has at least moderate infiltration potential.  Requiring new development and redevelopment to abstract some of the new stormwater generated and encouraging retrofitting to increase infiltration on existing sites would reduce runoff volumes and help reduce future downstream erosion in streams and channels or flooding in landlocked basins; minimize additional pollutant loading that would have been conveyed by that stormwater; and help maintain groundwater levels, preserving wetland hydrology and stream baseflows. 

Because the Minnehaha Creek subwatershed is almost entirely developed, there will be few new opportunities to implement abstraction.  The primary strategies will be providing for infiltration on infill development and redevelopment, and retrofitting existing development.  The phosphorus load reduction plan for Lake Hiawatha incorporates an expectation of a significant reduction of pollutant loading from existing development   Abstraction and infiltration are important tools in achieving that load reduction and to prevent further degradation of the other lakes, streams, and wetlands.

Desired Outcomes:  Increased infiltration, reduction in pollutant loading and volumes of runoff to supplement other goals.

Metrics:  Acre-feet of infiltration to meet nutrient loading reductions for water quality and volume reductions for water quantity goals.

Goal 1.1 - Minnehaha Creek

Increase abstraction and infiltration to reduce runoff volumes carrying pollutant loads to the lakes and streams in the subwatershed and to promote groundwater recharge.

Actions

  1. In consultation with LGUs through an appropriate rulemaking process, amend existing or establish new District rules requiring abstraction of the first one inch of rainfall on new permitted development and redevelopment.
  2. Work cooperatively with the LGUs in the subwatershed to identify areas suitable for regional infiltration areas.
  3. Construct regional infiltration basins on a cooperative basis with LGUs where additional infiltration is desired.
  4. Promote reforestation and revegetation with native plants to increase infiltration.
  5. Develop infiltration strategies appropriate to wellhead protection areas and areas of groundwater sensitivity.
  6. Provide technical assistance to LGUs and developers to foster low impact development and redevelopment that minimizes new impervious surface and provides for increased infiltration.
    1. Develop and distribute model ordinances and design standards that incorporate low impact design principles.
    2. Sponsor educational opportunities for LGU staff, developers, elected and appointed officials and other interested parties to provide practical information and opportunities for sharing experiences.
    3. Provide education and training opportunities, technical and planning assistance for property owners and LGUs on methods to reduce runoff from and increase infiltration on their property by incorporating BMPs into landscaping, infrastructure maintenance, and reconstruction.
    4. Encourage the use of infiltration as a Best Management Practice within landlocked basins of the subwatershed.
    5. Develop a small grant program to provide financial assistance to property owners desiring to retrofit their property with BMPs to increase infiltration.