4.1 Abstraction/Filtration

MCWD Goal 1 - Abstraction/Filtration

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

Discussion

Land development and the associated creation of new impervious surface increases 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.  Most of the Christmas Lake subwatershed is 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 the 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 abstraction and filtration in the Christmas Lake subwatershed is desirable for three primary reasons: 1) to recharge groundwater inputs and reduce pollutant loading into Christmas Lake; 2) to help prevent localized flooding in landlocked basins with no natural outlet; and 3) to protect the hydrology of discharge (groundwater-fed) wetlands in the subwatershed.  Many of those wetlands have high ecological values and are dependant on groundwater to maintain those functions and values.

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:

  1. Minimize additional pollutant loading that would have been conveyed by that stormwater.  The phosphorus load reduction plan for Christmas Lake assumes that permitted new development and redevelopment will achieve a much higher rate of phosphorus load removal than can be achieved through traditional stormwater management such as detention ponds.  Abstraction and infiltration are important tools in achieving the load reductions necessary to maintain the excellent water quality in Christmas Lake, and to prevent further degradation of the other water resources in the subwatershed;
  2. Reduce runoff volumes and help reduce future downstream erosion in streams and channels or flooding in landlocked basins; and
  3. Help maintain groundwater levels, preserving wetland hydrology and stream baseflows. 

Because the Christmas Lake 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 encouraging retrofits on existing development.

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 - Christmas Lake

Increase abstraction and infiltration to reduce runoff volumes carrying pollutant loads to Christmas Lake, minimize potential flooding in landlocked basins, and to promote groundwater recharge.

Actions

  1. In consultation with LGUs through an appropriate rulemaking process, amend existing or establish new District rules to increase stormwater requirements through consideration of abstraction of the first one inch of rainfall on new permitted development and redevelopment.
  2. Develop infiltration strategies appropriate to wellhead protection areas (if designated in the subwatershed in the future) and areas of groundwater sensitivity.
  3. 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. Develop a small grant program to provide financial assistance to property owners desiring to retrofit their property with BMPs to increase infiltration.
    5. Encourage the use of infiltration as a Best Management Practice within landlocked basins of the subwatershed.