The intent of this sizing criterion is to maintain groundwater recharge rates at development sites to preserve existing water table elevations and support natural flows in streams and wetlands. Under natural conditions, the amount of recharge that occurs at a site is a function of slope, soil type, vegetative cover, precipitation and evapotranspiration. Sites with natural ground cover, such as forest and meadow, typically exhibit higher recharge rates, less runoff and greater transpiration losses than sites dominated by impervious cover. Since development increases impervious cover, a net decrease in recharge rates is inevitable.
Recharge and/or infiltration criteria offer additional benefits, as they promote more on-site infiltration/filtration of stormwater runoff, and enable communities to offer stormwater credits that reduce the water quality storage volume. Recharge credits provide real incentives to apply better site design techniques at development sites that can reduce the size and cost of stormwater BMPs needed at some sites. To maximize recharge, designers should explore how to use pervious areas for infiltration early in the site layout process.
The recharge volume is considered to be part of the total water quality volume provided at a site and is not an additional CGP requirement (e.g., Vre is contained within Vwq). Recharge can be achieved either by a structural BMP (e.g., infiltration, bioretention, and filter), better site design techniques, or a combination of both.
There are currently no statewide recharge sizing requirements for regular waters in the state of Minnesota, although previous stormwater guidance has strongly promoted recharge and infiltration (MPCA, 2000 and MC, 2001). Also, infiltration can be used as one way to meet the state CGP requirement for permanent stormwater management. Recharge and infiltration are strongly encouraged through better site design and stormwater credits. There are three readily available options for how a community or stormwater manager could determine the amount of runoff to include as the Rev factor.
Since there are no current required state infiltration requirements, any of the three approaches could be used. Stormwater managers are cautioned to review local conditions and select a method according to what can logically be expected, keeping in mind that the goal is to match pre-development volumes of infiltration as closely as possible in most cases. However, if the potential for using site development to enhance or increase local infiltration exists, for example through an infiltration basin, managers might choose another method to increase infiltration expectations.
Infiltration and recharge of polluted stormwater runoff is not always desirable or even possible at some development sites. Therefore, most communities qualify their recharge and/or infiltration requirements to reflect special site conditions, protect groundwater quality, and avoid common nuisance issues. For example, the local review authority may require