Stormwater infiltration practices capture and temporarily store stormwater before allowing it to infiltrate into the soil. Design variants include; the infiltration basin, the infiltration trench, the dry well and the underground infiltration system. As the stormwater penetrates the underlying soil, chemical, biological and physical processes remove pollutants and delay peak stormwater flows.
Infiltration practices are applicable to sites with naturally permeable soils and a suitable distance to the seasonally high groundwater table, bedrock or other impermeable layer. They may be used in residential and other urban settings where elevated runoff volumes, pollutant loads, and runoff temperatures are a concern. In applications where the stormwater runoff has a particularly high pollutant load or where the soils have very high infiltration rates, a significant amount of pre-treatment should be provided to protect the groundwater quality. Sources that include potential stormwater hotsposts (PSH) should not be introduced to infiltration areas.
Infiltration practices may be located at the end of the treatment train or they can be designed as off-line configurations where the water quality volume is diverted to the infiltration practice. In any case, the practice may be applied as part of a stormwater management system to achieve one or more of the following objectives:
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA General Stormwater Permit for Construction Activity (MN R100001), commonly called the Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least 1 acre of new impervious area is being created, and the permit] stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, infiltration practices fall under the “Infiltration / Filtration” category described in Part III.D.1. of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases infiltration will be used in combination with other practices, standards are described for the case in which it is a stand alone practice.
The following terms are thus used in the text to distinguish various levels of stormwater pond design guidance.
REQUIRED: Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).
HIGHLY RECOMMENDED: Indicates design guidance that is extremely beneficial or necessary for proper functioning of the infiltration practice, but is not specifically required by the MPCA permit.
RECOMMENDED: Indicates design guidance that is helpful for infiltration performance but not critical to the design.
Of course, there are situations, particularly retrofit projects, in which an infiltration facility is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular infiltration facility, depending on where it is situated both jurisdictionally and within the surrounding landscape.
The narrow and versatile shape of infiltration trenches and dry wells makes them well suited for retrofit projects. For example, infiltration practices can be situated along the margin or perimeter of a developed site or roadway in many cases. They are particularly desirable as retrofit practices in watersheds or catchments that are targeting volume reduction practices to help minimize channel erosion.
Generally, infiltration should not be used to treat runoff from manufacturing or industrial sites or other areas with high pollutant concentrations unless correspondingly high levels of pre-treatment are provided.
The BMP design restrictions for special watersheds table below provides guidance regarding the use of infiltration practices in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.
|BMP Group||receiving water|
|A Lakes||B Trout Waters||C Drinking Water2||D Wetlands||E Impaired Waters|
|Infiltration||RECOMMENDED||RECOMMENDED||NOT RECOMMENDED if potential stormwater pollution sources evident||RECOMMENDED||RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride|
|Filtration||Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments||RECOMMENDED||RECOMMENDED||ACCEPTABLE||RECOMMENDED for non-nutrient impairments|
1Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
2 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies
Infiltration practices should remain effective water quality improvement systems for many years, even during winter conditions, if designed and constructed properly and it has been shown that hydraulic efficiency and infiltration rates can remain at levels used for design sizing. However, in cold climates, some special considerations are HIGHLY RECOMMENDED for surface systems to ensure sustained functionality and limit the damage freezing temperatures and snow and ice removal damage may cause.
One concern with infiltration in cold weather is the ice that forms both over the top of the practice and within the soil structure and can completely stop infiltration. To limit the effect of this problem, it is HIGHLY RECOMMENDED that the facility be actively inspected to ensure that it is properly drawing down before it freezes in the late fall. Adequate drawdown can be determined using one of the several field assessment techniques such as those recommended by the University of Minnesota Extension (see section on Assessment). If it is determined that stormwater runoff is not infiltrating prior to hard freeze, the BMP should be placed offline for correction in the spring.
Even if the infiltration properties of an infiltration practice are marginal for snowmelt runoff during the period of deep frost in the winter, the storage available in the facility will provide water quality benefit if it is dry entering the melt season. Routing the first highly-soluble portions of snowmelt (first flush) to an infiltration facility provides the opportunity for soil treatment (such as filtration, adsorption, microbial activity) of these soluble pollutants. Again, however, flow originating in an industrial area, a high traffic area where large amounts of salt are added, or another PSH should be diverted away from infiltration systems if pretreatment features have not been properly designed to handle such an increase in loading. Proprietary, sub-grade infiltration systems provide an alternative to standard surface based systems. Essentially, these systems provide an insulated location for pre-treated snowmelt to be stored and slowly infiltrated, or simply filtered and drained away if groundwater sensitivity is an issue. The insulating value of these systems adds to their appeal as low land consumption alternatives to ponds and surface infiltration basins.
For all BMP’s it is HIGHLY RECOMMENDED that snow and ice removal plans including predetermined locations for stockpiling be determined prior to or during the design process. Infiltration features cannot be used for significant snow storage areas as debris build-up, plant damage, and lower infiltration rates are likely to occur. Some snow storage unavoidable when BMP’s are adjacent to areas where snow removal is required but it is critical that the property owner and snow and ice removal contractor have identified other areas for large scale snow storage.
Excessive deicing agents have the potential to create a hot spot in some locations that could lead to reduced infiltration rates or concentrations that exceed surface water or groundwater standards. Locations such as busy intersections on slopes, parking garage ramps or on walkways near the entrances of commercial buildings are likely to be heavily treated with deicing agents to avoid slip and falls or vehicle collisions. This should be taken into consideration when siting any infiltration BMP.
For bioinfiltration features, dry swales with check dams, and tree trenches, special considerations regarding snow and ice storage and plant maintenance are required. Plant selection is critical to ensure that the damaging effects of snow and ice removal do not severely impact plantings or seedings. Even a small amount of snow storage can break and uproot plants requiring additional maintenance in the spring. Woody trees and shrubs should be selected that can tolerate some salt spray from plowing operations.
The amount of stormwater volume infiltrated depends on the design variant selected. Smaller infiltration practices (e.g. infiltration trenches) should either be designed off-line using a flow diversion, or designed to safely pass large storm flows while still protecting the infiltration area. In limited cases (e.g. extremely permeable soils), these smaller infiltration practices can accommodate the channel protection volume, Vcp, in either an off- or on-line configuration.
In general, supplemental stormwater practices will be necessary to satisfy channel and flood protection requirements when smaller infiltration practices are used. However, these practices can help reduce detention requirements for a site through volume reduction.
Due to their size, the larger infiltration practices (e.g. infiltration basins and underground infiltration systems) have the potential to provide greater water quantity benefits. Surcharge storage above the practice bottom is available for detention. Outlet structures can be sized to partially or fully accommodate larger storm peak discharge control while allowing the volume below the outlet to infiltrate.
Infiltration practices can remove a wide variety of stormwater pollutants through chemical and bacterial degradation, sorption, and filtering. Surface water load reductions are also realized by virtue of the reduction in runoff volume.
There are few data available demonstrating the load reductions or outflow concentrations of larger-scale infiltration practices such as infiltration trenches. Similarly, few sampling programs collect infiltrating water that flows through an infiltration system.
For properly designed, operated, and maintained infiltration systems, all water routed into them should be “removed” from stormwater flow, resulting in 100 percent efficiency relative to volume and pollutant reduction. For this reason, any infiltration BMP performance table should show all 100 percent entries for that portion of stormwater entering the infiltration system. This logic assumes that stormwater is the beneficiary of any infiltration system, but ignores the fact that pollution, if any remains after the internal workings of the infiltration BMP itself (see later discussion in this section), is being transferred into the shallow groundwater system. Good monitoring data on the groundwater impact of infiltrating stormwater are rare, but there are efforts underway today to document this, so future Manual revisions should be able to include some data updates.
Properly designed infiltration systems discussed later in this section will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours if under state regulation, or generally within 72 hours under certain local and watershed regulations. In no case should the by-passed volume be included in the pollutant removal calculation.
Data that are reported in performance literature for infiltration systems, unless reporting 100 percent effectiveness for surface water or documenting outflow water downward, are not accurately representing behavior, or are representing the excess flow (overflow) from a system. Design specifications in the following sections should prevent putting contaminated runoff and excess water beyond that which will infiltrate within the given time frame. Any runoff containing toxic material or excess volume that cannot infiltrate should be diverted away from the infiltration system and reported as inflow to another treatment device.
Follow guidance and carefully use of infiltration BMPs to make sure they are not transporting highly loaded or toxic contaminants into the groundwater system. These sections address the pollution remediation processes at work in infiltration systems to reduce or totally remove pollutants that move through them. However, extreme caution must be exercised and serious planning undertaken to assure that no highly contaminating material is routed into these BMPs. Of particular concern are toxic organics (gasoline, solvents) and high levels of chloride.
The drawings section contains details on how design and operations can either raise or lower the expected level of performance for infiltration BMPs.
The following general limitations should be recognized when considering installation of infiltration practices.
Please note that even though there are potential pollution and physical clogging problems with infiltration, it is one of the most important elements in the stormwater runoff treatment train. Fear of the limitations should not prevent well designed systems from being used.
As noted in various sections, discussion of BMP selection, the benefits associated with infiltration BMPs should only be accrued based on the amount of water actually passing through the BMP. Excess runoff beyond that designed for the BMP should not be routed through the system because of the potential for hydraulic and particulate over-loading, both of which will adversely impact the life and operation of the BMP. For example, an infiltration device designed to treat the first 0.5 inch of runoff from a fully impervious surface will catch about 30 percent of the volume of runoff in the Twin Cities. This means that 70 percent of the runoff volume should be routed around the filtration system and will not be subject to the removals reflected in the above tables. Attributing removal to all runoff just because a BMP is in place in a drainage system is not a legitimate claim.