The only sure way to eliminate stormwater pollution is to eliminate stormwater runoff. In recognition of this fact, Green Infrastructure (GI) and Low Impact Development (LID) practices have prioritized runoff reduction as a primary regulation for stormwater management. These practices have proliferated throughout the United States.
Surface infiltration is typically the first option considered for runoff reduction, as it is the most cost effective and reliable runoff reduction approach. However, there are often competing demands for land use in an urban environment therefore dedicating land space for surface infiltration is not always practical. Above ground infiltration may also be prohibited by soil conditions and low infiltration rates. When this is the case, the next step is to consider is subsurface infiltration.
Subsurface infiltration can provide many of the benefits of landscape based system, but without requiring dedicated land space. Subsurface infiltration systems are commonly comprised of a pretreatment component designed to remove sediment, trash and oil followed by plastic, metal or concrete storage units surrounded by permeable stone creating a high voids storage gallery. Infiltration systems are typically designed to support vehicular loading and to withstand lateral pressures from surrounding soil allowing the overlying land to be utilized for virtually any non-building application while maximizing land use.
Subsurface infiltration addresses both water quality and water quantity concerns while allowing for maximum land use, making it an ideal solution for runoff reduction practices.
In some cases, a hybrid solution may be the best approach. For example, adding chambers or perforated CMP to a bioretention cell can help make bioretention practical for sites with marginal soils. Runoff can infiltrate the bioretention cell and be temporarily stored in the chambers or CMP. This hybrid approach helps make GI and LID solutions practical in sites where it otherwise would not be practical all while reducing the footprint and depth of bioretention cells.