Low Impact Development Techniques:
Soil-Based Treatments
Many LID approaches rely on soil-based treatments. Soil-based treatments have been shown to be very effective in the removal of oil and grease, certain metals, and suspended solids. Soil-based systems can also be effective in the removal of phosphorous and certain types of pesticides, depending on the constituency of the base soil or the amended soil used in the site based feature.
Bioretention
Bioretention systems are small-scale detention facilities that provide pollutant removal by filtering out pollutants through the soil media and by phytoremediation (or assimilation of the pollutants by plants). Bioretention systems should be designed so that surface runoff from streets, sidewalks, rooftops or other hard surfaces enters the area at a point that will insure the maximum residence time possible. Depending on the circumstances, this may be through curb cuts that allow direct access or redirected to one end of the bioretention area. Bioretention areas serve two important functions, they slow the water down—dissipating energy—and they serve a primary filter capable of removing many urban pollutants through both mechanical and biological means. Proper siting of bioretention areas will depend on a number of factors including amount of space available, depth to an impermeable layer (such as a hardpan or bedrock), depth to perched water table, soil type and the type design of the develop project.
One cost-effective means of implementing bioretention systems is to integrate the absorption of parking lot runoff into a series of inter-connected landscape islands. Instead of typical landscape islands that are set higher than paved grade (and often require supplemental irrigation), these bioretention landscape areas are recessed, and the pavement graded so that surface flow is into, rather than away from, these areas. Even in small parking lots where there are no landscape islands, biofiltration of storm water can be achieved through the diversion of storm water runoff to a landscaped area at the perimeter of the lot. In addition to bioretention areas, two other options—sand filters and/or grass filter strips—may be considered for perimeter applications. In certain situations these options may be used in combination with bioretention areas. On clay soils underdrains may be necessary, directing flow to a conventional storm water conveyance or to a grass swale to achieve infiltration. In mountainous areas underdrains should be placed below the frostline. Bioretention systems have been shown to continue to function and drain even in harsh winter conditions.
The type of soil mix selected is an important consideration and will depend on anticipated pollutant loading, plant selection, type of soils at the site, climactic conditions, etc. There are some general guidelines that should be followed. Amended soils in bioretention areas should be relatively high in organic material (15-30%). The organic matter will allow for the sorption of many common urban pollutants and the sand will allow for slow but steady drainage of the cell. The use of geotech or geotextile fabric can be used above the aggregate layer to avoid fines or sediment filling in the void space. Geotech material should not be used to line the bioretention cell; this may disrupt some pollutant removal processes with the surrounding soil. Efforts have been made to establish anaerobic conditions to facilitate nitrite conversion, however, there is rarely sufficient residency time for this to be effective.
Plan selection is also a very important consideration. Local conditions will dictate the appropriate types of plans to include. Generally grasses, sedges, and various ground covers in combination with bushes and other appropriate low growing landscape appropriate plants should be considered. Plants native to an area can be a good choice, since they will be adapted to climate and native soil types. A mix of natives and adapted landscape plants may provide needed mix of rapid establishment, pollutant assimilation, drought and saturation tolerance, and possible salt tolerance. Trees and large shrubs should be avoided within the actual bioretention cell, to allow for cost effective and timely soil media removal when and if necessary. It is critical that the basic function and aesthetic elements of the urban landscape continue to be viable.
Rain Gardens
Two systems related to bioretention areas are so-called "rain gardens" and bioswales. Ran gardens are small depressions near homes and other buildings that collect runoff from a roof, driveway or yard and allow it to infiltrate into the ground. Generally, rain gardens are planted with shrubs or perennials and can be very colorful and aesthetically pleasing landscaped areas that provide other important environmental enhancements. Rain gardens can reduce the amount of pollutants that enter nearby lakes, ponds and streams. While each rain garden makes only a small contribution, the cumulative effect from a neighborhood of these small detention and infiltration areas can be substantial. Bioswales are shallow depressions created as opened storm water conveyance systems that are generally not as elaborately landscaped as bioretention systems and are primarly designed for transportation and infiltration of storm water.
