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Why Do It
Surface soil filters—also known as bioretention systems—capture, filter, and slowly release runoff from smaller storm events. This reduces flooding, protects streams from erosion. They are effective at removing a wide range of stormwater pollutants. They can be designed to remove nitrogen from stormwater, which is a harmful nutrient to coastal bays and estuaries. Surface soil filters have flexible configurations that can fit into existing landscape and developed areas. They appear as attractive lawn, shrubs, or small trees.
In South Portland, Surface Soil Filters are a recommended approach to meet Basic or Post-Construction Stormwater Management Plan permit requirements.
A surface soil filter consists of a specific soil media that allows stormwater to flow down through the system. Plants on the surface of the filter reduce stormwater volume by evapotranspiration. An underdrain releases water slowly from the bottom of the filter.
Systems can also be configured to reduce nitrogen pollution by incorporating an anaerobic (no oxygen) zone which contains wood chips or other carbon source. Nitrogen reduction is important if sensitive coastal bays or estuaries are downstream.
They are often vegetated with grass or landscape plantings. When installed over poorly draining soils, they can be equipped with a drainage pipe. If soils conditions permit, water can be allowed to infiltrate into the ground. An underground zone may contain wood chips or other carbon source to promote removal of nitrogen. Filters can be configured in a variety of ways and can be integrated into landscape islands, planters, medians, setbacks, and other locations with appropriate design.
Surface soil filters may be adapted to most soil conditions. The bottom of the soil filter should be above the seasonal high groundwater table. An impermeable liner may be necessary for some soils, groundwater conditions, or site uses. Download the detailed design specifications for details.
Ideally, soil filters are located in close proximity to the origin of the stormwater runoff and it is anticipated that these facilities would most often be distributed through a developed area.
Design and Installation
Surface soil filters should be designed by an engineer. They can be constructed using common materials. Some manufacturers have developed proprietary filter media and structures which may be used instead. Soil filter media composition is critical to the function of these systems and careful attention is required regarding construction materials and installation.
Mulch should be replaced yearly. The pre-treatment structure needs to be cleared of plant debris and sediment yearly. Plantings will have landscape maintenance needs such as weeding, pruning, and possible replacement of plants which are not thriving.