Bioretention System (Bio II)
About Bioretention System (Bio II)
General Information Performance Data Water Quality Treatment Process Design

BIO II is the second bioretention system installed at UNHSC and reflects the best 2005 design standard. Its design virtually eliminates clays from the bioretention soil mix and eliminates geotextile lining. It has demonstrated strong water quality treatment and water quantity management performance in all seasons.

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About Bioretention System (Bio II)

Bioretention systems are among the most common Low Impact Development (LID) stormwater approaches. Runoff flows into landscaped depressions, where it ponds and infiltrates the soil. The engineered soil mix and vegetation provide water quality treatment and infiltration similar to undeveloped areas. UNHSC has evaluated two such systems. The first initially displayed strong performance and then experienced hydraulic failure after ten months due to design flaws. (See discussion in “Design”) In 2005, UNHSC installed Bio II, a smaller, more affordable system that addresses these flaws, and thus far, demonstrates better infiltration and strong water quality treatment.

Where to Use It

Bioretention systems can be used throughout the United States. To achieve maximum volume reduction, they must be located in soils that accommodate infiltration, such as those in groups “A” (sand, loamy sand, or sandy loam with high infiltration rates) and “B” (silt loam or loam with moderate infiltration rates). Careful site analysis is required to design an effective, integrated network of these systems that allows infiltration throughout a site. They are most effective as part of a well-distributed network of runoff control measures. They can also be used as an end-of-pipe system. As with any infiltration or filtration system, when used in pollution hotspots or poor soils, they should be lined and outfitted with subdrains that discharge to the surface.


The acceptance of bioretention systems varies regionally. In areas such as Prince Georges County, Maryland—the birthplace of the bioretention system—regulations encourage their proliferation. In other areas, local acceptance may be hindered by lack of performance data, unfamiliarity with the design, and seasonal functionality. The hydraulic efficiency and concern over clogging has restricted their use in the past. However, improvements in the design specifications of the bioretention soil mix (BSM) and the elimination of fabric can address these concerns. The cost to install Bio II to treat runoff from a one-acre parking area was $18,000. This does not include maintenance expenditures, which may involve routine inspection and the periodic mowing of side slopes and replacement of vegetation, as needed. For more information about Bio II, please consult the UNHSC.