Pathogen (Indicator Bacteria) Removal in Stormwater BMPs

Indicator bacteria can be removed from or inactivated in surface waters and stormwater through a number of natural processes, such as ultraviolet light (from sunlight), sedimentation, filtration, drying, and predation. Urban stormwater can be treated by implementing stormwater Best Management Practices (BMPs), each of which provides some combination of treatment mechanisms. Unfortunately, little is known regarding how well stormwater BMPs remove indicator bacteria. A brief description of common BMPs and removal mechanisms pertinent to indicator bacteria removal are given below. It should also be noted that many BMPs (such as stormwater wetlands and wet ponds) attract animals, and may act as a source of indicator bacteria.

 

Dry Detention Basin

Dry detention basins are commonly used to meet the peak flow mitigation requirements present in many municipalities. They fill with runoff during storm events and temporarily detain it for 1 to 2 days. Stormwater slowly, but completely, drains from the system after the rain event. These systems are commonly designed to be dry in between storm events.

dry detention

Possible Indicator Bacteria Removal Mechanisms:

Likely potential for indicator bacteria removal: moderate

 

Wet Pond

Like dry detention basins, wet ponds (wet detention basins) are commonly used to meet the peak flow mitigation requirements present in many municipalities. Wet ponds have also been shown to reduce pollutants (sediment, nutrients, metals) in stormwater. They fill with runoff during storm events and temporarily detain it for 1 to 2 days. Excess stormwater slowly, but completely, drains from the system after the rain event. These systems are designed to maintain a "permanent pool" in between storm events.

wet pond

Possible Indicator Bacteria Removal Mechanisms:

Likely potential for indicator bacteria removal: moderate

 

Stormwater Wetland

Although stormwater wetlands can be used for peak flow mitigation, they are primarily considered a water quality BMP. Stormwater wetlands perform better than wet ponds or dry detention basins for a number of pollutants by employing natural treatment mechanisms. Stormwater wetlands are designed to maintain a permanent pool, but excess runoff stored during storm events drains from them slowly within 2-4 days.

wetland

Possible Indicator Bacteria Removal Mechanisms:

Likely potential for indicator bacteria removal: moderate

 

Bioretention

Biorentention areas are used for water quality improvement and are effective for a wide range of pollutants (nutrients, metals, sediment). These systems rely on infiltration, allowing runoff to pass through a sandy media before exiting. Bioretention areas are designed to be dry in between storm events.

bioretention

Possible Indicator Bacteria Removal Mechanisms:

Likely potential for indicator bacteria removal: good