Buried Treasures

Feb. 9, 2010

About the author: Fred Dotson is vice president of Cultec Inc. Dotson can be reached by e-mail at [email protected].

As commercial land development and urbanization increase the number of paved surfaces for the nation’s watersheds, more professionals are turning to underground storm water management systems.

These systems have proven to address concerns of increased impervious surfaces while allowing for the maximum development of available land. The use of one such best management practice (BMP)—subsurface retention/detention infiltration and storage chamber systems—has been growing rapidly.

Subsurface storm water chambers offer cost-effective installation.

Subsurface chamber systems can be used in almost any type of storm water situation. Infiltration chambers collect, store, treat and release storm water on site. They are capable of both retaining and detaining storm water. Pretreatment filters may be used prior to allowing the water to enter the storm water system, and further pollutant removal is accomplished by filtering through the soil.

Retention systems hold the water until the soil is again suitable for infiltration back into the ground. In detention systems, the chambers store runoff temporarily; an additional back-end filter may be used to treat the storm water before it is cast out off site. Detention systems are typically used on large building sites where infiltration is not desirable.

System Benefits

Subsurface chamber systems bring the following benefits to applications:

Maximum use of land. There are many benefits to using a subsurface system, and its allowance for the maximum use of land is an important one. Engineers are increasingly looking underground for storage and treatment options to leave space for additional buildings, parking and landscaping, especially in urban areas where land is at a premium.

Pollutant removal and groundwater recharge. The U.S. Environmental Protection Agency promotes using BMPs to remove pollutants from storm water and selecting practices capable of providing groundwater recharge.

Underground chamber systems remove a high percentage of phosphorus, nitrogen, lead, zinc, suspended solids and organic compounds from runoff through infiltration. They also offer significant groundwater recharge in areas with a high percentage of impervious surfaces. In addition, the systems allow for more controlled infiltration and replenish the surrounding soil and aquifer. Their high infiltrative capacity is especially important in arid regions because of the need for groundwater recharge.

Pollutant removal via infiltration is an importance subsurface system feature.

While designing these systems, professionals need to remember that there should be significant separation (2 to 5 in.) from the bottom of the infiltration system and the seasonally high groundwater table to reduce the risk of contamination. The systems should also be separated at least 150 ft from adjunct drinking water wells.

Safety. Underground solutions reduce potential liabilities inherent in aboveground water storage such as a wet pond, and they eliminate accessible standing water, a breeding ground for insects such as mosquitoes.

Cost-Effectiveness. Chamber systems are cost-effective to install because the units are stackable, easy to ship and do not require heavy installation equipment. They often are made with interlocking connections for a fast and straightforward installation. The high-density polyethylene used in many chambers is durable and corrosion resistant.

Pretreatment and maintenance. These systems require a combination of pretreatment measures—for example, a catch basin and sump to keep impurities and debris from entering the storage area. Maintenance is minimal, required only of the preliminary collection system prior to feeding the bed or other filtering devices that may be employed.

System Limitations

For all of underground infiltration’s benefits, it is important to note that a system might not be appropriate in areas where groundwater is a primary source of drinking water due to the potential for contaminant migration if pretreatment has not taken place. Additional concerns might include limited performance in areas with poorly permeable soil (e.g., clay) and reduced infiltration due to excessive sediment accumulation.

About the Author

Fred Dotson