Rain gardens and bio-filtration swales are an increasingly important part of the sustainability approach for new construction. They keep rainwater from flowing over impervious surfaces where it can pick up pollutants and carry them to water bodies. This benefit is more and more imperative when it comes to controlling non point source pollution. The Washington State Department of Ecology, for example, has reconfirmed surface runoff as the leading pathway for toxics to get into Puget Sound. (Phase 1 study, Phase 2 study).
Infiltration testing allows you to determine whether a swale will be successful at a given property. An infiltration test involves excavating a test pit (typically 5 feet by 5 feet). A hydrogeologist adds water to the pit, then records how much water is necessary to maintain the water level at the same level over a period of 10 to 12 hours. The hydrogeologist then stops the water flow and measures the drop in water level. The infiltration rate determined from a field test is called a field infiltration rate. To determine the design infiltration rate, the hydrogeologist then adjusts for a number of factors such as site variation, number of tests conducted, degree of long-term maintenance and influent pre-treatment/control, and the potential for long-term clogging from silt and bio-buildup.
Geotechnical laboratory testing is useful for rain gardens, which require a specific mix of soil types that helps scrub some of the contaminants from the rainwater before it reaches a major body of water, yet still allows the water to drain. Rain gardens are also an effective way to store water from large storm events and prevent it from overloading the storm drain and sewer systems in the public streets.