BioSwale Growing Medium

A bioswale or a bioretention is a landscape element that is designed to capture and filter contaminates and sediments from stormwater runoff. Types of water pollutants that a bioswale can mitigate include but are not limited to silt, organic and inorganic compounds, and pathogens. Common organic compounds are pesticides that are used in the landscape. Whereas, common inorganic compounds can be macronutrients like nitrates and phosphates from fertilization or heavy metals such as lead, cadmium and chromium.

Gail Materials has designed three basic types of bioswale mixes. We have a sand based mix which is considered a high percolation rate media. The second is a loamy sand blend which is a moderate percolation rate mix. Our third is a sandy loam mix that is a slow percolation rate media. We recommend that not too much trust should be put into laboratory percolation rates since they tend to report percolation rates that are excessively high and do not represent true “in field” conditions.   As a general rule, increasing the amount of silt and clay particles will decrease percolation rates. Therefore, if you want a bioswale that moves water at a fast rate then use the sand or loamy sand based mixes. If you wish to retain the water for a longer period then it is best to use the sandy loam mix.

In all of our blends, we include high quality humus which provides a carbon source as a value to filter water as well as increases beneficial bacteria and fungi for soil health and root growth. What makes our bioswale mixes special is the addition of a calcined diatomaceous earth called AXIS®. Many university studies have been performed with AXIS® and the benefits are multifold. AXIS acts a filter and can remove particles 0.5 microns and larger. Studies have shown that stormwater has a median particle size of 3 to 15 microns. In addition, AXIS® increases soil permeability and infiltration while increasing water holding capacity which reduces runoff. If your project is a LEED certified project then AXIS® can contribute up to 8 LEED points when it is used.

For all of our mixes, we have a top layer and a bottom layer. The top layer contains the organic matter with a suggested maximum depth of placement up to 18 inches. Below that depth, we have the same soil type along with AXIS® included but we eliminate the organic humus. Placing organic material to deeper depths can create anaerobic conditions which will harm roots. Also, when the organic matter decomposes it can cause the material to shrink from the intended level of installation.

AutoCAD details and Architectural Specifications are now also available on CAD Details. For further information please contact a Gail Materials representative.