Frequently Asked Questions

There are specific plans with most project outlined by engineers and architects specific to each project’s landscape.  However, the general idea is to place a [perforated drainage pipe, usually with a filter sock along the bottom of a small pond dug along that collects the stormwater.  The pond is typically 4 to 6 feet in depth.  The drainage pipe discharges outside of the pond but also has a T or connection for a vertical standpipe that will be above the surface level of the pond for overflow discharge.  The underdrain system is typically filled with #57 gravel and lined with a geotextile liner.  Above the gravel and drainage system the bioretention soil mix is placed usually 36 to 48 inches in depth and covered with a mulch layer.  During the installation process machinery and other equipment must not be used to compact the bioretention soil as it is installed.  This can be done with excavators positioned from the edge, slinger trucks, or in a manner that starts in sections and works your way out of the pond.  Select native plants are planted within the pond to uptake moisture and nutrients retained.  The inlets are lined with concrete or rip rap to prevent incoming runoff from eroding the edges of the pond.

Bioretention soil mix typically consists of sand, soil, and compost.  Some blends call for the addition of calcined clay products or mulch.  The mix can be determined in the plans by ratios of volume or a dry weight basis (which require some conversion).  Each component must meet certain specifications in addition to the final blend meeting criteria.  RSI uses large stockpiles of each component that has be tested by a soil lab to blend into a soil mix that is completely homogenous for consistency.  Each component is measured for an accurate blend.  The final bioretention soil mix is tested to make sure it meets the criteria for the project.  The blended soil can be delivered and placed into the cell or basin without contaminating it.  Mixing soil on-site can be a tedious process and limited to available soil that may not be suitable.  The worst-case scenario is for a soil to cause a system to fail.  RSI has decades of experience to make sure you have a quality bioretention soil mix that you can be confident will work effectively. 

Bioretention ponds, bioretention cells, and bioretention basins are common names referring the same systems.  Rain gardens are usually smaller versions of these systems.  The idea is to capture stormwater runoff, specifically the water from smaller, more frequent events.  Generally, this water contains the most contaminants of nutrients, oils/grease, and other debris.  The bioretention areas are landscaped with plants and contain engineered soil to help filter the runoff and reduce the quantity of the runoff.  A bioretention pond is designed to slowly release clean water back into the watershed over a longer period of time that benefits the environment.

Bioretention soil mix, or Bio Soil, is an engineered soil that is designed to be placed in bioretention areas that capture, retain, and treat stormwater runoff.  These areas are typically placed beside impervious surfaces such as buildings, parking lots, or roads so the runoff can be directed into the cell.  Rip rap or concrete flumes help dissipate the velocity of the runoff and a thin mulch layer on top along with specific plants are on the surface of the cell on top of the bioretention soil mix.  The bioretention soil is designed to have a specific range of infiltration for the water to move through the soil.  The organic material and soil’s ability to update nutrients and other contaminants acts like a filter for the stormwater.  The water is allowed to remain in the cell for uptake by the plants, infiltrate into the groundwater, or filtered before discharge.  The soil must meet certain criteria for physical and chemical characteristics like pH, infiltration, texture, P-index, nutrients, soluble salts, organic matter, and more.

Bioretention soil mix or biosoil is a soil designed to be placed in a bioretention cell.  It is blended to meet certain criteria such as pH, infiltration, texture, P-index, nutrients, soluble salts, and organic matter content.  The engineer, architect, or best management practice guidelines may change for each project.  The bioretention soil’s function is to treat stormwater runoff effectively.  The blend much be homogenous to make sure there are no inconsistent portions of the soil that cause the whole bioretention system to fail.

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