James Houle, Program Manager, University of New Hampshire Stormwater Center

Thomas Ballestero

Iulia Barbu

Tim Puls

Track: Research

Abstract

Bioretention systems, also known as “rain gardens,” are among the most common Low Impact Development (LID) stormwater technologies in use today.  This presentation highlights the results of over 10 years of Bioretention systems field testing and optimization, and the results of more recent laboratory column tests of various Bioretention soil mixes on the overall effectiveness for Phosphorus (P) and Nitrogen (N) removal.

Most bioretention systems have proven effective with respect to sediment and sediment-associated pollutants pollutant removal, however, the performance for dissolved nutrients (N and P) is more variable.  Standards for bioretention filter media mixes vary dramatically, though there are some commonly employed ingredients: loam, sand, compost and woodchips The examined bioretention soil mix compositions include different combinations and percentages of these materials.  In addition, filter media amendments such as water treatment residuals, iron, limestone sand, and basic oxygen furnace slag were tested for P removal.  The textural class of the soil proved to be important in P removal (loam based mixes were more effective than sandy soil mixes) due to lower permeability and therefore slower filtration rates. Improved P removal was also demonstrated by the addition of soil amendments containing Fe₂ and Al oxides. The amendment to soil mix ratios was optimized based on economic considerations. Lower P removal rates were demonstrated by soil mixes containing compost; it was found that the P leaching from the compost could significantly offset the amount of P removed by the media itself. Along with mix composition the use of internal reservoirs composed of stone were also evaluated for N removal in relation to holding time and the ratio of internal storage reservoir volume to water quality treatment volume.

These results provide insight into nutrient removal capabilities of a range of Bioretention soil mixes and design standards that could be particularly used in watersheds where nutrients are the primary water quality threat.