Purpose in bioswale soil mixes: Provide additional pollutant removal and other benefits
Suggested volume: 5 to 10% of total mix
What to look for: Which amendments to use will depend on characteristics of your base mixture of sand and compost.
Water treatment residuals
Water treatment residuals (WTR) are aluminum or iron oxides used in purifying water for drinking. Like clay and organic matter, they have a high surface area and can adsorb both negatively charged anions like PO3- (phosphate) and N03- (nitrate), as well as positively charged cations, such as metals. Even after their use in water treatment these compounds are still highly reactive.
While WTR are traditionally landfilled, they have been reused to reduce the mobility of phosphate in agricultural soils where phosphorus has built up overtime. WTR are also being explored as an additive to soil mixes to reduce nutrient loss and enhance contaminant removal in bioswales that use composts.
Results from one study indicate that WTR can be an effective method for both purposes, without reducing water infiltration rates significantly.
Wood chips can be added to increase the carbon to nitrogen (C:N) ratio in a soil mix, thus reducing the potential for nitrogen to leach from the system. Wood chips are often also added as a mulch layer over bare soil to reduce erosion and help soils retain moisture during dry periods. Wood chips, however, often have a low pH (are acidic). Additions of wood chips should therefore be monitored so that they don’t lower the pH of a bioswale soil enough to reduce its capacity to bind pollutants, such as metals. Compost, however, can provide buffering capacity against fluctuations in pH, reducing the risk of creating acidic conditions through wood chip amendments.
Perlite, vermiculite, or other porous, mineral materials may also be used in green infrastructure soil mixes. These volcanic materials are more porous than sand and can provide drainage attributes similar to sand, while at the same time offering increased water-holding capacity. Their ability to hold more water than sand comes from the higher surface areas provided by the particles’ nooks and crannies. Their bulk density is also lower than sand, meaning they are lighter and well-suited for mixes such as those used on green roofs where weight is an important consideration.