Abstract
Stormwater runoff from rainfall events can contain pollutants that are carried into nearby streams and lakes. One constituent of concern is phosphorus, which can be present in fertilizer used in agricultural and home gardening applications and in soil particles. One technique for treating stormwater is bioretention, which involves capturing runoff and infiltrating it through natural or engineered soil beds. The effectiveness of different engineered soil mixes and underdrain amendments at removing phosphorus was the focus of this research. Of particular concern was the ability of underdrain media to reduce the leaching of phosphorus from compost-based bioretention soil media (BSM). The BSM used in this study was composed of 60-70% sand and 30-40% of compost with an infiltration rate of at least 5 inches per hour. Seven, four-inch diameter columns were filled with BSM or BSM plus topsoil to a depth of 18 inches and an eighth was filled to 30 inches. Adding topsoil reduces the compost fraction in the BSM. Various underdrain materials, including pea gravel, crushed limestone, and activated alumina, were tested to investigate their phosphorus adsorptive potentials. Activated alumina is a synthetic aluminum-based adsorptive material used for phosphorus removal. It was incorporated into the underdrain of two columns as a 4-inch layer on top of a pea gravel base. The other underdrains were composed of 12 inches of gravel or limestone. The columns were dosed equally with a synthetic stormwater mixture at both low and high rates to simulate varied rainfall conditions. Most columns leached phosphorus from the BSM into the effluent. The columns with an activated alumina layer, however, showed little or no leaching and in some test runs net removal of phosphorus (compared to the influent concentration) was observed. When deeper BSM was used or the BSM was mixed with topsoil, phosphorus leaching increased. The unexpected results for the BSM plus topsoil columns can be explained by a high proportion of compost in the topsoil used. In some columns the media was washed with tap water prior to the experimental runs. Washing the columns removed some particles with a size larger than 0.45 microns, but not all. As a result, phosphorus leaching was reduced, but not eliminated, by media washing except in the columns containing activated alumina. The results from this laboratory experiment were incorporated into the design of a larger experimental project sponsored by the State of California with Proposition 84 funds.