Field-Scale Column Testing of Engineered Soils and Bioretention Configurations for the Removal of Nutrients and Lesser-Studied Constituents

Implementation of bioretention systems is constrained by lack of information on design and performance for three particular situations: (1) sites where nutrient export from bioretention facilities is a concern, (2) sites where groundwater contamination is a concern due to high groundwater or porous native soil, and (3) sites where information on local pollutants of concern is not available. Improved engineered soils and better bioretention designs could overcome these siting constraints.

The experimental goals of this project were (1) to test different bioretention configurations and soil/media mixtures that would minimize leaching of nutrients, and (2) to generate pollutant removal data for a wide range of constituents to better inform regulatory requirements including those for vertical separation to groundwater. Constituents studied included lesser-studied pollutants such as ammonia, TKN, organic carbon, mercury, PAHs, PCBs, aluminum, silver, cadmium, chromium, and selenium.

A number of field-scale experimental bioretention columns were constructed and operated at the campus of California State University, Sacramento. They incorporated a variety of improved media including dual layers of activated alumina overlain with various compost/sand mixes. The design variables tested included media conditioning, location of the underdrain, and outlet flow control. In addition, to directly test constituent movement during infiltration in soils, several columns were constructed with bioretention media on top of different soil types. All experimental columns were duplicated and included vegetation to ensure realistic simulation of full-scale systems.

Treatment performance was assessed during 6 storm events. Runoff from the campus was collected and spiked with specific constituents to ensure sufficient concentrations in the influent. The experimental columns were dosed at a constant rate and influent and effluent grab samples were collected and composited.

The results of these experiments are presented, and implications for using different soil/media mixes and improved bioretention designs to overcome siting constraints are discussed. This project was funded by the State Water Board Proposition 84 Grant Program.