Available at: https://digitalcommons.calpoly.edu/theses/1891
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Bioretention cells have become a commonly used green infrastructure technique to help infiltrate and remove contaminants from stormwater runoff. Bioretention cells are constructed from a layered or heterogeneous soil mixture designed to optimize their ability to infiltrate influent stormwater and remove contaminants carried by the water as it filters through the soil media. The soil mixture, composition, and planting vary depending the local regulatory agencies. As urbanization occurs across the United States, more natural land is converted from pervious surfaces, such as grasslands and forests, to impervious surfaces such as asphalt and concrete, to help reduce the impact of the runoff generated by this increased flow bioretention cells are an often-used method to treat stormwater. These impervious surfaces do not allow rainfall to infiltrate, and the water runs off into receiving water bodies such as rivers and streams as a non-point source pollutant. To help reduce pollutant loadings into receiving water bodies, Low Impact Development (LID) techniques were developed to reduce stormwater volume, peak flow, and contaminant loading rates. The bioretention cell is one of the most popular LID techniques and is comprised of a soil media that is either a layered or homogenous media, which is built following a regional agency’s standard. The performance of bioretention soil media is highly variable depending on the amount of each soil constituent present in the media.
This study compares five different soil mixtures from various agencies’ specifications to determine which media composition is most effective at removing total suspended solids (TSS) and nitrates, two of the most prevalent contaminates carried by stormwater. This study also compares mixtures’ hydraulic conductivity which determines the volume of water that the media can infiltrate and “treat”. To perform these tests, six columns of soil media were constructed with media depths of 91.5 cm (36 inches). Columns were dosed with either tap water (Phase I) or a synthetic stormwater blend (Phase II) to determine the amount of TSS and nitrate exported by each mixture. The soil mixture in each column was characterized to understand how soil characteristics effect the performance of the various media mixtures.
The bioretention soil media columns were all shown to be effective at removing influent TSS with an average removal rate of over 88% across all the columns, ranging from 99.9% removal to 73.6% removal. Most bioretention soil mixtures used in the test were shown to be ineffective at removing influent nitrates, with breakthrough of nitrate occurring after the first two pore volumes. Interestingly, the media with higher organic content were more effective at removing nitrates, with removal rates as high as 59.9% compared to the media with lower organic content. Hydraulic conductivity was also highly variable across the various soil media mixtures depending on the percentage of sand and fine media particles present in the media. Hydraulic conductivity ranged from a high value of 42 cm/hr to a low of 8.3 cm/hr. By comparing these results, a more effective bioretention soil media mixture can become agency standard and allow bioretention cells to have more consistent and better performance.