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Optimizing bioretention media with vegetation for stormwater treatment in Denver, Colorado
Calahan, Samuel E.
Calahan, Samuel E.
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2022
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Abstract
The total global population continues to swell, and this growth is especially rapid in the world’s urban centers. This has considerable consequences for urban hydrology as the built environment expands to facilitate urban life. Increased hydrologic flashiness, water pollution, and altered aquatic ecosystems tend to accompany expansion of the urban fabric, and city water managers need alternatives to more traditional gray infrastructure to mitigate these impacts.
Many such alternatives fall under the classification “green infrastructure” or “low-impact development,” mimicking natural systems to restore a site’s pre-development hydrology and gain a host of ancillary benefits not offered by traditional infrastructure. Bioretention systems, also known as porous landscape detention, use a small basin filled with porous media and planted with vegetation to detain and treat urban stormwater runoff, and have been recognized as an effective green infrastructure tool since the 1990s.
This research aims to design innovative geomedia for use in bioretention systems that can remove pollutants considered a priority by the City and County of Denver, sustain vegetation planted in the media, and help restore a site’s natural hydrology. Mesocosm experiments, which use large columns to bridge the research gap between bench-scale and field-scale testing, were conducted to evaluate system performance using three bioretention media amendments: biochar, clinoptilolite zeolite, and coconut coir. Four bioretention media blends were assessed in mesocosms to determine the ability of these materials to reduce the discharge of excess nutrients, metals, and bacteria found in stormwater runoff, and to sustain vegetation health and survivability during dry periods, compared to traditional bioretention media.
Two control blends, representing conventional media commonly used in the Denver region, were compared with two amended blends. 75 percent of one amended blend, called the Colorado School of Mines blend (CSM), consisted of conventional media, with biochar, zeolite, and coir amendments comprising the remaining 25 percent. The other amended blend, designed to provide greater water quality benefits and called the Water Quality blend (WQ), contained only 35 percent sand in addition to 30 percent biochar, 30 percent zeolite, and five percent coir. Each mesocosm was planted with three perennial Prairie Blues Little Bluestem (Schizachyrium scoparium) grasses and two perennial Pink Creeping Phlox (Phlox subulate) ground cover plants to assess whether the media amendments can improve vegetation survival during dry periods. Nine stormwater treatment (SWT) experiments were conducted using a synthetic stormwater representative of local storm runoff. One flush test was conducted using dechlorinated tap water to evaluate constituent output with little or no input. The SWT experiments and flush test were each conducted over roughly one hour, with water input at rates which simulated a large storm in the Denver area. Hydraulic conductivity, a common performance metric for bioretention media, was assessed for each blend to evaluate the rate of saturated flow through the media. Batch desorption tests were completed after the SWT experiments to evaluate pollutant retention. The SWT experiments were conducted over six months between May and October 2021; hydraulic conductivity tests were conducted over six weeks in January and February 2022; and batch desorption tests were conducted over one week in March 2022.
Results indicate that incorporation of the treatment amendments can improve overall pollutant bioretention performance. Slight but consistent improvements were observed for nitrogen (N), copper (Cu), and Escherichia coli (E. coli) in the CSM blend relative to the control blends: a 15 percent reduction of total nitrogen (TN) export, averaged over all trials, and a 93 percent reduction for trials three through 10; a 67 percent reduction in Cu export for a 5.2 percent increase in removal efficiency, averaged over all trials; and a 48 percent reduction in E. coli export for a 70 percent increase in log-reduction, averaged over all trials. However, both blends containing the treatment amendments tended to export nutrients, particularly phosphorus. To avoid nutrient export, media rinsing should be considered before or upon installation. Effluent from these rinses could be reused to recycle the nutrients leached, for example by irrigating the media. In assessing the vegetation performance, there were no clear differences between media blends. However, all blends succeeded in supporting vegetation. The CSM blend consistently retained a 10 to 25 percent higher absolute soil moisture content than the other blends, which was generally more than twice as much water as that retained by the other blends. Characterization of the CSM media independent of the SWT experiments suggests that the soil moisture it retains may also be more readily available for uptake by plants roots than water in the other blends.
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