Performance of stormwater bioretention mesocosms amended with biochar and fungi

Chelsea Mitchell

Washington State University

Co-Authors: A. Jayakaran and J. McIntyre

Current stormwater permitting regulations in the state of Washington do not include performance goals for the treatment of pollutants like certain organic contaminants (Polycyclic Aromatic Hydrocarbons – PAHs) and bacteria (fecal coliform, Escherichia coli). To inform Best Management Practices (BMPs) for the treatment of these contaminants in stormwater runoff, a mesocosm study is being conducted to assess the performance effects of adding biochar (BC) and/or fungi (Stropharia rugosoannulata) to a typical bioretention soil media (BSM, 60:40 sand:compost mixture) planted with Carex oshimensis. Twelve bioretention columns were constructed to test the following treatments in triplicate: 1) BSM, 2) BSM+BC, 3) BSM+Fungi, and 4) BSM+BC+Fungi. The bioretention columns were initially conditioned with clean water, and then over four months were dosed three times with stormwater runoff collected from a highway in Tacoma, WA. During each dosing event PAHs, Escherichia coli, Fecal coliform, Total Suspended Solids (TSS), and Dissolved Organic Carbon (DOC) were measured in the influent, bioretention column effluents, and a stormwater grab sample taken directly from the collection site. Stormwater grab samples contained 0.58-1.80 ug/L of Total PAHs, 22-600 CFU/ 100mL of E. coli, 400-700 CFU/100 mL of Fecal coliform, 35-158 mg/L of TSS, and 1.83-3.49 mg/L of DOC. PAHs were not detected in any of the effluent samples, indicating efficient removal of PAHs by all treatments. Indicator bacteria were frequently detected in bioretention effluents at concentrations higher than influent concentrations, indicating a net export of E. coli and Fecal coliform from the bioretention columns, though there were no significant differences in export of these bacteria across bioretention treatments. Removal of both TSS and DOC was highest for treatments containing biochar (BSM+BC and BSM+BC+Fungi). DOC removal was significantly higher than the reference treatment, BSM, for BSM+BC (Wilcoxon, p=0.00049) and BSM+BC+Fungi (Wilcoxon, p=0.001). Results thus far suggest that bioretention is an effective stormwater treatment process for PAH removal, but may export fecal indicator bacteria. This study will continue until spring 2021 with five more stormwater dosing events.

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5 thoughts on “Performance of stormwater bioretention mesocosms amended with biochar and fungi

  1. Thanks,Chelsea. It will be interesting to see if you can distinguish any significant additional benefit due to the fungi.


  2. Thanks for checking out my presentation, Grant!

    We will continue to monitor these columns through the fall, so I look forward to seeing whether the parameters we are measuring are significantly impacted by the fungi as well after continued dosing with stormwater.


  3. Thank you so much for taking the time to present this information– utilizing fungi and biochar in bioretention media is an interesting idea in itself, but I especially like that you pointed out that this study could help to inform policy about stormwater treatment!
    This content and study is really interesting, but I’m confused about the conclusion that you drew from your results. In your abstract, you said that bioretention may be a net exporter of fecal indicator bacteria, but the conclusion from your paper says that while results don’t indicate significance, biochar and fungi appear to reduce fecal bacteria. So my two questions are:
    could you explain your abstract conclusion versus what’s on your poster?
    what role does your statistical analysis play if, despite a lack of significance, you say that these treatments reduce fecal bacteria?
    I could follow everything else you explained really well and I thought you made this all so very clear! Thank you for your time!


    1. Hi Samantha, thank you for your thoughtful questions! I should have been more clear – in most of the bioretention effluent samples I collected, the fecal indicator bacteria (E. coli and fecal coliform) concentrations were higher than the influent, suggesting export of bacteria from the columns. Though all columns showed this export, the control columns (with no added biochar or fungi) exported greater quantities of bacteria than the treatment columns on average. I chose to mention this (even though the statistical tests were not significant) in acknowledgment that critical values are somewhat arbitrarily set (a=0.05 here), and tests for statistical significance, while very useful, are just one tool used in making sense of a data set. Does this answer your questions? I appreciate you engaging me in a thoughtful conversation about statistics!


      1. Yes, that all makes a lot of sense, thank you! I’ve only used generalize linear mixed models and AICc rankings to interpret my statistics particularly because I’ve felt P values are, just as you said, arbitrary. Thank you so much for your perspective!


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