Poster 237 – Click on poster below to view presentation from author.
Chelsea Mitchell
Washington State University
Co-Authors: Anand Jayakaran, Jenifer McIntyre
We evaluated the treatment of Polycyclic Aromatic Hydrocarbons (PAHs) and Fecal Indicator Bacteria (FIB) in highway runoff by bioretention. We compared treatment performance by Washington’s standard bioretention mix of 60% sand, 40% compost (by volume), and by three other blends amended with biochar and fungi. Biochar-amended mixtures contained 60% sand, 20% compost, and 20% biochar. Fungi-amended mixtures were inoculated with Stropharia rugosoannulata. Twelve bioretention columns were constructed in a greenhouse, each containing 18” of bioretention mix planted with Carex oshimensis overlaying a 20” gravel drainage layer. Columns were initially conditioned with clean water and then dosed with stormwater collected from a highway downspout in Tacoma, WA during 8 events over a 14-month period. Effluents for each column were analyzed for 23 PAHs, E. coli, Fecal coliform, dissolved organic carbon (DOC), and total suspended solids (TSS). To track the fate and transport of PAHs within the bioretention columns, soil cores were taken four time throughout the study. TPAHs were almost completely removed by all treatments across all storms, with removal rates ranging from 97-100% for 94 out of 96 samples. Compost appears to be a source of PAHs in bioretention soils, as biochar-amended soils initially contained half the soil TPAHs as columns with the standard 60:40 sand:compost mixture. We observed a net loss of TPAHs between 44-76% in bioretention soils which could not be explained by PAHs in effluent alone, suggesting that microbial bioremediation and/or plant uptake attenuated soil PAHs. Fungi amended bioretention soil showed higher TPAH losses (70-73%) than bioretention soil alone (60-71%). Influent concentrations of FIB spanned 3 orders of magnitude. E. coli and fecal coliform were initially exported, but all columns achieved some treatment after the first dosing event. DOC was exported from all columns, but export was reduced in biochar-amended columns.
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This is great work, Chelsea! It’s exciting to see work with both biochar and fungi in bioretention media. I have two questions: first, how did you quantify TPAHs? Second, why do you think you saw a reduction in fecal coliforms only when the columns both contained biochar and were inoculated? Any ideas on fungi x biochar interactions?
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Thanks for your interest, David! We quantified TPAHs using EPA standard method 8270D-SIM (GC/MS) for 24 PAH analytes. Great question regarding the biochar x fungi interaction for fecal coliform reduction. I have been puzzling over this finding myself. The Stropharia rugosoannulata fungi used in this study are capable of consuming fecal coliforms and producing antibiotics. The partial replacement of compost with biochar may reduce the ability of fecal coliforms to grow (biochar has less leachable nutrients and bioavailable carbon than compost) inside the bioretention media, and may also improve bacterial attachment to the media. My current hypothesis is that these processes may have an additive effect on fecal coliform attenuation. However, since we did not see strong effects of biochar or fungi amendments individually for fecal coliform removal, it is also possible that there is some interaction between fungi and biochar that we are currently not aware of. We are hoping to do some follow-up studies that could elucidate some of these internal processes.
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