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.