As the numbers of installed greenroofs continue to grow internationally, designing greenroof growing media to reduce the amount of nutrients in the stormwater runoff is becoming essential. Biochar, a carbon-net-negative soil amendment, has been promoted for its ability to retain nutrients in soils and increase soil fertility. This study evaluated the effect on water quality of greenroof runoff after adding biochar to a typical extensive greenroof soil. Prototype greenroof trays with and without 7% biochar (by weight) were planted with sedum or ryegrass, with barren soil trays for controls. The greenroof trays were subjected to two sequential 2.9 in/hr rainfall events using a rainfall simulator. Runoff from the rainfall events was collected and evaluated for total nitrogen, total phosphorus, nitrate, phosphate, total organic carbon, and inorganic carbon. Greenroof trays containing biochar showed lower quantities of nutrients in the stormwater runoff compared to trays without biochar. Biochar-amended soil with and without plants showed a 3-to 25-fold decrease in release of nitrate and total nitrogen concentrations, as well as a decrease in phosphate and total phosphorus concentrations release into the rainfall runoff. Phosphorus results from trays planted with sedum indicate that sedum interacted with both soils to cause a decrease of phosphorus in the greenroof runoff. In correlation with a visual effect in turbidity, biochar-amended soil showed a reduction of total organic carbon in the runoff by a factor of 3 to 4 for all soil and plant trays. Inorganic carbon was similar for all tests showing that inorganic carbon neither reacted with, nor was retained by, ii biochar in the soil. The addition of biochar to greenroof soil is an effective way to retain nutrients in a greenroof soil, reduce future fertilizer demands, and improve the water quality of the stormwater runoff by reducing nitrogen, phosphorus, and total organic carbon concentrations in the runoff water. iii ACKNOWLEDGMENTS I would like to gratefully acknowledge Dr. Graig Spolek, who opened up his lab to the idea of testing biochar, without his kindness, expertise and ongoing support this study would not have been possible. I also thank my advisor, Dr. Gwynn Johnson, for sharing her time and wisdom, and Dr. Chris Berger who presented an ever-kind presence. I want to express my gratitude to Dr. David Feinstein for his encouragement, numerous stimulating discussions, help with experimental design, and most precise and accurate advice. I am grateful to my fellow engineering students, in particular the students of the GBRL lab for their ongoing moral support and assistance with a variety of fun and tedious tasks;
