Current City of Seattle BSM specifications consist of 60% mineral aggregate and 40% fine compost consisting of nutrient–rich organic matter. This compost contains nitrogen, phosphorus, and metals such as copper, resulting in the export of nutrients downstream to receiving waters when underdrains are present. Utilizing water treatment residuals (WTRs) in compost-based BSM has the potential to reduce the export of nutrients while further reducing costs for stormwater treatment. Herrera and RKI produced this white paper for Seattle Public Utilities to summarize research findings and evaluate the effectiveness and potential consequences of using Water Treatment Residuals (WTRs) as a component in bioretention soil media (BSM) for improved phosphorus and copper removal.