The study was designed to evaluate the effectiveness of a stormwater retrofit at both the individual treatment installation level and system-wide in a 207-acre basin with primarily urban residential and commercial landuse. The retrofit in the Echo Lake drainage basin included installation of various best management practices (BMPs) including Filterra® and bioretention planter boxes (BPBs) along the highway. A detention tank system (DTS) was also installed to provide flow control for the majority of the basin prior to discharge to Echo Lake. This retrofit was selected for evaluation by this study because: (1) stormwater at the individual BMPs was accessible for sample collection both before and after treatment; (2) stormwater quality in this basin had been analyzed prior to the retrofit installations, providing a baseline for comparison; and (3) there is an ongoing, long-term water quality monitoring program for Echo Lake. These site qualities made it possible to address the following study objectives:

Individual BMPs
• Objective 1: Evaluate the effectiveness of individual enhanced stormwater treatment installations at reducing solids, nutrients, bacteria, metals, select organic contaminants and toxicity in highway runoff in Shoreline.
System-wide
• Objective 2: Evaluate the flow control benefits of the system-wide stormwater DTS, and any additional reduction of solids, nutrients, bacteria, metals and select organic contaminants that occur.
• Objective 3: Assess changes in stormwater quality in this system by comparing historical stormwater data to current stormwater quality before and after treatment.
• Objective 4: Identify if nutrient and bacteria levels have changed in Echo Lake over time, and how these changes correspond to changes in stormwater infrastructure in the contributing basin. The purpose of this analysis is to consider potential effects to the receiving water body for discussion, not to establish a causal relationship.