From May 2009 through June 2010, Herrera Environmental Consultants conducted hydrologic and water quality monitoring of a compost-amended biofiltration swale and a standard (control) biofiltration swale in the median of State Route 518 for the Washington State Department of Transportation. Herrera conducted this monitoring to obtain performance data that supports the issuance of a General Use Level Designation (GULD) for the compost-amended biofiltration swale from the Washington State Department of Ecology. This monitoring was performed in accordance with procedures described in "Guidance for Evaluating Emerging Stormwater Treatment Technologies; Technology Assessment Protocol – Ecology (TAPE)" (Ecology 2008).
This document is a technical evaluation report on the compost-amended biofiltration swale, prepared by Herrera and based on results of the monitoring described above. The goal of this report is to demonstrate satisfactory performance of the compost-amended biofiltration swale for issuance of a GULD for basic, enhanced (dissolved metals removal) and oil treatment.