The Washington State Department of Transportation (WSDOT) is responsible for maintaining more than 7,000 miles of highway, 48 rest areas, 24 maintenance facilities, and 19 ferry terminals. Biofiltration swales are one of the primary BMPs that WSDOT uses to treat stormwater runoff from the many highways and facilities throughout the state. The question is, how long can these swales last and still perform the intended treatment for which they were designed? The WSDOT Highway Runoff Manual (HRM) states that the effective life of a biofiltration swale is 5-20 years. For this study, WSDOT is monitoring two biofiltration swales beyond their effective age, each with different construction and highway location designs. Both swales are in Western Washington and have been monitored for over two years to determine their effectiveness for total suspended solids (TSS) removal. Data collected from this study may allow WSDOT to adjust its maintenance and replacement schedule of biofiltration swales, as well as help inform potential future swale longevity studies. If these swales are currently still functioning, should their effective lifespan be extended? Can these older swales be retrofitted to gain even more pollutant removal in the future?