Biofilters, often referred to as rain gardens, are landscape features that store and treat stormwater runoff. Surface runoff is directed into shallow, vegetated depressions with underlying layers of soil, sand, and gravel. They may have an underdrain that sends excess runoff to the storm sewer. These areas are designed to mimic a natural ecosystem where pollutant removal occurs through soil infiltration and plant uptake. They are commonly located within or adjacent to parking lots or in residential open spaces.