A method of mounting a grate adapter unit beneath a stormwater collection grate includes steps of providing a grate adapter unit having a plurality of outwardly extending mounting flanges, each mounting flange being adapted to fit a different size commercially common stormwater collection grate. The grate adapter unit is trimmed, either at the factory, at the contractor's facilities or at the installation site, at the desired location to select a desired one of the mounting flanges that is appropriate for the collection grate to which the grate adapter is being fit. The grate adapter unit is then mounted beneath the stormwater collection grate using the selected mounting flange. A stormwater remediation unit may be pre-mounted to a lower end of the grate adapter unit or mounted to the lower end after installation.