Abstract: Perlite particles, after being expanded and treated with a silicone have the ability to selectively absorb hydrocarbons while rejecting water on or in which the hydrocarbons may be present. A carefully designed cartridge having an apertured top and an apertured bottom permits runoff water to percolate through a body of particulate perlite so that the desired separation can be accomplished. The cartridge in which the perlite is contained is easily removed from a catch basin when the perlite has become spent. The cartridge may then be opened and the spent perlite removed, after which the cartridge may be refilled with fresh perlite and replaced in the catch basin.