Abstract: The present disclosure is directed to a method and apparatus for monitoring leaks from underground storage tanks and the like. One or more wells of specified depth are formed and each well is lined with plastic pipe which is perforated to admit migratory diffused hydrocarbon molecules from the soil into the pipe. A cap closes the upper end of the pipe and supports a cable which holds an absorbent cartridge. The cartridge has a perforate housing and the housing filled with beads of divinylbenzene which serve as a hydrocarbon, hydrophobic absorbent material. A test procedure is also set forth wherein absorbed hydrocarbons from test cartridges are correlated to background levels and the signature of a stored hydrocarbon product.