Abstract: A leak detection system for detecting leaks of a hydrocarbon from an underground tank uses a dye that is soluble in hydrocarbon, but not in water. At least one riser pipe extends from a point adjacent the bottom of the tank to the surface. The lower end of the riser pipe is perforated. A permeable container containing a quantity of the dye is placed in the bottom of the pipe. A line extends from the container to the top of the pipe for enabling an inspector to periodically pull the container to the surface to determine if the color of the container has changed, indicating leakage of hydrocarbon liquid.