Here is where I found this!http://www.amnh.org/learn/pd/sharks_rays/gallery_week4/
blood in the water!
A drop of blood...
Sharks Can Detect a Single Drop of Blood in the Ocean!
Sharks are often portrayed as having an almost supernatural sense of smell. However, reports that sharks can smell a single drop of blood in a vast ocean are greatly exaggerated. While some sharks can detect blood at one part per million, that hardly qualifies as the entire ocean. Sharks do, however, have an acute sense of smell and a sensitive olfactory system--much more so than humans. Sharks' nostrils are located on the underside of the snout, and unlike human nostrils, are used solely for smelling and not for breathing. They are lined with specialized cells that comprise the olfactory epithelium. Water flows into the nostrils and dissolved chemicals come into contact with tissue, exciting receptors in the cells. These signals are then transmitted to the brain and are interpreted as smells.
Because of the extreme sensitivity of these cells, as well as the fact that the olfactory bulb of the brain is enlarged, sharks can detect miniscule amounts of certain chemicals. This varies, of course, among different species of sharks and the chemical in question. The lemon shark can detect tuna oil at one part per 25 million--that's equivalent to about 10 drops in an average-sized home swimming pool. Other types of sharks can detect their prey at one part per 10 billion; that's one drop in an Olympic-sized swimming pool! Some sharks can detect these low concentrations of chemicals at prodigious distances--up to several hundred meters (the length of several football fields)ódepending on a number of factors, particularly the speed and direction of the water current.
Predation is not the only behavior in which olfaction plays a crucial role. Evidence exists that this keen sense of smell is also instrumental in sexual behavior. Males are able to detect pheromones produced by females, even in low concentrations, helping them locate potential mates.
The Sensory World of Fisheshttp://www.csuchico.edu/~pmaslin/ichthy/Snsry.html#start
This site presents the basics of shark's senses, including smell.
Topics in Shark Biology: The Nose Knowshttp://www.reefquest.com/topics/nose.htm
This site provides a thorough discussion of shark olfaction.