Since the Industrial age scientists and engineers have studied the peculiarities of fluid dynamics for its applications in machine engines and chemical factories. A recent, and rather counter-intuitive discovery in this realm comes from Professor Ernesto Altshuler at the University of Havana in Cuba. While brewing the traditional South American tea, ‘yerba mate’, Altshuler noticed a peculiar occurrence, sometimes the tea leaves in his cup would find their way upstream into the pot of hot water. Of course, as physicists, he and his research partners could not resist attempting to model this anomaly.
Through their research, the team at University of Havana discovered that small particulates are able to travel upstream when the distance the water drops is less that one centimeter in height. Water flowing up a stream seems like an impossible occurrence, but the scientists’ model shows that under these conditions, the stream forms small back-currents on its edges which can transport particulates in the opposite direction of the flow as shown by the following diagram.
Velocity Field of particles on the surface of the channel
This discovery has dangerous implications for chemical manufacturers and a variety of other industries, by showing that it is possible for deadly chemicals to stealthily enter an otherwise fresh water supply. One chemical, Aluminum hydroxide, a chemical often used in water purification processes, was implicated in a 1988 mass poisoning incident when dangerous amounts of the chemical found its way into tap water supplies, stripping copper, lead and other heavy metals from pipes. This back-flow process detailed by Altshuler shows that these chemicals could find their way into fresh water reservoirs and streams, contaminating them without the knowledge of the plant operators.
These possibilities for contamination must now be considered for many plants which deal with lethal chemicals, and will hopefully save lives or, at the least, a few brain cells.
References: S. Bianchini, A. Lage-castellanos, and E. Altshuler arXiv:1105.2585v1 [physics.flu-dyn]
Massachusetts Academy of Sciences Blogline 
Thanks for mentioning our little paper. However, I must correct a couple of points. Actually it was student Sebastian Bianchini (from Argentina!) who observed the effect by the first time. We only have a possible explanation for it, but –unfortunatelly– not a quantitative explanation. Finally, I am not quite sure that the water-tap contamination you mention is due to a similar effect…it seems a different scenario…do you have some more data on it?.