Chaos Wells are corrupted fountains that were created when the Burning Legion went to Kalimdor after destroying Lordaeron, Mannoroth and Tichondrius went to a glade in Ashenvale to consider using the Orcs to defeat Cenarius. Mannoroth proceeded to spill his burning blood into the Fountain of Life, corrupting it, causing anyone who drinks of it to be infused with the Blood of Mannoroth. When Cenarius attacked the Orcs, Grom Hellscream took a detachment to explore the forest. They found the Chaos Well and drank it, reinstating the blood pact between them and Mannoroth. The Orcs proceeded to use their new found power to kill Cenarius and the Night Elves.
In World of Warcraft Edit
Although much of Ashenvale isn't very open and isn't similar compared to the WC3 version/maps of Ashenvale, the Chaos Well is no where to be found. A few theories could be the chaos well depleted itself and crumbled, or the local Satyrs and Demons could somehow be related.
And although also not proven there is some evidence that supports that there may be more corrupted or chaos wells found in the Barrow Dens since there are corrupted creatures located there.