I recently had the chance to visit an old mining town called Nacozari de Garcia in Sonora, Mexico. I was very surprised at the mining disaster I found when I arrived.

Nacozari is a mining district with a long history of copper mining. About 50 years ago, when mining was performed in the area the town was built around, tailings (the material that was processed to take out the copper) was disposed of using a method called Valley Fill.

Valley Fill means exactly what it says - you fill the valley or other voids in the landscape with the tailings that come from the mine.

Disposing of mine tailing using the Valley Fill method is very problematic. In the case of Nacozari, 50 years later, there's a very large problem in the area because of the way that the mine tailings were disposed of. The problem is compounded even more by the housing development that had encroached upon the tailings. In some cases buildings have even been built over top of the tailings.

The copper in the rock that was processed was in the form of a sulfide mineral. A process called flotation was used to separate the sulfides from the surrounding rock. The flotation process requires that all the material to be processed be in very small pieces before the copper minerals can be separated from the surrounding rock. Since everything has been made into a fine material before being processed, the tailings are made up of pieces of rock that are the size of a grain of sand and smaller. Some of the material has even been ground so fine that it forms a sludge when water contacts it.

It's rare, if not unheard of, that a mine processes and recovers 100% of the target metals. Some of the metals aren't recovered and they're left in the tailings that are thrown away.

Through time, nature takes its course on the tailings and some of the minerals leach out. The picture attached to this article shows the copper leaching out of the tailings at one of the old Nacozari tailings deposits.

The problem with depositing tailings in the valley is that the valley is naturally where water accumulates. When water accumulates in the valley it seeps through the tailings and speeds up the leaching process.

The bigger problem with disposing of tailings in valleys is that the water that accumulates in the valley is usually fed into a larger accumulation of water somewhere down the landscape. Leached minerals from the mine tailings end up contaminating more and more water.

In the United States, Mountaintop Removal Mining commonly use the Valley Fill method to dispose of the tailings and overburden.

Once tailing are deposited into the valley it is a very expensive and complicated process to clean up the mess. There is no need use the valley fill method of mine tailings disposal these days.

If you'd like to see more pictures of the mine tailings disaster at Nacozari de Garcia please visit our photostream on Flickr.