A big hole in the ground

It is hard to get a grip on just how big the Grand Canyon actually is.  446km long, up to 29km wide and 1,800 metres deep make for a big hole.  The geology of the canyon is as spectacular as the view but its formation has been the subject of a lot of debate.  It is quite obvious that it has been formed by erosion from the Colorado River, but the rate of erosion has been the topic of much geological debate.  The most recent interpretation of the erosion is that although parts of the canyon formed up to 70 million years ago the whole drainage path of the Colorado developed through the length of the Canyon in the last 6 million years.

Poisoned water

The mineral above looks very lovely but it contains a deadly secret.  It is a sulphide mineral called Arsenopyrite and, as the name suggests it contains  the element Arsenic - which is poisonous.

The mineral is useful as an ore of Arsenic (a useful metal in several alloys) and also is a good indicator of the occurrence of gold in its host rock.  However, when groundwater is being extracted from rocks that contain arsenopyrite the water will be contaminated with this toxic element.  This is a big problem in some countries, notably Bangladesh and China where tens of millions of people can be affected.

China's Arsenic risk estimated

Hydrogeologists now have a means to predict where these problems might occur using remote sensing and GIS techniques which may help some of the areas at risk.  What do you think could be done to remediate the problems?