Tennessee coal sludge spill

I've avoided talking about the Tennessee coal sludge spill for a while - but since nothing else is happening, I should probably give it my attention.

After 40 years of hard work, freezing temperatures and very heavy rain in very short periods, one of the retaining walls in a coal ash slurry pond has gave way, causing a large coal slurry landslide. Two other ponds were not affected, and are apparently receiving the material as it is cleaned up.

1) This is a nationalised company.
2) I don't understand why the material is mixed with water to start with - it needs containing if this happens and may permit any water soluble metals to leech out.
3) We don't do it like that over hear - rather keep it dry so that it can't wash anywhere. I've seen in the past that it's been pumped back down abandoned mine shafts, or piled into abandoned quarry's. A lot is also sold to the construction industry (think breeze blocks and concrete).
4) There is a concern about leeching of metals from the fly ash - I've seen a probe at a power station monitoring the acidity of the ground below the oldest part of a fly-ash tip. I'm not sure of the results but I don't believe it was that bad.

The metals we're worried about are naturally occurring - they occur in the plants themselves. Plants concentrate them from soils into their roots / cells. As coalification takes place the trees are squeezed turning the plants into rocks and forcing out (amongst other things) oxygen and, in the very oldest coals, hydrogen. This concentrating effect can turn what was once around 2% ash (in the wood) to perhaps 15% ash (in the coal). Of course burning off the carbon / tars in the coal leaves the ash behind.
So these metals do occur naturally - and have been collected naturally (up until the coal was burnt).

I do have great sympathy for those people affected, and I'm glad no one was badly hurt. I still don't understand why they would use such a system.