Why Did the US Use Depleted Uranium Weapons in Syria?

The recent confirmation by the US that DU ammunition was used in two attacks in Syria in late 2015 raises a number of troubling questions.
Firstly, why was DU used? Has it been used again? Will it be used again?
Secondly, and no less important, what will happen next in order to mitigate any health or environmental risks the contaminated sites may pose?
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