I know this is nit-picking a bit but it's not contamination the turns ammonium nitrate into an explosive - it's always an explosive but one that is fairly difficult to detonate. However, if it is stored badly and allowed to clump together due to the ingress of moisture or if it is contaminated with something like diesel oil, it becomes much easier to detonate - that is the danger. Another danger is that it is a strong oxidizing agent so if anything inflammable comes into contact with it and catches fire, the fire will be very difficult to extinguish as the oxygen needed to continue combustion comes from the ammonium nitrate itself not from the atmosphere.
@Paulos_the_fog Sure. Thank you. For interest, the latest information (from today's local paper) usefully clarifies the contaminant as a mixture of "seawater and a small amount of hydrocarbons". I am no chemist. For me the aggravating factors of this saga are:
1. This is Russian cargo earning dollars for Putin 's missiles raining down on Ukrainian civilians.
2. Local Reform MP Rupert Lowe engaged mouth before the fact of contamination was out.
3. I find the decision to dump at sea disgraceful.