Also not local, also looking at Google Maps.

My guess would be the tank (circled in red) behind the chemical plant in the north-central part of the facility.  There's a large dike around it.  An article stated that neither the tank or the dike failed, but that an external transfer system did.  Perhaps it was at the rail line that goes down through the middle of the facility?  The UAN could have gone towards the river along the blue line.

In the second image, using Streetview, you can see the blue topped-tank behind the Hy-Vee (looking west towards the facility from Broadway St).

My 2 cents.

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