I think your own interpretation is quite correct. Clearly "aluvial" is a noun and the "aluvial del río" has been flooded with toxic sludge. There's plenty of other evidence out there that an "aluvial" is something a river has:
https://www.google.es/search?num=100&q="el aluvial del río"&...

And here's a definition:

"ALUVIAL (S.)
El nombre proviene del latín alluvies, avenida, riada.
1.-Suelo formado por depósitos recientes en zonas de valle y, con frecuencia, sometido a inundación por los ríos que aportan el material."
José Luis Rozas López, Diccionario de términos edafológicos (Ministerio de Medio Ambiente, España), tomo I (A-J)
http://www.magrama.gob.es/es/ministerio/servicios/publicacio...

So yes, it clearly means the alluvial plain or floodplain. I don't think there's another more suitable term. Of the two, which are often used interchangeably, I would use "floodplain" here, because it applies to the area the the river currently floods, whereas "alluvial plain" can apply to a larger area that it has flooded in the past:

"An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time."
http://en.wikipedia.org/wiki/Alluvial_plain