Is The Case of the Toxic Spell Dump set in the same universe as Poul Andersons Operation Chaos?
No. Although The Case of the Toxic Spell Dump was heavily influenced by Anderson’s Operation Chaos, as well as Robert A. Heinlein’s Magic, Inc. The Case of the Toxic Spell Dump does seem to be set in the same universe as Turtledove’s own Thessalonica. In Operation Luna (1999), Poul Anderson’s sequel to Operation Chaos, the main character relaxes by reading a mystery novel entitled The Case of the Toxic Spell Dump.