Non-uniform fading from one texture to another I'm just looking for broad suggestions here. I'm barely post-noob:)

I'm animating a pool of water which will start green and murky, then be magically transformed into clear and sparkly. It's pretty easy to just keyframe the material to change from one to the other, but what I'd like to do is this: have the texture change from a central point, expanding until the whole body is changed: ie, in mid transition, the murky green would have an expanding clear sphere at its centre.

What would be the simplest way to achieve this? If anyone can suggest a tutorial they think might help, I'd be very grateful.
Here is a simple setup how you can do this. I created a spherical gradient that will control the transition from one material to another:



And to control this gradient I use coordinates of empty.



You can use this setup to control textures to.  

And you can add some distortion or waviness for this effect using noize texture:



To animate noize move texture in Z-axis: