Custom Concrete interaction with water issue

Hi All, 

I'm working on adding some custom concrete to a mod, so far i have followed this guide: https://www.youtube.com/watch?v=AiMxOVIN-64

but  i want the powder to solidify when it hits the surface of the water, not the bottom of the body of water. I'm not sure how to do this as I think it would involve interaction with the falling block entity somehow?

does anyone have any advice on how I would do this?