My Greetings,
As you will see inside my profile, I'm used to work with this kind of projects, and by the way, you could use several things to retain the rainwater like Hydrogels.
Nevertheless, I will go down into the chase, because you need a consulting research work regarding the state of art of "nanotechnologies" inside this world of products to effectively retain rainwater.
Keep in Touch,
This is an easy task,
:)