Rock Wall Separation

I keep reading about people using foam and spray foam to separate their land from water....Doesn't it leach into the water??? I have some rocks to make a rock wall. I was told to use a piece of foam behind to insure a good separation from the water to the land...but all the people posting about foam are assuming you are making a fake rock...I have real rocks. I was just going to silicone the actual rocks into place but someone told me that wouldn't separate the water from land enough and water will get murky....but wont the foam leach toxins (that is worse lol) anyway. advice on separating land from water relatively naturally would be nice.

Bump: just had an idea...what if I got that liquid silicon stuff and painted the back of the rocks???