I have a pond which has a 4' wall made of two parts. The main wall is concrete blocks with a facing of variously sized horizontal flat rocks on the front. This wall also has a pipe along the top with valves that allows water pumped from the pond to run down the rock face of the wall. After many years of good use, I found that there is a small gap between a couple of the rocks wherein the water can run in and flow out the back and through he concrete block and on the ground. Thus emptying the water from the pond over time. I'd like to be able to put some type of sealant in the crack. Any suggestions?