I have an account and the owners have a 12'x12' concrete dog kennel. With two dogs in it but there is a divider separating the male from female. Around the concrete there is 2"-3" rock coming out 3' with brick border. My question is that the owners asked me what he could put around it, because the way the concrete slab in sloped all the dog pee drains to the front and pools in the rocks which after awhile it starts to really stinks and he rinses with the garden hose and when he does the run off water turns black. Any suggestions?