At face value, ignoring its role as a metaphor, it doesn't make sense - it's literally the opposite of what's happening. When you mix salt water and freshwater

At face value, ignoring its role as a metaphor, it doesn't make sense - it's literally the opposite of what's happening. When you mix salt water and freshwater together, you don't get more freshwater - you turn freshwater into salt water. Replace "fresh" with "clean" and "salt" with "dirty" to make it more obvious.