Well what I did when I created the map was to put a water source in each pond, BUT I lowered it as far as it would go as you can see by the pond on the left.
Sure it rises, and the one at the top right actually overflowed into the top left one for a while until I put a second pump into the large one at the bottom, that caused the top right one to drop its level which then caused the top left one to slowly drop down again to where the water source level was. - I am guessing that over time if no more waste goes into the top left one the "poo" will disappear like it does in rivers etc.
By the way, these ponds actually exist in real life in the city where I live, I made this map based on that city (sorta) so that's why they are there
