Fecal coliform can enter urban runoff from pet waste and other animals. The ponds are constructed with shallow water sections to allow ultraviolet light (sunlight) to naturally kill bacteria in the water. Natural organisms within the ponds feed on the bacteria and there is also natural die-off of bacteria during the 7-10 day retention time in the ponds. Studies show that the Natural Treatment System could remove an estimated 20% of the fecal coliform throughout the year.