This is a very simple model. In real life, it is rare that a pollutant would reach groundwater or surface water so easily, as long as the chemicals were applied in a responsible manner. Also, it rarely happens so quickly. In some cases, it takes years before the contaminated groundwater reaches the surface. An example of this is Lake Taupō. Even though measures are in place to reduce the nutrients going into the lake, nitrogen-rich groundwater will continue to feed it for many years to come.