Water contamination remains one of the largest and most important issues when dealing with the effects of urban sprawl on local water resources. Keeping water clean and drinkable becomes a larger challenge as more people flock to city centers and their outlying areas. This population-driven urban sprawl leads to contamination whether directly by trash or waste, or even indirectly from pressure moving soil and sentiment into the water, putting pesticides in the soil, or even an increase of acid rain. An urban population is going to result in water contamination, making the protection and filtering of local water resources even more important.
An obvious and well-noted effect of urban sprawl on water is the increased demand from local resources. Just as 100 people consume more water than do 10 people, 10,000 people will need a lot more water than 1,000 will. The pressure put on water resources can be enormous and even lead to the draining of natural wells and aquifers that once were adequate to support a smaller population. Sometimes multiple cities and their urban sprawls will pressure the same water resource, as can be seen with the Colorado River in the western United States.
The depletion of water resources can affect the natural environment, as well. If the water levels get too low or too polluted, fish populations can die, as well as animal populations that rely on the fish for food. Local fauna can also suffer from a lack of water and take on a sickly or dying look, which continues up and down the life cycle. Contamination can be just as bad as all-out depletion.
For urban developers a balance must be struck between managing and protecting water resources but also finding ways to encourage growth while urban sprawl is taking place. People still need places to live and often population growth can mean economic growth as well, but without water for drinking, sewage treatment and a host of other issues, there's no way to sustain the urban environment over the long term.