Mexico City is one of the most populous cities in the world. It is an old city with leaky sewage pipes build at the turn of the century. The leakage from the pipes mixes with poorly draining rainwater and is then used for irrigation.
The scarcity of water in the Mexico City area has mandated the continued pumping of too much water from the aquifer underneath the city causing it to sink into the ground, creating more broken and leaking pipes.
As the population has exploded over the last several centuries the lakes have been drained and the forests chopped down, reducing the humidity of the area.
An inadequate water supply leads directly to other problems. Every home and business in Mexico City has a cistern built on the roof to provide gravity pressure to the faucets and a larger cistern built underneath the home to store water.
This cistern system is necessary because Mexico City has only a single central water processing plant that pumps treated water to every home and business in the Mexico City area. This means that each residence or business receives a water supply for only a couple of hours each day. The water cisterns on the roof and underground fill with water when it is running. These cisterns then provide water until the next day when the water flows again.
The water sits a minimum of 22 hours each day before it is used and longer if all of it is not utilized before the cisterns fill again. The remaining water in the cisterns contaminates the new water. The stagnant water in the tanks, especially if they are further contaminated by insect infestation can easily make the water unsafe to drink.
As a result of these problems, affordable drinking water is supplied to residents of Mexico City by bottled water companies.