One of the main sources used to obtain fresh drinking water around the world are the aquifers underground. These are large rock and dirt structures that act as water filters. We obtain it by drilling wells into these aquifers and pumping the water up.
There is growing evidence to suggest that we are draining these aquifers faster than they can replenish their water supply. This is of course, a bad thing, because as the water levels deplete, the less fresh drinking water we have available.
Of the 37 largest aquifers on Earth, 21 of them are showing lower than normal levels of fresh water.
An aquifer is something that occurs naturally, so it is really difficult to try and "fix" this problem without making changes to our habits. The only way to really fix the water depletion problems would be to limit aquifer drilling.
There are other ways to obtain drinking water, such as filtering, cleaning, and treating salt water, fresh water, and recycling existing water, but people just tend to enjoy natural fresh water more than purified water because of the taste.
We will have to do something soon, or we could be faced with larger fresh water supply problems.
