The earth is a closed ecosystem. The water tha exists on the planet dates back to the creration time of our planet. This means that it get recycled continiously through the years and remains the same through the hydrological cycle.
The hydrological cycle is the vaporization process of the surface water of the earth and return to it through precipitation. But as is known, from the 20th century with the rapid industrial development and the extensive use of internal combustion engines, the continuous emission of pollutants has caused the air pollution phenomenon from which it could not miss the water.
So there is the phenomenon of acid rain, rain with presence in it acid dissolved pollutants, substances that are not physiological characteristics of clean air, but human activity products mainly burning fossil fuels such as oil and coal containing sulfur oxides and nitrogen.
The acid rain causes devastating effects on ecosystems, crops and cultural monuments, but even worse insult drinking water supplies with the admixture of contaminated water into them. So lakes, rivers and underground aquifers especially near cities, presenting content to airborne contaminants making them dangerous for consumption water.