Fresh drinking water comes mainly from rivers, lakes and reservoirs. a person who studies these is a limnologist. Lakes are ecosystems which can serve as sentinels of climate change. Global and warming and pollution damages the natural balance.
Water in "still", large bodies can become unfit for drinking purposes through a process called climatic eutrophication. It can get so bad that fish cannot survive there. People cannot drink such water. It is toxic.
Humans are to blame because eutrophication is when nitrogen and phosphorus from fertilizers is washed into lakes. Sewerage treatment plants add to the problem. Others factors are air pollutants, fuel products and general erosion.
These nutrients feed algae which float to the bottom when they die. Decomposition absorbs oxygen causing dead zones in waters. No life can live in hypoxic layers. Warming surface water stops the strata from being mixed in winter and autumn. Thus, the body of water remains lifeless.