Question

why is groundwater clean and safe to drink

Answer 1

See, when the water from rain or other means reaches land, it is absorbed by the ground. Then it goes from the natural filteration done by sand, rocks etc. and becomes pure and safe for drinking.

Answer 2

Water that is deep underground, such as that from an aquifer, is more likely to be pure and of drinking water quality than water that is just below the surface (usually referred to as "groundwater"). This relative purity has not changed despite the use of pesticides or fracking or the other horrible things that other perhaps well-meaning responders with political agendas would have you believe so you can join their bandwagon. A group that does excellent work, Engineers without Borders, has dug wells in Africa to supply good quality water to people there who were previously burdened with horrific disease rates from poor water quality. The villagers do not need to boil the well water (or treat it in any way) to make it safe to drink but they draw from the well as they need water and do not attempt to store it in tanks.