Purity is a word that is not very commonly used with confidence and certainity. In this day and age, what products could be be pure? Water is no different in this regard. Water is never found in a pure state in nature. Both groundwater and surface water may contain many constituents, including microorganisms, gases, inorganic and organic materials. But why is this? It's because your water is changing as it moves about the planet.
The chemical nature of water continually evolves as it moves through the hydrologic cycle. The kinds of chemical constituents found in groundwater depend, in part, on the chemistry of the precipitation and recharge water. Near coastlines, precipitation contains higher concentrations of sodium chloride, and downwind of industrial areas, airborne sulphur and nitrogen compounds make precipitation acidic.
One of the most important natural changes in groundwater chemistry occurs in the soil. Soils contain high concentrations of carbon dioxide which dissolves in the groundwater, creating a weak acid capable of dissolving many silicate minerals. In its passage from recharge to discharge area, groundwater may dissolve substances it encounters or it may deposit some of its constituents along the way. The eventual quality of the groundwater depends on temperature and pressure conditions, on the kinds of rock and soil formations through which the groundwater flows, and possibly on the residence time. In general, faster flowing water dissolves less material. Groundwater, of course, carries with it any soluble contaminants which it encounters.
When it comes to making the best choice for our health, water that has been treated effectively with modern treatment equipment and adhering to modern regulations is the best since there is no 'pure' source of water available to us. Even with remarkable natural purification processes at work, it's a safe bet to keep the water you drink in your home safe with the use of a good water filtration system, especially as most city's water pipes continue to age and it is difficult for utilities to upgrade every component within a distribution system.