The abundance of the waters, lakes and rivers is a typical feature of nature in Finland. The volume of water resources varies with the seasons and the geographical location.
The lakes are shallow: although they cover a large surface area, there is not that much water. Consequently, surface waters in Finland can easily become polluted. The Baltic Sea is also vulnerable to pollution.
Groundwater means water that is absorbed and collected underground. Most domestic water used in Finland is groundwater, because it is usually of higher quality than surface water. Treated surface water is used in the Helsinki metropolitan area, Turku region and Oulu in particular. Groundwaters can be polluted by soil extraction, road salting, fuel distribution, and agricultural and chemical accidents. Groundwaters can also become acidic.