Groundwater forms wherever water flows or trickles down into gaps, cracks, holes and pores within soils or rocks. Finland’s groundwater reserves are replenished in the spring when the winter snow and ice melts, and then often again in the autumn – typically the rainiest season.
Variations in water tables
Groundwater can be found in almost every part of Finland, but is particularly widespread in areas with extensive deposits of permeable sands and gravels formed during the last ice age. The depth of the water table may vary from less than a metre to more than thirty metres, but is typically about 2–5 metres below ground level.
Groundwater reserves can be significantly depleted, and the water table lowered, due to the excessive use of groundwater, or after major groundwork or excavation, as well as following droughts.
A valuable resource
In Finland, groundwater is widely used by local residents and by waterworks, since it is often much purer and better protected from contamination than the water in lakes and rivers. Groundwater can usually be safely consumed without any treatment.
Approximately 60% of the total water supply distributed by Finland’s waterworks consists of groundwater. This figure also includes water from artificially maintained reservoirs of groundwater fed from lakes and rivers.
Groundwater reserves in Finland do not normally suffer from contamination on a wider scale, since individual bodies of groundwater tend to be small. The risk of contamination is highest in areas where soils consist of coarser sands and gravels, which can be infiltrated by pollutants as well as water.
Considerable contamination may be caused locally where salts are used to de-ice slippery roads, on over-fertilised farmland, at garages and service stations where oils may accidentally enter the soil, and following accidents involving chemicals. In some cases, the harmful consequences of an accident may only become evident in the groundwater after many years.
Groundwater quality may also be reduced by the nature of the local soil or bedrock. In certain areas the groundwater can contain harmful concentrations of iron and manganese. Wells drilled into the bedrock may contain high levels of substances such as arsenic or radon in places.
Groundwater reserves are also vulnerable to acidification in the same areas where there have been acidification problems in lakes. Groundwater acidification generally only occurs after a long time lag, however, since it may take many years for the acidifying substances to penetrate down through the soil. Where groundwater becomes acidified, harmful heavy metals may be leached from the surrounding soils into the water.