The water rained on the ground seeps down the soil by gravity and at a certain depth fills the soil pores, cavities, hollows and gaps between soil particles. This is how water becomes groundwater. Aquifers divide into sections of various shapes and sizes where water volume, flow direction and flow rate vary due to the porosity and layered structure of the soil as well as the level of the bedrock. Water is gradually discharged out of springs and the littoral zones of bodies of water. Formations with a high level of groundwater flowing easily are called groundwater areas.
In Espoo, there are 11 groundwater areas important for our water supply. Information on the groundwater areas in Espoo can be found in the publication (in Finnish) “Espoon pohjavesialueiden suojelusuunnitelma [Protection plan of Espoo’s groundwater areas] (1/2015)” by the City of Espoo’s Environment Department.
In order to determine the groundwater level, permanent groundwater observation pipes are often installed at building sites, from which the water level can be regularly observed before, and often even after, construction.
Groundwater in clay areas
Fine-grained soils such as clay conduct water poorly. There are many clay areas in Espoo where water accumulates on the surface of the ground and runs into the sea as surface runoff. In this case, the clay layer, being impermeable to water, acts as a type of insulation almost preventing the penetration of rainwater into the soil. In clay areas, more coarse layers of soil are found under the clay cover. In a groundwater pipe installed in a coarse soil, groundwater may rise above the ground. In this case, the groundwater is pressurised. Pressurised groundwater is present in hillsides in large soft valleys, especially. When building on hillsides, problems may arise if the clay layer is excavated.
Groundwater in the rock
Under the layers of loose soil, we find the rock and rock fractures. There are usually plenty of fractures because the bedrock has been fractured during its extended geological history. From the soil, groundwater slowly enters and fills the rock fractures. The water found in rock fractures is called rock groundwater. In general, the yield of rock groundwater is greater in fracture zones or in their immediate vicinity.
Groundwater level variation and monitoring
In flat areas, the groundwater level sets evenly on the same level. The level fluctuates, falling when moving toward the shoreline. Annually, the groundwater level varies as a result of annual precipitation. In areas with large groundwater reserves, annual precipitation does not result in major variation. In many cases, small groundwater reservoirs are highly dependant on precipitation, and in dry periods their level falls sharply. This results in the emptying of wells drawing their water from them.
City of Espoo’s Geotechnical Unit monitors the groundwater level with some 800 observation pipes in the Espoo city area. This monitoring covers the main groundwater areas and most built areas and those designated for construction. The oldest groundwater data date back to over forty years. Groundwater monitoring provides valuable information for construction. Monitoring also provides information on the impact of construction on groundwater. As a result of construction, the groundwater level may fall, which in turn may lead to the depression of the ground. Wooden foundations used in construction in the past may be damaged as a result of the falling groundwater level.