Groundwater flow conditions in the coastal bedrock area of the Gulf of Finland

Abstract

The coastal area and archipelago of the Gulf of Finland mostly comprises bedrock terrain with a generally thin soil cover and represents a distinct hydrogeological regime. The bulk of the bedrock area consists of relatively unbroken blocks with small, hydraulically uniform systems. The direct groundwater flow from the blocks to the sea is restricted to the blocks bordering the sea. The blocks are crossed by faults and fractures, which locally form long broken zones inside the rock mass. A single fault can catch water from several blocks and also from the soil cover. Most groundwater in the coastal strip flows to the sea through the bedrock fault zones, which are thought to represent the most favourable flow conditions. The dimensions of hydraulically uniform horizons, hydraulic conductivities and hydraulic gradients of the fault zones are poorly known. The estimated groundwater flow distance from the land to the sea in the faults reaches 8-10 km, thought the average distance may only be around 2 km. In the block areas the flow distance is even shorter, down to about one kilometre. On the whole, the amount of groundwater discharging directly to the sea from this regime (excluding overlying sand and gravel deposits), which has a surface area of around 2100 km2 , is approximately 4 m3 s-1 . This is a third of the total direct discharge to the sea in the coastal areas of Finland.