Acidic sulphate soils leach metals into the Baltic Sea

There are areas in the Baltic Sea catchment basin where the chemical properties of the soil facilitate the transport of harmful substances into waterways. The most common problematic soil type is the so-called acid sulphate soil.

Soils which contain high levels of sulphur minerals are known as acidic sulphate soils. In such soils, sulphides are oxidised to sulphates. When this occurs, the soil pH drops to a very low level.

Sulphur compounds, on the other hand, dissolve environmentally harmful metals from soil minerals. When it rains, metals can continue to leach into waterways and from there to the sea.

Ostrobothnia particularly suffers from acidic soil and metals leaching into the water

Most of the acidic sulphate soils in the Baltic Sea catchment basin are located in Ostrobothnia. These former seafloor sediment layers of the ancient Littorina Sea are nowadays often cultivated or lie under layers of peat, forming the bottom of marshy areas.

Due to land uplift and the drainage of marshes, the acidic water and harmful metals dissolved therein are leached into waterways. Examples of water-borne metals include cadmium, zinc, and aluminium.

The amount of leaching depends a lot on the weather. If a long dry period is followed by heavy rainfall, the pH of the water will be very low. This will significantly increase the acid and metal load in waterways and the sea.

It has been estimated that many locally and regionally important fish stocks along the west coast of Finland have been severely weakened by such loading events. This toxicity interferes with the reproduction of migratory fish and causes fish kills.

Limiting the damage requires investments

The damages caused by draining fields can be reduced by controlled subsurface drainage. The harm caused by both peat production and forestry, in turn, can be reduced by new drainage regulation methods.

However, the cost of these methods slows down their implementation. This problem should also be addressed in environmental support schemes. Achieving a good quality water status, particularly in places like Ostrobothnia, also requires reducing the damage caused by acidic sulphate soils.