Published on: 19.1.2021

Information produced by Finnish Environment Institute

Sensitive groundwaters of coastal sulphate soils

While acid sulphate soils are common on Finnish coasts, fresh, good-quality groundwater may also be found in these areas. However, the water quality can easily deteriorate if the groundwater level drops during an exceptionally dry period.

Acid sulphate soils are found in ancient sea bottoms, where marine bacteria decomposed dead aquatic organisms lying on the seabed, consuming oxygen during this process. This created anoxic sediments containing sulphur and iron compounds, which today are on dry land.

Acid sulphate soils are most commonly found along the extensive land uplift coasts of the Gulf of Bothnia, but also in other low-lying coastal areas. Their anoxic layers are usually below the groundwater surface where they do not cause environmental problems, and the groundwater is perfectly potable.

This situation changes if the groundwater surface drops and the compounds containing sulphur and iron come into contact with air in the soil. A complex chemical process is triggered, the end products of which include sulphuric acid. The groundwater becomes acidic and its iron concentration increases, and it may also contain harmful concentrations of aluminium and heavy metals. If such groundwater is discharged into lakes and rivers, fish kills may be the result.

Drought and decreased groundwater levels are consequently a particular risk in areas with acid sulphate soils.

Image: © Tiina Tuomaala, Vastavalo