ACID SULFATE SOILS

For thousands of years, coastal areas of today’s Finland and neighbouring countries were submerged under the Littorina Sea. This early stage of the Baltic Sea existed between 7500 and 1500 BP and was characterized by brackish water that was both warmer and richer in nutrients than today. Conditions were thus favourable for primary production and microbial activity. The production, and consequent sedimentation and decomposition of fresh organic material brought reducing conditions to the bottom sediments of the Littorina Sea as oxygen was consumed in the process.

The sea water contained a lot of dissolved sulfate and the reducing conditions at the bottom were enough to reduce sulfur to sulfide, which precipitated primarily as various forms of iron sulfide. The characteristic black colour of the bottom sediments is due to iron monosulfides such as mackinawite. Another very stable form of iron sulfide is pyrite FeS2. As long as these sulfide-rich sediments remain under water they represent no danger to the environment. But they have been steadily rising from the sea as a result of the postglacial isostatic land uplift. As the sediments are exposed to the oxygen of the atmosphere, the sulfides are reoxidized and sulfuric acid is formed as a reaction product. For pyrite, the simplified process is described by

4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO42- + 16H+

In Finland, there are thousands of square kilometres of acid sulfate soils along the western coast and the occurrences are among the largest in Europe. Elsewhere acid sulfate soils are found in, e.g., Southeast Asia, West Africa, the Caribbean and especially in Australia where they have received a lot of attention.

However, the land uplift itself is so slow, with a maximum of approximately 8 mm per year on the Finnish west coast, that the environmental impact from the acid produced and metals leached is minimal. It is only when the groundwater table is lowered for agriculture or infrastructure that the problem becomes acute. In both cases a low and controlled groundwater table is required. As sediments with a high clay content and very low hydraulic conductivity are drained and dried, a largely irreversible soil shrinkage is one of the first physical changes that takes place. This leads to the formation of a permanent structure of cracks in the soil. The formed structure has a high hydraulic conductivity and allows for a very efficient drainage. It also allows atmospheric oxygen to enter the soil during the summer months when the groundwater table is low.




 

Furthermore, the drainage must be effective all the way from the soil to the sea. Here the land uplift comes into play again. The uplift is at its maximum close to the Finnish west coast and is much smaller further inland. This means that the drainage flow in rivers and streams slows down with time as the height difference between inland and coast is reduced. Thus, the risk for flooding requires recurring dredging of river mouths.

There is an on-going conversion of old sulfide-rich sediments into acid sulfate soils in areas affected by agriculture or infrastructure. The acidification of the drainage water brings with it leaching of metals from the soil minerals. Leached metals are e.g. cadmium, cobalt, nickel and zinc, but also aluminium which has been linked to large-scale fish kills in affected rivers. Aluminium is thought to precipitate as aluminium hydroxide on the gills and effectively suffocating the fish. The first documented fish kill on the Finnish west coast is from the river Kyrö and dated 1834. Lately, fish kills have appeared regularly with approximately 10-year intervals.

For an acid episode to occur in a river, one or more dry summers are required. A dry summer will lower the groundwater table and allow oxygen to enter the soil through empty cracks and pores.

Oxidation products in the form of e.g. sulfuric acid, and metals leached by the acid, remain in the soil until flushed out of the soil by heavy autumn rains. A snow melt in the spring that coincides with the thawing of the ground will allow meltwater into the soil and can thus flush out oxidation products of the soil to the same effect. Acid episodes with fish kills can therefore occur both in the autumn and the spring.

It has been estimated that in Finland acid sulfate soils release more metals to the environment than the industry as a whole. The impact is the worst on the west coast where the chemical and ecological status of many rivers and streams is poor despite many actions to correct the problem.


The probability of an area with acid sulphate soil (in Finland) >> Visit GTK Finland´s map here