OUR APPROACH

A large part of the acid sulfate soils in Finland are in agricultural use. These soils are very fertile once the plough layer has been limed. Earlier, the acidity was considered a problem only for agriculture and beginning in the 1930s the recommended treatment was liming of the plough layer and effective drainage. This treatment solved the agricultural problem but led instead to serious environmental problems. The acidity formed and the metals leached were transported with the drainage water to recipient water streams harming fish and other water-living organisms.

Liming of the plough layer does not improve the pH of the soil layer below and is therefore not a way to improve the quality of the drainage water. Liming of recipient water streams has been tried but has been showed to be costly and cannot, with few exceptions, be regarded as a solution to the acidification and metal contamination of these waters.

A direct way to minimize the oxidation and the ensuing formation of acidity and leaching of metals is to reduce the drainage depth, i.e., to raise the groundwater table. However, this would restrict the type of agriculture possible on the soil. Compared to open ditches, subsurface drainage will in most cases lower the groundwater table further and increase the oxidation. Controlled subsurface drainage that stores water in the field will at least partially remediate this, but a dry summer will still lower the groundwater table exposing sulfides to oxygen. Subsurface irrigation to maintain the groundwater table at the desired level is a solution if suitable water can be found nearby.

Experiments have also been made by surrounding a subsurface drainpipe with a mixture of quicklime (CaO) and soil or gravel. The idea was that the drainage water must pass this mixture on its way to the drainpipe. Metals will precipitate as the pH is raised considerably. In this method the acidified soil layer is limed, but only very close to the drainage.

In the PRECIKEM method the mitigation is aimed at the very sites where the acidification arises

The Finnish Ministry of Agriculture and Forestry and Ministry of the Environment published a joint publication, Guidelines for mitigating the adverse effects of acid sulphate soils in Finland until 2020, in 2011 outlining a national strategy. Here the need for liming deeper soil layers where the acidity arises was recognized, but it was concluded that this was not possible except in connection with the construction of subsurface drainage when diggings can be limed.

In the PRECIKEM project (Chemical precision treatment of acid sulfate soils to prevent the formation of acid), this restrictive conclusion was challenged. A subsurface drainage system allows for subsurface irrigation. By mixing a ultrafine-grained powder (median particle diameter < 2.5 µm) of limestone (CaCO₃) with the irrigation water, the resulting suspension can be pumped through the drainage system and out into the partly oxidized soil layer at drainage depth utilizing the soil’s pores and cracks. A stable structure of macropores with a high hydrological conductivity is formed in these soils as they are drained and dried. The treatment suspension can therefore easily spread in the soil.

The limestone treatment has three goals:

• to make conditions unfavourable for the acidophilic microbes that catalyse the oxidation of sulfides
• to neutralise already formed acidity
• to reprecipitate already dissolved metals

In the PRECIKEM II project (Precision chemical treatment of acid sulfate soils for the protection of waters in environmentally sustainable agriculture), peat was used in combination with limestone. The role of peat is:

• to work as an antioxidant by consuming oxygen before sulfides are oxidized
• to promote microbes that catalyse the reduction of iron and sulfur
• to immobilize dissolved metals.

As climate changes, more severe weather is expected in the form of dry summers, heavy rains and high drainage flows during winter. To counteract negative effects of especially drought in agriculture, the combination of three methods is envisioned:

• Controlled subsurface drainage to conserve groundwater.
• Subsurface irrigation to maintain a good level of groundwater.
• PRECIKEM treatments to protect deeper soil layers from oxidation during dry periods.