Naturally occurring deposits containing sulfidic materials are common along the coasts of the northern half of the Baltic Sea, especially in coastal areas that have been subjected to land uplift. Anthropogenic disturbances such as farming, construction of infrastructure, peat cutting, and forestry, have resulted in large quantities of sulfides becoming exposed to atmospheric oxygen. Exposure of the sulfidic materials to air initialises chemical reactions that produces sulfuric acid, creates acid sulfate soils with a pH between 3–4, and mobilises enormous quantities of acidity and soluble metals.
Mitigating the acidic and metal-rich drainage from an acid sulfate soil in agricultural use is challenging. Our approach is based on the fact that the soil layers at drainage depth contribute predominantly to the accumulation of oxidation and leaching products in the drainage waters. The oxidation process is most significant during dry periods, while the oxidation products and leached metals are subsequently flushed out during wet periods. In the end of the summer when the groundwater is well below drainage depth and the soil macropores are empty, a solution/suspension of treatment chemicals (finely ground calcium carbonate, calcium hydroxide, peat) is pumped into the soil layers via the control well and drainage pipes.
Prior to field experiments, column leaching experiments are performed in the laboratory. Suspensions are pumped through cylindrical soil samples from the soil horizon at 80–90 cm below the surface. In the outflowing solution, pH, EC and ORP are measured continuously. Selected metals and anions are analyzed from samples taken from the permeate
For the full scale experiments an advanced experimental field was built on Risöfladan on the west coast of Finland, at 63.045°N and 21.711°E, about ten kilometers from the city of Vaasa. The experimental field is divided into twelve 1-hectare subfields, each with its own drainage system consisting of a control well, collector pipe, and drainage pipes.
In the full scale experiments treatment suspensions are created by mixing ca. 100 m3 water from nearby Toby River with several hundred kilograms of chemicals in a mixing vessel, before being pumped into the subsurface soil via the control well and subsurface drain pipes. Treatment effects are studied by analyzing pH, acidity, EC, ORP, major anions (SO42- , Cl- , NO3- ) and selected metals (e.g. Al, Mn, Cd ) in the drainage waters.
Monitoring of the drainage waters showed that the short-term effects of the subsurface treatments were remarkably good. The best results were observed with CaCO3 doses of ca. 700 kg/ha. pH of the drainage waters increased from the original value of ca. 4 to 6 and remained at this level about one year. The drainage waters of the field had an acidity of ca. 4 mmol dm-3 which was halved due to the CaCO3 treatment. When considering metals, the treatments had the biggest effect on Al concentration. Some effects can be observed still, five years after the treatments, but some were obvious only during ca. 2 years.
The on-going Precision chemical treatment of acid sulfate soils for the protection of waters in environmentally sustainable agriculture (PRECIKEM II) project is funded by the European Agricultural Fund for Rural Development via the Rural Development Programme for Mainland Finland 2104 – 2020.
