Environmental impacts and acid loads from deep sulfidic layers of two well-drained acid sulfate soils in western Finland

Discharge from acid sulfate soils (a.s. soils) poses a threat to the living organisms of about 30 rivers in western Finland. Most agricultural a.s. soils in the country are well drained with subsurface piping at a depth of 1 m, resulting in oxidation of the sulfidic materials. Consequently, acidic solutes, hazardous to fish and other aquatic organisms, are effectively leached to watercourses. Many of the soils are not recognised as a.s. soils by international classification systems, such as the FAO-UNESCO system, Soil Taxonomy and the World Reference Base for Soil Resources (WRB), because the acidic horizon and sulfidic materials are too deep to be diagnostic. The purpose of this study was to (1) investigate the influence of sulfidic materials on the quality of drainage water where these materials are below the depth of the drainage pipes, (2) test the effectiveness of controlled drainage as a means to decrease the production of acidic solutes and (3) assess the criteria for a.s. soils in international classification systems against the actual characteristics of well-drained a.s. soils of Finland. Quality of drainage waters was monitored in 1998-2000 in two soils, which had sulfidic materials (total S 0.7-1.2%) below 110 cm (Mustasaari soil) and below 150-200 cm (Ilmajoki soil). The soils had been pipe-drained for 30 and 70 years, respectively. According to Soil Taxonomy, both soils were Sulfic Cryaquepts. In the FAO-UNESCO system, the Mustasaari soil was a Thionic Gleysol and the Ilmajoki soil was a Dystric Gleysol. The a.s. characteristics were not expressed in any of the classifications of the WRB system. The average acidity of drainage waters from the fields was 11.5 and 3.5 mmol l-1 at Mustasaari and Ilmajoki, respectively. In the Mustasaari discharge, the pH was always <4.0 and in the Ilmajoki discharge mostly <4.5. Controlled drainage, applied at Ilmajoki, slightly improved the quality of the drainage water. Discharge from these soils contributes substantially to the acidification of river water, where acidity >0.3 mmol l-1 and pH<5.5 are considered critical for fish survival. In well-drained soils, sulfidic materials, at least above depths of 200 cm, have a detrimental effect on the quality of discharge. Diagnostic criteria for a.s. soils, particularly in the WRB system, should be revised to include this kind of soil among the thionic taxa.