Tuesday, 11 July 2006

Controlled Drainage and Lime Filter Drains as means To Combat Drainage-Induced Adverse Environmental Impacts of Acid Sulphate Soils in Finland.

Ilona Bärlund1, Sirkka Tattari1, Mats Åström2, Markku J. Yli-Halla3, and Heikki Harmanen4. (1) Finnish Environment Institute, P.O.Box 140, 00251 Helsinki, Finland, (2) Department of Biology and Environmental Science, Kalmar University, 39182 Kalmar, Sweden, (3) Department of Applied Chemistry and Microbiology, University of Helsinki, Building D, P.O. Box 27, Helsinki, 00014, Finland, (4) Seinäjoki Polytechnic, School of Agriculture and Forestry, Ilmajoentie 525, Ilmajoki, 60800, Finland

The largest Acid Sulphate (AS) soil areas in Europe occur on the western coast of Finland. In terms of Soil Taxonomy, 15,000 ha of Typic Sulfaquepts, 50,000-115,000 ha of Sulfic Cryaquepts, some Sulfisaprists and about 200,000 ha of Dystric Cryaquepts (sulfuric horizon/ sulfidic materials below 150 cm) are in agricultural use, totalling in above 300,000 ha of agricultural land. According to the FAO and WRB systems, some 43,000-78,000 ha of them are Thionic Gleysols, the rest being mainly Dystric Gleysols/Cambisols. Usually associated with remnants of organic materials, the sulfidic materials are a mixture of iron monosulfide (FeS) and pyrite (FeS2) and they were accumulated during 7500 - 2500 BP. They have more recent top horizons, mostly with loamy or clay loam textures, which were never sulfidic. The soils make good agricultural land for small grains, grassland and also for sugar beet after artificial drainage and liming. Drainage has promoted oxidation of sulfidic materials. Jarosite and iron hydroxide coatings give rise to a strong structure in the subsoil, resulting in high water conductivity. The unripe reduced subsoil is massive and practically impermeable to water. The sulphur concentration of the sufidic materials seldom exceeds 1%. Yet, in these non-calcareous soils oxidation of sulfide results in severe acidification. Drainage waters have a pH around 4 and high concentrations of dissolved sulphate, aluminum (Al) and manganese. Also heavy metals, such as zinc and cadmium, are effectively mobilized upon oxidation-induced acidification. These solutes have adverse impacts on aquatic life. To find means to combat the off-site problems of AS soils, field experiments were carried out in two AS soils in Western Finland. In Ilmajoki (Sulfic Cryaquept/ Dystric Cambisol, 47 m above sea level), the sulfidic materials (0.7-1% S) were below 200 cm of soil surface. At Mustasaari (Sulfic Cryaquept/ Thionic Gleysol, a polder at sea level), sulfidic materials (0.9-1.2%) occurred below 110 cm. The experimental treatments included 1) conventional pipe drainage (control) with subsurface pipes installed at the depth of 1-1.2 m, 2) lime filter drainage (LFD) where (15 kg m-1) CaO was mixed with the backfill soil of the trench, 3) controlled drainage (CD), where a high groundwater level was maintained with weirs to retard oxidation of sulfides, but no water was pumped into the drainage system from outside sources, and 4) a combination of LFD+CD. 50-70 drainage water samples were collected from each plot in 1998-2002 at Ilmajoki and in 1999-2001 at Mustasaari and analyzed for pH, acidity, sulphate and total Al, at Ilmajoki also for dissolved (filtered 0.45 um) Al. The level of groundwater was also monitored. In Mustasaari, the drainage waters of the control plots were very acid (mean pH 3.7, acidity 11 mmol l-1) and high in sulphate-S (2300 mg l-1) and total Al (88 mg l-1). Due to the proximity of the sulfidic materials to the soil surface, evapotranspiration during summer was so high that even CD could not keep the sulfides submerged, resulting in large amounts of acidic solutes. Thus, the experimental treatments did not markedly improve the quality of drainage waters. In Ilmajoki, the drainage waters of the control plots were less acidic (average pH 4.3, acidity 3.9 mmol l-1), and lower in sulphate-S (440 mg l-1) and total Al (20 mg l-1, all of which was dissolved). Both LFD and CD, and particularly their combination (LFD+CD) substantially increased pH and decreased the concentration of dissolved Al. CD was quite successful submerging the sulfides, which were deep in the soil profile, and LFD had an additional effect neutralizing the water. Where sulfidic materials are close to the soil surface, the tested treatments were not successful in improving drainage water quality. Stopping drainage and re-establish the native high water table may be the only economically feasible way to reduce the environmental loading from these soils. Instead, in soils where sulfides are deeper, the LFD, CD and especially LFD+CD are potential means to combat the adverse off-site effects. Fortunately, these kind of AS soils constitute the majority in Finland.

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