Abstract
The main aim of this study was to determine, during extreme hydrological conditions, the source(s) of acids, sulphate and metals (alkali and alkaline earths) in the Munsala stream (western Finland) draining mainly acid sulphate soil, peat and till. Samples were collected at 6 sites along the main stem on 3 high-flow and 3 low-flow events, and were analysed for the required chemical and isotopic variables. The acid sulphate soils (located under farmland) had a large impact on the stream as indicated by pH values occasionally down to 4.0, moderately to strongly increased concentrations of inorganic solutes, and a high acid SO4 2- load characterised by negative d34S(sulphate) values. In addition, the forested areas underlain mainly with till and peat released low SO4 2- but low pH waters (down to at least 4.6) during high flows, indicating the importance of humic acids in controlling the pH. These humic acids flocculated abundantly in the middle/ lower reaches as a result of interaction with acid sulphate water. Therefore, not only the farmland acid sulphate soils but also the organic-rich soils/horizons in the forested areas contribute to water-quality deterioration.;
Highlights
The main aim of this study was to determine, during extreme hydrological conditions, the source(s) of acids, sulphate and metals in the Munsala stream draining mainly acid sulphate soil, peat and till
SO42− load characterised by negative δ34S(sulphate) values
The farmland acid sulphate soils and the organic-rich soils/horizons in the forested areas contribute to water-quality deterioration
Summary
The main aim of this study was to determine, during extreme hydrological conditions, the source(s) of acids, sulphate and metals (alkali and alkaline earths) in the Munsala stream (western Finland) draining mainly acid sulphate soil, peat and till. The geochemical atlas of stream waters in Finland and the results of several other hydrochemical studies show that a large number of small and medium-sized streams in the coastal areas of Finland are acidified and have exceptionally high concentrations of many metals (Edén and Björklund 1993, Palko and Yli-Halla 1993, Weppling 1993, Åström and Björklund 1995, Lahermo et al 1996, Edén et al 1999, Åström and Spiro 2000) It is known, that the source of these high acid and metal levels is widespread acidic soils commonly referred to as acid sulphate (AS) soil but classified mostly as Sulfic Cryaquepts or Typic Sulfaquepts
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