Abstract
During heavy rains and snow melting, acid sulphate (AS) soils on the coastal plains of Finland are flushed resulting in discharge of acidic and metal-rich waters that strongly affect small streams. In this study, the impact of AS soils occurrence and hydrological changes on water quality were determined for 21 rivers (catchment sizes between 964122 km2) running through an AS soil hotspot area in western central Finland. Water samples, collected at the outlet, during eight selected events, were analysed for pH, dissolved organic carbon, electrical conductivity (EC) and 32 chemical elements. Based on the correlation with percentage arable land in the catchments (a rough estimate of AS soil occurrences, as up to 50% of the arable land is underlain with these soils), it was possible to categorize variables into those that are enriched in runoff from such land, depleted in runoff from such land (only one element), and not affected by land-use type in the catchments. Of the variables enriched in runoff from arable land, some were leached from AS soils during high-water flows, in particular (aluminium, boron, beryllium, cadmium, cobalt, copper, lithium, manganese, nickel, sulphur, silicon, thorium, thallium, uranium, and zinc) and others occurred in highest concentrations during lower flows (calcium, EC, potassium, magnesium, sodium, rubidium and strontium). Molybdenum and phosphorus were not leached from AS soils in larger amounts than from other soils and thus related to other factors connected to the arable land. Based on the concentrations of potentially toxic metals derived from AS soils, the 21 rivers were ranked from the least (Lestijoki River, Lapväärtinjoki River and Perhonjoki River) to the most (Sulvanjoki River, Vöyrinjoki River and Maalahdenjoki River) heavily AS soil impacted. It has been decided that Vöyrinjoki is to be dredged along a ca. 20 km distance. This is quite alarming considering the high metal concentrations in the river.;
Highlights
Introduction has not been adequately investigatedA few studies focusing on a single river exist (Åström and Åström 1997, Edén et al 1999, Åström 2001b, Roos and Åström 2005), but studies covering and comparing several rivers at the same time are rare (Weppling 1993)
The aim of this study was to investigate the hydrochemistry of all the rivers in this area and to rank them from the least to the most affected by this environmentally unfriendly soil type
Acid sulphate soils are expected to occupy large areas under the agricultural land, but
Summary
Introduction has not been adequately investigatedA few studies focusing on a single river exist (Åström and Åström 1997, Edén et al 1999, Åström 2001b, Roos and Åström 2005), but studies covering and comparing several rivers at the same time are rare (Weppling 1993). Acid sulphate (AS) soils cause, in adjacent surface waters, high concentrations of sulphuric acid (pH minimum < 3.0) and many potentially toxic elements such as cadmium (Cd) (up to 25 μg l-1), nickel (Ni) (1 200 μg l-1), zinc (Zn) (2 500 μg l-1), cobalt (Co) (700 μg l-1), aluminium (Al) (300 000 μg l-1), manganese (Mn) (120 000 μg l-1) and thallium (Tl) (0.4 μg l-1) (Åström and Spiro 2000, Åström 2001a) Sundström et al (2002) roughly estimated for the AS soil occurrences in central western Finland, an annual discharge to the water environment of, among others, 80 tons Ni and 40 tons Co This is in the magnitude of ten times more than the corresponding discharge from the entire Finnish industry. The inevitable result of this stress is extensive and serious hydrobiological damage (Kjellman and Hudd 1996, Hudd 2000)
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