Acid sulphate soils (AS soils) release large amounts of acidity and chemical elements into the rivers in western Finland. A recent study by our research group showed that AS soils liberate more toxic metals into water courses than the whole Finnish industry together. The elevated concentrations of metals are transported to the estuary, where they are deposited as a result of higher salinity and pH, and slower water currents. The bottom sediments are therefore excellent tools to evaluate changes in element discharge into the environment over both temporal and spatial scales. The objective of this study was to track changes in sediment elemental distribution as a result of leakage from AS soil landscapes and to quantify the impact of AS soils from the drainage area to the sea.

The Vörå River is one of the most strongly AS soil affected rivers in Finland. It has a low pH (occasionally down to 4.0) and carries high amounts of many metals, which as a result of increasing pH and salinity, are precipitated and deposited in the bottom sediments in the estuary. The coastal area adjacent to this river was therefore selected for a bottom sediment and suspended particulate matter study. This study comprises sampling of four sediment cores and collection of particular matter with sediment traps, in a profile extending from the river mouth out to the open sea. A selective chemical extraction procedure was used in order to identify the chemical speciation within the sediment.

The results show that an appreciable amount of elements (e.g. Al, Cd, Co, Cu, La, Mn, Ni and Zn) are elevated in the surface and sub-surface of the sampled bottom sediments compared to the deeper sediment background levels. These elements are all known to be leached from AS soils. At a site, approximately 4 km from the river outlet the sediments had 5-100 times higher concentrations of Cd, Co, Mn, Ni and Zn compared to the background levels. These elevated concentrations are much higher than the corresponding concentrations in the sediment cores situated closer to the river mouth. The reason for this deposition pattern is that some elements like for example Al and Cu are pH sensitive, and are therefore deposited closer to the river mouth, while other elements like Cd, Co, Mn, Ni and Zn are transported further out in the estuary and are not deposited until the impact of sea water increases. In contrast, elements that are not abundantly leached from Finnish AS soils (e.g. Cr, Pb, Ti and V) display a consistent concentration pattern in all sediment cores. This leads to the conclusion that the elevated element concentrations in the surface layers are a result of leakage from AS soils.

The elevated concentrations of Cd, Co, Mn, Ni and Zn pose a significant environmental problem since: 1) episodic pH declines in the shallow estuary, caused by strong runoff from the river, might cause sudden release and relocation of the metals; and 2) as a result of the isostatic land uplift of the region (0.9 cm/year), these bottom sediments are going to be new land in a near future and will then consist of metal-contaminated land. The elevated concentrations of these metals at such a distance from AS soil environments suggest that the environmental impact of metal leakage extends far beyond the geographically adjacent environment. This poses a significant issue for land management.