CATERMASS – Final Report

Acid sulfate soils (AS-soils) are regarded as the most problematic soils in the world.
Finland has Europe’s largest areas of AS-soils (up to 3000 km2). These soils originate
from anoxic basins in the former Baltic Sea where sulfate reducing bacteria converted sea
water sulphate to sulphides in bottom sediments. Due to the strong land uplift (3-9
mm/year) these sulphide-bearing sediments have emerged above current sea-level. By
reclamation, burning of the peat cover and heavy liming they nowadays constitute some of
the most productive farm-lands in Finland. Meanwhile, due to these measures, the
groundwater level is strongly lowered during dry spells, enabling oxygen to penetrate the
soil. When exposed to oxygen, sulphides oxidize and produce sulphuric acid and make the
soil extremely acid (pH 2,5-4), which in turn mobilizes enormous quantities of metals
(including Al, Cd, Co, Ni and Zn) restored in the soil. Together with acidity, these metals
are flushed from the soils into drains and recipient estuaries during wet spells. Metal
discharges from AS soils are estimated to significantly exceed the corresponding
industrial discharges from the entire Finnish industry. This is also the case for cadmium,
one of the EU EQS directive priority substance metals. Moreover, cadmium
concentrations also exceed the environmental quality standards in many waterbodies
affected by AS soils.

In Finland, leaching of acidity and heavy metals from AS-soils is the most common cause
for bad or poor ecological and chemical status of surface water bodies, affecting more
than 30 coastal rivers and estuaries. Deteriorated or vanished fish stocks in numerous
rivers and estuaries is the most visible effect. Impacts of acid runoff vary according to the
quantity, quality and proximity of AS-soils and catchment characteristics of water bodies.
Climate change is likely to increase and widen the area of environmental damages unless
targeted mitigation measures are developed. High peak concentrations of toxic
compounds occur especially after long dry periods and subsequent heavy rainfalls. As a
result of climate change, these hydrological extremes are expected to become much more
common in river basins with small lake area and rapidly fluctuating discharges. For
aquatic ecosystems and fish stocks this means increased probability for exposure to toxic
metal compounds. The deeper layers of AS-soils also have high content of organic matter.
Drying and consequent mineralization of these organic storages would lead to gaseous
emissions (CO2, N2O, SO2).

The CATERMASS project (Climate Change Adaptation Tools for Environmental Risk
Mitigation of Acid Sulphate Soils) aimed at developing climate change adaptation tools
for the Finnish River Basin Districts to mitigate impacts of increased leaching of acidity
and metals from acid sulphate soils drained for agriculture and forestry. The overall
objective was to promote wide application of techniques and actions reducing acidity and
metal concentrations in drainage waters, thus enhancing achievement of EU
environmental objectives according to the Water Framework (2000/60/EC), Flood
(2007/60/EC), Habitat (92/43/EEC) and EQS (priority substances) directives. Especially,
our objective was to consolidate the knowledge base for adapting pollution control
methods to the changing precipitation, runoff and temperature conditions.