Abstract :
Concentrations, distributions and mobility of chemical elements were investigated in reduced sulfur-rich estuarine
sediments located in western Finland. The main objective was to determine the possible extent of metal leaching
when dredged masses of these sulfur-rich sediments are dumped on the land and thus exposed to air. When dredged,
the reduced sulfur in the sediments oxidises resulting in a lowering of pH, which in turn is expected to leach metals.
The study area is an artificial lake claimed from the Botnian sea in 1962. In this lake, several mass-kills of fish have
occurred, believed partly to be due to dredging. Two sediment samples Ž0 50 and 50 100 cm. were taken from 39
sampling points in the lake. These samples were leached in aqua regia Ž2:2:2 HNO3 HCl H2O. and analysed for Fe,
Al, Mg, Ca, K, P, Na, Mn, Zn, Ba, V, Sr, Cr, Ni, Cu, Co, As, Pb, B, Mo and Cd with ICP-AES. Sulfur and organic
carbon were analysed with Leco. In a controlled laboratory experiment, the sediments were allowed to oxidise for 1
year while moisturised with deionised water every month. The pH and conductivity were determined in the beginning
of the experiment Žreduced state. and in the end Žoxidised state.. In the supernatants in the oxidised states the
amount of leached metals ŽNa, Al, Mn, Zn, Sr, Co, Ni, Cu, Cd, Cr, Pb, U, Li, Rb and As. were determined with
ICP-MS. The sediments were found to contain low levels of toxic metals but, as expected, high concentrations of
sulfur. In the experiment, pH was lowered Ždown to 3.0. and the conductivity increased in all samples due to
oxidation and release of metal ions. The extent of leaching varied between 0.03% for As and 12.3% for Na. Critical
pH values, at which high amounts of metals begin to leach, were obtained graphically. These values varied between
4.8 ŽNi. and 3.3 ŽCr.. Not all elements were controlled by pH, e.g. Mn correlated well with its aqua regia leachable
concentration. In a planned dredging operation in the area some 23 300 t Ž10 500 m3. Ždry wt.. of sediments will be
dredged. The amounts of metals likely to be leached, according to the results from this study, are as follows Žkg.: Al
Ž1710., Mn Ž1230., Zn Ž59., Sr Ž39., Co Ž13., Ni Ž12., Cu Ž2. and less than 1 kg of Cd Cr As Pb.
