ASSESSING TOPSOIL HEAVY METAL CONTAMINATIONS DUE TO FORMER MINING ACTIVITIES

摘要

SUMMARY The leaching rate of heavy metals with drainage water is a significant criterion for assessing heavy metal contaminations of soils and associated risks of ground water pollution. However, the transport rates of heavy metals in soils are difficult to quantify. First of all monitoring is limited to small lysimeter plots and results are hardly transferable. Secondly the solid-liquid-transfer conditions in soils are highly variable, primarily due to the alternating retention time of percolating soil water. This paper aims to introduce a new method that allows to estimate the leaching rate of heavy metals from contaminated top soils for standardised transfer conditions. The method should provide an effective and easy to use tool for the assessment of contaminated soils on regional scale. The developed method is based on metal specific transfer functions which have to be determined experimentally. For laboratory tests 16 undisturbed soil columns were taken at different highly contaminated sites in the German States of Sachsen and Baden-Wuerttemberg. Parallel to the column tests the mobilizable metal contents of each of the tested soils were determined using different standardised extraction methods (DIN 38414, 19730, 19735). Measured metal concentrations of percolating soil water at unsaturated, steady-state flow conditions were plotted as a function of corresponding mobilizable metal contents of the tested soils. This comparison results in a set of empirical functions- one for each extraction method respectively metal species. Best fit was obtained using the NH_4NO_3-extraction (DIN 19730), where correlation coefficients range between r=0,62 and r=0,85. Since NH_4NO_3 mobilizable metal contents are available on regional scale in Germany the empirical transfer functions allow to calculate metal concentration of percolating soil water based on mobilizable metal contents as determined by simple extraction methods. As a first example heavy metal concentrations were calculated for the Mulde river basin in Saxony (eastern Germany) based on available NH_4NO_3 monitoring data of about 4600 topsoil samples. Leaching rates were calculated by multiplying the resulting metal concentrations with local groundwater recharge rates. In order to assess subsurface retention the calculated topsoil losses for Zn and Pb were compared with measured metal discharge rates of a selected river catchment resulting in a much greater retention of Pb than Zn.