MODELING LEACHING OF INORGANIC HG(II) IN A SCANDINAVIAN IRON-HUMUS PODZOL - VALIDATION AND LONG-TERM LEACHING UNDER VARIOUS DEPOSITION RATES

Increasing mercury contents are reported from freshwater systems and f ish in northern Europe and North America. Mercury input from soils is a major source with the leaching being affected by increased atmospher ic mercury deposition compared to pre-industrial times and by other en vironmental conditions such as acid rain. The results of a mathematica l model-calculation of vertical inorganic Hg(II) leaching in a Scandin avian iron-humus podzol under different atmospheric input rates of mer cury are presented. Leaching under background rain conditions was calc ulated to be considerably stronger than under acid rain conditions. In creasing fractions of deposited soluble of solute atmospheric mercury were leached from the O-f(h)-horizon with decreasing soil content of s oluble mercury under acid rain conditions; this effect was less pronou nced under background rain conditions. The steady state concentrations of soluble mercury of the upper soil horizons were calculated and com pared with the actual concentrations of total (= soluble + insoluble m ercury) and extractable (= estimate of soluble) mercury measured in th ese horizons. The results indicate that even if the deposition of airb orne mercury to soil is strongly reduced, the total mercury content of the soil decreases only slowly. It may take decades or even centuries before a new steady state concentration of total mercury is establish ed in the soil. The decrease of the mercury concentration in the Of(h) -horizon is probably largely dependent on the turnover of organic matt er, binding most of the deposited airborne mercury in an insoluble for m. Hence, present day mercury leaching is likely to be dominated by me rcury deposited during former times and temporarily retained in an ins oluble form in the organic matter.