MODELING POTENTIAL LONG-TERM RESPONSES OF A SMALL CATCHMENT IN LAPLAND TO CHANGES IN SULFUR DEPOSITION

The SMART model (Simulation Model for Acidification's Regional Trends) was developed to estimate long-term chemical changes in soil and lake water in response to changes in atmospheric deposition. Its major outputs include base saturation, pH, and the molar Al/Ca ratio for soils, and major cations and anions for soil and lake water. The model structure is based on the anion mobility concept and the charge balance principle. SMART was applied to the Christmas Lakes, a chain of small oligotrophic catchments in northeast subarctic Finnish Lapland (69 degrees 25'N, 29 degrees 11'E), located close to the border with Norway and 40 km from the Nikel smelter in Russia, a major source of sulfur emissions. The Christmas Lakes are less than 60 ha in size, have clear water, and are low in base cations and alkalinity. At present, the lakes are not very acidic, the pH being around 6.5 in autumn. The SMART model was calibrated to present soil and water quality data, using best estimates for historical deposition patterns for the period 1890-1990. The model applications in this paper are intended to demonstrate the range of potential catchment responses in northern Lapland for different assumptions concerning sulfur. The two calibrations of the model, assuming (1) a high sulfur deposition in combination with sulfate adsorption and (2) a lower present deposition with no adsorption, revealed that the present sulfur deposition has to lie between 0.4 and 0.8 g S/m(2)/year. The model results for both calibrations indicate that future sulfur deposition would have to be very low to stop and reverse the ongoing acidification. However, since the present base saturation is fairly high (> 50%), there is still time (several years) to reduce the sulfur deposition to values the system can tolerate.