EFFECTS OF DECREASED ATMOSPHERIC DEPOSITION ON THE SULFUR BUDGETS OF 2 DUTCH MOORLAND POOLS

The chemical composition of surface waters of two Dutch moorland pools and of incident precipitation, was monitored from 1982 to 1990. For t his period, sulfur and water budgets were calculated using a hydrochem ical model developed for well-mixed nonstratifying lakes. Total atmosp heric deposition of S decreased significantly after 1986 at both locat ions. A model describing the sulfur budget in terms of input, output a nd reduction/oxidation processes predicted a fast decrease of pool wat er SO42- concentrations after a decrease of atmospheric input. However , SO42- concentrations in the surface water was lowered only slightly or remained constant. Apparently a source within the lake caused the u nexpectedly high SO42- concentrations. The possible supply of SO42- fr om the sediment through regulation by (K-)Al-SO4 containing minerals o r desorption of SO42- from positively charged surfaces in the sediment was evaluated. Solubility calculations of pore water with respect to alunite, basaluminite and jurbanite indicated that SO42- concentration was not regulated by these minerals. lt is suggested here (1) that de sorption of SO42- from peaty sediments may account for the estimated S O42- supply provided that the adsorption complex is periodically recha rged by partial oxidation of the upper bottom sediments and (2) that b ecause of exposure of a part of the pool bottom to the atmosphere duri ng dry summers and subsequent oxidation of reduced S, the amount of SO 42- may be provided which complements the decreasing depositional SO42 - input. In future research these two mechanisms need to be investigat ed.