Impact of isostatic uplift and ditching of sulfidic sediments on the hydrochemistry of major and trace elements and sulfur isotope ratios in streams, western Finland

Acid soil drainage is of major economic concern for the arable crop farmers and fishermen in midwestern Finland. Almost all the farmland in this region consist of acid sulfate soils developed on sulfide-bearing fine-grained marine and lacustrine sediments. During rain storms in autumn and snowmelt in spring these soils are flushed, resulting in low pH and high metal concentrations in affected watercourses. A study of 40 first- and second-order streams during the 1997 autumn high water flow event demonstrated that (1) the pH in peat/till dominated catchments is controlled by organic acids (pH > 4.5), while in streams draining acid sulfate soil pH is controlled by sulfuric acid (4.5 > pH > 3.0); (2) the delta(34)S((sulfate)) values with range of -16 to +7 parts per thousand VCDT show two populations with medians of -6.2 and +3.2 parts per thousand, the first is dominated by original sedimentary sulfide and the second is most likely related to secondary sulfide formation in the acid sulfate soils; (3) the concentrations of Al, Ca, Cd, Cl, Co, Cu, Mn, Ni, Na, Zn, and SO42- are considerably higher in streamwaters in the investigated area than in headwater streams from the whole country as a result of extensive leaching of the acid sulfate soils; and (4) As, Ba, Cr, Fe, Ph, Sb, and V concentrations are controlled by soil and water pH, dissolved organic matter, and suspended particles and are not like the other trace elements, significantly enriched in the streamwaters.