Hydrological processes behind annual and decadal-scale variations in the water quality of runoff in Finnish catchments with acid sulfate soils

In this study we assess long- and short term temporal variations in the impact of acid sulfate (a.s.) soils on river water quality. We demonstrate how such variations depend on changes in hydrological conditions driven by land use, meteorological variations and potential changes in climate with important implications on mitigation strategies, water ecology and utilization of water resources. Quality of river water discharging into the Larsmo-Oja Lake in Midwestern Finland was studied by using long term water data collected during 1963-2009. Acid sulfate soils are extremely acidic soils (pH <4) that are known to discharge very large amounts of acidity and metals into recipient water courses, and this was also evident in the study area where extreme acidic events have occurred frequently. Looking at the whole study period, there was an abrupt and consistent decline in pH in the late 1960s and early 1970s in the main river (Esse River) that coincided with extensive drainage works that dropped the ground water level, enabling oxidation of sulfidic soils and transport of acidity to the rivers. Since then, there is a trend of decreasing acidic events and rising pH values, probably due to a continuous depletion of the acidic pool in the existing a.s. soils. In the short run, water quality varied greatly due to varying hydrological conditions between seasons and years. Generally, the impact from a.s. soils was highest during high runoff in autumn and spring, and therefore, neutralization of acidity in discharge water by liming would at such occasions be very demanding. The relationship between the runoff and water quality was, however, somewhat different during different seasons. As expected, dry summers (low ground water levels) were found to increase the impact from a.s. soils in the subsequent autumn, but only if runoff was high. Towards the end of the study period winters tended to become warmer with higher runoff and spring floods tended to occur earlier. Thus, events with bad water quality during the winter months have become more common and acidic spring surges occur earlier. Seen from the data in this study, it is obvious that potential changes in the future climate will have significant consequences on the impact from a.s. soils on water courses. (C) 2013 Elsevier B.V. All rights reserved.