Meteorological impacts on the water quality in the Pajuluoma acid sulphate area, W. Finland

Water samples (n = 354) from a small catchment (7.4 km²) covered by acid sulphate soils (pH < 4) were collected during all seasons and all types of hydrological conditions in 1990-2001. The electric conductivity (EC) and pH, i.e. the key indicators of acid sulphate soil impact in the current setting, were determined. Representative daily runoff and precipitation data was available for the whole study period. The 10th and 90th percentiles for EC and pH were 29-140 mS/m and 3.8-4.6, respectively. While the water quality varied remarkably from year to year, and even within seasons, some regularity was found. The water quality was generally worst in late autumn (water temperature < 5 °C) and in spring. Of all seasons the variations were clearly smallest in spring, indicating that most representative samples can be obtained in this season. There were significant correlations between autumn, early winter and spring water quality within hydrological years. Thus acid and metal surges in spring are somewhat predictable. At base flow conditions (runoff about 1 L/s km² or less), the water quality was relatively good in all seasons. Above base flow conditions, the impact of acid sulphate soils tended to slightly increase with increasing runoff and precipitation, especially in early summer, but not in late summer. No significant signs of dilution during flood conditions (up to 100 L/s km²) were found and neither were there any correlations to rising or falling limbs. The severity of individual summer droughts, which in theory should increase the oxidation of S and acidity in the soils, had little or no impact on the water quality in subsequent autumn and spring. On the other hand, there was a remarkable long-term increase in EC and a corresponding decrease in pH (starting in 1995) after a suite of several very dry summers. After that the water quality did not improve even if the dry summers were followed by some wet summers. This indicates that the temporary pool of readily leachable acidity in the soils is fairly large. Moreover, it indicates that the potential shift towards more extreme global weather conditions (with more severe dry spells) may have significant impacts on the water quality in midwestern Finland, a region that is heavily affected by acidity and metals from acid sulphate soils.