Long-term trends and variation of acidity, CODMn and colour in coastal rivers of Western Finland in relation to climate and hydrology

High acidity caused by geochemical processes and intensive land use of acid sulphate (AS) soils have continuously degraded the status of water bodies in Western Finland. Despite this, research on the long-term pattern and dynamics of acidification in rivers affected by acid sulphate soils is scarce. This study examined changes in alkalinity and pH value during the period 1913–2007 in nine large Finnish rivers discharging into the Gulf of Bothnia. In addition, patterns of CODMn and colour were analysed during the period 1961–2007. Relationships between pH, alkalinity, CODMn and colour and climate variables were also studied. In four rivers with no AS soil impact (Kokemäenjoki, Kemijoki, Iijoki and Oulujoki), critically low pH levels did not occur during the study period, whereas three rivers exposed to minor or moderate levels of runoff from AS soils (Lestijoki, Kalajoki, and Siikajoki) had all periods with critically low pH and alkalinity. The most severe acidity problems occurred in the rivers Kyrönjoki and Lapuanjoki, with extensive drainage of AS soils being the main reason for the low pH status. Maximum discharge was clearly related to the acidity status of many rivers during the autumn–winter runoff period, when a significant negative linear correlation was found between maximum discharge and minimum pH in the rivers affected by AS soils. There was also a more distinct relationship between maximum chemical oxygen demand (CODMn) and minimum pH in autumn runoff than in spring. CODMn levels significantly increased with increasing discharge in the rivers with no or minor AS soil impact. Climate change is predicted to increase river flow in general and winter discharge in particular, and therefore the acidity problems in affected rivers may increase in a future climate.