Impact of acid sulfate soil catchments on water quality in a lake in western Finland; trends and total metal load

Acid sulfate soils occur worldwide and can cause serious ecological damage by releasingacidity and toxic metals into watercourses. This study focused on short- and long-termchanges to the water quality of a lake in western Finland. Here, a decline in water qualitytook place in the late 1960s. Since then, events concerning poor water quality have frequently occurred. The annual load of some potentially toxic metals varied from hundredsof kilograms to thousands of tons, depending on the metal. The proportion of low-orderstreams draining the nearest field to the lake is only 7% of the catchment area, while theshare of the total metal load was estimated to be over 30%. This points to the importanceof monitoring small coastal catchments. High runoff conditions were proven to be moreimportant in terms of water quality than occasional summer droughts. During the pastdecade, acidic conditions have become more prolonged in the autumn, which is in line witha predicted scenario relating to climate change.