Characterization of acid sulfate soils and assessing their impact on a humic boreal lake

Due to acidity and metals from acid sulfate soils (a.s. soils), many watercourses in midwestern Finland have since 1960s regularly experienced short but intensive periods of poor water quality during high water flow. This has led to occasional massive fish kills and a significant decline in fish populations.However, many watercourses in northern Europe are often also characterized by low pH caused by naturally occurring organic acids, non-carbonate bedrock and previously a large input of acid rain. Therefore the role of acid sulfate soils has not always been properly addressed.This study was done on Larsmo and Öja Lake, a freshwater reservoir embanked from the alkaline Bothnian Bay in the 1960s. Since that, events of severe acidity and fish kills have been documented on several occasions. Severe but heterogeneous a.s. soils with relatively coarse grain sizes were found in the catchment. These soils were found to leach metals typical for a.s. soils such as Al, Cd, Co, Ni and Zn to recipient streams, but in contrast to other a.s. soils studied in Finland, some of them even seem to be leaching large quantities of Fe. Patterns of acidity, sulfate, sulfur isotopes, organic carbon and metals in rivers, streams and the lake revealed that (1) a.s. soils in the catchment are by far the most important source of acidity and metals in the lake i.e. the main reason for fish kills, (2) the load from discharge of organic (humic) acids and dredging operations is insignificant on the lake as a whole, (3) young near-field sediments comprising only 7% of the whole catchment accounted for roughly 20–30% of the total acidic load, (4) most of the acid/metal load entered the lake system from the south, and (5) by regulating outlets to the sea it seems to be possible to prevent this load from dispersing into the northern part of the lake system.

► We identify the cause of acidic and metal rich water in Larsmo and Öja Lake.
► Acidic metal surges are caused by sulfide oxidation in acid sulfate soils.
► Humic acids and dredge spoils are insignificant sources of acidity.
► Small streams in the near-field area cause an unexpected high share of the load.
► Flow regulation through the outlets from the lake can reduce the damage.