Long-term trends of water chemistry in mountain streams in Sweden – slow recovery from acidification

[摘要] The water chemistry of streams and precipitation in the province ofJämtland, northern Sweden has been monitored since the 1980s to studylong-term trends, occurrence of acid episodes, and effects of liming. Theacidity in precipitation increased in the 1970s, followed by a loss of acidneutralizing capacity (ANC) and low pH in the streams. Sulfur depositionbegan to decrease in the 1980s, until approximately 2000, after which thedecrease levelled out. Stream water sulfate concentration followed theprecipitation trend but decreased more slowly and since the late 1990s asubtle increase was observed. Sulfate concentrations in the snow typicallyhave been higher than or equal to the stream sulfate levels. However,during the period of rapid deposition decrease and also since 2005 streamsulfate has sometimes exceeded snow sulfate, indicating desorption ofstored soil sulfate, possibly because of climate-related changes in run-offroutes through the soil profiles, following shorter periods of frost.

From 1982 to 2000, total organic carbon (TOC) increased by approximately 0.1 mg L⁻¹ yr⁻¹.The mean trends in sulfate and TOC fromapproximately 1990 until today were generally opposite.

Acidic episodes with pH 4.0 at flow peaks occurred frequently in the unlimedstreams, despite relatively well-buffered waters at baseflow. To evaluatethe main causes for the loss of ANC during episodes, the changes in majorion concentrations during high flow episodes were evaluated. The mostimportant factors contributing to ANC loss were dilution of base cations(Na⁺, K⁺, Ca²⁺, Mg²⁺), enrichment of organic anions andenrichment of sulfate.Wetland liming started in 1985 after which the earlier observed extreme peakvalues of iron, manganese and aluminium, did not reoccur. The studied areais remote from emission sources in Europe, but the critical load of acidityis still exceeded. The long-term recovery observed in the unlimed streams isthus slow, and severe acidic episodes still occur.

[学科分类] 地球化学与岩石