Labile aluminium chemistry downstream a limestone treated lake and an acid tributary: Effects of warm winters and extreme rainstorms

The outlet from the limestone treated Lake Terjevann consisted mainly of well-mixed lake water (mean pH 6.1) during the ice-free seasons including the unusually warm winters of 1992 and 1993. However, during the ice-covered period acidic water (mean pH 4.8, mean inorganic aluminium (Ali) about 160 μg/l) from the catchment draining under the lake ice dominated. A downstream tributary was generally acid and rich in aluminium (mean pH 4.6, Ali about 230 μg/l). After an extreme rainstorm loaded with sea-salts cation exchange in the soil resulted in more than a doubling of the Ali concentration (reaching about 500 μg/l). It took 3–4 months until the Ali concentration returned to pre-event levels. During the ice-covered period, the acidic outlet and tributary waters resulted in acidic conditions below the confluence (pH < 4.8, Ali about 150 μg/l) while during the ice-free periods the more neutral outlet water resulted in higher pH and lower Ali concentrations (pH > 5.2, Ali about 95 μg/l). However, during the latter climatic conditions the water was most probably more harmful to fish due to hydrolysing and polymerizing aluminium. After the sea-salt event, the increased Ali concentration in the tributary made the zone below the confluence potentially more toxic (pH ∼ 5, Ali ∼ 250 μg/l). Expected global warming resulting in winter mean temperatures above 0 °C may eliminate the seasonal acidification of the outlet from limestone-treated lakes creating permanent toxic mixing zones in the confluence below acidic aluminium-rich tributaries. Besides, more frequent rainstorms as a consequence of global warming may increase the frequency of sea-salt events and the Ali concentrations in the mixing zones.