The impact of a single black snowfall on streamwater chemistry in the Scottish Highlands

Abstract

During snowmelt, ions fractionate into the first meltwaters1–3, giving rise to concentrated, often acidic, solutions4,5. This may cause transient acidification of streamwaters (the so-called 'acid flush') in poorly buffered upland catchments6, which can kill fish7 or, more commonly, cause recruitment failure8. The chemistry of snow and meltwater can affect the magnitude and duration of the 'acid flush', particularly when stream discharge is not dominated by pre-event water. Such conditions are found in upland Britain, in the Scottish Highlands9,10 The most acidic snowfalls in the Highlands are coloured or black11, and occur under specific meteorological conditions12. Here we present the streamwater chemistry during two acid-flush events with minimum recorded pH of 4.25 and 4.10. An acidic black snow (pH 3.4) fell in the 8-day period between the events. The meltwater composition during the first event was influenced by the composition of the existing snow-cover, whereas the second event shows the effect of the fall of polluted snow. These observations demonstrate that a single snowfall can strongly influence the composition of meltwaters. Hence, the large-scale distribution of ions in snowpacks must be considered in attempting to predict the magnitude of acidification during the thaw13 and the possible impact of acid rain on aquatic ecosystems. This work highlights how chance associations of episodic events (coloured snowfall and snowmelt) may amplify acidification in streamwater.