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

Abstract

During snowmelt, ions fractionate into the first meltwaters^1–3, giving rise to concentrated, often acidic, solutions^4,5. 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 Highlands^9,10 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.