Continuous measurements of riverine chemical constituents reveal the environmental effect of the 2014–2015 Bárðarbunga eruption in Iceland

The 2014–2015 Bárðarbunga eruption caused one of the most intense and widespread volcanic derived air pollution events in centuries. The volcanic plume was tracked all the way to central Europe. Volcanic gas and aerosol dissolved in precipitation and accumulated as aerosol and snow on land in Iceland. Here we report on the continuous monitoring of discharge and dissolved constituents of a direct runoff river located 85 km from the eruption site during the entire snowmelt period in 2015. Osmotic samplers collected continuous water samples so short-lived flood peaks would not be missed. Comparison between results from this 2015 study and data from 1998 to 2013 from the same river showed that average pH was 0.2 units lower in 2015 and the Cl and SO₄ average concentration were 36% and 38% higher than prior to the eruption, respectively. Integrated riverine fluxes of H⁺ and SO₄ were two to three times higher during summer 2015 than in the control years, and the Cl flux was 37% higher. The measured excess H⁺ flux was however only a small fraction of the calculated excess H⁺ flux stemming from excess SO₄ and Cl fluxes in 2015 which can be explained by ion-exchange in the catchment soil, water-rock interaction consuming protons within the catchment, and by elevated sea spray derived Cl and SO₄ due to relatively high wind speed during the 2014–2015 winter. In addition, the small H⁺ increase in the river water can be the cause of the less than expected excess SO₄ riverine flux, normalised to the total measured volcanogenic SO₂ and HCl emission fluxes (11 Tg/0.1 Tg), which underscores the slow oxidation of SO₂ to SO₄ in relatively dry weather conditions and reduced sunlight during winter in Iceland. Despite being sensitive to riverine pH and aluminium changes, freshwater ecosystem was not threatened by the eruption, since it did not cause large pH changes and the dissolved aluminium concentration was always low during the study period. This study provided the means to quantify the integrated chemical fluxes and the impact of volcanic emissions on a riverine system in the vicinity of the eruption site and the implication of increased chemical fluxes of solutes of volcanic origin on the system.