Influence of sea salt on stream water chemistry in an upland afforested catchment

The chemistry of bulk precipitation and stream water was monitored in an acidic afforested catchment at Llyn Brianne in upland Wales between 1985 and 1990. Throughfall, stemflow and soil water chemistry were also monitored between 1988 and 1989. Marine‐derived solutes dominated the ionic composition of precipitation and stream water, which had mean Cl concentrations of 113 μequiv. 1⁻¹ and 245 μequiv. 1⁻¹, respectively. The higher concentrations in stream water reflect occult and dry deposition on the forest canopy and the effect of interception and transpiration losses. Chloride variations in stream water (112‐454μequiv. 1⁻¹) were damped compared with bulk precipitation (28‐762μequiv. 1⁻¹) due to the mixing of event (‘new’) water with pre‐event (‘old’) water in the catchment soils. A storm episode monitored in the catchment in April 1989 was associated with high sea salt inputs and Cl concentrations in throughfall (1466μequiv. 1⁻¹) and storm runoff were exceptionally high (392μequiv. 1⁻¹). The Cl signal in stream water during the episode was consistent with an event (‘new’) water contribution to the storm response. However, a short‐term hydrochemical budget estimated that although Cl outputs from the catchment during the event (1.17 kg ha⁻¹) were equivalent to 8% of inputs in throughfall and stemflow, the storm runoff was equivalent to 32% of effective precipitation. This indicates that pre‐event (‘old’) water was the dominant source (> 75%) of storm runoff. Although sea salt inputs during the event had a marked impact on stream water chemistry, the anomalously high levels of acidity sometimes associated with sea salt events were not observed in this particular study.