Relationships between chloride and major cations in precipitation and streamwaters in the Windermere catchment (English Lake District)

SUMMARY. 1. The ranges of concentrations for pH, CV. Na⁺. K⁺Ca²⁺, Mg²⁺ are given for streams draining igneous rocks (Borrowdale Volcanics) and sedimentary muds, silts and shales (Silurian Slates) in the catchment of Windermere (230 km²). Impacts on the biota are briefly discussed. Relative contributions of inputs from various sources are examined: precipitation, rocks and soils, sewage, deicing salt used on highways./ 2. In bulk precipitation, ratios of Na⁺/CV (as μequiv. I^?1) ranged from 0.41 to 1.83 over a 2-year period. The overall volume-weighted mean ratio was 0.86, as in seawater, hut 2′4%(1975) and 5.7% (1976) of Cl^? was non-marine in origin, being balanced by H ⁺ (in winter), K and Ca²⁺ (in summer). In moorland headwater streams, CI^? is largely derived from precipitation: there is a pronounced annual cycle of midwinter high and midsummer low concentrations unrelated to stream discharge. Na* and K⁺ display similar cycles but Na⁺, Ca2⁺ and Mg²⁺ (and pH) are discharge-related due to leaching from rocks and soils: 10–20% Na+, 15–17% Mg²⁺ and 65–75% Ca²⁺ are so-derived whereas K+ comes from precipitation. Na⁺/Cl⁺ ratios in streams on Borrowdale Volcanics alter seasonally, with midsummer values >1. Lower values occur in streams on Silurian Slates where some CI^? is apparently derived from rocks. 3. Seasonal changes in streamwater concentrations of Cl, Na⁴ andK⁺are exponentially related to time. Instantaneous rates of change i day-1¹) are compared in relation to position in the catchment and inputs from anthropogenic sources. Deicing salt has raised (10–100-fold) the concentrations of Na^* and Cl^? in streams on mountain passes; the effects persist throughout the year.