Chemical composition of water‐bodies in the English Lake District: relationships between chloride and other major ions related to solid geology, and a tentative budget for Windermere

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Abstract
SUMMARY. Mean tarnwater concentrations of CI‐and other ions (Na+, K+, Ca2+, Mg2+, SO42‐+ NO3‐) decrease with increasing distance from the sea and altitude but are also related to solid geology. Lowest concentrations occur on slow‐weathering igneous rocks (Borrow‐dale Volcanics); minima of c. 100 μequiv. CI‐I‐1are similar to the volume‐weighted mean for bulk precipitation. At least 10–20% of Na+is leached from upland catchments, giving Na+/CI‐ratios greater than the equivalent ratio (0.86) in seawater and precipitation. Evapotranspiration at 20–30% annual rainfall accounts for CI‐concentrations in many tarns on igneous rocks but not all; some CI‐may be leached from the rocks in upland catchments and come from groundwaters at low altitudes near the sea. Sea‐spray has little influence on tarnwaters near the coast. On sedimentary rocks overall mean CI‐concentrations are 27–73% higher than equivalent means on Borrowdale Volcanics; concentrations of other ions are also higher. A 5‐fold to 10‐fold range of CI‐concentrations is not simply due to increased evapotranspiration. Na+/CI‐ratios are < 0.86, especially on Skiddaw Slates where tarnwaters contain 10% or more excess CI‐balanced by Ca2+, apparently derived from groundwaters rich in CaCI2. On other sedimentary rocks (Silurian Slates and Carboniferous, Triassic and Permian series) tarn waters display a similar but less pronounced excess of CI‐and Ca2‐+relative to Na+. Some extra CI‐may be captured by dry deposition on vegetation but a portion, perhaps 15–30% of the total, apparently comes from the rocks or from groundwaters via deep aquifers in contact with seawater or connate water. Anthropogenic sources on the catchments are also considered: deicing salt used on highways in winter accounts for a 27% increase of CI‐in Windermere South Basin over a 20‐year period.