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

SUMMARY.1 Mean tarnwater concentrations of CI ‐ and other ions (Na + , K + , Ca 2+ , Mg 2+ , SO 4 2‐ + NO 3 ‐ ) 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 ‐1 are 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. 2 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 Ca 2+ , apparently derived from groundwaters rich in CaCI 2 . On other sedimentary rocks (Silurian Slates and Carboniferous, Triassic and Permian series) tarn waters display a similar but less pronounced excess of CI ‐ and Ca 2‐+ 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. 3 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.