Long‐term and seasonal changes in the chemical composition of precipitation and surface waters of lakes and tarns in the English Lake District

Abstract

SUMMARY. The composition of bulk (wet and dry) precipitation in 1975 and 1976 was similar to that found 22 years previously. In 1975, mean values for nine precipitation samplers at one site (Wraymires) ranged from pH 4.3 to 4.5; similar values were obtained in 1976. Samplers covered with fine‐mesh plastic gauze caught substantially more Ca2+ and K+ than open samplers, but pH and SO2‐4+ NO3‐ concentrations were similar in open and covered samplers. In precipitation, c. 50% of H+ was balanced by NO3‐ and c. 50% by SOi; 80% of the SO2‐4 was balanced by Ca2+ and Mg2+.Conccntrations of major cations (H+, Na+, K+,Ca2+, Mg2+) and anions (CI‐, NO3‐ SO2‐4 and alkalinity [Alk—largely HCO3‐]) in upland water‐ bodies were similar to those found in precipitation, but pH levels were generally higher and above 6.0 m some tarns. At lower altitudes, on base‐rich roeks and soils, Ca2+ and Alk become dominant. Results of a survey of lakes and tarns in 1974–78 are compared with a survey in 1953–56 and published data (chiefly for pH and Alk) for 1947–50, 1932 and 1928. Comparisons are also made with other measurements of Alk in three productive lakes (Blelham Tarn, Esthwaite Water and Windermere) for 1936–39 and 1945–80. Winter levels of NO3‐N, PO4‐P and Si are given for these lakes; although the first two have increased during the late 1960s and the 1970s there has been no significant change in the last. NO3‐ and probably some SO2‐4. In productive lakes a substantial (c. 50%) rise in mean Alk occurred during the late 1960s and the 1970s, possibly related to increased winter levels of NO3‐N and PO4‐P derived from sewage and fertilizers. In this period the maximum pH levels reached in summer were notably high, sometimes exceeding pH 10. The rise in Alk, conductivity and pH of surface waters is influenced by climatic factors (a decade of drier years), sewage input and biological productivity within the lakes.Considerable seasonal fluctuations in the concentrations of major ions, a characteristic feature of surface waters in the English Lake District, are illustrated and some implications for cation‐anion balance briefly discussed. Mid‐winter concentrations are usually high forNa+, K+, Cl‐. NO3‐and low for Ca2+, Mg2+, Alk. SO2‐4.Alkalinity. pH and conductivity of Lake District tarns and lakes show no signs of acidification during the period 1928–80. On the contrary, productive lakes have become more alkaline and some unproductive low‐alkalinity (< 100 μ‐equiv. 1‐1) lakes also show signs of alkalization, with increased mean concentrations of Na+. Ca2+ and Mg2+, balanced by Alk.