Freeze‐out of salts in hard‐water lakes1

A 1Syear series of daily measurements of pH and total alkalinity recorded at the water intake of a municipal water-treatment plant on Clear Lake, Iowa, demonstrated that rapid chemical changes occur annually as the ice cover melts. Additional measurements of total and calcium hardness on this and three other eutrophic, hard-water lakes showed that total and calcium hardness concentrations declined rapidly as the main ice tiass was destroyed. This decline was not caused by chemical or biological precipitation of carbonates, but by a dilution effect of melting ice. The magnitude of the declines could be predicted from measurements of hardness under the ice, lake volume, and volume of ice. Studies of natural lakes and farm ponds have shown that water hardness and alkalinity levels are generally higher in winter under an ice cover than in spring or summer and that the magnitude of the winter to spring-summer decline is greater in eutrophic than in oligotrophic waters (Thomas 1953; Forney 1957; Jones and Bachmann 1974; Barica 1977). Mortimer (1941), Greenbank (1945), Forney (1957), and others have suggested that the higher values under the ice are the result of the resolubilization and release of carbonates from the bottom sediments as oxygen concentrations and redox potentials decrease and as carbon dioxide concentrations increase due to microbial decomposition. This resolubilization and release process presumably is similar to that in the hypolimnion of a lake during summer stratification (cf. Mortimer 1941; Hutchinson 1957). Reductions in water hardness and alkalinity levels generally have been attributed either to chemical precipitation of carbonates induced by a loss of equilibrium carbon dioxide during lake mixing or to biogenic precipitation of carbonates as equilibrium carbon dioxide is withdrawn during photosynthesis (Forney 1957; Hutchinson 1957; Otsuki and Wetzel 1974). An alternative hypothesis is suggested by the work of Barica (1977) and Rigler (1978), who have shown that reductions and increases in lake volume associated with ice formation and disappearance can account for the observed increases and decreases in water hardness and alkalinity levels in some lakes. This is based on the observation that, as lake ice forms, major ions tend to be frozen out so that the ice is relatively pure while the salts that-have been excluded are added to the remaining water. -Our study was stimulated by inspection of a 15-year series of daily measurements of pH and total alkalinity recorded at the water intake of a municipal watertreatment plant at Clear Lake, Iowa. We found significant changes in these variables from winter to summer, especially at the time the ice melted. Our objective was to examine this and some other Iowa lakes to determine which of these hypotheses best explained the decreases in salt concentrations observed at the time of spring icemelt. We were also interested in determining whether the winter increases could be accounted for by biological action or physical freeze-out and how rapidly the salt concentrations declined as the ice cover disappeared. l Journal Paper J-10674 of the Iowa Agriculture and Home Economics Experiment Station, Ames (Project 2483). Supported in part through a grant from the Office of Water Research and Technology (Project A-063-IA), US. Department of the Interior, >lS authorized by the Water Research and Development Act of 1978. 2 Present address: School of Forest Resources and Conservation, University of Florida, Gainesville 32611. 970 We thank the personnel of the municipal water treatment plants at the towns Freeze-out of salts in lakes 971 Table 1. Morphometric data and average summer chemical conditions for the study lakes (Jones and Bachmann 1978).