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The use and misuse of pure water PVT properties for lake waters

Naturevolume 266pages707708 (1977) | Download Citation



ALTHOUGH dissolved substances affect the thermodynamic properties of lake waters,1,2 many limnologists still consider lake water as pure water3–8. This can be a good assumption if used with caution: lake water is by no means pure water, especially when the precise pressure, volume, temperature (PVT) properties are considered. PVT properties of lake water can be determined from an equation of state for sea water provided that the total mass fraction of dissolved salts in seawater and lake water are equated9. In addition, the total mass fraction of dissolved salts affects calculations concerning the temperature of maximum density and thus stability10. Therefore, in the absence of direct PVT measurements on a particular lake, it is at present most appropriate to use an equation of state for seawater when examining effects of PVT properties in lakes. We present here a short equation of state determined with a precision of 2 × 10−6 g cm−3 in density over the appropriate range for most fresh water lakes, 0–0.6‰ salinity, 0–30 °C, and 0–180 bar. The equation of state is based on the 1 atm pure water density and compressibility equations of Kell11, the pure water temperature and pressure effects of Fine and Millero12, and the effects of dissolved salts of Millero et al.13 and Chen and Millero14: where dp and d0 are the densities of lake water at applied pressures P and 0 (sea level), respectively, t is the temperature in °C, S(‰) is the salinity: where gT is the total grams of dissolved salt in 1 kg of lake water15. The values of gT usually increase with depth due to the increase of HCO3, Ca2+ and nutrients16. The small changes in composition in a lake can be accounted for by making the appropriate changes in gt. Further, Millero9 and Millero et al.17 showed that the densities calculated from the seawater equation of state for Lake Tanganyika water and artificial river water agree with the measured values to within ±2×10−6g cm−3 despite the large composition difference between the two waters. This verifies the validity of our equation of state for fresh waters.

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  1. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, 33149



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