Département Ressources aquatiques et eau potable

Molecular diffusion is a key transport process for noble gases in water. Such diffusive transport is often thought to cause a mass-dependent fractionation of noble gas isotopes that is inversely proportional to the square root of the ratio of their atomic mass, referred to as the square root relation. Previous studies, challenged the commonly held assumption that the square root relation adequately describes the behaviour of noble gas isotopes diffusing through water. However, the effect of diffusion on noble gas isotopes has only been determined experimentally for He, Ne and Ar to date, whereas the extent of fractionation of Kr and Xe has not been measured. In the present study the fractionation of Kr and Xe isotopes diffusing through water immobilised by adding agar was quantified through measuring the respective isotope ratio after diffusing through the immobilised water. No fractionation of Kr and Xe isotopes was observed, even using high-precision noble gas analytics. These results complement our current understanding on isotopic fractionation of noble gases diffusing through water. Therefore this complete data set builds a robust basis to describe molecular diffusion of noble gases in water in a physical sound manner which is fundamental to assess the physical aspects of gas dynamics in aquatic systems.

We applied coarse spectral analysis to more than 2 decades of daily near-surface water temperature (WT) measurements from Müggelsee, a shallow polymictic lake in Germany, to systematically characterize patterns in WT variability from daily to yearly temporal scales. Comparison of WT with local air temperature indicates that the WT variability patterns are likely attributable to both meteorological forcing and internal lake dynamics. We identified seasonal patterns of WT variability and showed that WT variability increases with increasing Schmidt stability, decreasing Lake number and decreasing ice cover duration, and is higher near the shore than in open water. We introduced the slope of WT spectra as an indicator for the degree of lake mixing to help explain the identified temporal and spatial scales of WT variability. The explanatory power of this indicator in other lakes with different mixing regimes remains to be established.

Long-term changes of 14 water constituents measured in continuously and water discharge proportionally collected samples of four Swiss rivers over a period of 39 years are analyzed using several statistical techniques. Possible drivers and causes for the identified trends and shifts are explained by consideration of catchment characteristics and anthropogenic activities. Water temperatures increased by 0.8–1.3 °C, whereas water discharges remained largely unchanged. Concentrations of alkalinity, total hardness, Ca, and Mg regulated by dominant carbonate lithologies in catchments increased by up to 10%. We attribute this change to an increase in the partial pressure of CO₂ in the subsurface, provoked by increasing temperatures. Re-oligotrophication processes in lakes also influence the behavior of alkalinity and silicic acid. In contrast to concentrations, most loads did not change significantly, due to their large variances. Therefore, no changes in overall weathering rates of carbonate rocks can be detected. The outgassing of CO₂ in rivers from the place of carbonate dissolution to measurement stations amounts up to 6% (mean) of CO₂ sequestered (mean 1.1 mol m⁻² a⁻¹) by the weathering of rock minerals. Changes in alkalinity/Ca/Mg ratios indicate an increase in calcite precipitation over time. Total nitrogen concentrations and loads peaked at the end of the 1980s and then decreased up to 50%, while NO₃ concentrations showed almost no changes. This dynamic matches the changes in the agricultural N balance. Concentrations and loads of Na and Cl increased up to 60% due to an increase in the various uses of rock salt.