The geochemistry of the Rhine and the Rhone. 5, Synthesisand conclusions

Abstract

Every month 2 samples of the Rhine and 1 sample of the Rhone are analysed for about 20 chemical and physical variables at 7 stations by two monitoring agencies.This paper presents some of our conclusions on the accuracy of these programmes and on the solubility of the calcium/carbonate/sulphate/phosphate system. In total we have analysed 916 data sets ; they were screened for imprecision before being used.At all stations the water is saturated with CaCO₃ , but the "apparent" solubility product depends on the pH. The regression line can be described by IP_ca = 8.73 x 10^-15. pH^15.667.In both the Rhine and the Rhone an acidification is seen and an increase of the sulphate concentration when going downstream. We have shown that the increase in sulphate concentration is due to the disposal of CaSO₄. This addition of sulphate influences the quotient Ca/HCO₃. The data fit very well the theoretical regression line Ca/HCO₃ = 0,5(SO₄) + 0.5.The acidification originates from the mineralization of organic matter. For 6 stations dowstream from the great lakeswe found that the two rivers are saturated with hydroxy-apatite, the -log of the ionic product being 49.9 (Rhine ; σ = 0.6 ;N = 259) and 50.0 (Rhone ; σ = 0.3 ; N = 479). The o-phosphate concentration does not therefore depend on the phosphate loading of the rivers, but on the Ca concentration, the temperature and the pH. This has great influence on the transport of phosphate through the delta's of the two rivers. The disturbance of acidity and buffering capacity influences heavy metal behaviour as well.