Evolution of the chemical composition of the Garonne River water during the period 1971-1984 / Evolution de la composition chimique des eaux de la Garonne entre 1971 et 1984

Abstract The evolution of the chemical composition of the Garonne River water was studied for the period 1971–1984. Long-term trends have been detected and quantified for all the major elements using concentration-time relationships. An increasing concentration is observed for most of the elements while the pH exhibits a small decrease. The large increases of NO− 3, SO2- 4, Cl− and K+ in the river water are related to a marked increase in fertilizer applications. Ca2+, Mg2+ and HCO− 3 increases are not so evident. The coefficient of calcite saturation is too low to explain this change but rather it can be attributed to an increase in carbonate dissolution resulting from the nitrification of N fertilizers in the basin. A decrease in pH, more particularly during low flows, is attributed to oxidation of organic matter which is reflected by an increase in COD.