Middle Term Evolution of Water Chemistry in a Karst River: Example from the Loue River (Jura Mountains, Eastern France)

Plotting multiyear chemographs in a karst river may display evolutional trends in water quality. In recent decades, different factors could explain for instance phosphate decrease and nitrate increase. In the Jura karst area, different springs and rivers not only exhibit variation of anthropogenic molecule concentrations, but also evolution of major element concentrations, that results in a change in electrical conductivity. Over a 30-year period, average electrical conductivity of the Loue River, which is totally supplied by karst springs, has increased from 260 to 470 µS/cm. Such an 81 % variation is only explainable by increases in major components, i.e. calcium and hydrogenocarbonate ions, which are the almost exclusive by-products of karstification processes in the Jurassic limestones of the Jura Mountains. Indeed, no direct anthropogenic cause may be invoked for such an evolution. An increase of dissolution throughout the system is necessarily correlated to an increase in the carbon dioxide transfer, resulting from an increase in dissolution and/or production rate.