The Dissolved Load of the Loire River: Natural Inputs and Anthropogenic Activities Contributions

Abstract

The Loire river, one of the largest watersheds in France, has been monitored just outside of the city of Orleans from 1994 to 1996 and 150 km downstream Orleans, in the village of Brehemont from 1996 to 1998. The sampling frequency ranged between 2 days and 2 weeks according to the river flow. The location of the sampling point at Orleans represents 34% of the total Loire watershed with 76% silicate rocks and 24% carbonate rocks. The second location at Brehemont represents 26% more of the total watershed, draining only additional carbonate rocks. This study reports on the temporal distributions of major and trace elements and Sr isotopic measurements on the dissolved load of the Loire river at these two points of the watershed. Natural and anthropogenic inputs to the dissolved load have been distinguished and the exportation rates due to rocks weathering and anthropogenic inputs can also be compared. The temporal fluctuations of major and trace elements in the dissolved load can be divided into 3 types of geochemical behaviour. They exhibit characteristics of mixed waters as shown by the positive correlation between discharge and 878r/86Sr ratio. The first kind, present during the low flow, is concentrated in major and trace element and is characterised by a low isotopic Sr ratio. It may be related to groundwaters inputs and anthropogenic activities. The second kind, present during high flow, is more diluted and may be related to rain water and weathering inputs. The mass-balance approach (Meybeck, 1983; Drever and Hurcomb, 1986) shows that the dissolved load is composed by the mixing of atmospheric inputs, rock weathering and human activities. In order to estimate each component contribution, the rain water influence is subtracted from the river water composition, using chloride as the atmospheric reference (Meybeck, 1983) and using the rain water database collected between 1997 and 1998 near the second sampling location. The atmospheric inputs range from 3 _ 1% for Mg 2§ and Ca 2§ species to 33 _+ 9% for Na § specie. All residual chloride amounts in these atmospheric corrected samples have an anthropogenic origin because there is no evaporitic rock on the Loire watershed. The atmospheric corrected samples were plotted in X vs CIdiagrams (where X represents other major elements) in order to determine rock weathering and anthropogenic activities signatures in the dissolved