Dissolved load of the Loire River: chemical and isotopic characterization

The Loire River, with one of the largest watersheds in France, has been mon itored just outside the city of Orleans since 1994. Physico-chemical parame ters and major and trace elements were measured between 2-day and 1-week in tervals according to the river flow. The sampling site represents 34% of th e total Loire watershed with 76% silicate rocks and 24% carbonate rocks, Elements are transported mainly in the dissolved phase with the ratio of to tal dissolved salts (TDS) to suspended matter (SM) ranging between 1.6 and 17.4. Chemical weathering of rocks and soils are thus the dominant mechanis ms in the Loire waters composition. The highest TDS/SM ratios are due to di ssolved anthropogenic inputs. The database shows no link between NO3- conte nt and river flow. The Na+, K+, Mg2+, SO42-, and Cl- concentrations are see n to decrease with increasing discharge, in agreement with a mixing process involving at least two components: the first component (during low flow) i s concentrated and may be related with input from the groundwater and sewag e station water, the second component (during high flow) is more dilute and is in agreement with bedrock weathering and rainwater inputs. A geochemica l behaviour pattern is also observed for HCO3- and Ca2+ species, their conc entrations increase with increasing discharge up to 300 m(3)/s, after which , they decrease with increasing discharge. The Sr isotopic composition of t he dissolved load is controlled by at least five components - a series of n atural components represented by (a) waters draining the silicate and carbo nate bedrock, (b) groundwater, and (c) rainwaters, and two kinds of anthrop ogenic components. The aim of this study is to describe the mixing model in order to estimate the contribution of each component. Finally, specific export rates in the u pper Loire watershed were evaluated close to 12 t year(-1) km(-1) for the s ilicate rate and 47 t year(-1) km(-2) for the carbonate rate. (C) 2000 Else vier Science B.V. All rights reserved.