Cadmium transport in the Lot-Garonne River system (France) -: temporal variability and a model for flux estimation

Fluvial cadmium (Cd) inputs into the Gironde estuary via the Lot-Garonne River system were evaluated using nine years of daily measurements of water discharge and total suspended sediment content (TSS) combined with monthly analyses of dissolved and particulate Cd concentrations. This study demonstrates that dissolved and particulate Cd concentrations in the Lot River have strongly decreased in the early 1990s. However, dissolved and particulate Cd concentrations in the Lot River are still higher than those in the Garonne River. In 1992-1998 mean annual Cd concentrations are more or less constant and thus, annual Cd fluxes mainly depend on hydrology. Daily total Cd fluxes range from 0.26 to 966 kg day(-1) in the Lot River and from 0.31 to 1360 kg day(-1) in the Garonne River, downstream their confluence. During dry years, Cd fluxes at La Reole are dominated by inputs from the upstream Garonne River, whereas during wet years they are controlled by inputs from the polluted Lot River watershed. The relation between particulate and dissolved Cd fluxes depends on TSS concentrations, which appear to be mainly controlled by the succession of floods and low waters reflecting short-term basin-scale climate variability. Cadmium concentrations, fluxes and the ratio between the particulate and dissolved Cd fluxes depends on the absolute value and instantaneous evolution (strong relative increase or decrease) of discharge. An empiric model permits deducing intervals of daily dissolved Cd fluxes from discharge, hydrological key situation and related intervals of 'typical' dissolved Cd concentrations. In 1992-1998 the Lot-Garonne River system shows close relations between annual dissolved and particulate Cd fluxes. These relations allow deducing annual particulate Cd fluxes using annual dissolved Cd fluxes obtained from the model. The validation a posteriori shows that our empiric model accurately reproduces the annual Cd fluxes estimated from the field data with uncertainty lower than 25% for dissolved fluxes and lower than 30% for the particulate fluxes. Consequently, the empiric model permits estimating annual dissolved and particulate Cd fluxes in the Lot-Garonne River system, from daily discharge data, without Cd analysis. Cadmium fluxes obtained from the model may be used as approximate values for gross fluvial Cd inputs into the Gironde estuary. (C) 2003 Elsevier B.V. All rights reserved.