A cadmium budget for the Lot-Garonne fluvial system (France)

Routine measurements of river discharge and total suspended sediment concen tration (TSS) are combined with regular analyses for particulate and dissol ved cadmium to produce a box model that allows us to propose a cadmium mass balance for the Lot-Garonne man-influenced river system (8400 km(2)). Near ly half the cadmium in the Garonne river is supplied by the tributary Lot r iver. Cadmium input onto the Lot river comprises wet deposition from the at mosphere, molecular diffusion at the sediment-water interface, surface-wate r runoff and discharge from the leaching of waste at a zinc refining plant. Approximately 85% of the cadmium in the Lot river is derived from anthropo genic origin. Cadmium in the industrial discharge is 80% dissolved and 20% in the particulate phase (4.2 and 1.1 t yr(-1), respectively). Total inputs are estimated at 4.81 t yr(-1) and 1.54 t yr(-1) for the dissolved cadmium and for the particulate phase, respectively. Budgeting estimates an output onto the Garonne river of 0.54 t yr(-1) for the dissolved cadmium (about 8 %) and 6.13 t yr(-1) for the particulate cadmium (about 92%) indicating tha t downstream sediment-associated cadmium fluxes are enhanced by the 4.27 t yr(-1) removed from solution and the 0.32 t yr(-1) remobilized by the erosi on of sediment blanketing the Lot river bed. These figures are found to be comparable with those generated by a dilution model which suggests that 97% of dissolved cadmium is taken up by the particulate phase over 0.5 km down stream from the primary anthropogenic source.