『Abstract
Heavy metal (Zn, Cd, Cu and Pb) mass balances in the Lot-Garonne
fluvial system have been established for 1999 and 2000. The mean
annual discharges of these years are close to the mean discharge
of the previous decade. The estimated annual dissolved and particulate
fluxes in this model watershed integrated daily input from diffuse
and point sources, diffusive fluxes at the water/sediment interface,
changes in the dissolved-particulate partition and changes in
sediment stock. Cadmium, Zn, Cu and Pb entering the Gironde estuary
via the Garonne River (11-14 t a-1 of Cd; 1330-1450
t a-1 of Zn; 126-214 t a-1 of Cu and 127-155
t a-1 of Pb) are mainly transported in the particulate
phase and the major part (i.e. 〜74 to 96％ for Cd, 〜60％ for Zn,
〜50 to 60％ for Cu and 〜80％ for Pb) is transported by the Lot River.
The main anthropogenic heavy metal point source is located in
a small upstream watershed (Riou-Mort River) accounting for at
least 47％(Cd), 〜20％(Zn), 〜4％(Cu) and 〜7 to 9％(Pb) of the total
heavy metal inputs into the Garonne River, although it contributes
only 1％ of the discharge. Mass balances for 1999 suggest that
under mean annual hydrologic conditions on the basin scale, the
heavy metal budget of the Lot-Garonne fluvial system is balanced
and that the stocks of Cd [200 t; Environ. Tech. 16 (1995) 1145]
and Zn in the Lot River sediment are constant under mean discharge
conditions. Heavy metal input by molecular diffusion at the sediment
surface represents an important component of dissolved metal inputs
into the system (e.g. 30％ for Cu). Except for Cu, these dissolved
inputs are totally removed from solution by SPM. Based on the
generally constant Zn/Cd (〜50) concentration ratio in sediment
cores from the polluted Lot River reaches and the sediment stock
of Cd [200 t; Environ.Tech. 16 (1995) 1145], the present day Zn
stock in the Lot River sediments has been estimated at about 10,000
t. In addition to the mobilization of river-bed sediment and associated
heavy metals by intense floods, local human activities, including
river-bed f\dredging, may strongly modify the heavy metal budget
of the river system. In 2000, the dredging-related remobilization
of polluted Lot River sediment released 2-6 t Cd. This additional
Cd point source was estimated to account for 15-43％ of the gross
inputs into the Gironde Estuary.』
1. Introduction
2. Methodology
2.1. Sampling sites and methods
2.2. Sample analyses
2.3. Flux calculations
3. Results and discussion
3.1. Water discharge and SPM
3.2. Heavy metal concentrations
3.2.1. River water
3.2.2. Suspended particulate matter
3.2.3. River sediment
3.3. Discharge, SPM and heavy metal budget of the Lot River (1999-2000)
3.3.1. Watershed background inputs
3.3.2. Point source (Riou-Mort watershed) inputs
3.3.3. Diffusive heavy metal fluxes from the sediment
3.3.4. Lot River internal reactivity
3.3.5. Contribution to gross fluxes into the Gironde Estuary
4. Conclusions
Acknowledgements
References