A cadmium budget for the Lot-Garonne fluvial system (France)
Summary (2 min read)
Introduction
- The Lot-Garonne-Gironde fluvial system can be considered as a natural model for pollution in non-ferrous metals such as Cd, Zn, Ba, As (Roux & Simonet, 1987).
- As oysters are very sensitive to this pollution, the Marennes-Oleron oyster farming located ca. 30 km to the north of the mouth of the Gironde estuary is considered at risk (Jouanneau et al., 1993).
- This plant was founded in 1840 and is located near Decazeville, a town on the RiouMort that tributes to the Lot river ca. 200 km upstream from the Lot-Garonne confluence.
- Several large hydroelectric dams which were built from 1945 to 1960 in the upper reaches of the Lot valley, together with the 62 smaller storage structures located downstream from the Riou-Mort confluence, decrease the magnitude of winter and spring floods (Massio, 1976).
Sampling and methods
- Because the Lot river is the major polluting tributary of the Garonne, and then of the Gironde, it was necessary to obtain a representative estimation of its total suspended sediment (TSS) fluxes.
- Samples from Temple (Lot) were collected automatically using a Buhler PB-MOS sampler, whilst samples from the five remaining sites were collected manually.
- At both sites, these tests showed that variations in TSS concentrations due to depth and distance from the bank did not exceed 6% and 2.3% during flood and base flows, respectively.
- Each sample was left on a hotplate until the residue was completely dissolved and then brought to 5 ml in a volumetric flask using double-deionized water.
- The results of the cadmium analysis for 79 suspended sediment and 99 water samples collected during year 1996 are reported in Tables 3 and 4.
Results
- River discharge, total suspended sediment (TSS) and cadmium concentrations are the major parameters for the estimation of dissolved and particulate matter fluxes.
- Note that the higher TSS concentrations correspond to the higher river water flows, as previously recorded for numerous rivers (Meybeck, 1992).
Discussion
- As the frequency of TSS sampling is high, the quantification of the annual TSS fluxes (FT SS) in the Lot-Garonne system is based on the assumption that the TSS concentration remained constant between two consecutive samples (i.e. day after day).
- Considering the errors due to the sampling and analytical methods and those deriving from the Qriv determination, the precision is about 15% for theFTSS at Temple (Lot), Port Pascau and La Reole .
- As the daily mean TSS fluxes are well known, the annual particulate cadmium fluxes can be estimated from the assumption of the constant concentration given by Meybeck et al. (1994) as follows: = 63651 FTSS .
- The results calculated for 1996 show that about 48% of cadmium flux through the Garonne is contributed by the Lot river.
- This result is comparable to others obtained since 1990 (Lapaquellerie et al., 1996).
Sources of cadmium
- The annual atmospheric cadmium flux (1994) was estimated by determining the cadmium concentration of rain water collected by two rain gauges installed at Temple (downstream Lot) and at Castelnau Lassout (upstream Lot) .
- These results are comparable to those obtained from two short surveys in February and June 1994 made by the “Agence de l’Eau Adour-Garonne” (1995).
- Furthermore, the authors note that only about 20% of the cadmium is transported by the Riou-Mort river in particulate forms.
- Using a corrected temperature diffusion coefficient of 3.3 10−6 cm2 s−1 for cadmium (Lerman, 1979), dissolved cadmium diffusion fluxes of 25 10−6 and 9.2 10−6 ng cm−2 s−1 were computed from the concentration gradients obtained between the interstitial and the deep water at Cajarc and Temple, respectively.
Cadmium mass balance
- This result can be tested by a simple model that shows the change of the cadmium concentrations as a function of dilution from the Riou-Mort to the Gironde estuary.
- The parameters for calculation are presented in Table 6, and the calculated concentrations are compared to those measured in Figure 4.
- This comparison shows that the calculated dissolved cadmium concentrations are higher than those measured from the Riou-Mort to La Réole, whereas the measured concentrations are higher than those calculated in the estuary.
- They are the natural cadmium delivered by the Lot catchment (i.e. 0.44 t yr−1), the cadmium released from the sediment blanketing the Lot river bed (i.e. 0.32 t yr−1), the Riou-Mort cadmium discharge which is the value given by the dilution model (1.11 t yr−1), and the cadmium which is deduced to be removed from solution (4.26 t yr−1).
- This value is clearly comparable to that determined using the box model.
Conclusion
- The annual river discharge for the Lot-Garonne fluvial system during 1996 is representative of the longerterm mean.
- The flux estimates presented in this study are based upon regular high frequency measurement for a large study basin and algoritihms recommended by Meybeck et al. (1994).
- These algorithms are similar to those used for British rivers (Webb et al., 1997).
- Within a distance of 0.5 km from this anthropogenic source, about 97% of dissolved cadmium input is taken up by the suspended solid phase.
- These are between the Riou-Mort and Bouillac on the Lot river, and from La Réole to Bordeaux at the GaronneGironde fresh/salt water interface.
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