Water quality in the Gironde estuary: 1 - data processing, 2 - Synthesis. Contribution to the Gironde estuary white paper

The main problem of the Gironde estuary is a pollution by the cadmium. During this last ten years, the good quality of the water is proved by the analysis of the main physico-chemical and biochemical parameters and bacteria. But since 1989, straight to the Blayais power station, the BOD₅ varied between 2 and 3,5 mg.l^-1 and a diminution of the dissolved oxygen has been observed; it is probably caused by the accumulation of biodegradable organic matter in the maximum turbidity zone during years characterized by low river flows. For average river flow, urban discharges takes part for 20 % in total fluxes of BDO₅; in 1991, this part attempts 35 %. During the last 7 years, the pluri-annual evolution of the nitrate concentrations measured in the Garonne river shows an increasing of 18 %. This river is situated in conditions of probable eutrophication since some rates of chi a attempt 100 μg.l^-1. (author)