Role of the dams on the River Lot on two types of pollution:point-sources (cadmium) and non-point sources (phosphorus)

The concentrations of cadmium and bioavailable phosphorus were determined in sediments samples of a large section of the River Lot (a 248-km section of 57 artificial stretches and dams). Phosphorus inputs include an important agricultural watershed (diffuse sources) and main point-sources such as towns. Cadmium pollution is mainly due to an industrial activity in the upper reach: high concentrations existed some years ago, the pollution is now lower. The concentration data related to a lithological cartography of the river bottom (sand/gravels/mud) and of the thickness of soft deposits, provide a longitudinal assessment of Cd and P storage in the whole river. The results show the location in the river where the two elements are associated to the sediment muds which are mainly stored in the largest dams situated in the lower section of the river. The sedimentary amounts represent potential sources (i) of metallic pollution in case of river flood or dam emptying (e.g. 69 tons of Cd in the last lower dam), (ii) for eutrophication processes in the lower course (near 10000 tons of bioavailable-P in the last five dams). The cadmium flux leaving the watershed of river Lot is still important and remains greater than the industrial source (strongly reduced since 1986) of the pollutant located in the upper part of the river.