Lead is a toxic heavy metal that can be deposited in bed sediments, usually associated to particulate matter of iron and manganese oxides. Thus, sediments become a route of exposure to Pb for aquatic organisms. Benthic organisms ingest and come in contact to particulate, dissolved and sediment-bound Pb. Lead associated with sediment fractions that exhibit cation-exchange capacity or that are easily reduced is generally more bioavailable than that associated with other fractions. Furthermore, changes in ambient environmental conditions can increase the bioavailability of Pb associated with inorganic solid phases and organic matter (CCME 1999). The bottom sediments in the North Western area of the Río Negro river, Uruguay, are composed of sedimentary rock deposits from the Gondwana continent. The reducing environment and particle size might be responsible of the characteristic water color. The clay fraction is a laminated structure of phylosilicate with SiO4 and alumina groups. The minerals are montmorillonite and illite of 2:1 structure, alternated with Aluminum gibbsite. The sequence of sedimentation which took place under oxidative conditions during the Carboniferous Period and started with extended deposits of tillits, associated to varvic lutite, sand and sea level deposits, indicative of circumpolar conditions. Permian Period sediments, which were placed under reductive conditions, are rich in fossil vegetation and bituminous lutite, which composes their carbonous fraction.