Uranium and thorium isotopes in the rivers of the Amazonian basin: hydrology and weathering processes

• Published in: Hydrological processes 2003-01, Vol.17 (1), p.17-31
• Main Authors: Marques Jr, Aguinaldo N., Al-Gharib, Iyad, Bernat, Michel, Fernex, François
• Format: Article
• Language: English
• Subjects: 210Pb; Amazon river; Amazonian basin; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Hydrology; Hydrology. Hydrogeology; Isotope geochemistry; Isotope geochemistry. Geochronology; isotopes; Mineralogy; Silicates; South America, Amazonia; thorium; uranium-series; Water geochemistry; weathering
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.1112

Two expeditions (October 1989 and May 1992) were carried out to two points of the main Amazon River channel and four tributaries. The Solimões and Madeira rivers, taking their origin in the Andes, are whitewater rivers. The Negro River is a typical acid, blackwater river. The Trombetas River flows through bauxite‐rich areas, and is characterized by low concentrations of dissolved humic substances. The 238U, 234U, 232Th and 230Th activities were recorded from dissolved, suspended particulate phases and river bank sediments. The latter were analysed for their 226Ra, 228Ra and 210Pb contents, and also subjected to leaching with 0·2 M hydroxylamine–hydrochloride solution to determine the concentrations of radionuclides bound to amorphous Fe hydroxides and Mn oxides and hydroxides. The dissolved U average concentration in the Amazon system is ten times lower than the mean world river concentration. The uranium concentration observed at Óbidos in the lower Amazon (0·095 µg L−1), where the U content in the river bank sediments and suspended matter is lowest, suggests U release from the solid phase during river transport. About 485 t of U are transported annually to the Amazon delta area in dissolved form, and 1943 t bound to suspended particulate matter. Total U and Th concentrations in the river bank sediments ranged from 1·59 to 7·14 µg g−1 and from 6·74 to 32 µg g−1, respectively. The highest concentrations were observed in the Trombetas River. The proportion extracted by means of the hydroxylamine solution (HL) was relatively high for U in the Trombetas river bank sediment (31%) and for Th in the Solimões sediment (30%). According to the alpha recoil effects, the 234U/238U activity ratios of the Andean river waters and downstream Amazon water (Óbidos) were >1, but were