Transport of U and Th from a river affected by acid mine drainage (AMD) to the Atlantic Ocean

Carlos Ruiz Cánovas¹, Manuel Olías¹, Francisco Macías¹, María Dolores Basallote², Eduardo Navarrete³ and Juan Mantero⁴

¹Department of Earth Sciences & Research Center on Natural Resources, Health and the Environment. University of Huelva, Campus El Carmen, E-21071, Huelva, Spain manuel.olias@dgyp.uhu.es; carlos.ruiz@dgeo.uhu.es, manuel.olias@dgyp.uhu.es, carlos.ruiz@dgeo.uhu.es, francisco.macias@dgeo.uhu.es

²Department of Ecology and Coastal Management, Institute of Marine Sciences of Andalusia, CSIC, E-11510, Puerto Real, Cádiz, Spain mdolores.basallote@csic.es

³Andalusian Environmental and Water Agency, C/ Johan G. Gutenberg, 1 - Isla de la Cartuja, 41092 Seville, Spain  eduardo.navarrete@juntadeandalucia.es

⁴Department of Applied Physics II, ETSA, University of Seville, Av Reina Mercedes 2, 41012 Seville, Spain, manter@us.es

 

This work studies the behavior of Uranium (U) and Thorium (Th) in the Tinto River during the hydrological year 2017/2018. The dissolution of surrounding rocks due to extreme acidity conditions, generated during the oxidation of sulfides, induces the release of high concentrations of U and Th into the water. Maximum dissolved concentrations of 57 μg/L of U and 61 μg/L of Th were determined in this study, coinciding with the first precipitation events of the hydrological year due to the washout of evaporitic salts covering the riverbed and mining areas during the dry period, as well as the transport of sulfide oxidation products. Subsequently, a progressive decrease in concentrations was observed, reaching values close to 5 μg/L by February 2018, which are much higher than those found in freshwaters.

With the arrival of intense rains in March and April 2018, a significant decrease in concentrations (<1 μg/L) occurred, primarily due to dilution. Both elements exhibit quasi-conservative behavior due to the low pH values in the river (2.1-4.2), preventing the precipitation of Al mineral phases and the incorporation of U and Th into them. Although the precipitation of Fe mineral phases at these pH values is intense, the adsorption/coprecipitation processes of U and Th onto these phases seem to be limited by the formation of sulfate complexes (Th(SO₄)₂), ThSO₄²⁺, UO₂SO₄). The transport of U and Th by particulate matter is very limited, although there appears to be a correlation with Al. An increase in concentration of U and Th is observed during the mixing of these acidic waters with ocean waters due mainly to the formation of carbonate complexes which enhances the mobility of U and Th in the estuarine domain.