Evaluation of the impact produced by the Huelva phosphogypsum stacks on their estuarine environment

Resumo

The manufacturing of phosphoric acid from the chemical attack of the phosphate rock with sulfuric acid (wet process), generates phosphogypsum (PG) as a waste. PG is mainly composed of gypsum, but also contains high levels of pollutants such as heavy metals, acids, natural radionuclides, and some others trace elements. The five phosphoric acid plants included in the chemical industrial complex of Huelva generated around 100 Mt of PG from 1968 to the end of 2010 when its productive activity stopped. These wastes are stored in large piles on the salt marshes of the Tinto River covering an area about 1000 ha, close to the urban centre of Huelva (SW of Spain). The PG stacks were directly disposed on the salt marsh sediments without any type of insulation, and in large proportion of their extension they are currently without any type of cover layer, being exposed to the external agents. This management of PG has generated its weathering and significant emissions of pollutants into their surroundings. The main objective of this Thesis has been to evaluate the impact produced by the Huelva PG stacks on their estuarine environment, focusing on the impact produced into the underlaying salt marsh sediments and the waters of the Tinto estuary. Sediment samples from different depths were taken, which were selected from the seven cores collected at different locations of the stacks, and the pollutant elements and natural radionuclides were analysed. On the other hand, water samples from the Odiel and Tinto rivers and their respective estuaries were collected monthly throughout a year and the activity concentration of U-Th isotopes and 210Po was determined. In addition, the behaviour of heavy metals and natural radionuclides when the acidic leachates from the stacks reach the marine environment was studied by means of mixing experiments. The results of this Thesis showed that the salt marsh sediments act as a “barrier” for the pollutants coming from the PG stacks and the infiltration of leachates is very limited, reaching a maximum depth of about 50 cm. The pollution by 230Th, 226Ra and 210Pb is mainly restricted to the first 20 cm of sediments due to mixing processes, while U-isotopes reach a bit higher depths (up to around 50 cm) in relation to leaching processes due to their lower reactivity and higher concentration in the polluted leachates. These results are of great relevance in the design of the restoration project of the PG stacks, and specifically for the new perimeter channel that is planned to build, suggesting that it should reach a depth of at least 1 m below the base of the stacks, to ensure the complete collection of leachates and prevent their release in the Tinto River estuary. The waters of the Odiel and Tinto rivers showed concentrations of U-Th isotopes and 210Po from one to three orders of magnitude higher than unperturbed continental waters due to the strong impact produced by the acid mine drainage (AMD). The studied radionuclides show a clear seasonal behaviour in these rivers, with three well-differentiated stages during the year. A non-conservative behaviour of the analysed radionuclides in the estuaries was demonstrated due to coprecipitation/adsorption processes produced by the increase of pH. It was also observed the significant radioactive impact produced by the polluted outflows from the PG stacks into the Tinto estuary, mainly during the rainiest months, increasing the concentration of U-isotopes and 210Po in the particulate phase. Finally, the mixing experiments provided relevant conclusions. The acidic PG leachates show heavy metals and natural radionuclides (U-isotopes and 210Po) concentrations from two to three and from four to five orders of magnitude higher, respectively, than unperturbed aquatic systems. Major elements and some heavy metals as Mn, Ni, Cd, As, Sb and Co showed a conservative behaviour during the neutralisation of the leachates with seawater, remaining in the liquid phase, while other ones as Al, Fe, Cr, Zn, Cu and Pb precipitated and/or were adsorbed onto the solid phase. The U-isotopes and 210Po showed a clear non-conservative behaviour probably due to coprecipitation/adsorption processes onto the formed precipitates. These precipitates exhibit heavy metal and natural radionuclide (U-isotopes and 210Po) concentrations from one to three orders of magnitude higher than unperturbed soils, demonstrating the serious environmental impact produced by the stacks into their surroundings.