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Phosphogypsum (PG) is disposed worldwide in large stacks usually placed in coastal zones, as in the case of Huelva (SW of Spain), where around 100 Mt of PG are stored on the salt marshes of the Tinto River estuary covering a surface of about 1000 ha. This management generates the weathering of PG, and due to its high acidity (pH ≈ 2) and pollutant load can provoke significant emissions into their surroundings. In this work were evaluated by laboratory experiments the effects of pH increase in the behaviour of heavy metals and natural radionuclides during the mixing of phosphogypsum leachates with seawater. The acidic phosphogypsum leachates showed concentrations of heavy metals from two to three orders of magnitude higher than natural continental waters, and natural radionuclides (U-isotopes and 210Po) from four to five orders of magnitude higher 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, 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, but while 210Po reached a total removal at pH ≈ 7, U- isotopes after a total removal at pH ≈ 5 returned into the liquid phase due to redissolution/desorption processes at near neutral pH. The formed precipitates, mainly composed by iron phosphates particles, showed heavy metal and natural radionuclide concentrations from one to three orders of magnitude higher than unperturbed soils. All these facts demonstrate the serious environmental impact produced by the PG stacks into their surroundings and the urgency of effective restoration measures. ; This work was partially supported by Spanish Ministry of Economy and Competitiveness [project number, CTM2015-68628-R], the Spanish Ministry of Science, Innovation and Universities [project number, EQC2018-004306-P], the Regional Government of Andalusia [project number, FEDER2018-UHU-1255876] and the Spanish Ministry of Education [grant number FPU15/00646].

A common method to measure radon exhalation rates relies on the accumulation chamber technique. Usually, this approach only considers one-dimensional gas transport within the soil that neglects lateral diffusion. However, this lateral transport could reduce the reliability of the method. In this work, several cylindricalshaped accumulation chambers were built with different heights to test if the insertion depth of the chamber into the soil improves the reliability of the method and, in that case, if it could limit the radon lateral diffusion effects. To check this hypothesis in laboratory, two reference exhalation boxes were manufactured using phosphogypsum from a repository located nearby the city of Huelva, in the southwest of Spain. Laboratory experiments showed that insertion depth had a deep impact in reducing the effective decay constant of the system, extending the interval where the linear fitting can be applied, and consistently obtaining reliable exhalation measurements once a minimum insertion depth is employed. Field experiments carried out in the phosphogypsum repository showed that increasing the insertion depth could reduce the influence of external effects, increasing the repeatability of the method. These experiments provided a method to obtain consistent radon exhalation measurements over the phosphogypsum repository. ; This research was supported by the Spanish Ministry of Science, Innovation and Universities, by the project 'Fluxes of radionuclides emitted by the PG piles located at Huelva; assessment of the dispersion, radiological risks and remediation proposals' (Ref.: CTM2015-68628-R) and the Junta de Andalucía Regional Government by the project "Basic processes regulating the fractionation and enrichment of natural radionuclides under Acid Mine Drainage" (Ref.: UHU-1255876). Special thanks are given to Antonio Padilla for his invaluable technical support and know-how.