Master Thesis

Potential mobilization of pollutants from mine-water affected coastal marsh soils with sea level rise: insights from the RíoTinto and Río Odiel estuarine system

Erica Oldani (10/2019-01/2021)

Support: Britta Planer-Friedrich, Andrea Colina Blanco

As a result of global Sea Level Rise (SLR), by the year 2050, exceptional tidal surges are predicted to impact the coastal marshlands of the Río Tinto and Río Odiel estuary in Andalusia, Spain. Risk assessment of the consequences of inundation-induced changes in water and soil geochemistry (more reducing conditions, higher salinity) is especially interesting due to the large metal(loid) load stored in these Acid Mine Drainage (AMD) affected marshlands. The hypothesis of enhanced desorption and remobilization of mining-associated heavy metals (Cu, Zn, Pb, Cd), toxic metalloids (As), rare earth elements (La), and radionuclides (U) was tested by anoxically incubating layers of seven marsh soil cores with seawater (100% SW) or brackish water (20, 50, 80% SW) over 65 and 85 days, respectively. Possible short-term effects of a permanent 100% SW inundation scenario were evaluated by analysing the solutions after incubation, while mid-term effects were estimated by studying the hypothetical metal(loid)s binding and fraction (re)distribution in the soils before and after flooding using a Sequential Extraction Procedure (SEP). Results showed an acidification of the water phase (pH change from 8.2 to 3.2), and a net release of dissolved iron (up to 1.8 g L-1 ) and toxic metal(loid)s exceeding irrigation and coastal waters quality standards adopted at national and European level, reaching the following maximum concentrations: Cu (115 mg L -1 ) > Zn (22 mg L-1 ) > As (1 mg L-1 ) > Pb (380 µg L-1 ) > Cd (190 µg L-1 ) > La (75 µg L-1 ) > U (50 µg L-1 ). Hotspots of metal(loid)s pollution were identified within the estuary area. The lower Tinto domain was found critical for remobilization of toxic As, Cd and U, and the uppermost Tinto domain for Zn, Pb, La, Al, and Fe. Desorption was enhanced by increasing seawater-to-river water ratios for all elements, except for As, which showed an opposite behaviour. Comparison between mobilized loads and the original soil exchangeable metal pool (as determined by SEP), indicated generally low mobilized percentages (<60%), with Cd and Pb showing the highest (up to 100%) and the lowest (<0.5%) mobilized percentages, respectively. Both correlation and hierarchical clustering analysis, suggested competitive desorption and reductive dissolution of Mn, Fe and P minerals as main geochemical drivers for mobilization, while metal sulfur mineral re-precipitation is expected to limit the net mobilization. Soil pH significantly correlated to aqueous pH after inundation and could therefore be an easy-to-measure predictor for floodwater acidification and net metal(loid) mobilization in future risk assessment studies.