Metal pollution in estuaries represents a serious environmental challenge, especially in areas affected by
industrial and mining activities. This study investigates the metal partitioning and availability of rare
earth elements (REE), Y and other trace metals (Ag, Tl, U and Cs) in the Ria of Huelva estuary (SW Spain),
strongly affected by mining and industrial activities. A 30 h monitoring campaign was performed collecting
periodic water samples and deploying diffusive gradient in thin films (DGTs) devices to determine
the main factors controlling metal availability. The dissolved concentrations of U (3118e3952 ng/L) and
Cs (284e392 ng/L) were in the same order of magnitude than those reported in other estuaries and
coastal waters worldwide, however, REE (26e380 ng/L), Y (15e109 ng/L), Ag (14e307 ng/L) and Tl (29
e631 ng/L) concentrations exceeded these values for the same salinities. Unlike most metals (i.e. Ag, Tl, U,
Cs), which were mainly found in the dissolved form (87e100% of total), REE and Y were found in the
particulate phase (22e36% of total). Metal lability was mainly related to the concentration in the water
column following this order: U>REE>Y>Ag>Tl. A similar binding mechanism was observed for Tl and Cd,
due to its chemical affinity. This relationship between chemical properties and absorption by DGT-resin
was also observed for REE (and Y), Rb and Sr, which may cause bioaccumulation upon persistent
exposure, considering the ability of these metals to cross the biological membranes. The lability of metals
predicted by geochemical codes did not coincide with absorption of labile metals by DGTs due probably
to the instability of complexes in contact with the DGT membranes, the inability of metals to form
thermodynamically stable complexes or the absorption of colloids. From this work it can be concluded
that DGT passive sampling should complement traditional sampling to monitor metal availability in
aquatic environments.
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