Thallium dynamics in the Weser estuary (NW Germany)

Despite its toxicity and versatile geochemistry, not much is known on the distribution, speciation and behaviour of Thallium (Tl) in the marine coastal environment, notably in estuaries. We here contribute to the discussion by presenting dissolved (<0.45 μm) and particulate data of Tl, Fe, Mn, as well as dissolved organic carbon (DOC) and dissolved Si for two transects through the Weser estuary during summer (July 2013) and winter (early March 2014).The results show that both, dissolved Tl (70-130 pM) and DOC (100-1000 μM) are elevated against average river concentrations, show the same distinct seasonality, decrease with increasing salinity and positively deviate from conservative mixing. Compared to that, dissolved Si displays almost conservative mixing with seawater while dissolved Fe and Mn are quickly removed in the estuarine turbidity maximum (ETM) classically located in the low salinity region of the estuary. The data further suggest that both, dissolved Tl and DOC exhibit only little loss at the ETM, and that Tl does not seem to cycle with Fe and Mn as was suggested for other estuaries. Moreover, reactive particulate Tl (as calculated from excess Tl enriched versus the lithogenic background) on the particles amount to only 20-50% of the dissolved Tl concentrations. This is in agreement with earlier studies underlining the low particle reactivity of Tl. However, the strong correlation between dissolved Tl and DOC is new and suggests a combined cycling through the estuary. Albeit being indirect, our observations support earlier suggestions of a possible association of Tl with organic ligands, and that Tl tends to remain in the dissolved phase when entering the ocean.