Mercury distribution in Douro estuary (Portugal)

Determinations of dissolved reactive and total dissolved mercury, particulate and sedimentary mercury, dissolved organic carbon (DOC), particulate organic carbon (POC) and suspended particulate matter (SPM) have been made in the estuary of river Douro, in northern Portugal. The estuary was stratified by salinity along most of its length, it had low concentrations of SPM, typically <20 mg dm−3, and concentrations of DOC in the range <1.0–1.8 mg dm−3. The surface waters had a maximum dissolved concentration of reactive mercury of about 10 ng dm−3, whereas for the more saline bottom waters it was about 65 ng dm−3. The surface waters had maximum concentrations of total suspended particulate mercury of 7 μg g−1 and the bottom waters were always <1 μg g−1. Concentrations of mercury in sediments was low and in the range from 0.06 to 0.18 μg g−1. The transport of mercury in surface waters was mainly associated with organic-rich particulate matter, while in bottom waters the dissolved phase transport of mercury is more important. Lower particulate organic matter, formation of chlorocomplexes in more saline waters and eventually the presence of colloids appear to explain the difference of mercury partitioning in Douro estuarine waters.