<p>The Douro River lower basin water quality was studied regarding its concentration on 18 trace elements (Be, Al, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, Ba, Tl, Pb and U, measured by ICP-MS). Other physicochemical parameters, such as pH, conductivity, dissolved oxygen and water temperature were also determined in situ.</p>
<p>To take into account the expected spatio-temporal changes and to look for anthropogenic influences on trace element levels, samples (n=88) were collected at 11 sampling sites in four sampling campaigns (October 2007; January, March and July 2008), in both low and high tides, in order to evaluate spatial, seasonal and tidal changes.</p>
<p>A multivariate approach – principal component analysis – was used to investigate interelement correlations and the variability observed in the different data sets.</p>
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<p>According to aquatic life limits (CCME, 2011), the quality of Douro River water was found acceptable. Except for DO, where a significant number of samples presented levels below 5.0 mg l<sup>-1</sup>, Cr(VI) (mean±sd = 3.09±1.54; median = 2.82 ppb) and Se (10.9±10.1; 7.74 ppb) all the other parameters measured fell well within acceptable limits. Occasional high levels were found for most trace elements, reflecting sporadic and local inputs. Important spatial differences in trace element levels were also found. Except for Be, Al and Mn, most metals tended to increase in the downstream direction. Some trace elements related with agriculture practices (Zn, Cu and Ni) were higher in samples collected on sampling sites located at the middle part of the studied area and were highly correlated, reflecting an eventual common source. Important seasonal differences in trace element levels were also observed, e.g., October samples were generally characterized by higher Ni, Cu, Zn and Pb levels.</p>
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