Abstract
Lead (Pb) is a metal of special importance because of its long history of commercial and industrial use, global atmospheric contamination accelerated by the use of gasoline additives, and health effects, with children being especially vulnerable. Global atmospheric Pb pollution reached its zenith in the 1970’s, but subsequent impacts on freshwater aquatic systems are poorly understood. Employing metal-free sampling and handling protocols, we show that snowmelt from the Athabasca bituminous sands region is an insignificant source of dissolved Pb to the Athabasca River (AR). Total Pb in the AR is low, and almost entirely in particulate form. Lead in the suspended solids in the AR exactly follows thorium (Th), a conservative lithophile element, and a linear regression of Pb against Th (Pb = 1.6 × Th + 0.0; R2 = 0.99) yields a slope identical to the Pb/Th ratio in the Upper Continental Crust. In the “dissolved” fraction, the Pb/Th ratio is equivalent to that of deep, open ocean seawater; and dominated by colloidal forms. Taken together, these results show that the efforts of recent decades to reduce anthropogenic Pb to the environment have been successful: Pb loading to the river can now be explained predominantly by natural processes, namely erosion plus chemical weathering.
Highlights
Lead in the Athabasca River (AR) is clearly dominated by natural inputs, mainly erosion, to such an extent that an anthropogenic component is difficult to discern
From October 7 to 17, 2014 using acid-cleaned polypropylene (PP) bottles, raw surface water samples were collected from thirteen sites along the main stem Athabasca River (AR) starting upstream of Fort McMurray and travelling downstream a distance of approximately 125 km
Water was collected from five tributary streams draining into the AR and groundwater from five sites (Fig. 1)
Summary
We developed a precise mechanical procedure to recover each polytetrafluoroethylene (PTFE) filter membrane from its plastic housing (Fig. S4) along with an effective protocol to digest the particles recovered (SI). The Pb concentrations in these suspended solids (Figs S5A and B), whether they were recovered from the main stem of the AR (8.6 ± 5.6 μg g−1, n = 13) or the tributary streams (4.7 ± 2.8 μg g−1, n = 5), are similar to the values for sedimentary rocks[20] which constitute the Western Canadian Sedimentary Basin[54] and are similar to the average Pb content of sandstones and limestones (7 μg g−1 Pb)[55] These Pb values in the suspended solids in the AR are similar to the Pb concentrations in the suspended particles (2.5 μg g−1) reported in Siberian rivers draining basalts[56]. In contrast to the AR, the average Pb concentration reported in the suspended solids of world rivers (25–100 μg g−1) is far higher[59,60,61]
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