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Among the long list of contemporary environmental contaminants, lead (Pb) is one of the most problematic potentially toxic trace elements because atmospheric contamination by this metal became a global phenomenon1,2. The geochemical cycle of Pb has been impacted by anthropogenic activities more than any other metal3. The use of leaded gasoline around the world from ~1920 to ~1970 was the last but by far the largest episode of the most significant Pb sources to the environment after smelting and refining of Pb and other non-ferrous metals, coal combustion and cement production4. Environmental impacts have been studied extensively, and evidence of Pb in both polar regions suggested no area on the earth’s surface has been unaffected by anthropogenic Pb1. However, the phase-out and eventual elimination of leaded gasoline in North America and Europe since 1970 and 1980, respectively has precipitated a dramatic decrease in the concentrations of atmospheric Pb4,5. While the fall in air Pb concentrations has been accompanied by a corresponding drop in human blood Pb6, there is no known safe level and blood Pb values are now “seen by society as evidence of its commitment to its own health”7.
While declining air Pb concentrations have been documented in environmental archives affected exclusively by atmospheric inputs such as polar snow and ice as well as ombrotrophic (i.e. rain-fed) peat bogs8,9,10 the consequences for surface waters have received less attention. Anthropogenic Pb inputs to global oceans were documented by Clair Patterson11 and declines in Pb concentrations in surface seawater subsequent to the phaseout of leaded gasoline have been clearly illustrated for the Atlantic Ocean by Ed Boyle12,13. In respect to freshwaters, pioneering work on Pb in surface and groundwaters in the 1970’s documented the significant contributions of anthropogenic Pb to aquatic ecosystems at that time14. It became clear from these studies that the key to understanding the biological significance of this additional Pb was to distinguish between dissolved and particulate forms: it is the dissolved (<0.45 μm fraction) that is potentially mobile, bioaccessible, bioavailable and toxic14. Coincidentally, almost all of the anthropogenic Pb released to the atmosphere from high temperature combustion processes is in the sub-micron fraction15. Thus, as far as potential impacts of Pb on human and ecosystem health is concerned, in both air and water, it is the sub-micron fraction which has the greatest relevance. However, studies show that within the conventional dissolved fraction (<0.45 μm) not all the Pb is bioavailable and it can be partitioned into colloidal and mainly ionic (truly dissolved) Pb16,17,18. Ionic Pb is the only fraction that can directly impact the aquatic life19. If it were not for the various industrial inputs, almost all of the Pb would be in the particulate fraction, both in the atmosphere14 as well as in water20, with wind and water erosion, respectively, being the dominant sources of Pb to the surficial environment.
The accurate and precise determination of Pb in the dissolved fraction of natural freshwaters, however, poses several considerable challenges. First, the natural abundance of dissolved Pb in freshwaters is extremely low. Pioneering work from the Patterson lab showed long ago that dissolved Pb in remote streams of the Sierra Nevada watershed is commonly below 15 ng/l21. The low concentrations of Pb pose two additional difficulties: 1), the need for tremendous analytical sensitivity, and 2), the grave risks of contamination which are encountered at virtually every stage of the sampling, handling, and analysis process22,23,24,25,26. While some labs have understood and met these challenges, most have not27. In the intervening period there have been many studies of Pb in rivers and lakes28,29,30,31,32,33,34,35,36,37,38,39,40,41, but there are remarkably few studies providing reliable data for the dissolved fraction. Studies employing the clean lab procedures and protocols developed for polar snow and ice as well as sector-field inductively-coupled plasma mass spectrometer (SF-ICP-MS) have reported Pb values in the dissolved fraction of surface waters and groundwaters in the range of a few ng/l or less8,42,43. Any studies of dissolved Pb in surface waters remote from industrial activity, therefore, need to be able to reliably measure Pb well below the part per trillion (ng/L) concentration range. Studies reveal that the dissolved Pb fraction (<0.45 μm) consists of both colloidal and free (ionic) forms, but it is the ionic Pb fraction that is biologically significant for aquatic organisms19. Depending on the relative importance of the colloidal fraction, ionic forms of Pb may be far less abundant than the fraction traditionally defined as “dissolved” i.e., <0.45 μm. The reliable determination of ionic forms of Pb in natural waters, therefore, presents considerable analytical challenges.
To better develop an understanding of the contemporary geochemical cycle of Pb in a major river system and to determine the impact of mining and upgrading activities of the Athabasca bituminous sands on dissolved Pb in the AR, we employ the state-of-the-art metal-free ultra-clean analytical sample collection, handling and analytical procedures developed at the University of Heidelberg for measuring Pb and Pb isotopes in Arctic snow and ice4,44. These ultra-clean sample collection, handling and analytical procedures have been successfully employed at the SWAMP lab, University of Alberta45. We compare total, particulate (>0.45 μm), dissolved (<0.45 μm), colloidal (1 kDa to 0.45 μm) and mainly ionic (<1 kDa) Pb concentrations in the lower reaches of Athabasca River (AR, ~1500 km) in northern Alberta, one of the largest rivers in Canada (http://www.awc-wpac.ca/). The section of the AR selected for study is in the heart of the industrialized region where open pit mines and upgraders of the Athabasca Bituminous Sands (ABS) are found. It was claimed that these commercial activities are a significant source of Pb to air and waters of this region46, so samples were collected upstream, downstream, and within the industrialized zone. The main objective of this study is to quantify the impacts of human activities on Pb dissolved in the river. From October 7 to 17, 2014 water samples and suspended solids were collected from thirteen sites along the main stem of the AR, upstream of Fort McMurray and along a transect ~125 km downstream, as well as from five tributary streams and five groundwater sampling locations (Fig. 1). In addition to the water and suspended solids, in March, 2015 snow samples were also collected from peat bogs in the area to characterize Pb in contemporary atmospheric deposition (Fig. S1 and Table S1).
Dissolved Pb concentrations for the main stem and tributaries from this sampling campaign are taken from Shotyk et al.45, and the rest of the data produced in this study. Sampling, handling and Pb determination in surface and groundwaters are described in detail in the supporting information (SI).