Sub-part per trillion levels of lead and isotopic profiles in a fjord, using an ultra-clean pumping system

A depth-discrete, ultra-clean sea water sampler was developed by intercepting the sample prior to the pump, in a protected-port pumping system. This development allowed precise sampling and measurements of very low levels of dithizone-extractable (`solubleʹ) Pb, as low as 0.5 ng/kg ( 2 pM), in a stratified fjord in northwestern British Columbia, Canada. Samples collected during the pulse of freshwater input or `freshetʹ conditions of June 1982 were at least three times higher in Pb than those collected in October 1982. By then, the bottom water had been renewed by deep shelf water of oceanic origin and the water column was markedly clear of suspended matter. Isotopically, the data fell into two distinct fields reflecting the composition of their sources or mixing thereof. Surface waters and the entire June 1982 profile appeared to be dominated by the very radiogenic Pb (206Pb/207Pb=1.25–1.29) of the Nass River, whereas the ultra-low Pb water lies clustered on a mixing line between the radiogenic input and some source with a 206Pb/207Pb ratio less than 1.19. This source could either be contamination from the shipʹs aura of gasoline Pb, 206Pb/207Pb=1.143, or contributions from the deep shelf water itself. Deep water samples in the northeastern Pacific about 1000 km to the southwest of the study area, had 206Pb/207Pb ratios of 1.18–1.19 which is in agreement with the presumed end member above. In addition, Pb contamination during sampling was not considered to be a significant factor owing to the consistency of the concentration data, even at levels approaching the quantitation limit. Further evidence was provided by consistency in isotopic data on samples collected near a high-Pb source with an isotopic composition distinctly different from gasoline Pb. For the subsurface, the most likely model to explain the levels of soluble lead and its isotopic composition would be a conservative one in which deep shelf water of 0.5 ng/kg levels, with 206Pb/207Pb 1.185, would have inflowed over the sill past station 2 and then mixed with the 1.5 ng/kg levels of the radiogenic fjord water in a 4:1 proportion to produce the 0.8 ng/kg levels seen at the actively mixed sites, such as station 4. While this pumping system is valuable at delineating mixing in a stratified regime, the practicalities of pumps and tubing diameter limits this sampling approach to no more than 500 m.