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 intercepti ng the sample prior to the pump, in a protected-port pumping system. This d evelopment allowed precise:sampling and: measurements of very low levels of dithizone-extractable ('soluble') Pb, as low as 0.5 ng/kg (similar to 2 pM ), in a stratified fjord in northwestern British Columbia, Canada. Samples collected during the pulse of freshwater input or 'freshet' conditions of J une 1982 were at least three times higher in Pb than those collected in Oct ober 1982. By then, the bottom water had been renewed by deep shelf water o f oceanic origin and the water column was markedly clear of suspended matte r. Isotopically, the data fell into two distinct fields reflecting the comp osition of their sources or mixing thereof. Surface waters and the entire J une 1982 profile appeared to be dominated by the very radiogenic Pb (Pb-206 /Pb-207 = 1.25-1.29) of the Nass River, whereas the ultra-low ph water lies clustered, on a mixing line between the radiogenic input and some source-w ith a Pb-206/Pb-207 ratio less than 1.19. This source could either be conta mination from the ship's aura of gasoline Pb, Pb-206/Pb-207 = 1.143, or con tributions from the deep shelf water itself. Deep water samples in the nort heastern Pacific: about 1000 km to the southwest of the study area, had Pb- 206/Pb-207 ratios of similar to 1.18-1.19 which is in agreement with the pr esumed end member above. In addition, Pb contamination during sampling was not considered to be a significant factor owing to the consistency of the c oncentration data, even at levels approaching the quantitation limit. Furth er evidence was provided by consistency in isotopic data on samples collect ed 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 similar to 0.5 ng/kg levels, with Pb-206/ Pb-207 similar to 1.185, would have inflowed over the sill past station 2 a nd then mixed with the similar to 1.5 ng/kg levels of the radiogenic fjord water in a 4:1 proportion to produce the similar to 0.8 ng/kg levels seen a t 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 similar to 500 m. (C) 1999 Elsevier Science B.V. All rights reserved.