The geochemical cycling of stable Pb, 210Pb, and 210Po in seasonally anoxic Lake Sammamish, Washington, USA

Abstract

The geochemical processes controlling the behavior of stable Pb, 210Pb, and 210po in seasonally anoxic Lake Sammamish, Washington were identified from water column distributions and box model calculations. Total (sum of dissolved and particulate) inventories of stable Pb, 210Pb, and 210Po increased in the whole lake during the latter part of the oxic stage of the lake and were attributed to diffusion from sediments. Large decreases in the total inventories of these elements occurred during the transition from oxic to anoxic conditions, and the lowest inventories were observed during the sulfidic stage of stratification. The cycling of stable Pb and 210Pb during oxic periods appeared to be linked to Fe cycling while 210Po cycling was more closely linked to the cycling of Mn. The behavior of stable and radioactive Pb and, possibly, 210Po during anoxia was influenced by sulfur cycling. Thermodynamic calculations indicated that dissolved Pb concentrations might be controlled by PbS precipitation during anoxia. Flux balance calculations during stratification indicated that atmospheric deposition was the major source of both 210Pb and 210Po to the lake and fluvial input was more important for 210Po than for 210Pb. 210Pb and 210Po supplied by atmospheric deposition were scavenged and removed by sedimentation from the epilimnion. Residence times with respect to scavenging and sedimentation in the epilimnion were 1–3 and 617 days, respectively, for 210Pb and 9–22 and 9–26 days for 210Po. Model results in the hypolimnion indicated that at certain times the radionuclides diffused out of the sediments and at other times into the sediments. Residence times with respect to sedimentation of particulate nuclides in the hypolimnion were 2–124 days for 210Pb and 8–48 days for 210Po. The behavior of 210Pb and 210Po in Lake Sammamish was compared to that in B~' kford Reservoir, Massachusetts. Although the comparison indicated similarities (e.g., links to Fe and Mn cycling, larger scavenging rates for 210Pb than for 210Po), the major differences between the systems were that atmospheric inputs were greater than fluvial inputs, activities of the radionuclides were lower, and the presence of sulfide in the hypolimnion during anoxia affected the cycling of stable Pb, 210Pb, and 210Po in Lake Sammamish.