CONTROLS ON DIAGENETIC MOBILIZATION OF HEAVY METALS IN MARINE AND LACUSTRINE SEDIMENT CORES, SOUTH PUGET SOUND, WASHINGTON

Tacoma area sediments and soils contain elevated levels of As, Pb, Cu, Zn, Cd and other metals as a result of emissions from the ASARCO smelter, which operated in Tacoma from 1890-1985. Numerous surveys, primarily of soil, have delineated the geographic extent of this contamination. However, less/little is known regarding variation of contaminant levels with depth at individual sites. The present study utilizes sediment cores from seven sites (four lacustrine and three marine) to investigate three main questions: (1) To what extent are heavy metals redistributed within the sediment column after deposition? (2) Are there differences in metal mobility between lacustrine and marine settings? (3) Is it possible to use heavy metal concentration profiles to correlate cores from different sites? Cores up to 4.5 m in length were collected with piston or gravity corers, sub-sampled at 1 cm intervals, subjected to 20% HNO3 leach, and analyzed for Fe, Mn and trace metals (by ICP-OES) and organic content (by LOI). Cores from Lake Louise and Gravelly Lake, located upwind from the smelter, and Lake Killarney, located downwind, all show elevated levels of metals within the top ~20 cm. In all lakes, metals co-vary closely with each other and with Fe (and to a lesser extent, Mn) implying an important role for Fe-Mn coatings in sequestering the metals. In contrast, cores collected from Quartermaster Harbor, a marine site downwind from the smelter, show similar up-section increases in metals, but lower maximum metal concentrations and weaker correlation between Fe, Mn and trace metal levels. Previous Pb-210 dating suggests a deposition rate of 0.2 cm/year in Quartermaster Harbor, implying that the entire emission history of the smelter is captured in the top ~30 cm of the core. We attribute the higher near-surface metal concentrations at the upwind sites to remobilization and upward transport of metals in lacustrine systems. Factors that may facilitate remobilization in lacustrine settings compared to marine settings include more reducing conditions within the sediment column and lower pH.