BEHAVIOR 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 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 four sites (two lacustrine and two 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 (62-98 cm in length) were collected with gravity or piston corers, sub-sampled at 1 cm intervals, subjected to 20% HNO₃ leach, and analyzed for Pb, Cu, Mn, and Fe (by AAS) and organic content (by LOI). Cores from American Lake, located upwind from the smelter, show elevated levels of metals within the top ~45 cm. The extent of this enrichment is described with an enrichment factor (EF = [metal]_max/[metal]_background). For Pb at American Lake, EF = 30 with Pb_max = 810 ppm. In both lakes studied, Cu and Pb 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 from Quartermaster Harbor, a marine site downwind from the smelter show similar up-section increases in Cu, Pb but with lower EF (14) and Pb_max (483 ppm). In addition, there is no apparent correlation between the behavior of Fe or Mn and heavy metal levels in this core. We attribute the higher near-surface metal concentrations and greater EF at the upwind site to remobilization and upward transport of metals in the lacustrine system. Factors that may facilitate remobilization in lacustrine settings compared to marine settings include more reducing conditions within the sediment column and lower pH.