Environ. Sci. Technol. 1985, 19, 854-861
Received for review December 14,1984. Accepted March 21,1985. Although the research described in this article has been funded wholly or in part by the U.S. Environmental Protection Agency through Contract 68-02-4033 to Northrop Services, I m -
Environmental Sciences, it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
Chlorinated Hydrocarbon Cycling in the Benthic Nepheloid Layer of Lake Superior Joel E. Baker and Steven J. Elsenrelch" Environmental Engineering Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, - Program, Minnesota 55455
Thomas C. Johnson and Barbara M. Halfman Duke University Marine Laboratory, Pivers Island, Beaufort, North Carolina 285 16
rn The dynamics and composition of the benthic nepheloid layer (BNL) in western Lake Superior were studied with respect to the cycling of trace hydrophobic organic compounds. The BNL was enriched in t-PCB, p,p'-DDE, and hexachlorobenzene (HCB) but deficient in dissolved organic carbon relative to surface waters. Resuspension events in midsummer resulted in a 50% increase in the PCB burden in the water column. Seasonal cycling of PCB congeners was strongly dependent on their degree of chlorination, with heavier chlorinated congeners lost from the water column (tlIz = 17-28 days at two sites). Steady-state t-PCB concentrations in the BNL were maintained over the summer by transport of lighter chlorinated congeners from the underlying sediments. DDE is an effective tracer of sediment resuspension in Lake Superior. Recycling of trace organic pollutants in the BNL serves to increase their residence times in the lake. Introduction Studies of the behavior of trace organic pollutants (e.g., PCBs, DDT, etc.) in large lakes historically have shown that the atmosphere has been a net source and the sediments a net sink (1-4). As atmospheric and riverine inputs of PCBs and DDT to the Great Lakes decrease (3,5-7), their hydrospheric burdens are expected also to decrease. However, seasonally driven processes of lake mixing and sediment resuspension (8,9) may increase residence times of such compounds in the water column. Episodic fluctuations in contaminant concentrations due especially to sediment resuspension may also have important short-term consequences such as exposure of contaminants to spawning fish. The response time of fish, water, and the atmosphere to diminished inputs is remarkably fast, however, with residence times on the order of a few years in the Great Lakes ecosystem ( 2 , 3 ) .The objectives of the present study were (1)to quantify the seasonal variability in concentration, speciation, and burden of selected hydrophobic organic compounds in western Lake Superior, (2) to determine the influence that sediment transport and resuspension exerts on the internal cycling of hydrophobic chlorinated hydrocarbons, and (3) to examine the water column's response rate to these seasonal changes. Although the benthic nepheloid layer (BNL) is a wellcharacterized feature of oceanic systems (e.g., see ref 10-13), little is known of its characteristics in freshwater environments. It is likely, however, that formation and maintenance of the BNL play significant roles in recycling sedimentary contaminants. The nepheloid layer is a zone 854
Environ. Sci. Technol., Vol. 19,
No. 9, 1985
extending above the bottom sediments with measurable gradients of particles, heat, and chemical species. Particles in this region are small (
