SAMPLING METHODS AND APPROACHES USING RADIONUCLIDE TRACERS IN THE STUDY OF SEDIMENT RESUSPENSION AND CROSS MARGIN TRANSPORT IN NEARSHORE OF THE LAURENTIAN GREAT LAKES

Preliminary results from an investigation of the U-238/Th-234 parent/daughter pair in the nearshore environments of Lake Michigan show that short-lived, naturally-occurring radionuclides are useful in determining the time scales of sediment transport processes on the orders of days to weeks. While the long term, depositional sink for some new particles entering the lake, either from rivers or shoreline erosion, has been well documented, the aggregate time scale of the processes by which these particles move from source to sink is not well quantified. These relatively “new” particles may undergo numerous episodes of resuspension and redeposition before making their way through the coastal margin and entering the long term focusing processes active in the depositional basins of the lake. These episodic events can be large in scale and magnitude, resuspending as much or more material than is permanently deposited in the lake on an annual basis. Th-234, a particle reactive radionuclide with a 24 day half-life, would appear to be well suited as a tracer for following rapid particle movement, particle residence times in the overlying water, and alongshore transport processes. The remotely operated vehicle (ROV) deployed technique for collection of surface sediments residing in nondepositional (hard bottom) lakebeds and some preliminary time series results for transient excess Th234 inventories in the nearshore of Lake Michigan are described.