J. Val Klump¹, James T. Waples¹, David N. Edgington¹, Kent A. Orlandini², Kim Weckerly¹, Don Szmania¹, and Richard A. MacKenzie^1
1University of Wisconsin-Milwaukee and ²Argonne National Laboratory

Objectives:

1. Investigate the dynamics of resuspension by quantifying changing particle residence times in both the water and the sediments of the nearshore plume zone of the southern basin
2. Estimate alongshore mass transport rates for these particles
3. Couple transport rates with particle composition changes to estimate transformation rates

Approach:

• Inventories of particulate and dissolved Th-234 (1/2 life = 24 d), a particle reactive, naturally occurring radionuclide, and its parent, U-238, have been measured in the water column and sediments of the nearshore region of southern Lake Michigan from Milwaukee to St. Joe (1998-1999)
• Water column and sediment inventories were also measured on a cross margin transect from 10 to 40 meters off Milwaukee for three 10 day time series (Mar-May 2000), along with simultaneous sediment trap and tripod deployments and CTD casts.
• Suspended sediment concentrations and activities of particulate and dissolved Th-234 were measured on a daily basis from late Jan 2000 to late May 2000 from water taken from the Milwaukee water filtration plant (MWFP) intake at 20 meters, ancillary data on particle size distribution, turbidity and conductivity were collected by the MWFP.
• The basic approach relies on the fact that the parent isotope U-238 behaves as a conservative, non-reactive element in lake water (i.e. is in solution) with a relatively constant concentration and a very long half-life. Its daughter, Th-234, is particle reactive and short-lived (24 day half-life). Rapidly scavenged by particles in the lake, excess Th-234 generated in the overlying water column can be detected on the bottom only if particle settling rates are relatively rapid. Hence the depletion or the enrichment of Th-234 in the water or on the bottom can be used to calculate relevant short term particle residence times. One such type of calculation is shown in Fig. 1.
• Horizontal transport and particle movement may also be estimated from alongshore focusing and enrichment of excess Th-234.

Results:

• In general, particle residence times determined from sediment inventories (calculation shown above) decrease in a counter-clockwise trend, in keeping with the general concept of sediment transport from sources on the western side of the basin to the major depositional sink region off of St. Joe (Fig. 2).
• Temporal variations in nearshore (10 m to 40 m) particle behavior show a familiar bimodal increase in the suspended particle load, with the highest suspended sediment concentrations occurring in March. The "instantaneous" residence time of particles in the water column generally fell between 10 and 20 days, with exceptions occurring during the months of April (64 days) and October (32 days). One hypothesis is that increased residence times may be associated with increased diatom abundance associated with spring and fall phytoplankton bloomes. Particle settling velocities varied by a factor of 3 from 0.7 to 1.8 meters per day. "Annual" resuspension rates for each month were calculated as the product of the particle settling velocity and the suspended sediment concentration. Resuspension rates peaked during the month of March at > 4 g m^-2 d^-1 , nearly two orders of magnitude higher than average net accumulation rate in the lake. (Fig. 3).