Sediment Resuspension and Transport: Radionuclide Tracer Studies
J. Val Klump1, James T. Waples1, David N. Edgington1, Kent A. Orlandini2, Kim Weckerly1, Don Szmania1, and Richard A. MacKenzie1
1University of Wisconsin-Milwaukee and 2Argonne National Laboratory
- 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
- Estimate alongshore mass transport rates for these particles
- Couple transport rates with particle composition changes to estimate transformation rates
- 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.
- 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
- 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).
Eadie (sediment trapping); Robbins (transport into a depositional zone); Lesht (tripod measurements of
hydrodynamics); Hornbuckle (HOC resuspension); Schwab (modeling); Milwaukee Water Filtration Plant