U-238/Th-234 Tracer Studies of Sediment Resuspension and Horizontal Transport in Nearshore Lake Michigan J. Val Klump 1, James T. Waples 1, Kent A. Orlandini 2, David N. Edgington 1, Kim Weckerly 1, Don Szmania 1, & Richard A. MacKenzie 1 With the collaboration of : Brian Eadie 3, John Robbins 3, Barry Lesht 2, Dave Schwab 3 1 University of Wisconsin-Milwaukee 2 Argonne National Laboratory 3 NOAA GLERL

Th-U disequilibria: application to studies of particle dynamics Basin scale distribution and particle transport: Q: can we track particle movement on a time scale of weeks ? how much mass is available ?? how long do particles remain in the water column ?? can we estimate the direction & rate of horizontal advection ??

1 2 n-1 mean flow Cross-margin transport during a “plume event” n+1 Zone of final deposition shore open lake Plume = new sediment inputs = particle transport Org C < 0.5% Cs-137 = 0 Pb-210 = 0 Org C > 5 % Cs-137 ~ 5 pCi/g Pb-210 ~ 20 pCi/g

Seasonal changes in spatial distributions U 234 Th excess 234 Th derived from water column half life = 24.1 days rapid particle scavenging, deposition, & resuspension T res = I water col. J sediment = [ 234 Th ] part z water  I sed = fCi m -2 fCi m -2 d -1 = days

vehicle sampling sample filtering Sampler mk1

4 5 6 pump Sampling sequence 1 2 3

mass – pool of resuspendible sed depth alongshore

as move from west to east: Inventories  Particle residence times  T res = I water /J sed = I water / I sed

“downstream” enrichment of Th-234 Inventories  Particle residence times 

f = I [ 234 Th ex ] / I [ 238 U] sediment suspended part.dissolved 1 = secular equil w/ 238 U

“Two Box” transport model:  x = distance A1A1A1A1 A2A2A2A2

1. Estimate transport times: I supported = V  U-238 = A  z  U-238 U-238 = 100 fCi L -1 z = 20 meters V 1 (source) ~ 1800 km 2  0.02 ~ 3600 km 3 V 2 (sink) ~ 425 km 2  0.02 ~ 8.5 km 3 V 1 /V 2 = A 1 /A 2 ~ 4.2 May 99: f 1 = 0.88 f 2 = 1.17  t ~ 38 days Aug 99: f 1 = 0.79 f 2 = 1.46  t ~ 23 days ? transport velocity ~ 100 to 150 km  30 days ~ 3 – 5 km/day

2. Estimate of source area: if dist. ~ 100 km & velocity ~ 4 km d -1 (B. Lesht)  25 days ~ one ½ life for f 1 = 0.85 f 2 = 1.45  V 1 /V 2  6 i.e. the longer it takes the greater the source area D. Schwab & D Beletsky

3. Estimate of mass transport required: for 234 Th ex of 50%, i.e. f = 1.5 area of temporary sink ~ 500 km 2 z = 20 meters  excess Th-234 ~ 0.5 Curies ~ 1 x dpm if [Th-234] susp part ~ 35 pCi g -1 ~ 80 dpm g -1  a rapid transport “event”  13 x 10 3 MT ( w/o decay )  25 x 10 3 MT ( one ½ life ) i.e. the longer it takes the more you need  ~ 1% of total susp. particulates in the plume  “half lives” to transport amt ~ total annual deposition ? ~ several years ?? [~ 4-8 yrs]

Sediments in the Lake = the ultimate sink via burial Where are they being buried ? How quickly do materials move from source to sink ? ~ years to decades Chart of sediment thickness

Special thanks to: Dave Lovalvo, the R/V Neeskay NSF-CoOP NOAA COP