U-238/Th-234 Tracer Studies of Sediment Resuspension and Horizontal Transport in Nearshore Lake Michigan J. Val Klump1, James T. Waples1, Kent A. Orlandini2, David N. Edgington1, Kim Weckerly1, Don Szmania1, &

Richard A. MacKenzie1

With the collaboration of:Brian Eadie3, John Robbins3, Barry Lesht2,Dave Schwab3

1University of Wisconsin-Milwaukee 2Argonne National Laboratory 3NOAA GLERL

Th-U disequilibria: application to studies of particle dynamics

Basin scale distribution and particle transport: 1998-99

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 finaldeposition

shore

open lake

Plume

= new sediment inputs

= particle transport

Org C < 0.5%Cs-137 = 0Pb-210 = 0

Org C > 5 %Cs-137 ~ 5 pCi/gPb-210 ~ 20 pCi/g

Seasonal changes in spatial distributions -- 1998-99

238U 234Th

excess 234Thderived from water column

half life = 24.1 days

rapid particle scavenging, deposition, & resuspension

Tres = I water col.

J sediment

= [ 234Th ]part z water

Ised

=fCi m-2

fCi m-2 d-1= days

vehicle

sampling

samplefiltering

Sampler mk1

4

5

6

pump

Sampling sequence

1

2

3

sediment inventory vs. depth

resuspendible sediment inventory, g/m2

0 10000 20000 30000 40000

dept

h, m

eter

s

0

10

20

30

40

50

60

Sep 98Apr 99 May 99Aug 99

mass – pool of resuspendible sed

0 10 20 30 40 50

mas

s in

vent

orie

s, g

/m2

0

10000

20000

30000

40000

west ------> east

alongshore sed inventories

depth alongshore

0 10 20 30 40 50

Th-

234

inve

ntor

ies,

fCi/c

m2

0

500

1000

1500

2000

2500

3000

3500

0 10 20 30 40 50

part

icle

res

iden

ce ti

me,

day

s

0

200

400

600

800

as move from west to east:

Inventories Particle residence times

Tres = Iwater/Jsed = Iwater/ Ised

Box ---> "downstream"

1 2 3 4

Th

-23

4 In

v.,

fCi/c

m2

0

100

200

300

400

500

Box ---> "downstream"

1 2 3 4

Pa

rtic

le r

esi

de

nce

tim

e,

da

ys

0

50

100

150

200

“downstream” enrichment of Th-234

Inventories Particle residence times

"downstream" enrichment of exTh-234

Box ---> "downstream"

1 2 3 4

I e

xT

h-2

34/I U

-23

8

0

1

2

3

post plume

f = I[234Thex]/ I[238U]

Box ---> "downstream"

1 2 3 4

focu

sin

g f

acto

r

0

1

2

3

Box ---> "downstream"

1 2 3 4

focu

sin

g f

acto

r

0

1

2

3

Box ---> "downstream"

1 2 3 4

focu

sin

g f

acto

r

0

1

2

3

sediment suspended part. dissolved

1 = secular equil w/ 238U

“Two Box” transport model:

x = distanceAA11 AA22

f n = I Th-234 / I U238water

An = area of nth box

Vn = volume of nth box zn = depth of nth box U238 = U238water = 100 fCi/ L

Th–234n=1 exported = (1-f 1)* A1 * U238 * z1

Th-234 n=2 = Thsupported + Thimported = A2 * U238 * z2 + [(1-f 1)* A1 * U238 * z1] exp(-t) = V2 * U238 + [(1-f 1)* V1 * U238 ] exp(-t) f 2 = [Th-234n=2/ A2 ] / U238 * z2 = 1 + (1-f 1)*(V1/ V2)*exp(-t) transport time to attain observed downstream excess f rom observed upstream deficiency :

t = -1/ * ln[(f 2-1)/ (1-f 1)/ (V1/ V2)] mean transport velocity = x / t

1. Estimate transport times:  

Isupported = V U-238 = A z U-238

 U-238 = 100 fCi L-1

z = 20 meters  V1 (source) ~ 1800 km2 0.02 ~ 3600 km3

V2 (sink) ~ 425 km2 0.02 ~ 8.5 km3

  V1/V2 = A1/A2 ~ 4.2

May 99: f1 = 0.88 f2 = 1.17

  t ~ 38 days Aug 99: f1 = 0.79 f2 = 1.46

  t ~ 23 days

? transport velocity ~ 100 to 150 km 30 days ~ 3 – 5 km/day  

)1(

)1(ln

1

11

22

fI

fIt

2. Estimate of source area:   

if dist. ~ 100 km & velocity ~ 4 km d-1 (B. Lesht) 

25 days ~ one ½ life   for f1 = 0.85 f2 = 1.45

  V1/V2 6

i.e. the longer it takes the greater the source area

t) )exp(f(1

1f

V

V

Sink

Source

1

2

2

1

D. Schwab & D Beletsky

3. Estimate of mass transport required:  for 234Thex of 50%, i.e. f = 1.5 area of temporary sink ~ 500 km2 z = 20 meters  excess Th-234 ~ 0.5 Curies ~ 1 x 1012 dpm  if [Th-234]susp part ~ 35 pCi g-1 ~ 80 dpm g-1

a rapid transport “event” 13 x 103 MT (w/o decay) 25 x 103 MT (one ½ life)

  i.e. the longer it takes the more you need

~ 1% of total susp. particulates in the plume 50-100 “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

www.uwm.edu/Dept/GLWI