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
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