Vertical and Horizontal Mixing in Green Bay
18
by use of Radon-222 and D2 O
#441
Shelby L. LaBuhn1, J. Val Klump1, Pat D. Anderson1
1School of Freshwater Sciences, University of Wisconsin –Milwaukee, Milwaukee, WI 53204
Abstract: Two natural tracers were used for studying water mass movements and boundary layer exchange mechanisms, in Green Bay, Lake Michigan, a semi-enclosed freshwater embayment with a history of
hypereutrophication and recurring seasonal hypoxia. Stable oxygen and hydrogen (D218O) samples are used to track water masses in a system that is dominated by 2 end members: nutrient rich riverine inflow
and mesotrophic open lake waters. The extent of water mass mixing coupled with seasonal stratification is key in determining the onset and duration of hypoxia. Coupled to this are the processes of benthic
metabolism and sediment-water chemical exchange. Distributions of radon-222, in the water column and in sediment porewaters of the bay, serve as a conservative tracer for estimating boundary layer
exchanges, both at the sediment-water and air-sea interfaces. Incoming, cold Lake Michigan hypolimnetic water can be traced into the southern bay, where they appear to be the origin of southerly progressing
hypoxic intrusions and bottom waters as they pass over organic-rich anoxic sediments that consume oxygen and release methane at relatively high rates.
Study Site
Vertical Mixing
Horizontal Mixing
Radon-222 is a geochemical tracer that primarily originates in marine and aquatic sediments from the alpha
decay of radium-226. This noble gas diffuses from the sediment into the water column and eventually the
atmosphere. 222Rn fluxes can be quantified and used to study other dissolved constituents.
Stable isotopes of water, D218O, can be used to trace water sources with
distinct signatures, such as Fox River (FR) and Lake Michigan (LM)
exchange within Green Bay.
GB Bottom Water
7/29 - 8/1/2013
-44
Looking for Missing
Sources/Sinks
Box Model
100
-46
67
79
53 43
D/H (per mil)
Diffusion ≈ negligible
Vertical transport <5%
during stratification(1.)
21
-48
17
-50
31
9
Lake Michigan
-52
Flow in
or pulse
Non-Steady State:
Js -JT ≠ λIWC
5
Flow out
or pulse
-54
-7.2
Js avg
•
•
The FR has a lighter delta signature,
common of land-based sources
LM has a heavier signature,
especially in the hypolimnion
Q: When an excess inventory (Iex) exists, can it be used as a
bottom water label for horizontal movement?
Calculations for 222Rn Fluxes
Jw = λ*I
Jsed=Ds*ф*dC/dz OR (Ds*λ)1/2 * (Ceq – C0)
λ  222Rn Decay Constant (.181 day-1)
I  Inventory =Activity (dpm/m3)*depth (m)
Ds Diffusion Constant, assumed from Ref3
Ф Sediment porosity (cm3/cm3)
Unaccounted Excess Bottom Water
Radon-222
2013
Site
Imeas
Jsed
(dpm/m2) (dpm/m2/day)
GB-5
2695
GB-6
8353.5
GB-9
16675
GB-13 1042.5
GB-17
2883
GB-21
2654
GB-43
7950
GB-43 7944.75
GB-43
3927
GB-53
12037
GB-53
8068
GB-79
50220
GB-80
6910
GB-83
29130
GB-100 51060
181.471
181.471
181.471
165.865
87.073
247.448
687.706
687.706
687.706
687.706
687.706
315.850
315.850
315.850
315.850
-6.6
-6.4
-6.2
-6.0
-5.8
Time to decay Pulse
Imeas to Ised Length
Ised
Iex
(dpm/m2)
(dpm/m2)
(day)
km
1002.602
1002.602
1002.602
916.379
481.068
1367.115
3799.483
3799.483
3799.483
3799.483
3799.483
1745.029
1745.029
1745.029
1745.029
1692.398
7350.898
15672.398
126.121
2401.932
1286.885
4150.517
4145.267
127.517
8237.517
4268.517
48474.971
5164.971
27384.971
49314.971
5.463
11.713
15.532
0.712
9.893
3.665
4.079
4.075
0.182
6.371
4.160
18.562
7.603
15.552
18.653
10.926
23.426
31.064
1.425
19.785
7.330
8.158
8.151
0.365
12.742
8.321
37.123
15.207
31.105
37.306
Bottom water samples provide
path of water movement in
hypolimnion-which is useful for
investigating hypoxic occurrences
July 2013 Green Bay
Bottom Water
August 2013 Green Bay
Bottom Water
D218O can help confirm the connection between incoming, cool Lake Michigan
water (upper left) and reduced dissolved oxygen concentrations (left).
A: Sometimes. It is likely to more applicable in Northern Bay
where excess inventories are more prevalent.
Left: A map visualization of data
presented in the above table,
indicating that a majority of the
excess bottom water radon occurs
in the northern portion of the bay
and in regions of the southern
bay.
-6.8
O18/O16 (per mil)
•
A fraction of 222Rn
diffuses to the water
column, creating a
vertical gradient that
is referred to as a
“deficiency.”
-7.0
References
1. Loewen, M.R. et al. 2007. Can. J. Fish. Aquat. Sci.
2. Moore , J.R. et al. 1973. University of Wisconsin Sea
Grant Technical Report.
3. Martens, C.S. et al. 1980. Science.
2013 Porewater Radon-222 Fluxes
^
Right: Porewater fluxes are
highest near Chambers Island,
where there are known
manganese deposits. These high
fluxes may be partially
responsible for dampening the
“excess” signal in the mid-bay.
*
**
*
^
Ref. 2.
**
More Lake Michigan water in the southern bay (*,**) may
mean more hypoxia (^).
NOAA Center for Sponsored
Coastal Ocean Research
Coastal Hypoxia Research Program
NOAA-NOS-NCCOS