GROUNDWATER DEPENDENCE OF WETLANDS: A TRACER
APPROACH
Methods
• Hydraulics
• Seepage meters
• Water balance
• Environmental tracers
• radon
• oxygen-18, deuterium
• ion chemistry
Approach
Tracers will be useful for identifying groundwater inflow to
surface water if:
1. Concentrations of the tracers in surface runoff and
groundwater are distinct
2. Tracers are conservative (do not undergo chemical
reactions once in
the wetland)
The proportion of
groundwater is determined
by comparing surface water,
groundwater and wetland
concentrations.
Environmental Tracers
Radon
• unreactive gas, only source is from decay of uranium
(and radium) in aquifer materials
• negligible concentrations in atmosphere
• radioactive, half-life 3.8 days
• zero concentration in rainfall (runoff), high
concentration in groundwater
Chloride
• relatively unreactive
• low concentrations in rainfall, higher concentrations
in groundwater
Radon
Increased
Groundwater
Inflow
Increased
Residence
Time
radioactive decay
gas exchange
evaporation
Increased
Residence Time
Increased
Groundwater Inflow
Chloride
Groundwater
Radon
X
Groundwater,
Short Residence Time
Surface Water,
Surface Water,
Short Residence Time
Long Residence Time
X
Ground Water,
Long Residence Time
Chloride
Rainfall
Groundwater
Reconnaisance Survey
• Radon and chloride concentration measured in 37 wetlands and 25
bores.
• Runoff chemistry not measured, although rainfall data is available for
chloride. Runoff concentrations of radon assumed to be zero.
• Sampling took place in November 2004 and February 2005.
• Duplicate samples
collected from 3 wetlands.
10
(c)
PicP
EP1
GSw
ToppWSB
HTF2
CockL (1)
0.1
DRSD
GordL
222
Rn (Bq/l)Feb2005
1
034
DMS
TM1
GGD
0.01
TWH (2)
GrS
DRSG
WWZ2
RndS(1)
LBLL TWH
GGB
7SisB
RUS A
GHL A
WWZ
LEdward RUS B
TMM
MtMI
0.01
0.1
222
1
Rn (Bq/l)Nov2004
10
Radon
Groundwater,
Short Residence Time
Surface Water,
Surface Water,
Short Residence Time Long Residence Time
222
Rn (Bq/L)
8
6
4
2
Ground Water,
Long Residence Time
Chloride
(a)
HTF1
PicP
PicP
EP1
EP3
GSw EP1
HS AD
0.4
DRSZ
0.3
GSw
ToppWSB
ToppWSB
0.2
DRSG
WWZ2
HTF2
TWH (2)
GrS
CockL (1)
TWH (1) TWH (2)
GrS
DRSG
HNU
CockL (2)
0.1
LBLL
DRSD
WWZ2
HSY
HTF2
7SisB 7SisA GordL
LBLL
RUS
A
CSW
CockL (1)
DRSD
GGB
TWH
(1)
GHL
(1)
GordL 7SisB
GGB
WWZ
TMM GGD
034
RUS A 034
DMS(2) DMS
TM1
MtMI
GGD GHL
RUS
B B TM1TMM
RUS
MtMI WWZ
0.0
10
100
RndS
LtBool
RndS
GHL B
LEdward
LEdward
1000
-
Cl (mg/L)
LLeake
LLeake
10000
Tracer Mass Balance
Steady state, advective model:
k
E
Is, cs
Q
V=Az, c
Ig , c g
∂V
= I s + I g − Q − EA = 0
∂t
∂cV
= I s cs + I g c g − Qc − kAc − λVc = 0
∂t
c=
I s cs + I g c g
I s + I g − EA + kV + λV
tr =
Is + Ig
V
Ionic Tracers
• if we choose conservative, unreactive ionic tracers (e.g., chloride),
then λ = 0, k = 0
c=
I s cs + I g c g
I s + I g − EA + kV + λV
c=
• unknowns Is, Ig
I s cs + I g c g
I s + I g − EA
Radon
• unreactive gas, only source is from decay of uranium (and radium) in
aquifer materials
• negligible concentrations in atmosphere
• cs = 0
c=
c=
• unknowns Is, Ig
I s cs + I g c g
I s + I g − EA + kV + λV
I g cg
I s + I g − EA + kV + λV
Simultaneous Solution
• requires two tracers
• measure c1, c2, cs1, cs2, cg1, cg2
• estimate E, k, z
• λ is known
• solve for Ig and Is
Rule S A
Rule S B
Grants S
Round S
Cockatoo L
Honans Scrub AD
Honans Scrub U
Honans Scrub Y
Lt. Bool Lagoon
Picaninnie Ponds
Green S
Gordon Lagoon
Topperweins S
Border S
7 Sisters A
7 Sisters B
Lake Edward
Honans Scrub TF1
Honans Scrub TF2
Woolwash Z
Woolwash Z2
Diag. Rd. S D
Diag. Rd. S Z
Cole S West
Trail Waterhole
Deadman S
Greenrise L
Ewens Pond
Lt. Blue L
Diag. Rd. S G
Gran Gran B
Gran Gran D
Mt. McIntyre PS
The Marshes M
Blue Tea Tree S
Lake Leake
Ig/Is
Ig/V (d -1)
100
(a)
10
1
0.1
0.01
0.001
0.0001
100
(c)
10
1
0.1
0.01
Moderate Dependence
High Dependence
Diagonal Road Swamp G
Diagonal Road Swamp Z
Blue Tea Tree Swamp
Cole Swamp West
Deadman Swamp
Ewens Ponds 1
Ewens Ponds 3
Diagonal Road Swamp D
Gran Gran B
Green Swamp
Gran Gran D
Greenrise Lake
Grants Swamp
Honans Scrub AD
Honans Scrub TF2
Honans Scrub TF1
Little Blue Lake
The Marshes M
Honans Scrub U
Honans Scrub Y
Trail Waterhole
Woolwash Z
Little Bool Lagoon
Low Dependence
Round Swamp
Border Swamp
Topperwien Swamp
Cockatoo Lake
Piccaninnie Ponds
Gordons Lagoon
Woolwash Z2
Lake Edward
Lake Leake
Mt. McIntyre Perched Swamp
Rule Swamp A
Rule Swamp B
Seven Sisters Swamp A
Seven Sisters Swamp B
Is this too good to
be true?
Uncertainties in Model
• spatial variability of groundwater chemistry
Further Uncertainties in Model
• spatial variability of wetland water chemistry
• radon diffusion from sediments
Radon Diffusion from Sediments
Radon Production in Sediments
Radon Emanation Measurements
Radon (Bq/L)
0.01
0
20
γ
c=
λε
0.1
1
10
100
−1
⎛ ⎛
⎞
⎛
⎞ ⎞⎟
D
λε
λε
⎜1 − ⎜1 +
⎟ exp⎜ −
z ⎟⎟
⎜
⎜ ⎜ k + λb ⎟
⎟
D
⎝
⎠
⎝
⎠
⎝
⎠
Depth (cm)
40
60
80
100
Green Swamp
120
γ = 0.003 Bq/cm3/day
Radon-222 (Bq/L) in Honan South
24/05/06
0.35
0.33
0.50
0.23
0.39
0.35
0.08
0.18
0.13
0.09
0.21
0.19
0.34 0.26
0.22
0.29
0.67
0.74
20m
0.59
Radon-222 (Bq/L) in Honan South
25/07/06
0.32
0.49
0.14
0.58
0.26
0.17
0.57
0.42
0.50
0.17
0.75
0.12
0.59
0.42
0.17
0.06
0.20
0.07
0.19
0.15
0.19
0.16
0.24
0.19
0.26
0.23
0.30
0.79
0.23
0.12
0.26
0.11
20m
0.31
0.34
0.95
0.45
1.05 1.05
0.40
0.90
0.48
0.62
0.32
0.72
0.11
0.54
0.84
0.41
0.87 1.03
0.85
0.92
EC (uS/cm) in Honan South
surface water, 25/07/06
1063
1034
996
1048
1029
1056
1017
1021
1047
1007
1034
966
1031
1055
998
772
874
901
968
973
910
932
928
898
896
898
1010
1011
986
1000
970
919
975
962
872
974
828 811
948 874
942
805
828
941
922
20m
1011
908
916
900
951
937
922
854
Conclusions
• radon and chloride provide a valuable tool for
determining groundwater input to wetlands
• diffusional input is relatively small, and can be
corrected for or measured
• limited mixing poses problems in very shallow
lakes
• radon cannot distinguish between different
groundwater sources (e.g., perched versus
regional groundwater)