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Tracing anthropogenically-driven groundwater discharge into a coastal lagoon from Brazil
Isaac R. Santos; William Burnett; Richard Peterson; Jeffrey Chanton Department of Oceanography, Florida State University, Tallahassee, FL, USA
Felipe Niencheski; Carlos F.F. Andrade; Idel B. MilaniDepartment of Chemistry, Fundaçao Universidade do Rio Grande, Rio Grande, Brazil
Axel Schmidt; Kay Knoeller Helmholtz Centre for Environmental Research, Leipzig, Germany
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(1) Density driven circulation(2) Tidal pumping(3) Wave setup (4) Fresh groundwater discharge
Robinson et al., 2007
Submarine groundwater discharge driving forces
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Hypotheses1 - The annual irrigation creates extreme conditions that seasonally change groundwater discharge into Mangueira Lagoon
2 - Dredging of irrigation canals alters the groundwater fluxes
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Patos
Mangueira
MirimAtlanticOcean
Brazil
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222Rn survey results – Aug/2006
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0
300
600
900
1200
ML GW
Co
nd
. (m
S/c
m)
0
1
10
100
1000
ML GW
222 R
n (
dp
m/L
)-6
-4
-2
0
2
ML GW
d18O
0.0
0.1
0.2
0.3
0.4
0.5
ML GW
CH
4 (m
M)
0
100
200
300
400
500
ML GW
22
4R
a ex (
dp
m/m
3)
01234567
ML GW
22
3R
a (d
pm
/m3)
-25
-19
-13
-7
-1
5
ML GW
d2H
0
50
100
150
200
ML GW
22
6R
a (d
pm
/m3)
Groundwater and Lagoon water tracer concentrations
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222Rn time series – Vitor Barbosa Canal
0 3 6 9 12 15 18
22
2R
n
0
8
16
24
32
Win
d S
peed
0
3
6
9
Winter – Aug/2006
0 4 8 12 16 20W
ind
Spe
ed0
3
6
9
22
2R
n
0
2
4
6
Con
duct
ivity
0.30
0.35
0.40
0.45
0.50
Summer – Jan/2007
Time (h)
Pumps on Pumps on
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Modeling 222Rn-derived groundwater discharge
Approach 1: Steady state
Jben + 222I226 - 222I222 - Jatm - Jhor = 0
Inputs - Outputs = 0
Approach 2: Non-steady state (irrigation canals disturbed by pumping)
dI222/dt = 222I226 + Jben - 222I222 - Jatm - Jhor
Inventory rate of change = Inputs - Outputs
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Offshore 222Rn transects
(A) Vitor Barbosa Canal - Winter
kh=781 m2/day
Distance from Pump House (m)
0 100 200 300 400 800 1000 1200
22
2R
n (d
pm/L
)
0
10
20
30
40
50
Con
duct
ivity
100
200
300
400
500
(B) Rancho Grande Canal - Winter
kh=1879 m2/day
Distance from Pump House (m)
0 100 200 300 400 800 1000 1200
22
2R
n (d
pm/L
)
0
2
4
6
8
10
Con
duct
ivity
100
200
300
400
500
222Rn (dpm/L)Conductivity (mS/cm)
(C) Abandoned Canal - Summer
kh=2620 m2/day
Distance from Pump House (m)
0 100 200 300 400
22
2R
n (d
pm/L
)
0
1
2
3
4
5
Con
duct
ivity
100
200
300
400
500
(D) Rancho Grande Canal - Summer
kh=2043 m2/day
Distance from Pump House (m)
0 100 200 300 4002
22R
n (d
pm/L
)0
2
4
6
8
10
Con
duct
ivity
100
200
300
400
500
600
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Determining mixing losses
y = -0.0152x + 10.613R2 = 0.9619
0
2
4
6
8
10
12
0 200 400 600 800
Distance offshore (m)
Ln
(2
22R
n)
hKm
m: slope of the log-linear curveKh: mixing coefficient (m2/d): decay constant
Determine mixing coefficients from 222Rn and conductivity transects
02
2
PCx
C
x
CKh
Neglecting advection and Production:
Mixing coefficients Kh in th order of 103 m2/day
Use iterative approach to adjust 222Rn concentrations along the transect
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Estimated advection rates
Time series ID IRn222 Jatm Jmix IRa226 Jben Adv. Rate
dpm/m2/d dpm/m2/d dpm/m2/d dpm/m2/d dpm/m2/d cm/d
Winter (August 2006)TS 1 Oliveira Canal 2876 3009 266 14 6138 5.3TS 2 Oliveira Beach 145 174 43 4 358 0.2TS 3 Santa Marta Canal 105 450 61 5 611 0.4TS 4 Santa Marta Beach 42 71 7 4 116 0.0TS 5 Vitor Barbosa Canal 9364 15508 223 12 25083 21.8TS 6 Rancho Grande Canal 3525 25239 64 9 28819 25.0TS 7 Vitor Barbosa Beach 68 197 16 4 277 0.1TS 8 Mirim Lagoon Harbor 220 408 60 9 678 0.5
Summer (January 2007)TS 1* Oliveira Canal** 202 1360 37 6 24259 21.0TS 2 Oliveira Beach 12 107 7 2 124 0.0TS 3* Santa Marta Canal*** 54 816 77 4 4344 3.7TS 4 Santa Marta Beach 12 103 5 3 117 0.0TS 5* Vitor Barbosa Canal 136 2151 7 3 32059 27.8TS 6* Rancho Grande Canal 260 960 37 5 17500 15.1TS 7 Vitor Barbosa Beach 46 65 28 5 135 0.0
* Irrigation pumps on during most of the time - see Figure 7. Fluxes based on equation (2)
** In the summer, the time series was deployed ~100m closer to the pump house than in the winter
*** Santa Marta Canal was dredged a few hour before our sampling in the summerMixing contributes an average of ~6% (maximum of 12%) of the total 222Rn losses
Advection rates in the irrigation canals from the south are much higher
Extrapolated water fluxes (~50000 m3/day) represent only 2% of the direct rainfall
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Major ions in Mangueira Lagoon and Groundwater
GWGW
GW
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Groundwater-derived ion fluxes
0
20
40
60
80
100
F Cl SO4 HCO3 Na K Ca Mg
Rel
ativ
e co
ntr
ibu
tio
n (
%)
Rainwater
Groundwater
F Cl SO4 HCO3 Na K Ca Mg
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Conclusions
1) No major seasonal changes in groundwater advection rates driven by irrigation
2) Advection rates in the canals were ~2 orders of magnitude higher than along the lagoon shoreline contribution ~80% of total groundwater inputs.
3) In spite of the small volume contribution (<2% of total water inputs), groundwater should not be neglected in dissolved species budgets
4) Anthropogenic changes in the water cycle may have a major effect on groundwater discharge in some coastal environments.