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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. Milani - PowerPoint PPT Presentation
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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
(1) Density driven circulation(2) Tidal pumping(3) Wave setup (4) Fresh groundwater discharge
Robinson et al., 2007
Submarine groundwater discharge driving forces
Hypotheses1 - The annual irrigation creates extreme conditions that seasonally change groundwater discharge into Mangueira Lagoon
2 - Dredging of irrigation canals alters the groundwater fluxes
Patos
Mangueira
MirimAtlanticOcean
Brazil
222Rn survey results – Aug/2006
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
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
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
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
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
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
Major ions in Mangueira Lagoon and Groundwater
GWGW
GW
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
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.