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.