Submarine Groundwater Discharge into the ocean off Mangueira Lagoon via 222Rn and Ra isotopes:

Preliminary findings and perspectives

Isaac R. Santos; William BurnettDepartment of Oceanography, Florida State University, Tallahassee, FL, USA

Felipe Niencheski; Karina Attisano; Carlos Andrade; Idel MilaniDepartment of Chemistry, Fundação Universidade do Rio Grande, Rio Grande, Brazil

Southwestern Atlantic Ocean Margin WorkshopMontevideo, Uruguay16-22 November 2008

Outline

1) Area background

2) Tracing SGD via 222Rn and radium isotopes

3) Is the La Plata River Paleocanal a preferential SGD pathway?

Patos

Mangueira

Mirim

Brazil- SGD Fe inputs ~10% of atmospheric Fe inputs into the entire South Atlantic (Windom et al., 2006)

Higher SGD than North of Patos Lagoon?

-Mangueira Lagoon level is higher than Patos Lagoon level-Lagoon closer to the ocean-Sediments more permeable (shell deposits)

Sandy coastal plain

Former lagoon bed deposits may act as confining units

Alongshore distribution of silicate (10 m depth)

- Patos Lagoon and La Plata River are the obvious nutrient sources

- No N-S trends suggest an additional source – Widespread, diffuse SGD?

Silicate in M data from Attisano, Niencheski, et al., 2008

222Rn as a SGD tracer

Platform with a continuous Rn monitor system

Push-point piezometer for groundwater sampling

Non-steady state box model

Assume wave action was constant during deployment

Concheiros Hermenegildo

222Rn time series

-Background 226Ra: ~70 dpm/m3

-Excess radon in spite of breaking waves

-Modeled SGD rates 2.22.4 cm/day

Radium alongshore sampling

Radium along-shore transect

-Fresher seawater in the winter

-No seasonal radium changes

- Some enriched 223Ra and 224Ra samples in the south

- Are the shell deposits a preferential flow path?

Concentrations in dpm/100L

Seawater residence time (t) :

gwRat 224ex

224 DepthRaSGD

Advection rates:

Modeling SGD from radium isotopes

223224

223

224

223

224

1*ln t

obs

i

Ra

Ra

Ra

Ra

Radium in beach groundwater

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35

Salinity

Ra

-22

4ex

(d

pm

/10

0L

)

0

2

4

6

8

10

0 5 10 15 20 25 30 35

Salinity

Ra

-22

3 (

dp

m/1

00

L)

Beach groundwaterSurf zoneOffshore transect

C*gw

Estimated SGD inputs (cm/day)

Overall averages: 3.1 cm/dayArea: 220 km long; 10 km wide

Total SGD into the area: ~800 m3/s

Patos Lagoon Flux: ~2400 m3/s

Is SGD a new source of nutrients (fresh SGD) or purely a recycling mechanism (saline SGD)?

Summer 2.1 1.6 2.2 1.7 6.8 5.1 3.7 2.2Winter 0.8 0.4 2.2 1.1 4.3 2.2 2.4 2.1

223Ra 226Ra Average224Ra

0

5

10

15

20

0 30 60 90 120 150 180

Distance from shore (km)

Ra

-22

4 (d

pm

/10

0L

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 30 60 90 120 150 180

Distance from shore (km)

Ra-

223

(d

pm

/10

0L)

Offshore radium transect

-Onshore radium source, likely SGD

-Mixing coefficients >3000 m2/s (too high!). Violates assumption of no inputs offshore?

-Peak at ~100 km indicates an additional source or advection of nearshore waters

-85

-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

Campos et al., 2007.

La Plata paleocanal

-70 km long

-20 km wide

- 30 m deep

-53.0 -52.5 -52.0 -51.5 -51.0

Longitude

-34.3

-33.8

-33.3

-32.8L

atit

ud

e

1

3

5

7

9

11

13

15

Silicate in bottom waters

- SGD or sub-Antarctic water advection onto the shelf?

Paleocanal

slope

M

Summary

1) Radium is the preferred SGD tracer as we cannot model how waves influence radon evasion

2) SGD is potentially a major source of water and nutrients to the Albardão area and the entire SWAOM margin

3) Shell deposits and the La Plata River Paleocanal may represent a preferential flow path for SGD