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

1. Southwestern Atlantic Ocean Margin Workshop Montevideo, Uruguay 16-22 November 2008 Submarine Groundwater Discharge into the ocean off Mangueira Lagoon via 222Rn and Ra isotopes: Preliminary findings and perspectives Isaac R. Santos; William Burnett Department of Oceanography, Florida State University, Tallahassee, FL, USA Felipe Niencheski; Karina Attisano; Carlos Andrade; Idel Milani Department of Chemistry, Fundação Universidade do Rio Grande, Rio Grande, Brazil

2. Outline • Area background • Tracing SGD via 222Rn and radium isotopes • Is the La Plata River Paleocanal a preferential SGD pathway?

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

4. Sandy coastal plain

5. Former lagoon bed deposits may act as confining units

6. 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 mM data from Attisano, Niencheski, et al., 2008

7. 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

8. 222Rn time series Concheiros Hermenegildo • Background 226Ra: ~70 dpm/m3 • Excess radon in spite of breaking waves • Modeled SGD rates 2.22.4 cm/day

9. Radium alongshore sampling

10. - Some enriched 223Ra and 224Ra samples in the south - Are the shell deposits a preferential flow path? Radium along-shore transect • Fresher seawater in the winter • No seasonal radium changes Concentrations in dpm/100L

11. Radium in beach groundwater C*gw Modeling SGD from radium isotopes Seawater residence time (t) : Advection rates:

12. Estimated SGD inputs (cm/day) Overall averages: 3.1 cm/day Area: 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)?

13. 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

14. La Plata paleocanal • 70 km long • 20 km wide • - 30 m deep Campos et al., 2007.

15. Silicate in bottom waters - SGD or sub-Antarctic water advection onto the shelf? mM Paleocanal slope

16. 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