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Submarine Groundwater Discharge into the ocean off Mangueira Lagoon via 222 Rn and Ra isotopes: Preliminary findings and perspectives Isaac R. Santos;

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Presentation on theme: "Submarine Groundwater Discharge into the ocean off Mangueira Lagoon via 222 Rn and Ra isotopes: Preliminary findings and perspectives Isaac R. Santos;"— Presentation transcript:

1 Submarine Groundwater Discharge into the ocean off Mangueira Lagoon via 222 Rn 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 Southwestern Atlantic Ocean Margin Workshop Montevideo, Uruguay 16-22 November 2008

2 Outline 1)Area background 2)Tracing SGD via 222 Rn and radium isotopes 3)Is the La Plata River Paleocanal a preferential SGD pathway?

3 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)

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  Mdata from Attisano, Niencheski, et al., 2008

7 222 Rn 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 Concheiros Hermenegildo 222 Rn time series -Background 226 Ra: ~70 dpm/m 3 -Excess radon in spite of breaking waves -Modeled SGD rates 2.2  2.4 cm/day

9 Radium alongshore sampling

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

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

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 m 3 /s Patos Lagoon Flux: ~2400 m 3 /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 m 2 /s (too high!). Violates assumption of no inputs offshore? -Peak at ~100 km indicates an additional source or advection of nearshore waters

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

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

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


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