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 November 2008
Outline 1)Area background 2)Tracing SGD via 222 Rn 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 Mdata from Attisano, Niencheski, et al., 2008
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
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
Radium alongshore sampling
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
Seawater residence time (t) : Advection rates: Modeling SGD from radium isotopes Radium in beach groundwater C* gw
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)?
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
Campos et al., La Plata paleocanal -70 km long -20 km wide - 30 m deep
Silicate in bottom waters - SGD or sub-Antarctic water advection onto the shelf? Paleocanal slope MM
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