HR: 1400h
AN: H53A-01    [Abstracts]
TI: Land or Ocean? Tracing Major Submarine Groundwater Discharge Driving Forces
AU: * Santos, I R
EM: santos@ocean.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AU: Burnett, W C
EM: wburnett@mailer.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AU: Dimova, N
EM: dimova@ocean.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AU: Mwashote, B
EM: mwashote@ocean.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AU: Peterson, R
EM: peterson@ocean.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AU: Chanton, J
EM: chanton@ocean.fsu.edu
AF: Department of Oceanography, Florida State University, Tallahassee, FL 32306, United States
AB: The influence of submarine groundwater discharge (SGD) on nutrient and trace element cycling in the coastal ocean has been increasingly recognized, but little information about its driving forces is available. The current definition of SGD includes submarine fresh groundwater discharge (FSGD - the terrestrial component) and recirculated saline groundwater discharge (RSGD - the marine component). While FSGD is driven by the hydraulic gradient, RSGD incorporates recirculated water caused by wave set up, tidally-driven oscillations, current-induced pressure gradients, and convection. Since these terrestrial and marine driving forces are usually superimposed, it is difficult to separate their relative contributions. We report quasi continuous (<1h time steps), one year-long observations of the groundwater tracers CH4 and 222Rn, and coastal groundwater levels at a coastal site on the Gulf of Mexico. We have applied selected parts of this dataset to separate the relative contributions of the marine and terrestrial components of SGD. Even though 222Rn and CH4 have different geochemical behaviors, the significant correlations (p<0.01) between them indicate a common source for these constituents. The activities of 222Rn in the coastal water ranged from 1 to 15 dpm/L, usually showing higher values during the spring tide. Two processes may explain this pattern: enhanced tidal pumping force when tidal amplitudes are greater and/or longer tidal periods (occurs when the mixed semi-diurnal tide approaches diurnal character), which would provide more time for tracer regeneration within the sediments. We applied a mass balance based on 222Rn to estimate total SGD rates. We assume that the temporal variability of the total inventory of 222Rn in the water column is balanced by the difference between its total inputs (SGD and 226Ra decay) and total losses (atmospheric evasion and mixing with low concentration offshore waters). Radon-derived advection rates are highly variable, but typically around ~10 cm/day. These results were in good agreement with observations made during intensive experiments, when we also deployed seepage meters along a shore-normal transect. To derive the terrestrial component of SGD, we used continuous observations of groundwater levels in a 3-m deep, freshwater well located onshore, 15m away from the high tide mark. The local water table responds very quickly (on a scale of hours) to rainfall events. While immediately after rain events the water table drops relatively rapidly (5-8 cm/day), it drops at a slower rate of 0.5-2 cm/day after several hours. We assume that the observed rate of decay of the water table is equivalent to FSGD, after accounting for local porosity (<0.3) and neglecting evapotranspiration. The comparison of both approaches indicate that the terrestrial component accounts for <10% of total SGD in our study area. This is consistent with direct measurements of salinity of the water emanating from the sediments. Based on a simple mixing model of seawater and freshwater, we estimated that FSGD may range from 0 to 18%, in good agreement to our 222Rn model - hydrological approach. We thus conclude that marine forces are the dominant SGD drivers at this site.
DE: 1818 Evapotranspiration
DE: 1828 Groundwater hydraulics
DE: 1830 Groundwater/surface water interaction
DE: 1832 Groundwater transport
DE: 1836 Hydrological cycles and budgets (1218, 1655)
SC: Hydrology [H]
MN: 2007 Joint Assembly