Keynote Summaries
On GERM 3, La Jolla, March 6-9, 2001

Dynamics and Fluxes of Fresh Water Input into the Oceans 

Speaker : Billy Moore
Student Writer : Matt Evans

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Summary

Unlike the direct input of fresh groundwater to the ocean, this study focuses on chemical reactions that occur as groundwater and seawater mix in coastal aquifers. In many ways this system is similar to an estuary with the river component replaced with fresh groundwater.  In coastal aquifers there is a mixing of seawater and freshwater and a significant number of chemical reactions are occurring.  Coastal aquifers have relatively high total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) and carbon concentrations (Table 1) and therefore any significant flux of water from these aquifers to the ocean will strongly affect coastal water nutrient, trace metal, and carbon budgets.  This study attempts to quantify the chemical effect of subterranean groundwater discharge (SGWD) to the open ocean and focuses on a section of the South Atlantic Bight between Cape Fear and the Savannah River and may not be representative of all coastal areas.

Radium isotopes, both short lived (²²³Ra, t_1/2 = 11.4 days and ²²⁴Ra, t_1/2 = 3.6 days) and relatively long lived (²²⁶Ra, t_1/2 = 1600 years) were used to quantify the exchange rate of coastal waters with the open ocean (²²⁶Ra and ²²³Ra) and the offshore flux of material from the subterranean system (²²⁶Ra).  These tracers are useful as the [Ra] in subterranean waters is high relative to ocean surface waters and they do no react strongly in the coastal oceans.

Shore-perpendicular profiles of the short lived Ra isotopes are consistent with eddy-diffusion rather than advection.  Using these profiles one can estimate eddy diffusion coefficients (exchange rates) of ca. 400 m²/s from coastal oceans to the surface open oceans.  This exchange rate can be combined with offshore concentration gradients (using ²²⁶Ra) to determine the tracer flux to the open ocean.  Five ²²⁶Ra shore-perpendicular profiles were measured off Charleston and Winyah Bay (South Carolina) (Figure 1) and show a steady decrease in concentration from ~25 dpm on the inner shelf to the open ocean value around 8 dpm at about 45 km off shore.  This translates to a ²²⁶Ra gradient = 2.6 dpm/m³/km.  If we then multiply by the eddy-diffusion coefficient we obtain a value of 1x 10⁵ dpm/m²/d.  Therefore the S²²⁶Ra flux from coastal to open oceans for the 320 km stretch of coast ´10 m water depth  = 3 ´10¹¹ dpm/d.

The crux to the technique lies in the assumption that the system is in steady state and therefore this flux from coastal to open ocean must be balanced by input from the subterranean fluids, rivers, sewers, or other sources. The riverine source is only 3% of the flux required by the offshore flux estimate (1´10¹⁰ dpm/d). If other sources are as small, the subterranean estuary flux must be approximately 3 ´10¹¹ dpm/d. If subterranean estuary waters contain 7 dpm ²²⁶Ra/L (100 ´ocean water), the discharge must be 500m³/s or around half of the river flow for the area

Now by measuring other components of interest (i.e. TDN, TDP) in the subterranean fluids, the fluxes of reactive species (nutrients, metals) to the coastal ocean due to the subterranean estuary can be calculated.  These are summarized below (Table 2) and show that SGWD is a significant source of nutrients to the open ocean.

Figures

Figure 1. Shore-perpendicular profiles of ²²⁶Ra.