Application of 226Ra, 228Ra, 223Ra and 224Ra in coastal waters to assessing coastal mixing rates and groundwater discharge to oceans

The fate of dissolved material delivered to the coastal ocean depends on its reactivity and the rate at which it is mixed offshore. To measure the rate of exchange of coastal waters, we employ two short-lived radium isotopes,223Ra and 224Ra. Along the coast of South Carolina, shore-perpendicular profiles of 223Ra and 224Ra in surface waters show consistent gradients which may be modeled to yield eddy diffusion coefficients of 350-540 m2s-1. Coupling the exchange rate with offshore concentration gradients yields estimates of offshore fluxes of dissolved materials. For systems in steady state, the offshore fluxes must be balanced by new inputs from rivers, groundwaters, sewers or other sources. Two tracers that show promise in evaluating groundwater input are barium and 226Ra. These tracers have high relative concentrations in the fluids and low-reactivity in the coastal ocean. Applying the eddy diffusion coefficients to the offshore gradient of 226Ra concentration provides an estimate of the offshore flux of 226Ra. Measuring the concentrations of 226Ra in subsurface fluids provides an estimate of the fluid flux necessary to provide the 226Ra. These estimates indicate that the volume of groundwater required to support these fluxes is of the order of 40% of the surface water flow. (author)