Multiple Isotopic Tracers for Study of Coastal Hydrological Processes

This study focused on the combined use of radon and radium isotopes as tracers of near-shore geophysical processes including submarine groundwater discharge (SGD), water exchange rates, and atmospheric evasion. Methods were developed for easier measurement of long-lived radium isotopes in natural waters and for continuous radon surveying over larger areas in the coastal zone. These tracer techniques were used to study the mentioned processes at study sites in Shelter Island Sound (New York), the Gulf of Thailand, and Apalachicola Bay, Florida. Groundwater fluxes calculated for Shelter Island using isotopic techniques produced results consistent with those measured directly via seepage meters. Groundwater discharge in the Chao Phraya Estuary (Thailand) was shown to be in the range of 2 to 20 m3/s, small compared to river discharge but much higher than seepage rates measured in nearby locations. An experimental assessment of Rn-222 evasion to the atmosphere was performed using radon and Ra-224 profiles in the Chao-Phraya Estuary in Thailand. The different trends in radium and radon isotopes measured in the estuary provided an estimate of atmospheric exchange that agreed with a theoretical approach. Short-lived radium isotopes were applied in a seasonal study of water residence time in Apalachicola Bay, Florida. The water transport within different sectors of the bay was evaluated as a result of advective (estuarine) and dispersive physical mixing. The distribution of the radium isotope ratios was used to determine apparent radium ages of the water within the bay. The results clearly showed how the water-residence time in the bay changes seasonally and that the winds and tides influence the water circulation in the bay. The radium tracer approach gave turnover times of 6 to 12 days in Apalachicola Bay during the studied periods.