Natural Radionuclide Applications for Riverine and Coastal Marine Investigations

Naturally-occurring radioisotopes are ubiquitous in nature, and as such, there are many opportunities for researchers to use them as environmental tracers. Their associated radioactive decay rates provide an inherent time clock that often makes these radioisotopes more effective chemical tracers than other substances, since they can help assess the temporal component of a particular process. Radioisotopes were employed in this study to examine different coastal oceanographic processes that have far reaching consequences. A review of various measurement methods for assessing Ra-226 concentrations, a valuable tracer employed throughout most of this dissertation, is presented. This is followed by applying natural radioisotopes to study various oceanographic processes in China, Hawaii, and a river system in the southeastern United States. Acrylic fibers impregnated with manganese-dioxide ('Mn-fiber') have become a valuable tool for concentrating dissolved radium for oceanographic applications. Several techniques have been outlined in the literature describing the measurement of Ra-226 on Mn-fiber via its gaseous daughter, Rn-222. We describe procedures for three radon-based non-destructive measurement techniques for Ra-226 on Mn-fiber (via RAD7, RaDeCC, and Rn emanation line systems) and perform an intercomparison among them, using the standard technique of gamma-spectrometry as a reference. We find that all methods agree in terms of the measured activity, with the respective correlation coefficients (r) between any two different methods ranging from 0.78 to 0.93. The methods vary in their advantages, with the Rn emanation line and RaDeCC techniques offering the best measurement uncertainties and lowest minimum detection activities, while the RAD7 method is the least operator-demanding technique. In the reported field activities, radium isotopes were mapped around the mouth of the Yellow River in China to examine the transport rates of Yellow River water through the estuary and into the Bohai Sea during three different field excursions, covering a large range in the discharge patterns of the river. Using radium isotope ratios, horizontal transport rates ranging from 1.4 to 4.7 cm/s were found throughout the delta and apply over most of the discharge range of the river while exhibiting no seasonal variability. Time series analyses of radium isotopes and Rn-222 were also used near the Yellow River delta to assess submarine groundwater discharge (SGD) rates. Modeled vertical SGD rates in this area varied between 4 and 20 cm/day during the sampling periods, thus delivering significant volumes of groundwater containing elevated concentrations of nutrients to the coastal zone. We combine these results to show that the dissolved freshwater nitrate inputs (riverine and SGD) around the Yellow River delta cannot be directly transported to the central Bohai Sea (where increasing nitrate levels have been documented) before being biologically or otherwise removed. Rn-222 was the main tracer employed to examine groundwater discharge along the leeward coast of the Big Island of Hawaii. Here, geological formations act to direct coastal aquifer waters to point-source discharges, forming buoyant plumes of freshwater extending out into the coastal waters. A box model was created using mass balance of radon, salt, and water to assess the discharge rates of these plumes. The point-source inputs were found to discharge thousands of cubic meters of brackish water to the coastal zone each day, ranging from 1100 m3/day to 12,000 m3/day of total water fluxes. The purely freshwater components of these discharges varied from 630 m3/day to 8600 m3/day. Considering possible sources of nutrient and industrial contamination in this area, these discharges can significantly affect the local reefs and ecosystems along this coast. A particle tracing project was performed in the Apalachicola-Chattahoochee-Flint River system in the southeastern United States. The river system empties into Apalachicola Bay in northwest Florida, a biodiverse and economically valuable ecosystem to the state of Florida. We applied various naturally-occurring radionuclides as tracers to reveal suspended particle transport behavior under both base flow (June 2006) and high discharge conditions (February 2007). Potassium-40 activities are used to assess the lithogenic/crustal fractions of each suspended sediment sample (ranging from 4-60%), whereas the organic fraction ranges from 4 to 32% by mass. Particulate radium isotopes (namely Ra-226 and Ra-228) were employed to trace the origin of the suspended particle flux to Apalachicola Bay. During low discharge, the Flint River dominates the particulate flux to Apalachicola Bay (70%), whereas the Chattahoochee River contributes 30% of the flux and the Apalachicola River is net depositional. During high discharge, the Chattahoochee River contributes the majority of the suspended particles (56%), with the Apalachicola River contributing 30% and the Flint River only providing 14% of the particulate flux. The Be-7/Pb-210 activity ratios were used to assess the residence time of suspended particles in upstream reservoirs (5.2 days in Lake Blackshear and 60 days in West Point Lake), and also to examine open channel transport velocities (~ 13 cm/sec in the lower Flint River) during high discharge.