Distribution of 223Ra and 224Ra in the plumes of the Mississippi and Atchafalaya Rivers and the Gulf of Mexico

Two naturally occurring, short-lived radium isotopes (223Ra and 224Ra) provide unique tracers of riverine plumes as they mix into the ocean. Here, we apply these isotopes to studies conducted at the mouths of the Mississippi and Atchafalaya Rivers in November 1993 and May 1994. During each study, plume waters contained unsupported activities of short-lived radium isotopes for distances of over 75 km from the Mississippi River mouth. These isotopes may be used to calculate apparent ages of the plume since the water was last in contact with sediments at the river mouth. The ages provide a unique time history of water in the plume. Plume ages derived from the isotope ratios indicate that the plume moved considerably faster in May compared to November. During May 1994, there were significant enrichments of all radium isotopes in the bottom waters near the Mississippi mouth. The suspected source of these enrichments is the discharge of salty groundwater into the bottom water. Besides supplying radium isotopes, these discharges may provide significant inputs of nutrients and other materials.