Radium Sampling Methods and Residence Times in St. Andrew Bay, Florida

Abstract

Activity ratios (AR) of radium isotopes have been used with success to constrain estimates of water ages and to approximate residence times in coastal waters. We compared two common radium sampling methods (grab sampling and stationary moorings) to estimate water ages and the residence time of St. Andrew Bay waters in northwest Florida, USA. Both sampling methods utilize manganese dioxide fibers (“Mn fibers”) to adsorb dissolved radium from the water column. Grab samples capture radium activities at a discrete time while moorings integrate radium activities over longer deployments. The two methods yielded similar results in this study and thus both approaches are useful for water age comparisons and residence time approximations. However, since radium often varies as a function of tidal stage, deploying moorings over a complete tidal cycle is the preferred approach. An estimated residence time for North Bay and West Bay of 8–11 days was approximated using ARs for both ex²²⁴Ra/²²³Ra and ex²²⁴Ra/²²⁸Ra. Some complications were introduced as St. Andrew Bay is a tidally dominated, rather than a river-dominated bay system where this method has previously been applied. The largest freshwater source to this bay system is from a man-made reservoir, with an average freshwater flow of only 20 m³ s⁻¹. The activity concentrations and ARs measured by both sampling methods suggest that while the reservoir is the prominent radium source, it is not the only radium source. Nonetheless, a tidal mixing model applied to the western half of the system yielded an approximate flushing time of 10–12 days, similar to that derived from our radium-based water age approach.