Modeling radium fate and transport in a subterranean estuary

Ann Mulligan, Marine Policy Center
Matthew Charette, Marine Chemistry & Geochemistry

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Abstract

Radium isotopes have been used extensively in the past 10-15 years as tracers of submarinegroundwater discharge from coastal aquifers. A basic assumption in using these tracers is thatradium behavior in the subsurface is relatively straightforward and easily predictable. Recent work has shown that radium behavior in the subsurface is highly sensitive to groundwater salinity however. In fresh water radium is highly sorbed onto aquifer solids and is relatively immobile. Conversely, radium is more highly dissolved in saline water and therefore more mobile in saline groundwater. Interestingly, simple mixing calculations between a low radium low salinity groundwater and low radium high salinity seawater suggest that sorbed radium is a maximum at intermediate salinities. This unexpected result suggests that numerical modeling will be extremely useful in unraveling complex radium dynamics in coastal aquifers, particularly on seasonal time scales, when the salinity in the groundwater changes in response to changes in inland recharge. This project represents a first attempt at transient spatial modeling of radium fate and transport in a coastal aquifer and will be a significant step forward in unraveling the complexities of radium behavior. Results from this work will help us understand observed radium distributions within and flux from coastal aquifers and will help reduce the largeuncertainties in radium-based estimates of submarine groundwater discharge rates. This project also represents a first step toward more complex geochemical modeling of coastal aquifer systems.