COI Funded Project: Modeling radium fate and transport in a subterranean estuary


Funded Project 2010:
Radium isotopes have been used extensively in the past 10-15 years as tracers of submarine
groundwater discharge from coastal aquifers. A basic assumption in using these tracers is that
radium 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 large
uncertainties in radium-based estimates of submarine groundwater discharge rates. This project
also represents a first step toward more complex geochemical modeling of coastal aquifer