GROUNDWATER TRANSPORT AND RADIUM VARIABILITY IN COASTAL POREWATERS

Submarine groundwater discharge (SGD) is an important source of nutrients and other solutes from coastal aquifers to estuaries and coastal oceans. The four isotopes of radium (²²³Ra, t_1/2=11.4 d; ²²⁴Ra, t_1/2=3.66 d; ²²⁶Ra, t_1/2=1600 y; and ²²⁸Ra, t_1/2=5.75 y) have been essential tracers for estimating SGD, but spatial and temporal variations in isotope activity make it difficult to determine the groundwater endmember for these methods. In this study, we developed a groundwater flow and transport model to calculate SGD and identify controls on observed spatial and temporal variations Ra isotope activities in groundwater at a salt marsh island at North Inlet, South Carolina. Groundwater age was also simulated to provide an independent measure of groundwater transport. A primary goal of the modeling was to test prior statistical correlations that suggested that spatial and temporal variations in Ra activity are controlled by variations in groundwater transport.

Model experiments confirmed that groundwater transport is an important control on spatial variations in Ra activity, particularly through lower flow rates in a surficial mud layer than in an underlying sandy aquifer and greater tidal exchange near the marsh creeks than in the marsh interior. Spatial variations in Ra activity were also influenced by variations in sediment grain size, which affects the fraction of Ra that becomes mobile. Simulations showed that decreased SGD during seasonal highs in sea level caused measurable increases in groundwater age, but these variations failed to reproduce observed temporal variations in Ra activity. Instead, results from this marsh island suggest that temporal variations in measurements of ²²³Ra and ²²⁴Ra were primarily caused by variations in Ra sorption coefficients. Temporal variability in Ra activities was largely captured by the model when the distribution coefficient governing Ra sorption was allowed to vary as a function of temperature. The remaining discrepancies between simulated and measured Ra suggest that temporal variations in Ra activities are influenced by redox-controlled variations in Ra sorption. Our results corroborate existing conceptual models of flow within layered intertidal salt marshes and highlight the value of combining numerical and tracer methods to estimate SGD.