GROUNDWATER TRANSPORT AND RADIUM VARIABILITY IN COASTAL POREWATERS

Radium isotopes (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) are important tracers of groundwater movement and discharge in coastal systems, but spatial and temporal variability in porewater Ra activity can strongly affect flow estimates. To better understand the factors affecting Ra variability in coastal systems, a field and modeling study was conducted at an island in North Inlet Salt Marsh in Georgetown, South Carolina. Porewater and surface water samples were collected from November 2009 to February 2011, for Ra, salinity, temperature, pH, and redox. Sediment samples were collected for analysis of bulk ²²⁸Ra and ²²⁶Ra activity. Prior hydraulic head observations from piezometers on the island were used to generate independent estimates of groundwater fluxes and to calibrate a numerical model of groundwater flow for the island.

Porewater Ra activities decreased with depth below the marsh surface and increased with distance from creeks. These patterns were consistent with variations in porewater residence time associated with increases in permeability and sediment grain size with depth, and greater tidal exchange near the creeks. Temporal Ra variability in porewater was largely consistent with variations in groundwater transport due to changes in mean water level. Higher mean water resulted in less groundwater discharge and longer porewater residence times. Higher mean water was also associated with greater average porewater Ra activity. The ²²⁸Ra/²²⁶Ra activity ratios measured in the confined aquifer near the tidal creeks indicated that the confined aquifer is the primary source of Ra from this site to the tidal creeks. Our field results highlight the importance of understanding the hydrology of any coastal system when interpreting radium measurements and of identifying and sampling the correct porewater end-member(s) for Ra budgets.