Groundwater transport and radium variability in coastal porewaters

Abstract

Radium isotopes (223Ra, t1/2 = 11.4 d; 224Ra, t1/2 = 3.66 d; 226Ra, t1/2 = 1600 y; and 228Ra, t1/2 = 5.75 y) are considered excellent tracers of groundwater movement and discharge in coastal systems. However, spatial and temporal variability in porewater radium activity have raised questions about the accuracy of these tracers. To better understand the factors affecting radium variability in coastal systems, measurements of porewater and surface water radium activity were made at an island in North Inlet Salt Marsh in Georgetown, South Carolina, from November 2009 to February 2011. Water salinity, temperature, pH, and redox potential were also recorded, and sediment samples were collected for analysis of bulk 228Ra and 226Ra activity. Hydraulic head observations during 2007–2008 from piezometers on the island were used to generate independent estimates of groundwater fluxes.Porewater radium activities decreased with depth below the marsh surface and increased with distance from the creek banks. Salinity measurements were lower and redox potential higher near the marsh creeks. The stratigraphy of the island is typical of intertidal wetlands in the southeastern U.S., with a mud layer overlying a confined sandy aquifer; the observed patterns in porewater radium, salinity, and redox potential were consistent with (1) shorter porewater residence times in the permeable sand aquifer than in the low-permeability mud, (2) differences in grain size between the mud and sand, and (3) greater tidal exchange near the creeks. Temporal variations in porewater radium activity were not associated with salinity, pH, and redox potential although temperature provided significant control (P < 0.05, r2 < 0.47) over variations in 228Ra and 226Ra activity. Lower mean sea water levels resulted in greater calculated groundwater discharge and were also associated with lower average porewater 224Ra and 223Ra activity, in that groundwater discharge variations strongly affected short-lived radium activity at this site. The 228Ra/226Ra activity ratios in the surface water and porewater signified that the confined aquifer, rather than the surficial mud, was the primary source of radium to the surface water. Our results highlight the importance of understanding the hydrology of any coastal system when interpreting radium results. It is also essential to identify and measure the correct porewater end-member(s) (i.e. source aquifers) when calculating radium budgets.