Testing the use of Naturally Occurring Radium Isotopes to Quantify Groundwater Discharge and Recharge

Natural distributions of radium isotopes are increasingly being used to evaluate water and solute transport in a variety of environments. Using these isotopes to trace groundwater flow is complicated, and typically, many simplifying assumptions are made to permit application of analytical or numerical models to the data. However, many of these assumptions have not yet been adequately treated, and the validity of the technique has been questioned. In this study, radium isotopes are used to determine vertical exchange through peat sediments overlying karstic limestone at two different sites in the Florida Everglades. One site is located in an area of groundwater recharge, and the other is in an area of groundwater discharge. Here we present results from the two sites and examine the errors associated with assumptions about the vertical homogeneity of radium production, sediment porosity and dry bulk density, sediment versus pore water partitioning of the radium (K_D), and ionic strength of the pore water. Many of these sediment characteristics have interrelated effects on the radium profiles. For instance, an increase in chloride concentration with depth will cause a downward decrease in K_D. Dissolved radium activities in such a zone will increase with depth and may resemble profiles caused by upward advection in the absence of salinity variation. Similarly, an increase in dry bulk density with depth may cause an increase in volume-weighted radium production with depth, in which case the radium profile could erroneously suggest vertical advection. The impact of these factors was modeled to assess the reliability of this technique for this environment, and in general as a tracer of groundwater flow.