Four naturally occurring isotopes of radium (^{223,224,226,228}Ra) have a range in half-life that extends from a few days to over 1,600 years. Unique geochemical attributes make these radium isotopes ideal to examine sediment/water interface exchange processes in coastal waters. Here we present radium isotopic data from a recent field excursion to Florida Bay, a heavily-impacted coastal system in south Florida.

Figure 1. Florida Bay. Click on image to open larger picture (20.2k).

Florida Bay is a shallow, brackish, semi-enclosed water body that receives most of its limited freshwater supply from the Everglades, principally by surficial water runoff through Taylor Slough/Trout Creek. Because the entire region is underlain by highly porous limestone and due to other hydrologic constraints, there is the possibility that ground water flow may be a significant input into Florida Bay. To evaluate the extent of such a subsurface contribution, radium isotopes were determined in two shallow wells, a seepage meter site, and a series of water column samples across a salinity gradient away from Taylor Slough.

All four radium isotopes were at least an order of magnitude greater in the two shallow well samples than in the water column samples. For example, ²²⁶Ra ranged from about 0.50 dpm L^-1 at a salinity of 5 to over 13 dpm L^-1 in Well B (salinity = 47.2). Isotopic radium ratios reveal that the well waters (i.e., marine ground water) are geochemically distinct from surficial waters and are regenerated on a time-scale of several days (i.e., ²²³Ra/²²⁴Ra). Results indicate that this radium quartet can be used effectively in Florida Bay to examine the exchange of surficial water and ground water.

Land-sea interfaces are complex biogeochemical reactors. In such environments, allochthonous/autochthonous source functions are the sum of RIVERINE, OCEANIC, ATMOSPHERIC, GROUND WATER and IN SITU inputs. Ground water discharge can be a large component of terrestrial runoff and is controlled in part by aquifer recharge, hydraulic conductivity, permeability, transmissivity, evapotranspiration and tidal forcing. Submarine ground water discharge (SGD) can be either fresh water, marine or a mixture of both. The nature of SGD yields information on the history of ground water evolution/transport. In Florida Bay, subsurface flow appears to be quite widespread, at least partially of marine origin, and strongly dependent on tidal forcing and local precipitation. In this system, the radium quartet offers detailed time-dependent information on the evolution and transport of subsurface flow and hyporheic exchange.

Next: Background Information

Related information:

SOFIA Project: Tracing the Mixing of Groundwater into Coastal Waters Utilizing a New Radiometric Technique: Radium Isotope Systematics to Look at the Geologic Control of Aquifers