226Ra and 228Ra in the mixing zones of the Mississippi and Atchafalaya Rivers: indicators of groundwater input

Naturally occurring isotopes of radium were measured in the Gulf of Mexico in the discharge zones of the Mississippi and Atchafalaya Rivers during 1993 and 1994, and a model was developed to help interpret the relative contributions of different end-members to the mixing zones. Samples collected from below the pycnocline were enriched in 226Ra and 228Ra, much beyond activities expected for surface samples of similar salinities. The elevated bottom water radium activities could not be supported by diffusion from bottom sediments, nor from resuspension and desorption of regenerated radium. They appear to originate from subterranean seepage of radium-rich groundwater which, while seeping through the sediments, also enriches these sediments in radium (by adsorption). From the measured riverine dissolved and particulate loads, the total 226Ra flux was estimated to be 1.1×1014 dpm year−1 (disintegrations per minute per year) from the Mississippi River and 0.5×1014 dpm year−1 from the Atchafalaya River. The 228Ra flux was estimated to be 1.3×1014 dpm year−1 from the Mississippi River and 0.6×1014 dpm year−1 from the Atchafalaya River. However, surface waters in both river plumes contained radium activities exceeding the maximum activities expected from river and ocean inputs. There is evidence that the frequency with which radium-rich water from below the thermocline gets mixed with the surface water in the plume depends to a large extent on local weather conditions and the depth of the water column.