Award: OCE-0961970

“Collaborative Research: Groundwater Discharge, Benthic Coupling and Microalgal Community Structure in a Shallow Coastal Lagoon”

Project Outcomes Report: Florida State University

Project OCE0961970

Intellectual Merit

This study demonstrated the utility of radon and radium isotopes in identifying areas where submarine groundwater discharge (SGD) is qualitatively important as well as quantifying SGD rates.  Both resistivity and radioisotope measurements suggested that the strongest groundwater source was at the east end of the study site, Little Lagoon, Alabama, an area where toxic diatom blooms occur commonly.  We surveyed the lagoon several times making continuous measurements of temperature, salinity and radon (²²²Rn), which is much higher in groundwater than surface waters so serves as a groundwater tracer.  Figure 1 shows an example of the results from one of these surveys.  The aerial distribution of both radon and salinity can be used to interpret where groundwater inputs are important.  The areas with relatively high radon and low salinity are most likely influenced by groundwater inputs.  After these surveys were completed, follow up sampling was able to focus on the areas of interest identified by the tracers.

We used natural radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra) to estimate the residence time of the lagoon waters (about 11 days on average) and to identify two different groundwater sources (deep and shallow aquifers).  We constructed a three-end-member mixing model to calculate the fractions of lagoon water samples contributed by the two different groundwater sources as well as by the Gulf of Mexico.  Figure 2 shows how the ²²⁸Ra/²²⁶Ra activity ratios are very different between the shallow and deep groundwater sources.  Combining the estimated fractions with a ²²⁶Ra mass balance, we were able to estimate an overall average shallow and deep groundwater discharges of 1.22±0.53 and 1.59±0.20 m³ s^-1, respectively (total discharge = 2.81±0.57 m³ s^-1).  SGD rates based on the Rn mass balance model ranged from 0.60 to 2.87 m³ s^-1 and displayed a temporal pattern in general agreement with the water table elevation in the area.

We observed well-defined relationships between nutrients and chlorophyll-a (Figure 3) during periods when there was a diatom bloom (April 2010) and when no bloom was present (March 2011).  A well-defined relationship between radium (groundwater-derived) and nutrients was clear during the April 2010 sampling, while no relationship existed between the same parameters during the drier March 2011 period when no bloom occurred.  A multivariate analysis was applied to examine possible relationships between SGD, nutrient concentrations, and other environmental factors.  Results indicated that groundwater discharge is a likely driver of algal blooms in Little Lagoon.

Broader Aspects

During the course of this project, two graduate students received training, in both the laboratory and the field, of the latest techniques for collection, measurement and interpretation of natural radioisotopes.  One of these students (Ni Su) earned a Ph.D. based in part on the results she collected during this project.  That student is now a postdoctoral researcher.  Another student (Kristen Eller) finished a M.S. based partly on results from the project and she is now employed as an Environmental Specialist by the Florida Department of...