Task 2: Groundwater/Surface Water Influence on Wetlands and Benthic Habitats

The Tampa Bay Estuary Program (TBEP) has identified freshwater discharge into Tampa Bay as a critical issue in maintaining the proper salinity and water quality of the bay and thus the health of a variety of benthic habitats. A combination of shallow seismic and resistivity mapping will aid in locating point sources of groundwater inflow into the bay. Anecdotal evidence suggests that a direct linkage between areas of seagrass die-off and submarine water discharge exists. Radon and a suite of isotopes will be used to quantify groundwater/ surface water exchange rates and thus will quantify the flux of groundwater into the bay. Nutrient and elemental measurements will be used to determine quality of groundwater and surface water. Long-term (up to 20 years ago) freshwater inflow, salinity, and nutrient patterns will be evaluated to aid in reconstructing historical conditions of wetlands. These data will be utilized to examine linkages of water quality and Tampa Bay ecosystem health, e.g., seagrass sustainability.

Groundwater /Surface Water
In the two study localities described in Task 1 (Fig. 1) we will examine aspects of groundwater influences in the bay on ecosystem health. These influences include groundwater inflow and withdrawal. Specifically we will

identify areas of groundwater inflow at both sites
quantify amount of groundwater at each location
determine the quality of the groundwater through the long-term flux of selected
constituents.

The mapping exercises performed in Task 1 (specifically shallow seismic mapping) along with resistivity mapping will provide insight into potential point source locations for groundwater inflow and withdrawal at each location.

Task 2A: Integrated Studies on the Subsurface and Surface Hydrogeology of Study Sites

Task Participants:
Peter Swarzenski (USGS/GD), Mario Fernandez (USGS/WRD), Yvonne Stoker (USGS/WRD), Paul Carlson (FMRI), Don Axelrad (DEP), Darryl Felder (ULL), Paul Klerks (ULL), Victor Levesque (USGS/WRD), and Paul Boetcher (USGS/WRD).

Resistivity profiling at the Alafia River site and Terra Ceia Aquatic Preserve site near the Braden River will be performed and provide information on areas of submarine groundwater inflow. Radon measurements will provide information for aquifer water discharging into the Bay. Current conditions of salinity and freshwater inflow will be determined. These data will be used to evaluate watershed influence on wetland and bay ecosystems.

Task Summary and Objectives:

Identify subsurface freshwater sites within the Bay.
Resistivity maps generated (in cooperation with USGS/WRD) will be used in conjunction with shallow seismic data (from Task 1) to identify potential sites of subsurface freshwater.
Quantify exchange rates across the sediment-water interface.
Measure nutrients and solutes (SO4, NH4, PO4, SiO2, TN, TSN, TSP, TP), conservative constituents (Cl) and select isotopes (Ra, Rn, Sr) to identify unique subsurface water masses and their exchange rates by using custom multi-samplers (‘mini-piezometers’). Nutrient flux measurements will be coordinated with proposed efforts by FMRI (Paul Carlson), DEP (Don Axelrad), USGS/WRD, and ULL (Darryl Felder and Paul Klerks).
Evaluate salinity and nutrient patterns at study sites and compare with freshwater inflow.
Historical information (USGS/WRD) on salinity, nutrients and freshwater inflow associated with the Alafia River site will be compiled and examined to reconstruct changes in these parameters. This information will be used to calibrate salinity data derived from salinity proxies in cores and aid in establishing levels of confidence for use of salinity proxies for prehistorical salinities. Historical nutrient data will be compared to historical information on changes in productivity derived from productivity proxies in cores to aid in establishing the relationship between nutrient levels and primary productivity.

Task 2B: Collection and synthesis of long-term nutrient data

The Tampa Bay Estuary Program (TBEP) has identified nutrients and nitrogen loading to the bay as a critical issue, and restoration efforts are focused on reducing the total nitrogen input. Information on nutrient input into the bay derived from groundwater sources (from Task 2A) will be used in conjunction with nutrient flux data from tributaries located near the study sites to refine and verify nutrient loading estimates for the Alafia River study area.

Nutrient flux data from the Alafia River will provide information on the timing and magnitude of nutrient inputs to Tampa Bay. This information will assist USGS/BRD scientists in wetland studies at the mouth of the Alafia River and provide verification data for TBEP nutrient loading estimates for Tampa Bay to help guide future management decisions on methods of reducing nutrient inputs to Tampa Bay.

Task Summary and Objectives:

Nutrient Data: Collect nutrient data at the Alafia River near the mouth of the river during discharge measurements to determine nutrient fluxes. This information can also be used to evaluate whether nutrient regression models developed during the early 1990’s can be used to estimate current nutrient fluxes.
Long-term Loads: Using existing and newly acquired data, long-term (10 years) loads of selected nutrients will be calculated.