Loxahatchee Project Background

The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series (²²²Rn, ^{223,224,226,228}Ra) were studied during high and low discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge (SGD) in estuarine transport.

The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (<5 mg/L) close to zero salinity and increased several-fold (~18 mg/L; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet.

Figure 2. Cypress forest in the Loxahatchee River Estuary.

Figure 3. Mangrove forest in the Loxahatchee River Estuary.

Surface water-column ²²²Rn activities were most elevated (>28 dpm/L) at the freshwater endmember of the estuary and appear to identify regions of the river most influenced by the discharge of fresh groundwater. Activities of four naturally occurring isotopes of Ra (^{223,224,226,228}Ra) in this estuary and select adjacent shallow groundwater wells yield mean estuarine water-mass transit times of less than 1 day; these values are in close agreement to those calculated by tidal prism and tidal frequency. Submarine groundwater discharge rates to the Loxahatchee River estuary were calculated using a tidal prism approach, an excess ²²⁶Ra mass balance, and an electromagnetic seepage meter.

Average SGD rates ranged from 1.0 to 3.8×10⁵ m³/d (20-74 L/m²d), depending on river-discharge stage. Such calculated SGD estimates, which must include both a recirculated as well as fresh water component, are in close agreement with results obtained from a first-order watershed mass balance. Average submarine groundwater discharge rates yield NH₄⁺ and PO₄^-3 flux estimates to the Loxahatchee River estuary that range from 62.7 to 1063.1 and 69.2 to 378.5 µmol/m²d, respectively, depending on river stage. SGD-derived nutrient flux rates are compared to yearly computed riverine total N and total P load estimates.

Conclusions

• All three SGD methods (tidal prism, Ra, EM seepage meter) provide internally consistent results
  
• Near continuous ²²²Rn results confirm that most groundwater discharges upstream
  
• Streaming resistivity profiling results provide information as to the position/extent of the freshwater/saltwater interface
  
• Role of SGD in estuarine biogeochemical transport is investigated (for example, SPM, U, DOC, Ba)

Click on the images below to open a printable version of the USGS Report in Adobe Reader.

OFR 2004-1226 (808 KB PDF)

Submarine ground water discharge and its role in coastal processes and ecosystems http://sofia.usgs.gov/publications/ofr/2004-1226/

FS 2004-3117 (1.16 MB PDF)

Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Biscayne Bay, Florida http://sofia.usgs.gov/publications/fs/2004-3117/

OFR 2004-1369 (1.03 MB PDF)

An autonomous, electromagnetic seepage meter to study coastal groundwater/ surface-water exchange http://sofia.usgs.gov/publications/ofr/2004-1369/