Identifier

etd-07022008-050958

Degree

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

Dynamic balances between fresh groundwater, saline groundwater, and surface water control the physics and chemistry of subterranean estuaries. Investigations were performed in a subterranean estuary on physical processes contributing to fresh groundwater and saline water mixing, vertical and lateral positioning of this mixing zone, and how this mixing affects spatial and temporal distributions of 234,238U, 226Ra, and 222Rn. The subterranean estuary is located in an unconfined aquifer beneath Indian River Lagoon, Florida, USA, where I could examine redox responses to altered flow regimes. Continuous groundwater and surface water level measurements suggest a strong hydrologic connection between this lagoon and the groundwater-aquifer system. Periodic forces (e.g. tides, frontal passages) represent a minor contribution to water level variations; however, forcing by tropical cyclones creates large variations in mixing within the seepage outflow. Hurricane Wilma and Tropical Storm Tammy caused hydraulic gradients to reverse, causing lagoon water to recharge the aquifer and shifting the seepage face and subterranean estuary landward about seven meters. Additionally, seasonal distributions of dissolved 222Rn, 226Ra, and 234,238U in pore and surface waters revealed sensitivity to temporal and spatial mixing and geological heterogeneity. Processes affecting the distribution of these elements include U redox cycling, heterogeneous production of 222Rn from sediments, and 226Ra release during Mn-hydroxide reduction and/or surface exchange. Uranium cycling within the subterranean estuary resulted in a flux of approximately 54 μmol U m-2 y-1 to the lagoon. A one-dimensional 222Rn transport model was used to quantify fresh and marine sources to submarine groundwater discharge (SGD) through incorporation of heterogeneous production, diffusive, advective, and nonlocal transport mechanisms, and Monte Carlo simulations. Model-based volumetric estimates of fresh and marine SGD components yield ranges of 1.01 to 1.85 and 1.69 to 3.43 m3 d-1 m-1 of shoreline, respectively, suggesting fresh SGD contributes approximately one-third of total discharge measured within this subterranean estuary. The 30% fresh component in discharge and the uranium source to coastal lagoon demonstrates SGDs role in global ocean freshwater and elemental inputs. This study highlights the significance of distinguishing fresh and marine groundwater sources and the hydrogeological and chemical complexity of these dynamic subterranean mixing zones.

Date

2008

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Cable, Jaye E.

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