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 <sup>234,238</sup>U, <sup>226</sup>Ra, and <sup>222</sup>Rn. 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 <sup>222</sup>Rn, <sup>226</sup>Ra, and <sup>234,238</sup>U 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 <sup>222</sup>Rn from sediments, and <sup>226</sup>Ra 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<sup>-2</sup> y<sup>-1</sup> to the lagoon. A one-dimensional <sup>222</sup>Rn 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 m<sup>3</sup> d<sup>-1</sup> m<sup>-1</sup> 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.
| Identifer | oai:union.ndltd.org:LSU/oai:etd.lsu.edu:etd-07022008-050958 |
| Date | 02 July 2008 |
| Creators | Smith, Christopher Gerald |
| Contributors | Cable, Jaye E., Blanford, William J., Gambrell, Robert P., Li, Chunyan, Martin, Jonathan B., Tsai, Frank Tsung-Chen |
| Publisher | LSU |
| Source Sets | Louisiana State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lsu.edu/docs/available/etd-07022008-050958/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached herein a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to LSU or its agents the non-exclusive license to archive and make accessible, under the conditions specified below and in appropriate University policies, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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