The dissertation reports a series of hydrological studies that relates to seawater intrusion in a karst aquifer. In the
Woodville Karst Plan (WKP), karst conduit system is well developed and directly open to the Gulf of Mexico that allows seawater intrudes
into the aquifer. The Spring Creek Springs Complex and Wakulla Spring, located in a marine estuary and 11 miles inland, respectively, are
the two major groundwater discharge spots and connected through subsurface conduit network in the Woodville Karst Plain (WKP), North
Florida, USA. Investigation of seawater intrusion in the Woodville Karst Plain is the objective and motivation of this dissertation work.
Potential evidence of the longest documented seawater intrusion through conduit network in the Woodville Karst Plain (WKP) is found by the
data analysis of electrical conductivity and chemical measurements. Five periods of increased electrical conductivity have been observed
in the karst conduits supplying water at Wakulla Spring, one of Florida's largest first magnitude springs. A composite analysis of
rainfall, electrical conductivity and geochemical data provides strong evidence that the increases in conductivity are directly tied to
saltwater intrusion occurring at the Spring Creek Springs through the conduit network. This interpretation is supported by the conceptual
model established by prior researchers, and represents the first and longest documented case of saltwater intrusion through conduit
network in the WKP. Several numerical models are able to simulate the density-dependent seawater intrusion issue. On the other hand,
discrete-continuum numerical models are designed to simulate groundwater flow and solute transport in a dual-permeability karst aquifer.
However, none of the pre-existed code or model is able to deal with the two issues together. Therefore, a hybrid discrete-continuum
numerical model of Variable-Density Flow and Solute Transport - Conduit Flow Process (VDFST-CFP) is developed as a new modeling method
that provides more accurate simulation of seawater intrusion in a coastal karst aquifer with conduit network. Darcy-Weisbach equation is
applied to simulate non-laminar groundwater flow in the conduit system. Density-dependent groundwater flow with appropriate density terms
in both the conduit and porous media systems are analytically derived, then coupled with transport equations and solved numerically using
finite difference method with an implicit iteration procedure. Two synthetic two-dimensional cases are developed to validate the newly
developed VDFST-CFP model by comparing with other numerical models. The VDFST-CFP model improves simulations of density-dependent
seawater/freshwater mixing processes and exchanges between the two domains. In comparison with the discrete-continuum models, Darcy
equation of the continuum numerical models overestimates the flow rate but the VDFST-CFP is accurate under turbulent flow condition. The
pros and cons of model uncertainties, assumptions, conceptual simplifications and numerical techniques of the VDFST-CFP are discussed.
Several studies of numerical modeling have been done as an important method to evaluate seawater intrusion in coastal karst aquifers with
conduit network, since field observations are usually insufficient. A regional groundwater flow cycling numerical model is developed to
provide a general understanding of the flow regime that controlled by seawater/freshwater interaction in the WKP, using a
discrete-continuum CFPv2 model. Non-laminar flows in conduits and flow exchange between the two domains are coupled in the hybrid
numerical model. The time-variable salinity and equivalent freshwater head at the submarine spring have significant impacts on
seawater/freshwater interaction and discharges of springs. Simulated results match well to measurements with correlation coefficients
0.891 and 0.866 at Spring Creeks Springs and Wakulla Springs, respectively. The impacts of sea level rise on regional groundwater flow
field and the relationship between the two springs are evaluated as well by the numerical model. The controlling factors of seawater
intrusion in a dual-permeability are evaluated by local and global parameter sensitivity analysis with a two-dimensional SEAWAT model,
which also estimates the extents of seawater intrusion in the WKP. The local sensitivity study indicates the salinity at the submarine
spring is the most parameter to simulations in both the conduit and porous medium, which are effective to all parameters near the mixing
zone. The results of global sensitivity analysis exhibit similar pattern with the local study but are different for some parameters due to
their non-linear relationship to the simulations. Simulations in the porous medium are sensitive to not only matrix parameters but also
conduit conditions because of the conduit-matrix interaction. Dispersivity is important in a homogeneous porous medium model but is no
longer significant in an advection-dominated karst system. The effects of the identified important parameters on the extents of seawater
intrusion are quantitatively evaluated by examining the variation of salinity at the submarine spring with rainfall recharge, sea level
rise and longer simulation time under an extended low rainfall period. / A Dissertation submitted to the Department of Earth, Ocean and Atmosphere Science in partial
fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2016. / February 16, 2016. / Data analysis, Karst aquifer, Non-turbulent conduit flow, Numerical modeling, Seawater intrusion / Includes bibliographical references. / Bill Hu, Professor Directing Dissertation; Ming Ye, University Representative; Stephen Kish,
Committee Member; Doron Nof, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_360486 |
| Contributors | Xu, Zexuan (authoraut), Hu, Bill X. (professor directing dissertation), Ye, Ming (university representative), Kish, Stephen A. (committee member), Nof, Doron (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Earth, Ocean, and Atmospheric Science (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (124 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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