Data Analysis and Numerical Modeling of Seawater Intrusion Through Conduit Network in a Coastal Karst Aquifer

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.