GROUNDWATER FLOW IN THE UPPER CASTLE HAYNE AQUIFER: NUMERICAL SIMULATIONS AND STRONTIUM ISOTOPIC INVESTIGATIONS

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Samantha Eileen Kofroth (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: ABSTRACT GROUNDWATER FLOW IN THE UPPER CASTLE HAYNE AQUIFER: NUMERICAL SIMULATIONS AND STRONTIUM ISOTOPIC INVESTIGATIONS by Samantha Eileen Kofroth August 2019 Director of Thesis: Dr. Terri L. Woods Major Department: Geological Sciences The Castle Hayne Aquifer System (CHAS) is the most extensively used aquifer system in the North Carolina Coastal Plain. Heavy groundwater withdrawals from the Upper Castle Hayne Aquifer (UCH) have occurred since the 1960's and have caused changes in groundwater flow regimes. These changes require full understanding because they can lead to degradation of one of North Carolina's most valuable freshwater resources. A Principal Component Analysis explained 86.82% of the variance in groundwater chemistry in the UCH. The Component Loadings indicated strong correlations between the changing concentrations of many chemical variables and suggested that influx of saline water into the UCH and limestone dissolution have major impacts on water chemistry. The main goal of this project was to investigate groundwater movement within the UCH aquifer of North Carolina by comparing the results of a strontium-isotope-based , two-component , mixing model to the results of a computer-simulated flow model. The Sr-study suggests sources of water entering the UCH based on 87Sr/86Sr ratios and [Sr] concentrations. The Sr data utilized in this study were obtained from previous studies. A groundwater flow model was developed using Visual MODFLOW. The model was calibrated to current conditions , which include heavy pumping of the UCH. The simulated UCH equipotential map showed that groundwater in the UCH is moving toward the Nutrien phosphate mine from all directions. The map also showed a cone of depression that is similar the observed equipotential map generated from the 2017 water level data. Particle tracking simulations performed with MODPATH under current pumping conditions , and using effective porosities of 15% , 26% and 37 % for the UCH , yielded minimum total travel times from Earth's surface to well screens of 730 years , 825 years , and 920 years , respectively , and groundwater velocities of 0.002-22 , 0.0014-13 , and 0.001-9.15 (feet/day) , respectively. MODPATH simulation results performed under pumping conditions sometimes conflicted with the conclusions drawn from the Sr-study , except where the Sr-analysis indicate downward flow from the Surficial Aquifer through the overlying units. Therefore , alternative non-pumping simulations were performed. During non-pumping simulations , groundwater-flow vectors showed upward groundwater movement from the Lower Castle Hayne Aquifer (LCH) and the Beaufort Aquifer (BF) going into the UCH , for samples that plotted below or to the left of the calculated , two-component , Sr-mixing line. This means that vertical movement of water from units below the UCH could have occurred prior to pumping. Overall , the results of this study indicate that heavy groundwater withdrawals have extensively altered groundwater flow patterns across the North Carolina Coastal Plain. Further research , including the development of a model calibrated to pre-pumping conditions is required to fully understand the groundwater processes that have caused the current Sr-signatures.

Additional Information

Publication

Thesis

Language: English

Date: 2019

Keywords

Strontium isotopes, geochemical mixing model, groundwater flow model, Upper Castle Hayne Aquifer, Visual MODFLOW Classic, North Carolina Coastal Plain Aquifer System.

Subjects

Email this document to