Date of Award
1-1-2025
Document Type
Dissertation
Degree Name
Ph.D. in Engineering Science
First Advisor
Gregg R. Davidson
Second Advisor
Andrew M. O'Reilly
Third Advisor
Gregg R. Davidson
School
University of Mississippi
Relational Format
dissertation/thesis
Abstract
Groundwater age is a useful tool for characterizing various aspects of groundwater availability. No aquifer-wide groundwater age characterization was completed for the Mississippi River Valley alluvial aquifer (MRVA) before this study. Using 3H, 14C, SF6, and 3Hetrit concentrations in groundwater samples from the MRVA and some underlying aquifers of the Mississippi embayment, this study estimated age distributions of samples with lumped parameter models (LPMs) in the U.S. Geological Survey (USGS) TracerLPM software and investigated the factors controlling groundwater age in the MRVA. To better understand how hydrogeologic heterogeneity affects groundwater age, each sample’s age distribution coefficient of variation [Cv(LPMAD)] was compared to hydrogeologic heterogeneity metrics (HHMs) of the sample’s capture zone based on resistivity [proxy for hydraulic conductivity (K)] models of two different scales, to check for relations expected based on groundwater flow principles. Major findings included the following:
1. The MRVA’s configuration in an area likely impacts the spatial distribution of age there.
2. Groundwater sample mean ages throughout the MRVA were found to be influenced by depth, soil saturated K, lithologic potential for hydraulic connection between the MRVA and an underlying unit, and whether an aquifer or confining unit underlies the MRVA in the sampling location.
3. In some regions of the MRVA, groundwater sample mean ages exhibited different relations with recharge rates and pumpage rates.
4. Most groundwater samples had high Cv(LPMAD) and high coefficient of variation of flow path resistivity coefficients of variation, suggesting that differences among flow paths in their degrees of heterogeneity led to dispersion via mixing between molecules (on adjacent flow paths with different degrees of heterogeneity) that likely move in different directions and, during unsteady flow, at different rates from each other. These flow paths do not necessarily have different effective K values and are likely adjacent to each other (part of the same assemblage coming from the same recharge zone).
5. Comparisons of Cv(LPMAD) to HHMs based on resistivity models of two scales suggested that the greater level of detail in the finer-scale model caused a bias of some metrics that needed to be filtered out to make the model meaningful.
Recommended Citation
Gratzer, Michael, "Groundwater age estimates for the Mississippi River Valley alluvial aquifer and comparisons with hydrogeologic heterogeneity" (2025). Electronic Theses and Dissertations. 3285.
https://egrove.olemiss.edu/etd/3285