Date of Award

2026-05-01

Degree Name

Master of Science

Department

Geological Sciences

Advisor(s)

Lin Ma

Abstract

Water movement controls how solutes are transported and stored in arid Critical Zone (CZ) systems. In natural drylands, low precipitation and high evapotranspiration lead to long term salt accumulation in the CZ, whereas irrigation increases water input and increases downward solute transport. This study investigates how agricultural practices alter subsurface hydrology and geochemistry by comparing two deep CZ cores from the southwestern United States: a 50 m deep core beneath a flood irrigated pecan farm named 5R farm, in the Rio Grande Valley in Tornillo, Texas, and a 97.5 m deep core from the Red Lake Playa in the Jornada Basin, New Mexico. Lithologic logging, geophysical data, X-ray fluorescence (XRF), and pore water chemistry were used to characterize subsurface structure and solute distributions. Chloride mass balance (CMB) was applied to estimate groundwater recharge. A one-dimensional mobile and immobile transport model was used to evaluate chloride transport and storage behavior within the 5R farm profile. Results show that different soil textures strongly influence water and solute behavior in both systems. The 5R farm has thinner, discontinuous clay layers, near saturated and saturated conditions through much of the profile, and greater vertical connectivity, while the Red Lake Playa has thicker, more continuous clay and gypsum rich units that limit flow. Pore water profiles show these differences, with uniform ion concentrations in the 5R farm core and high near surface concentrations that decrease with depth in the playa, indicating irrigation driven redistribution in the 5R farm and long-term accumulation in the Red Lake Playa. In the 5R farm, carbonate-rich intervals are observed within fine grained layers and may reflect precipitation of dissolved calcium associated with long-term irrigation and earlier CZ processes. These carbonate rich zones may reduce pore connectivity and contribute to localized solute retention and redistribution. The Red Lake Playa shows gypsum rich zones associated with sulfate rich pore water under evaporative conditions, where low recharge and long residence times allow solutes to become concentrated and remain preserved over long timescales. CMB results show higher recharge in the 5R farm (252 ± 149 mm yr-1) compared to the Red Lake Playa (0.44 ±0.28 mm yr-1). The chloride timescales show these differences as well, with the 5R farm core showing ~79 years of solute redistribution, while the Red Lake Playa shows long term accumulation over ~13,600 years, including evidence of past wetter conditions. Transport modeling shows that chloride movement is not controlled by downward water flow alone but is influenced by exchange between mobile and immobile water domains. The results further suggest that irrigation both introduces new dissolved salts and redistributes salts already stored within the subsurface, rather than complete flushing the profile. This study showed that recharge, saturation conditions, subsurface heterogeneity, an mobile-immobile exchange together influence solute transport, while the subsurface structure controls solute storage in arid CZ systems. In the 5R farm increased water input increases vertical connectivity and redistributes salts downward, but they are not fully flushed due to retention in fine grained layers where observed carbonate accumulations may reduce permeability. In the Red Lake Playa, it has low recharge and strong evaporation, which leads to accumulation of salts, in gypsum and clay rich layers These findings are important when looking into anthropogenic water inputs on subsurface processes and the implications for groundwater recharge, salinity evolution, and long-term water resource sustainability in arid environments.

Language

en

Provenance

Received from ProQuest

File Size

92 p.

File Format

application/pdf

Rights Holder

Angie Lynette Cano

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