Date of Award

2025-12-01

Degree Name

Master of Science

Department

Environmental Sciences

Advisor(s)

Lin Ma

Abstract

Irrigation in agricultural systems alters hydrological cycles by redistributing surface water and groundwater, modifying Critical Zone elemental cycles, and impacting water quality and availability. Here, I focus on understanding agrohydrologic processes in Dryland Critical Zones and the impacts of land-use changes and climate variability in the extensively irrigated agricultural Upper Snake-Rock Watershed in semi-arid south-central Idaho. The Snake River originates in Wyoming and flows across southern and central Idaho. The Snake River supplies irrigation water to the Kimberly and Twin Falls areas of Idaho, the focus of this study, which is characterized by intensive agricultural activity. With the increasing pressure of food production for future population growth, monitoring water quantity and quality is a top priority. Many existing projects have provided valuable information for understanding agrohydrological processes in the study area. However, these processes are still not fully understood, particularly how subsurface flow in the shallow soil zones and the upper fractured basalt aquifer systems return excess water and nutrients from agricultural fields back to the Snake River. In this study, different types of agriculture-related water samples were collected from both irrigation seasons and non-irrigation seasons from multiple locations along the flow paths of the regional ground water systems. Sample locations include Snake River, agricultural irrigation canals, regional groundwater wells, and underground tunnels. Water chemistry parameters (salinity, pH, alkalinity, major and trace elements) and isotope ratios of 87Sr/86Sr and 234U/238U were measured in these samples to identify and quantify the agrohydrological signatures and improve our understanding of these processes. My study revealed that 87Sr/86Sr and 234U/238U ratios showed changes from irrigation to non-irrigation seasons and along the flow paths from east to west. Along with the correlation with concentrations of HCO3, lower 87Sr/86Sr and 234U/238U ratios revealed the presence of increased basalt weathering underneath agricultural fields. Furthermore, ions like Mg, Ca, and Na showed higher concentrations during non-irrigation seasons and correlated well with HCO3, providing evidence of increased chemical weathering and potential carbonate precipitation. Trace metals and major elements of interest for water quality concerns were also examined in this study. Trace elements like As, B, Cu, and Ni were released into water from the increased weathering of basalt during the agrohydrological processes. Low level of agricultural anthropogenic pollutants (P and NO3) was also observed in these water samples from most likely from fertilizer runoff, and their behaviors were consistent with their mobility factors in the environment. To further examine the water chemistry, a long-term dataset of major elements from 2016-2024 collected by the USDA Kimberly branch was analyzed and compared to the collected samples in this study. A long-term trend of variations in major element concentrations and elemental ratios (such as Ca, Cl, Mg, Na, NO3, and P) showed similar correlation with the major elements released during increased basalt weathering, providing insights into the long-term impacts of the agricultural activities and developments in this region.

Language

en

Provenance

Received from ProQuest

File Size

81 p.

File Format

application/pdf

Rights Holder

Jennifer Herrera

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