Groundwater of Pahvant Valley, Millard County, Utah
Authors: Gavin Greg , Inkenbrandt Paul , Schlossnagle Trevor , Molinari Rebecca 
Published: November 2024

Pahvant Valley, located in Millard County, Utah, encompasses 1610 square miles and includes several small towns, agricultural districts, hot springs, and biologically important wetlands, all heavily reliant on groundwater. This study, conducted by the Utah Geological Survey during 2022 and 2023, aims to define Pahvant Valley’s water recharge and discharge estimates, characterize its primary hydrogeologic units, and describe groundwater recharge and discharge areas. The research includes the collection of groundwater and surface water samples to estimate flow paths, sources of recharge and discharge, and residence times. Additionally, a water-level campaign was conducted in early March 2022 to create an updated potentiometric surface map for the region. Pahvant Valley’s groundwater system comprises three main aquifers: the valley-fill aquifer, the volcanic aquifer of the Tabernacle Hill and Ice Springs lava flows, and the Black Rock Desert volcanics. For this study, we delineated these aquifers into three conceptual groundwater zones based on hydrogeologic, geochemical, and potentiometric characteristics. Results of this study indicate significant groundwater level declines, particularly in agricultural areas, driven by overextraction and reduced recharge. Groundwater levels have declined by an average of 26 feet since 1986 and some areas have experienced declines of up to 160 feet. The study emphasizes the crucial role of streamflow from the Pahvant Range in recharging the valley-fill aquifer, with stable isotope and chemical analyses confirming that stream discharge significantly contributes to groundwater recharge. Additionally, groundwater quality varies across the valley and increased total dissolved solids could affect water usability in some areas. The analysis of irrigation practices reveals a significant shift in the early 1990s from flood irrigation to pivot irrigation, which led to increased and more consistent crop density and health, as indicated by Normalized Difference Vegetation Index (NDVI) data. From 1992 to 2021, NDVI values rose substantially in both magnitude and duration, reflecting higher crop yields over time. This increase in crop density and vitality resulted in higher evapotranspiration per acre, while the extended duration of greenness demonstrated stable yields regardless of surface water availability. As irrigation shifted from surface water to groundwater sources, numerous high-capacity wells were established to meet demand. Consequently, groundwater pumping in areas no longer reliant on surface water led to declines in groundwater elevations. These decreases in groundwater levels have been linked to land subsidence, with Interferometric Synthetic Aperture Radar (InSAR) analysis detecting up to 5 inches of ground deformation in the Meadow area between 2014 and 2022, closely associated with changes in groundwater levels. This study underscores the critical balance between groundwater extraction and recharge, the effects of irrigation practices on water use, and the importance of continuous monitoring and management to ensure sustainable groundwater resources. The findings highlight the need for sustainable groundwater management practices to maintain agricultural productivity and ecological health in Pahvant Valley.