5-10 From Snowmelt to Aquifer: Isotopic Characterization of Recharge to Shallow Groundwater in Cache Valley, Utah

Session: Advances in Mountain Hydrology: Connecting Cryosphere, Surface, and Subsurface Processes

Presenting Author:

Luke Alder

Authors:

Alder, Luke A.¹, Newell, Dennis L.², Neilson, Bethany T.³, Vaughn, Derrick R.⁴, Yoklavich, Tyler⁵, Hill, Devon⁶, Tennant, Hyrum⁷, Blau, Bryan⁸

Abstract:

Cache Valley, Utah-Idaho is a semi-arid intermontane basin that depends heavily upon groundwater to meet municipal, agricultural, and industrial demands. The region's groundwater system generally consists of shallow unconfined and confined aquifers to depths of ~60 m, underlain by a deeper, principal aquifer extending to ~400 m. Aquifers in central Cache Valley near Logan, Utah are primarily recharged from three main sources: mountain-block recharge, canal seepage, and surface water losses along the mountain-front. A large portion of mountain-front contributions to aquifers likely originate from the Logan River, one of the major tributaries to the Bear River flowing from the Bear River Range. However, the relative contribution of each of these recharge sources to the uppermost aquifers remains undetermined.  

In this study, we collected 30 samples from shallow (≤ 60 m) wells within 2 km of the Logan River to assess hydrologic connectivity between the river and underlying shallow aquifers. We use ion chemistry and stable isotope ratios (δ¹⁸O, δ²H, δ¹³C) as tracers to estimate source-contributions and infer flow paths. Preliminary median δ-values from canals (δ²H = –127.2‰; δ¹⁸O = –17.00‰), the Logan River  (δ²H = –127.4‰; δ¹⁸O = –16.99‰), and wells (δ²H = –123.8‰; δ¹⁸O = –16.46‰) are isotopically similar, suggesting that Logan River and canal infiltration contribute to groundwater recharge in this system. D-excess values from canals (8.6‰), the Logan River (8.6‰), and wells (7.5‰) further support this connection, although slightly lower d-excess in wells may reflect mixing with other groundwater inputs. Additionally, δ¹³C of dissolved inorganic carbon (DIC) shows greater variability among sampled sources (canals: –8.86‰; river: –9.09‰; wells: –11.18‰), indicating that additional subsurface biogeochemical processes or water–rock interactions are taking place. 

These preliminary results provide evidence of surface water contributions to shallow groundwater. We anticipate end-member mixing analysis and k-means clustering using δ-values will further differentiate isotopic signatures associated with mountain-front recharge, canal seepage, and deeper groundwater inputs. Results from this study will refine understanding of recharge mechanisms in karst-influenced basins and support sustainable groundwater management in Cache Valley amid increasing population growth and climatic variability. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8589

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