A geochemical and isotopic assessment of hydraulic connectivity of a stacked aquifer system in the Lisbon Valley, Utah (USA), and critical evaluation of environmental tracers

This study investigates hydraulic connectivity of a stacked aquifer system in the Lisbon Valley of southeastern Utah (USA), within the Paradox Basin, where numerous faults may act as conduits or barriers to cross-formational and intraformational flows. Hydraulic connectivity of the Burro Canyon Aquifer (BCA) and Navajo Aquifer (NA) is assessed using major ion chemistry, trace elements, and a suite of isotopes including δ18O, δD, δ34S-SO4, δ18O-SO4, 3H, δ13C-DIC, 14C, and 87Sr/86Sr. These environmental tracers are also examined using principal component analysis (PCA) to gain insight into which groups of tracers are related and provide the most definitive information about hydraulic connectivity. All geochemical and isotopic results show distinct groupings between the aquifers, suggesting minimal hydraulic connection. Groundwater from the BCA is meteoric in origin, has residence times that indicate recharge during the Holocene, and has a geochemical and isotopic signature reflective of the carbonate lithology. Groundwater from the NA inherits its signature from the host eolian sandstone, with possible minor input from underlying salt-derived brines, and is characterized as meteoric in origin, with residence times that indicate late Pleistocene recharge. PCA further contrasts groupings between the aquifers, while showing many tracers are related depending on lithology, residence times, and redox conditions. Finally, anomalous groundwater chemistry encountered at three wells highlights the benefits of using a multitracer approach to characterize complex hydrogeochemical processes.