GEOCHEMICAL AND ISOTOPIC ASSESSMENT OF REGIONAL GROUNDWATER FLOW AND AQUIFER CONNECTIVITY IN THE LISBON VALLEY, UTAH

We collected and analyzed geochemical and isotopic samples from a series of wells within the metal-rich Lisbon Valley of the Paradox Basin in southeastern Utah, in order to assess groundwater flow pathways and residence times. The principal aquifers of interest are the Cretaceous Burro Canyon Aquifer (BCA), composed of stream-lain sandstones and carbonates, and the Jurassic Navajo Aquifer (NA), an eolian sandstone. All geochemical and isotopic results show that these distinct aquifers are not strongly hydrologically connected under current natural hydrologic conditions. The upper BCA has a calcium-sulfate signature with high TDS and higher concentrations of important ore-forming trace elements, such as Cu, Fe, Co, Mn, and U. Near-zero to negative values of δ³⁴S-SO₄ and δ¹⁸O-SO₄ are consistent with sulfide oxidation in the mineralized aquifer. The BCA has a less radiogenic ⁸⁷Sr/⁸⁶Sr ratio, but higher concentrations of Sr²⁺, reflective of the carbonate lithology of the aquifer. Corrected radiocarbon ages in the BCA of 3,300-11,000 BP coupled with δ¹⁸O and δD values in the range of modern precipitation are indicative of recharge occurring during the Holocene. The lower NA contains sodium-bicarbonate groundwater with low-to-moderate TDS, while positive values of δ³⁴S-SO₄ and δ¹⁸O-SO₄ and lower concentrations of SO₄^2- are characteristic of eolian sandstones; alternatively, low DO and low concentrations of SO₄^2- indicate an anoxic environment, which would be conducive to bacterial sulfate reduction, which can also produce positive values of δ³⁴S-SO₄ and δ¹⁸O-SO₄. Groundwater in the NA is generally more radiogenic (higher ⁸⁷Sr/⁸⁶Sr) than the BCA, but has lower concentrations of Sr²⁺, characteristic of the higher silicate content of the NA. Corrected radiocarbon ages in the NA of 15,000-36,000 BP, coupled with δ¹⁸O and δD values lower than modern precipitation including snow, are indicative of Late Pleistocene recharge when the climate of the Four Corners area of the U.S. southwest was 5 to 7°C cooler than today. Corrected radiocarbon ages and stable water isotope values suggest recharge to both aquifers occurs at the La Sal Mountains, flowing southeast along a long lateral flow path to the Lisbon valley; distinct hydrogeochemistry in each aquifer suggests vertical flow is minimal.