Using Water Isotopes and Solute Chemistry to Investigate the Hydrology of Surface Water in the Cienega Creek Watershed

Abstract

Analysis of water stable isotopes and solute chemistry can reveal the source of water in surface water systems, and the nature of the hydrologic connection between surface water and local and regional groundwater. The Cienega Creek Watershed (CCW) in semi-arid southeastern Arizona contains two stretches of “Outstanding Arizona Waters”, rare wetlands (cienegas), and riparian ecosystems. Surface water and shallow alluvial aquifer water within CCW is threatened by climate change, groundwater pumping, and a proposed open pit copper mine. This study uses water stable isotopes (18O, 2H, 34S, 13C), solute chemistry, and tritium to investigate the seasonal sources of water in the wetlands, Cienega Creek, and riparian aquifer within the Las Cienegas National Conservation Area (LCNCA) in the Upper CCW. Seasonal samples of streams, springs, and shallow groundwater from piezometers along the length of Cienega Creek were analyzed. Wells tapping into basin groundwater were also sampled. Major ion chemistry, stable water isotopes, and tritium suggest that surface water (e.g., Upper Cienega Creek and wetlands in the Upper CCW) and groundwater in the underlying alluvial aquifer is a mixture composed primarily of basin groundwater, recharged at the Santa Rita mountain front prior to the 1950’s, with a smaller component of more recent recharge. Solute chemistry and tritium results indicate that recharge of the shallow alluvial aquifer does not occur via bank storage of recent precipitation along Upper Cienega Creek. Sulfate concentrations and δ34S and δ18O values of sulfate indicate that an alternate groundwater source originating from the Whetstone Mountains contributes to surface water in Upper CCW between Coldwater Springs and The Narrows. The apparent reliance of surface water and the shallow alluvial aquifer on basin groundwater implies that surface water and shallow groundwater in LCNCA could be impacted by changes in the regional water table. This insight, as well as the baseline hydrologic data that this research provides, will aid future research efforts and help inform land managers interested in the preservation of surface water within the LCNCA.