Origins and Residence Times of Water Supporting O'Donnell Creek Cienega in Southeastern Arizona

Abstract

Cienegas are freshwater wetlands that are unique to and one of the rarest habitats in the arid Southwest. They are often located near springs and perennial streams and provide habitat to a host of flora and fauna that rely on the seasonal to perennial saturated soils to standing or gently flowing waters. One cienega of particular interest is the O’Donnell Creek Cienega (OCC) located within the northeastern foothills of the Canelo Hills of Santa Cruz County in southeastern Arizona. This wetland is home to several threatened and endangered species, including the federally listed endangered orchid, the Canelo Hills ladies’ tresses (Spiranthes delitescens), which is found in only four cienegas in southeastern Arizona. The threats of increasing temperatures associated with climate change combined with groundwater decline because of anthropogenic habitat modification and groundwater pumping pose a threat to this and other Southwestern cienegas. In order to provide land managers and landowners with information to help protect cienegas, baseline hydrologic data is needed to understand how to sustain and even restore them. The goal of this study is to characterize the water types, recharge mechanisms, and flow paths supporting OCC. To address these issues, groundwater from domestic wells, shallow groundwater, surface water, and precipitation samples were collected in the summer and winter rainy seasons of 2021, from a radius of ca. 8 km around OCC. Fifty-four samples were analyzed for major ion chemistry, stable isotopes (18O, D), and field parameters (pH, DO, EC, and temperature). A subset of 15 samples were analyzed for radioactive isotopes (14C) with corresponding 13C-DIC, and 13 samples for 3H. Results show basin groundwater samples had small amounts of tritium (BDL to 1.9 TU), ranging from less than modern precipitation to below detectable limits, indicating predominantly older groundwater. Groundwater samples analyzed for 14C show a range of values (29.9-88.8 pMC). Waters sustaining the saturated conditions of OCC come from groundwater discharge via the shallow alluvial aquifer. These waters show a range of 18O and D isotopic signatures (-9.9 to -7.5 and -63.8‰ to -50.8‰, respectively) reflecting both winter and summer precipitation inputs at higher elevation mountain fronts. The apparent age of groundwaters found in the wetland range from ca. 60 to 500 years old, with a smaller signal of older groundwaters in the study area up to ca. 5700 years old. Water samples collected in OCC, and both upgradient and downgradient of OCC have differing 18O and 2H values, indicating alternate groundwater flowpaths in the study area influenced by the Sawmill Canyon fault and complex heterogeneity of subsurface geologic materials. It is hoped that these results will help to inform land managers on the best approaches to conserve water resources sustaining OCC under future climate projections.