Using Isotopes and Solute Tracers to Infer Groundwater Recharge and Flow in the Cienega Creek Watershed, SE Arizona

Abstract

The Cienega Creek watershed (CCW) of southern Arizona contains springs and wetlands (cienegas) that support several threatened and endangered species and two registered “Outstanding Arizona Waters” reaches. The lack of baseline scientific hydrologic studies in the CCW leaves important land management questions unanswered, such as how increases in urbanization, ranching, agriculture, or possible mining could impact groundwater resources? To help address these questions, this study investigates the hydrologic connection between recharge in the Santa Rita mountain system and groundwater in basin-fill aquifers, and the source water for the wetlands near Cienega Creek. Groundwater samples were collected from springs (feeding cienegas), wells, and piezometers completed in basin-fill sediments and shallow alluvial aquifers along a broad transect from the Santa Rita Mountains eastward across the basin to Cienega Creek. Samples were analyzed for major ion chemistry, stable isotopes (δ18O and δD of water, δ13C (DIC), δ34S(SO4) and δ18O(SO4)) and age tracers (3H, 14C). Results indicate springs are dominantly sourced year-round from basin groundwater, and δ18O values and sulfate to chloride ratios indicate little influence of summer monsoon floodwaters. The low sulfate concentrations and δ34S values of basin groundwater and springs are typical of local rain water values, and/or indicate small contributions of gypsum dissolution and pyrite oxidation, consistent with the lack of appreciable sulfate sources in basin sediments. Stable water isotopes in groundwater samples across the study area indicate recharge occurred from summer and winter precipitation at approximately 1700 ±200m (mountain front) and higher elevations (mountain block). Most of the groundwater samples analyzed for tritium are below modern precipitation values for the region, and 14C values are low (3.3-84.7 pMC), which indicates most recharge occurred prior to the 1950’s, even at the mountain front. The lack of recent recharge in shallow alluvial aquifers beneath the washes and near Cienega Creek suggests that groundwater throughout the basin is a relatively old resource, and future increases in groundwater capture or pumping may impact surface waters, including cienegas.