Using isotopes and solute tracers to infer groundwater recharge and flow in the Cienega Creek Watershed, SE Arizona

Rachel S. Tucci1, Jennifer McIntosh1, Andrew Salyon2, Ron Tiller2, Jeanmarie Haney3

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2Desert Botanical Gardens, Phoenix, Arizona

3The Nature Conservancy, Cottonwood, Arizona

Reaches of Cienega Creek and Davidson Canyon in the Cienega Creek Watershed (CCW) are registered as “Outstanding Arizona Waters,” and wetlands in the Las Cienegas National Conservation Area support several threatened and endangered species. The lack of baseline hydrologic and water quality studies in the CCW leave important land management questions unanswered, such as how increases in urbanization, ranching, agriculture, or possible mining might impact groundwater resources. To help address these questions, this study investigates the hydrologic connection between recharge in the surrounding mountain blocks and basin groundwater, which is contained in alluvial and Cretaceous aquifers and wetlands (cienegas) in the central portion of the basin. Specifically, we aim to determine: (1) What are the flow paths, seasonality and elevation of groundwater recharge? (2) What is the residence time of groundwater across the basin? And (3) What is the source of water in the cienegas, and the possible influence of monsoon floodwater recharge?

Groundwater samples from domestic water supply and previous mining exploration wells, springs, and alluvial aquifers were collected 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, δD, δ13C, δ34S) and radioactive isotopes (3H, 14C). Initial results indicate springs and alluvial aquifers are dominantly sourced year-round from basin groundwater, and δ18O values and sulfur to chloride ratio values indicate little infiltration of summer monsoon floodwaters. Most of the basin groundwater samples analyzed for tritium are below detection limit or lower than modern precipitation values for the region, and 14C values were low (3.33-77.09 pMC), which indicates recharge occurred prior to the 1950’s. The low sulfate concentrations and δ34S of basin groundwater, springs, and alluvial aquifers are typical of local rainwater values consistent with the lack of sulfate sources in basin sediments. The lack of recent recharge even in shallow alluvial aquifers beneath the washes and near Cienega Creek suggests that groundwater throughout the basin is a fossil resource, and that future increases in groundwater pumping may impact the cienegas.

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