Natural Tracer Study to Constrain Transit Times and Flowpaths of Groundwater from Davidson Canyon to Lower Cienega Creek

Abstract

Multiple reaches of Cienega Creek and Davidson Canyon Wash, located in the Cienega Creek Watershed, Arizona, have been designated as “Outstanding Arizona Waters.” These surface waters, riparian areas, and underlying groundwater in alluvial basins are under pressure from threats of increasing groundwater pumping, land use, climate change, and potential mining. Yet, little information is known about the regional hydrogeology, which is important for accessing and protecting the sustainability of natural resources in the area. This study investigates the hydrologic connection between the Santa Rita Mountains and lower Cienega Creek, along the Davidson Canyon sub-watershed. Davidson Canyon is an intermittent stream with seasonally sustained baseflows and is a large tributary to Lower Cienega Creek. This research aims to: (1) identify areas of recent recharge; (2) determine the relative age and transit time of groundwater; (3) determine the seasonality of recharge; and (4) use hydrochemistry and isotopes to evaluate flowpaths and mixing of groundwater and interaction with surface water. To address these aims, groundwater and surface water samples were collected seasonally in 2017-2018 and analyzed for major ion chemistry, stable isotopes (δ18O, δD, δ13C, δ34S) and radioactive isotopes (3H and 14C). Results indicate groundwater located in bedrock aquifers in the Davidson Canyon sub-watershed contains a mixture of young (<70 years old), and older (up to 23,000 years old) waters, and is recharged by both winter and summer precipitation. The groundwater in the Empire and Santa Rita Mountains appears to have obtained sulfate from precipitation and pyrite oxidation. Downgradient these waters have dissolved gypsum in the basin-fill aquifer closer to the Davidson Canyon Stream outlet. Groundwater located in the basin-fill aquifers in Lower Cienega Creek is a mixture of young (<70 years old), and older (up to 2,040 years old) waters, and is primarily recharged by winter precipitation. These waters have sulfur isotope signatures indicative of gypsum dissolution and are more chemically evolved downgradient in the basin-fill aquifer. Davidson Canyon flows are predominantly sustained by groundwater discharged from the shallow alluvial aquifer (<10 years old groundwater), recharged by summer and winter precipitation. Lower Cienega Creek surface waters are a mixture of modern (<50 years old) and older waters and are primarily recharged by summer precipitation. Lower Cienega Creek is sustained by local groundwater from the basin-fill aquifers, recent recharge to the shallow alluvial aquifer, and contributions from Davidson Canyon. The source of water in Lower Cienega Creek and Davidson Canyon changes throughout the summer monsoons and winter rainy seasons. Following the first summer or winter precipitation events, there is an initial pulse of older, more chemically evolved groundwater into the stream, followed by inputs of shorter residence time, more dilute waters.