Environmental Isotope Geochemistry in Groundwaters of Southwestern Arizona, USA, and Northwestern Sonora, Mexico: Implications of Groundwater Recharge, Flow, and Residence Time in Transboundary Aquifers

Abstract

The usefulness of environmental isotopes (δ18O, δ2H, 3H, and 14C) and other chemical tracers in hydrogeological research was evaluated in three case-studies in two different and adjacent basins located in southwestern Arizona, USA and northwestern Sonora, Mexico. In this dry region, located in the Sonoran Desert, riparian and wetland ecosystems have been replaced by irrigated fields and the local aquifers are prone to overdraft. The remaining functional ecological systems support a variety of terrestrial, avian, and aquatic fauna as well as a diversity of vascular plants that contrast with the surrounding desert. These ecosystems heavily rely on and compete with human settlements for water resources that are expected to decline as climate warms (Barnett, 2008). Detailed hydrogeochemical studies that address the origin of aquifer recharge and groundwater residence time are needed to understand the impacts of increased groundwater use and expected intensified drought in the area (Seager et al., 2007; Ault et al., 2016). In Appendix A, we use environmental isotope data and major ion chemistry from surface and groundwaters in the lower Colorado River to understand aquifer recharge mechanisms and the geochemical evolution of groundwaters in this transboundary aquifer. We find that locally-recharged groundwaters are dominated by Na-Cl of meteoric origin and mix with Colorado River waters of Ca-SO4 composition beneath the Yuma and San Luis Mesa. Low 3H and 14C data are consistent with bulk residence times of 5,700 corrected 14C years B.P. In Appendix B, we use environmental isotope data and major ion chemistry from surface waters, groundwaters, and precipitation in the Gran Desierto wetlands, Sonora to establish aquifer recharge mechanisms, water origin, flow paths, and groundwater residence time on these enigmatic spring-fed wetlands. We find that local recharge originates as winter precipitation, but is not the main source of water in the pozos. Instead, the pozos are fed by evaporated Colorado River water following flow paths created by the Altar Fault. The environment that allowed recharge to the aquifer feeding the pozos no longer exists, and the pozos are now vulnerable to major groundwater pumping and development in the area. In Appendix C, we use environmental isotopes and water chemistry to distinguish water types, recharge mechanisms, and residence time along several reaches of the Sonoyta River and Quitobaquito Spring. We find that areas located up gradient from the Sonoyta River are supported by local recharge which corresponds to water from the largest 30% of rain events mainly occurring during winter. Quitobaquito Spring is supported by 1) a mix of modern recharge and Pleistocene-aged groundwater or 2) Sonoyta River water supplying water through a suggested fault system connecting the spring to the alluvial aquifer beneath the. Ionic ratios seem to support the latter explanation. Curent drought conditions and groundwater use have reduced or eliminated recharge to the spring. This work shows that environmental isotopes (δ18O, δ2H, 3H, and 14C) are among the most useful suite of tracers in groundwater and surface water in southwestern Arizona and northwestern Sonora. The distinctive isotopic composition derived particularly from altitude effects permits a clear identification of water sources in high-relief basins in the Basin-and-Range Province. The results demonstrate the dependence of winter recharge in the area and the importance of high elevation recharge in the lower elevations of the basin.