The San Pedro River at Cascabel, Arizona: Stable Isotopes, Surface Water Sources, and Ecological Sustainability

Stable O and H isotopes distinguish groundwater in a shallow aquifer (< 10 m beneath the bed of the San Pedro river) from groundwater in a deep aquifer (about 30 m beneath the river, under clay) and groundwater in tributary washes. The shallow aquifer receives recharge from summer flood water of the river, and locally from tributary washes. The deep aquifer receives recharge from precipitation at the flanks of the basin, and is the main source of domestic and irrigation water at Cascabel. Summer flood water varies greatly in isotope composition, reflecting either (1) the composition of precipitation immediately prior to the flood, or (2), at other times, the isotope composition of water stored in the shallow aquifer or in the river banks. Base flow is perennial 9-11 km upstream of Cascabel, and extends downstream in winter after transpiration ceases in riparian trees. Isotopes in base flow indicate that the water is mainly the evaporated equivalent of deep aquifer or tributary aquifer water, but with possible transient input from summer flood water. Base flow is critical to sustaining the riparian environment. Base flow can be extended in space and time by increasing the availability of deep aquifer or tributary aquifer water, which can be achieved by limiting the withdrawal of irrigation and domestic water.