IDENTIFYING SOURCES OF GROUNDWATER RECHARGE IN ORAIBI WATERSHED, NAVAJO RESERVATION, ARIZONA: AN ISOTOPIC AND HYDROGEOLOGIC APPROACH

Water is critical in the American arid southwest, including the Navajo Nation where groundwater supports livestock, farming, and the livelihoods of both Navajo and Hopi Tribal members. Groundwater availability is determined by precipitation and infiltration, and by geologic controls like fractures and permeability on recharge and permeability contrast in stratigraphy. This study aims to improve the understanding of surface-water and groundwater interactions in a single watershed known as Oraibi Wash. The Oraibi Watershed covers an area of 1,896 square kilometers and is a tributary to the Little Colorado River. For much of the southwestern U.S., there are two main ways groundwater is recharged; by rainfall and snowmelt.  To help understand the recharge process, we integrate geologic analysis with stable water isotopes of 𝛿D and 𝛿18O to identify sources of groundwater recharge along Oraibi Wash. In this study, I explored the geologic features associated with spring emergence. Strike and dip measurements were collected in the field and from previous studies of the Black Mesa Basin to construct a geologic cross section along Oraibi Wash. The cross section along with field observations identified both stratigraphically and structurally controlled springs. I also estimated the relative amounts of recharge supported by winter and monsoon seasonal precipitation using stable water isotopes. Springs and wells samples both clearly show the importance of winter precipitation recharge. Also springs that were stratigraphically controlled had a wider range in isotopic variability whereas structurally controlled springs had a narrower isotopic range. Implications for these new findings indicate that winter recharge of groundwater will be critical for Navajo communities along Oraibi Wash. Changes in precipitation due to climate change that impact winter precipitation will have profound effects on groundwater in Oraibi Wash, and consequently, also on the Native communities living along this drainage.