Summary of Isotope Research on Sources of Perennial Flow in the San Pedro River
Jan. 22, 2021
The WRRC is pleased to feature the following article summary prepared by the author, UArizona researcher (retired) Chris Eastoe.This study used measurements of the ratios of stable (non-radioactive) isotopes in the hydrogen and oxygen of water molecules. Hydrogen exists in nature as two stable isotopes (hydrogen-2 with mass 2 and hydrogen-1 with mass 1). Oxygen exists as three, of masses 18, 17, and 16. The relative amounts of hydrogen-2 and hydrogen-1, and of oxygen-18 and oxygen-16, were compared in different water samples. The data continue to provide information about the sources of water (e.g., whether water fell as rain at high or low altitudes) and about changes that have affected water on the ground or in an aquifer (e.g., evaporation or mixing of different kinds of water).
Some major points of the study are:
- The reaches of the San Pedro River that have perennial (year-round) surface water are critical to the existing rich riparian ecosystem. Management of the river basin with the best interests of the ecosystem in mind requires an understanding of the sources of the perennial water. Five reaches with perennial water between the international border and Redington were studied (see map).
- Measurements of stable oxygen and hydrogen isotopes enable us to distinguish three sources: (a) water flowing from upstream in or just beneath the river channel; (b) summer monsoon floodwater stored in the bed and banks of the river; and (c) water from the mountainous flanks of the valley.
- Perennial flow in the reach south of Sierra Vista (Area 1) is a combination of sources (a) and (c).
- All three sources contribute to the perennial reach near St. David (Area 2), where the source (c) is deep basin groundwater.
- In Cascabel, near Benson Narrows (Area 3), source (c) predominates. The water is probably from Red Rock Creek.
- At Cascabel, near Gamez Road (Area 4), sources (a) and (c) supplied surface flow that had disappeared by 2019. Source (c) was from Hot Springs Canyon.
- Perennial flow near Bingham Cienega at Redington (Area 5) prior to the early 2000s was of source (c) and appears to have flowed to the river through a limestone aquifer that has been quickly depleted under drought conditions.
- Impermeable sills of rock in the riverbed divide the valley into four separate groundwater basins that are becoming increasingly isolated from each other as perennial flow volumes decrease. Decrease in flow has occurred because of long-term drought in combination with pumping of groundwater.
- Increased urban growth upstream of Benson is likely to degrade the perennial reaches near Sierra Vista and St. David.
Eastoe, Christopher J., Sources of Perennial Water Supporting Critical Ecosystems, San Pedro Valley, Arizona, Environmental and Engineering Geoscience, Vol. XXVI, No. 4, November 2020, pp. 463–479.
For more information, contact Chris Eastoe at eastoe@email.arizona.edu or via his website https://www.geo.arizona.edu/Eastoe.