Dissolved oxygen systematics in the Tucson Basin Aquifer

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Abstract
Dissolved oxygen (DO) is ubiquitous in Tucson basin groundwater; however, its distribution is systematic and distinct facies are apparent. Dissolved oxygen concentrations approach a minimum of 15% of the saturation limit in a chemically immature recharge facies and then rapidly increase downgradient. Groundwater throughout most of the central basin remains nearly saturated with dissolved oxygen for thousands of years during the course of its chemical evolution. DO concentrations were lowest where the water table was highest, indicative of the oxidation of phreatophyte detritus. Transverse dispersive influx of dissolved oxygen from the vadose atmosphere into the shallow phreatic zone represents an inferred process controlling the electrochemical evolution of groundwater within this aquifer. Traditional models which describe a progressive reduction of the electrochemical state of groundwater along the flow path cannot be directly applied to the Tucson basin which is a “dispersive influx‐dominated” system.