234U/238U DISEQUILLIBRIUM AS AN ENVIRONMENTAL TRACER OF GROUND-WATER DISCHARGE TO SPRINGS IN THE UPPER VERDE RIVER HEADWATERS REGION, NORTH CENTRAL ARIZONA

Discharge springs from two basin-fill aquifers and the regional Paleozoic limestone aquifer at the headwaters of the Verde River each have distinctive ²³⁴U/²³⁸U activity ratios (ARs). The headwaters region drains the transition zone between the Colorado Plateau and the Basin and Range physiographic provinces in north-central Arizona. ARs and U concentrations can be used to evaluate water sources and their mixing. The marine carbonate (Paleozoic aquifer) had the lowest uranium concentrations (0.6 to 0.8 µg/L) and highest ARs (4.0 to 9.2), suggestive of slow leaching of U over long time periods and preferential dissolution of ²³⁴U by decay-related alpha recoil. In contrast, the basin-fill aquifers are characterized by relatively constant AR of 3.0+0.3 and variable U concentrations. Each basin-fill aquifer represents the integration of many inputs from recharge area springs that drain a variety of upland rock types (Precambrian granite and gneiss, carbonate, sedimentary, and volcanic rocks), with igneous and metamorphic rocks containing the most uranium. U concentrations ranged from about 1.0 µg/L in wells near large ephemeral streams, to 4.2 µg/L near the outlet of the two basin-fill aquifers, where ground water is in contact with younger igneous rocks (30-Ma volcanic latite and 5-Ma basalts) with the dominant AR of 3.0+0.3. ARs of 15 springs in the first 2 km of the Verde River vary between 2.5 and 3.7, with U concentrations from 0.8 to 5.0 µg/L—indicating numerous distinct seeps, each with a unique flow path. The final AR and U concentration of the river were 3.7 and 3.1 µg/L, representing a flow-weighted composite of multiple seeps. Rapid, high precision measurements of uranium-series isotopes by quadrupole inductively coupled plasma mass spectrometry can greatly expand applications of uranium-series isotopes as environmental tracers.