DETERMINATION OF GROUND-WATER FLOW PATHS USING STABLE ISOTOPES AS GEOCHEMICAL TRACERS: UPPER AND MIDDLE VERDE RIVER WATERSHEDS, ARIZONA, USA

Stable isotopes of δD and δ¹⁸O were used as conservative tracers to identify spatial and temporal variability of recharge and flow paths of ground water that discharges into the Verde River, one of Arizona’s last remaining perennial rivers. Analysis of ground water for stable isotopes, major ions, and trace elements provided insight into potential source waters for the Verde River. More specifically, these chemical constituents allowed for the identification and quantification of source-water contributions to the river. In addition, two- and three-component mass-balance equations incorporating stable-isotope data were used to construct a conceptual model of ground-water flow for the upper and middle Verde River watersheds study area. Mixing calculations indicate that ground water discharging at springs in the headwaters of the Verde River is predominantly from regional aquifers in Big and Little Chino Valleys but includes water from higher-elevation sources in the northwestern part of the study area. An abrupt change in the chemical signature of the Verde River base flow occurs about 48 km downstream from the headwaters where ground water from the highest-elevation sources in the study area discharges to the river. A smaller portion of ground water from low-elevation sources in the southeastern part of the study area enters the river about 135 km downstream from the headwaters. Recharge to ground water that discharges to the river is predominantly from winter precipitation on the basis of temporal trends of stable isotope data. Identifying and quantifying the various source waters to the Verde River provide an improved understanding of how ground water moves and sustains perennial base flow in the river on a regional scale.