Determination of ground-water flow paths using stable isotopes as geochemical tracers: upper and middle Verde River watersheds, Arizona, USA

Abstract

Stable isotopes of hydrogen and oxygen, major ions, and trace elements were used as geochemical tracers to identify ground-water flow paths and potential source waters that eventually discharge into the Verde River, one of Arizona's last remaining perennial rivers. Identifying and quantifying the various source waters to the Verde River provides an improved understanding of how ground water moves and sustains perennial base flow in the river on a regional scale. More specifically, these chemical constituents allow 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. A stable-isotope end member value was calculated for four ground water regions, which were delineated based on geologic structure, hydrology, and chemical characteristics of the ground water. Mixing calculations indicate that ground water discharging as 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 about 48 km downstream from the headwaters indicates where ground water from the highest elevation sources in the study area discharges to the river. Ground water from low elevation sources in the southeastern part of the study area enters the river about 135 km downstream from the headwaters. The spatial and temporal variability of recharge was determined using stable-isotope measurements from precipitation and ground water samples within the watershed (fig.l.l ). Stable-isotope values from winter precipitation samples are within the range of stable-isotope values measured in ground-water samples throughout the study area. Summer precipitation samples from North American monsoons are enriched with respect to ground-water values and therefore are only a small contributor to ground-water recharge. Recharge to ground water that discharges to the river is predominantly from winter precipitation on the basis of temporal trends of stable-isotope data.