CHEMICAL AND PHYSICAL CHARACTERISTICS OF GROUND-WATER DISCHARGE NEAR THE SOUTH RIM OF GRAND CANYON, ARIZONA

Springs near the south rim of Grand Canyon are an important resource of Grand Canyon National Park and for Native Americans in the region. The springs are used as sources of drinking water by hikers, offer refuge to wildlife, and help sustain riparian ecosystems. Population growth on the Coconino Plateau has increased the demand for ground-water resources, and the increased demand potentially could affect the sustainability of spring discharge.

During 1999-2003, the USGS, in cooperation with the National Park Service, collected water samples from 46 sites associated with the Redwall-Muav Limestone aquifer near the south rim of Grand Canyon. Samples were collected at 32 springs, 9 creek sites, and 5 wells, and were analyzed for major ions, trace elements, and selected isotopes to determine baseline water quality and identify potential recharge areas and flow paths of ground-water flow to the springs. Rock samples and well cuttings from the major stratigraphic units of Grand Canyon were collected and analyzed for mineralogy and selected isotopes. Discharge from the springs is monitored at streamflow-gaging stations on tributaries of the Colorado River near the south rim of Grand Canyon in Grand Canyon National Park, and on the Havasupai Indian Reservation.

The chemical composition of water from springs, creeks, and wells was similar among most samples. Water chemistry at Blue Spring and Mohawk Canyon Spring at the margins of the study area, however, was distinctly different from that at the other sites. Tritium and carbon isotope analyses indicate that water less than 50 years old is absent at several springs and all of the wells. Most springs discharge a mixture of younger and older waters. Oxygen and hydrogen isotopic compositions varied little among samples, and for most sites the isotopic data plot close to the local meteoric water line. Flow paths are differentiated between the eastern part of the study area where 87Sr/86Sr values for water from springs and creeks correlate with 87Sr/86Sr values for rocks of Permian age. Farther west, 87Sr/86Sr values for water correlate with 87Sr/86Sr values for rocks of Mississippian and Pennsylvanian age. Trend analysis of discharge data from the gaging stations near the south rim of Grand Canyon shows minor changes in seasonal flow patterns and base flow during the past 10 years.