Présentations Colloques

Oral Presentation
Session 8.04: Isotopic and residence time tracers
Kreamer David
Are the Springs of the Grand Canyon At Risk? - Groundwater Exploitation and the Hydrogeology of the Grand Canyon, USA
Grand Canyon National Park exemplifies a problem facing many western National Parks in the United States - groundwater pumping outside of the Park boundaries may influence springs and within the Grand Canyon. These springs support unique wildlife and their diminishment or eradication would have profound effects on Grand Canyon ecosystems. Groundwater pumping associated with business and municipal growth south of Grand Canyon National Park is expanding, and there is evidence to suggest that these wells tap the same groundwater system that supplies canyon springs. The impact of pumping wells on spring flow can be better determined if the age of groundwater, emerging at the springs, is known, because water's subsurface travel time gives critical insight to the source and direction of groundwater flow. In order to investigate the age of groundwater in the Grand Canyon, field and laboratory measurements were made by researchers at University of Nevada, Las Vegas, in concert with the National Park Service, the Grand Canyon Association, the Desert Research Institute, and the U.S. Environmental Protection Agency. Tritium concentrations are less than 6 TR (18 pCi l) in all spring water sampled below the South Rim between the Little Colorado River and Havasu Canyon. This indicates that this water fell as rain or snow and entered the subsurface before the aerial nuclear weapons testing which began in the early 1950's. Absence of chlorofluorocarbons (CFCs) in this water further indicates that this water may predate the introduction of CFCs as refrigerants in the early 1930s. Elevated uranium isotope disequilibrium ratios further suggest that groundwater travel times may even be longer. Long travel times for groundwater indicate a connection between the springs and the increasing depleted regional groundwater system, a situation which could have serious consequences to the ecosystems in the Grand Canyon.
United States