U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Use of Chlorofluorocarbons, Dissolved Gases and Water Isotopes to Characterize Ground-Water Recharge in an Aquifer Contaminated by Acidic, Metal-Laden Wastewater

By P.D Glynn, E. Busenberg, and J.G. Brown

This report is available in pdf format: Glynn.pdf 179KB

ABSTRACT

Chemical and isotopic analyses of ground waters sampled from the Pinal Creek Basin, near Globe, Arizona, between 1991 and 1998 provide valuable information on this highly transient ground-water flow system. Improved knowledge of the flow system and of the recharge processes affecting it is essential in predicting the chemical evolution and migration of the extensively contaminated waters in the basin. Data for dissolved nitrogen and argon indicate that most of the ground-water recharge occurs very rapidly during floods in the winter and early spring. Ground-water samples collected in 1991 have chlorofluoro- carbon ages that generally increase with depth and distance downgradient in the metal- and acid- contaminated ground waters. The ground-water ages calculated from chlorofluorocarbon-11 concentra tions are reasonable-3 to 15 years for acidic ground waters and 20 to 30 years for neutralized, contami- nated ground waters. Ground waters sampled in 1993 have chlorofluorocarbon ages as much as 8 years younger than the waters sampled in 1991. Ground waters sampled in 1996 and in 1998 show that the age of waters in the acidic zone of the system increased by as much as 7 years. Deuterium and oxygen-18 isotope contents measured in the ground waters and their correlation with the specific conductance of the sampled ground waters support the hypothesis that Webster Lake was a major source of metal and acid contami- nation of ground water in the Pinal Creek Basin. The ground-water ages presented here, however, cannot be used to determine the age of the ground-water solutes introduced by the copper mining and refining operations because the introduced solutes are affected by water-rock reactions and because of remaining uncertainties concerning the application of the chlorofluorocarbon dating technique in this extensively contaminated and highly transient ground-water system.