title: Geochemical imaging of flow near an artificial recharge facility, Orange County, California.
abstract: Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium-3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where approximately 2.5 x 10(8) m3 (200,000 acre-feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at approximately 6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other "time sensitive" contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.
authors.affiliation: Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA. jfclark@geol.ucsb.edu
authors.forename: Jordan F
authors.initials: JF
authors.lastname: Clark
authors.forename: G Bryant
authors.initials: GB
authors.lastname: Hudson
authors.forename: M Lee
authors.initials: ML
authors.lastname: Davisson
authors.forename: Greg
authors.initials: G
authors.lastname: Woodside
authors.forename: Roy
authors.initials: R
authors.lastname: Herndon
mesh_terms: D002140:California; D004784:Environmental Monitoring / Q000379:methods; D005740:Gases; D055593:Geological Phenomena; D005844:Geology; D008962:Models, Theoretical; D014867:Water / Q000737:chemistry*; D014872:Water Movements; D014881:Water Supply
chemical_list: D005740:Gases; D014867:Water