1 A Comparison of Three Methods for Determining Travel Times Near a Large Artificial Recharge Facility Dror Avisar 1 , Jordan F. Clark 2 , Jeni McDermott 2 & G. Bryant Hudson 3 1 Department of Geography and Environmental Sciences, Tel Aviv University, Israel (droravi@post.tau.ac.il) 2 Department of Geological Sciences, University of California, Santa Barbara, CA 93106 USA (jeni_mcdermott@umail.ucsb.edu, jfclark@geol.ucsb.edu) 3 Chemical Biology and Nuclear Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA (hudson5@llnl.gov) ABSTRACT Hydrogeologic calculations, transient tracers and deliberate tracer experiments are established methods of determining ground water travel times. A two-year long deliberate tracer experiment using sulfur hexafluoride (SF 6 ) was conducted at the Montebello Forebay (LA County, CA) artificial recharge site to determine travel time to wells within 150 m of spreading ponds. SF 6 was detected at seven monitoring wells, all screened within 50 m of the ground surface and at nine of the eighteen production wells. Travel time was best correlated with screen depth. At four of the nine wells with SF 6 detections, hydrogeologic travel times were less than 0.3 years (16 weeks) and are in basic agreement with the SF 6 results. However, at the other five wells, the hydrogeologic travel times were estimated to be more than 4 years, significantly longer than indicated by the tracer data. Transport through low permeability layers due to gaps or leakage must be occurring. At all wells where SF 6 was not detected, the hydrogeologic travel times were greater than 3 years. At the production wells, tritium/ 3 He apparent ages were greater than 10 years, indicating mixing of young and old ground water caused by the long well screens. KEYWORDS Artificial recharge, geochemical tracer, ground water travel time, SF 6 , Tritium/ 3 He dating INTRODUCTION Quantification of subsurface residence times and flow paths are important criteria for managing artificial recharge sites. This basic hydrologic information is needed for evaluating subsurface water quality changes that may result from in situ biogeochemical reactions and mixing of water from different sources. It is also needed for validation of numerical models of flow (e.g., Thomson et al., 1999). Ground water travel time near artificial recharge sites is not simple to define as flow paths to wells can be numerous and convoluted. This especially true at facilities, such as the Montebello Forebay in Los Angeles, California, where percolation from large spreading basins is the principle method of recharge. Ground water travel times can be determined using hydrogeologic calculations or modeling, transient tracers, and deliberate tracer experiments. Each of these methods determines the flow of ground water differently and thus, provides complimentary information. Ground water travel times can be estimated using Darcy’s law and inferred flow paths. These flow paths are determined based on the depth of well perforation, aquifer stratigraphy, pump capacity, and distance to recharge location. Probable travel times are estimated using local hydrogeologic data, these inferred flow paths, and known hydraulic gradients (Bookman-Edmonston, 1994). Determining residence time for shallow ground water is also possible using environmental tracers such as tritium/ 3 He (Schlosser et al., 1989; Cook and Solomon, 1997). Tritium, a radioactive isotope of hydrogen, was released in large quantities in the late 1950s and early 1960s during above ground nuclear bomb testing. Although there is a natural (cosmogenic) source, most tritium in the