A COMPARISON OF HYDROGEOLOGIC MODELS, TRITIUM/³HE, AND DELIBERATE TRACER EXPERIMENTS TO UNDERSTAND GROUND WATER RESIDENCE TIME

The examination of subsurface residence times and flow paths of artificially recharged ground water are important criteria for understanding and monitoring water quality changes that may result from the mixture of different water sources and in situ biogeochemical reactions. Ground water travel time is not simple to define as flow paths from the recharge locations to extraction points can be numerous and convoluted. This is especially true for water extracted at monitoring and production wells. Established methods of determining travel times include hydrogeologic modeling, transient tracers and deliberate tracers. Each of these methods determines the flow of ground water differently, and thus, provides complimentary information. Sulfur hexafluoride (SF ₆ ) gas tracer was injected into the Rio Hondo and San Gabriel recharge basins at the Montebello Forebay (LA County, CA) artificial recharge site in early 2003. During this two-year experiment, SF ₆ has been detected at seven monitoring wells. All of these wells have screens within 140 ft of the ground surface. The maximum groundwater SF ₆ concentration observed was approximately 15% of the mean concentration in the surface water and approximately 20% of the concentration of the closest recharge basin (RH1). The monitoring well results indicate that SF ₆ was successfully transferred from the surface water to the groundwater during percolation at the spreading basins. SF ₆ tracer has been detected at nine of the seventeen production wells sampled during this study, indicating that the travel time of recharge water to some of these wells is less than two years. At four of the nine wells with SF ₆ detections, the hydrogeologic travel time determined by Bookman-Edmonston Engineering were less than 10 weeks and are in basic agreement with the SF ₆ travel time. However, at the other five wells, the hydrogeologic travel times were estimated at greater than 200 weeks, significantly longer than the indicated by the tracer data. Leakage through low permeability layers occurs, leading to the earlier tracer arrivals. At all wells with no detected tracer, the hydrogeologic times were greater than three years. All tritium/ ³ He ages are greater than 10 years for the production wells, indicating mixing of young and old ground water, resulting in an over-estimate of ground water age.