STABLE-ISOTOPE CHARACTERIZATION OF THE IMPACTS OF ARTIFICIAL GROUND-WATER RECHARGE

Stable isotopes of deuterium and oxygen-18 of surface and ground water , together with anion concentrations and hydraulic gradients, were use d to interpret mixing and flow in ground water impacted by artificial recharge. The surface water fraction (SWF), the percentage of surface water in the aquifer impacted via recharge, was estimated at different locations and depths using measured deuterium/hydrogen (D/H) ratios d uring the 1992, 1993, and 1994 recharge seasons. Recharged surface wat er completely displaced the ground water beneath the recharge basins f rom the regional water table at 7.60 m to 12.16 m below the land surfa ce. Mixing occurred beneath the recharge structures in the lower porti ons of the aquifer (>12.16 m). Approximately 12 m downgradient from th e recharge basin, the deeper zone (19.15 m depth) of the primary aquif er was displaced completely by recharged surface water within 193, 45, and 55 days in 1992, 1993, and 1994, respectively. At the end of the third recharge season, recharged surface water represented similar to 50 percent of the water in the deeper zone of the primary aquifer simi lar to 1000 m downgradient from the recharge basin. A classic asymmetr ical distribution of recharged surface water resulted from the recharg e induced horizontal and vertical hydraulic gradients. The distributio n and breakthrough times of recharged surface water obtained with stab le isotopes concurred with those of major anions and bromide in a trac er test conducted during the 1995 recharge season. This stable isotope procedure effectively quantified mixing between surface and ground wa ter.