Large-scale tracing of ground water with sulfur hexafluoride

Sulfur hexafluoride (SF6) was injected into a 9 km reach of the Santa Ana River in Orange County, CA, over a period of two weeks. The entire how of this river, which averaged 2.8 m(3) s(-1), percolated into the ground in the field area. The tracer was monitored at wells near the river to determine subsurface Row patterns and flow times with an accuracy much greater than could be achieved using numerical simulations of groundwater flow. During the experiment, SF6 effectively tagged 3.7 x 10(6) m(3) of water. The tracer plume was mapped in the subsurface for 18 months and indicates that linear ground-water velocities averaged about 2 km year(-1) The tracer reached two wells adjacent to the river (about 200 m away) within three weeks, giving evidence that SF6 was not retarded significantly relative to the ground-water flow. This is in agreement with previous laboratory experiments.