Defining transport near ASR operations using sulfur hexafluoride gas tracer experiments

Direct measurements of solute transport rates near ASR sites are needed to evaluate potential water quality changes and validate numerical models of flow. Sulfur hexafluoride (SF₆) tracer experiments are ideally suited for this purpose because SF₆ is non-reactive, not retarded, easily measured at low concentrations (<1 pmol l⁻¹), and inexpensive. It can be used to tag large quantities of water (>10⁶ m³) over periods of weeks. The introduction of this tracer into the water supply is challenging because it is a gas and is lost from water into air. The most efficient injection method is the release of SF₆ from gas permeable silicon tubing. Once in the surface water, its concentration must be monitored so that its input function into the groundwater aquifer is known. An experiment conducted in Orange County, California demonstrates that recharged water can be traced for more than four years.