Environmental Tracers for Tracking Managed Aquifer Recharge Through the Navajo Sandstone Aquifer

Victor M. Heilweil, U.S. Geological Survey, Salt Lake City, Utah

Sand Hollow Reservoir in southern Utah (U.S.A.) was constructed in 2002 to provide surface-water storage and artificial recharge to the underlying Navajo Sandstone Aquifer to meet water demands associated with rapid population growth in this arid region. The reservoir is an off-channel facility that receives water from the nearby Virgin River. The reservoir, when full, covers an area of 600 hectares with a storage volume of 74 million cubic meters. Recharge to the underlying sandstone is estimated to range from 0.25  to 4.3 million cubic meters per month. Ongoing annual environmental tracer sampling began in 2001at observation wells located between 20 and 1,500 m downgradient of the reservoir to monitor the movement of recharged water. These environmental tracers include specific conductance, general chemistry, dissolved oxygen, total dissolved-gas pressure, chloride/bromide ratios, dissolved noble gases, tritium, and chlorofluorocarbons. Elevated specific conductance and total dissolved-solids concentrations are partly attributed to the flushing of vadose-zone salts that had accumulated within the basin during the Holocene due to arid conditions and low natural recharge rates. Elevated total dissolved gas pressures are associated with the initial entrapment of air in the sandstone pores as saturation occurred beneath the reservoir. Chlorofluorocarbons and tritium/helium have a modern signature in the surface-water reservoir, in contrast to the older natural groundwater of the aquifer; these age tracers are also being used to show the transit of managed aquifer recharge through the aquifer. Understanding the groundwater residence time of this managed-aquifer recharge is critical to the management of this aquifer storage and recovery. Preliminary data indicate that water recharged from the reservoir has moved about 1 km downgradient of the recharge facility since its inception in 2002.