A Multiple Tracer Approach for Tracking the Movement of Artificial Recharge through Desert Sandstone

Sand Hollow Reservoir in southwestern Utah is operated conjunctively for surface-water storage and managed aquifer recharge. Since its completion in 2002, more than 86 Mm3 of water have artificially recharged the underlying Navajo Sandstone Aquifer. This water moves down-gradient towards the Virgin River about 4,000 m away, unless intercepted by production wells. Understanding ground-water travel times in Sand Hollow is important because current state permitting stipulates that recharge must be recovered within 10 years unless longer average residence times can be proven.

Because the native ground water is geochemically similar to the source water to the reservoir (Virgin River water originating predominantly from Navajo Sandstone within Zion National Park), a set of unique tracers including general and trace ions, dissolved gases, tritium, and stable isotopes are being utilized to evaluate artificial recharge travel times. Ionic tracers include Cl, Cl:Br ratio, and total dissolved solids; values for these constituents in the reservoir water are about twice that of native Sand Hollow groundwater. Prior to the reservoir, Cl and other naturally occurring salts accumulated during the Holocene Period in the vadose zone of Sand Hollow and flushing of these salts provides another unique tracer: Cl concentrations have increased by almost an order of magnitude at nearby monitoring wells (much higher than recharge water). Air entrapment in the sandstone beneath the reservoir during saturation of the vadose zone provides another unique tracer. In tandem with rising Cl:Br ratios, total dissolved gas pressures and dissolved oxygen in nearby observation wells have increased to more than 3 times atmospheric concentrations (>2,300 mm Hg and 23 mg/L, respectively). Tracer breakthrough times of about 4 years at monitoring wells within 50 m from the reservoir indicate potential travel times to the Virgin River may be on the order of hundreds of years.