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The Las Vegas Valley Water District in Nevada, USA, has operated an artificial recharge (AR) program since 1989. In summer 2001, observations of gas exsolving from tap water prompted a study that revealed total dissolved gas (TDG) pressures approaching 2 atm with a gas composition that it is predominantly air. Measurements of TDG pressure at well heads and in the distribution system indicated two potential mechanisms for elevated TDG pressures: (1) air entrainment during AR operations, and (2) temperature changes between the winter recharge season and the summer withdrawal season. Air entrainment during pumping was investigated by intentionally allowing the forebay (upstream reservoir) of a large pumping station to drawdown to the point of vortex formation. This resulted in up to a 0.7 atm increase in TDG pressure. In general, the solubility of gases in water decreases as the temperature increases. In the Las Vegas Valley, water that acquired a modest amount of dissolved gas during winter artificial recharge operations experienced an increase in dissolved gas pressure (0.04 atm/°C) as the water warmed in the subsurface. A combination of air entrainment during AR operations and its amplification by temperature increase after recharge can account for most of the observed amounts of excess gas at this site. [ABSTRACT FROM AUTHOR]

Simultaneous short-pulse injections of two tracers (sodium bromide [Br–] and fluorescein dye) were made in a losing reach of Snake Creek in Great Basin National Park, Nevada, USA, to evaluate the quantity of stream loss through permeable carbonates that resurfaces at a spring approximately 10 km down drainage. A revised hydrogeologic cross section for a possible flow path of the infiltrated Snake Creek water is presented, and the results may inform water management in the region. First arrival and peak concentration of the two tracers occurred at 9.5 and 12.7 days after injection, respectively. Fracture transport simulations indicate that Br– preferentially diffuses into immobile regions of the aquifer, and this diffusive flux is likely responsible for the major differences in the breakthrough curves. When considering the diffusive tracer flux, total apparent Br– and fluorescein dye recoveries were 16.9–22.1% and 21.7–24.3%, respectively. These findings imply that consideration of diffusive flux and long-term monitoring in fracture-dominated flow may support accurate quantification of tracer recovery. In addition, the apparent power law slopes of the breakthrough tails for both tracers were steeper at early times than have been attributed to heterogeneous advection or channeling in meter-scale tests, but the late-time Br– power law slope becomes less steep than has been attributed to diffusive exchange. These deviations may reflect fracture transport patterns that occur at larger scales. [ABSTRACT FROM AUTHOR]

Stream reach mass-balance can be used to estimate groundwater age at scales larger than individual wells. Atmospherically derived SF6 was used as an age-dating tracer in reach mass-balance analysis of six stream reaches in Nebraska, USA. The goal was to estimate the flow-weighted mean SF6 concentration in the groundwater discharge to each reach (Cgw) and the groundwater mean transit time (MTT) based on Cgw. The Cgw in one reach (1.48 fmol/kg) was consistent with a MTT of 27 years. High uncertainty in Cgw prevented plausible MTT estimates in the other reaches. Monte Carlo analyses indicated that low rates of groundwater inflow to the stream reaches (Qgw) led to high uncertainty in Cgw. This issue can affect the success of any reach mass-balance analysis, for age dating tracers or other solutes. To improve the likelihood of success in future applications of reach mass-balance, two new planning metrics are proposed for use, separately or together: the ratio of groundwater recharge rate (R) to stream density (D) within a watershed, and the long-term average Qgw/Qd in a stream reach (where Qd is stream discharge at the downstream end of the reach). In comparison to the Nebraska study site (R/D = 107 m2/year), more accurate estimation of Cgw was achieved in a previous SF6 study in North Carolina (USA) with greater R/D (231 m2/year). Larger Qgw/Qd is associated with more accurate outcomes in reach mass-balance, and long-term average Qgw/Qd in stream reaches may be estimated by GIS analysis of watershed areas and/or stream lengths. [ABSTRACT FROM AUTHOR]