Excess air during aquifer storage and recovery in an arid basin (Las Vegas Valley, USA)

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.     

Comparison of a Manual and an Automated Tracking Method for Tibetan Plateau Vortices

Tibetan Plateau vortices(TPVs) are mesoscale cyclones originating over the Tibetan Plateau(TP) during the extended summer season(April–September).Most TPVs stay on the TP,but a small number can move off the TP to the east.TPVs are known to be one of the main precipitation-bearing systems on the TP and moving-off TPVs have been associated with heavy precipitation and flooding downstream of the TP(e.g.,in Sichuan province or over the Yangtze River Valley).Identifying and tracking TPVs is difficult because of their comparatively small horizontal extent(400–800 km) and the limited availability of soundings over the TP,which in turn constitutes a challenge for short-term predictions of TPV-related impacts and for the climatological study of TPVs.In this study,(i) manual tracking(MT) results using radiosonde data from a network over and downstream of the TP are compared with(ii) results obtained by an automated tracking(AT) algorithm applied to ERA-Interim data.Ten MT-TPV cases are selected based on method(i) and matched to and compared with the corresponding AT-TPVs identified with method(ii).Conversely,ten AT-TPVs are selected and compared with the corresponding MT-TPVs.In general,the comparison shows good results in cases where the underlying data are in good agreement,but considerable differences are also seen in some cases and explained in terms of differences in the tracking methods,data availability/coverage and disagreement between sounding and ERA-Interim data.Recommendations are given for future efforts in TPV detection and tracking,including in an operational weather forecasting context.     

Tectonometamorphic evolution of the Himalayan metamorphic core between the Annapurna and Dhaulagiri, central Nepal

The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma ⁴⁰Ar/³⁹Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.     

Data reporting norms for 40Ar/39Ar geochronology

Data reported in ⁴⁰Ar/³⁹Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all ⁴⁰Ar/³⁹Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses).     

Sensitivity of large-scale atmospheric analyses to humidity observations and its impact on the global water cycle and tropical and extratropical weather systems in ERA40

Reanalysis data obtained from data assimilation are increasingly used for diagnostic studies of the general circulation of the atmosphere, for the validation of modelling experiments and for estimating energy and water fluxes between the Earth surface and the atmosphere. Because fluxes are not specifically observed, but determined by the data assimilation system, they are not only influenced by the utilized observations but also by model physics and dynamics and by the assimilation method. In order to better understand the relative importance of humidity observations for the determination of the hydrological cycle, in this paper we describe an assimilation experiment using the ERA40 reanalysis system where all humidity data have been excluded from the observational data base. The surprising result is that the model, driven by the time evolution of wind, temperature and surface pressure, is able to almost completely reconstitute the large-scale hydrological cycle of the control assimilation without the use of any humidity data. In addition, analysis of the individual weather systems in the extratropics and tropics using an objective feature tracking analysis indicates that the humidity data have very little impact on these systems. We include a discussion of these results and possible consequences for the way moisture information is assimilated, as well as the potential consequences for the design of observing systems for climate monitoring. It is further suggested, with support from a simple assimilation study with another model, that model physics and dynamics play a decisive role for the hydrological cycle, stressing the need to better understand these aspects of model parametrization.     

Monazite–xenotime thermochronometry: methodology and an example from the Nepalese Himalaya

Monazite-xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U-Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677-535 °C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the ~25.4-24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded ²⁰⁷Pb/²³⁵U dates that are statistically indistinguishable from the ²⁰⁷Pb/²³⁵U dates of coexisting xenotime crystals, permitting the application of both quantitative Y-partitioning and semi-quantitative Nd-partitioning thermometers as a cross-check for internal consistency. One of these sub-populations of accessory minerals, with a mean ²⁰⁷Pb/²³⁵U date of 22.364ǂ.097 Ma, provides inconsistent Y-partitioning (641ᆻ °C) and Nd-partitioning (515-560 °C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO₂) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO₄ binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean ²⁰⁷Pb/²³⁵U date=22.11ǂ.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535ᇅ and 525-550 °C, respectively. This consistency strongly suggests that the sample experienced a temperature of ~535 °C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by ~100 °C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U-Pb discordia with indistinguishable upper intercept ages (465.5NJ.7 and 470ᆟ Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 °C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite-xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.     

Quantifying stream-loss recovery in a spring using dual-tracer injections in the Snake Creek drainage,Great Basin National Park,Nevada, USA

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.