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.
The two neighboring Suvasvesi North and South impact structures in central‐east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents 40Ar/39Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. 40Ar/39Ar step‐heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in 40Ar/39Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system. 相似文献
Questions concerning the influence of soil type and crop cover on the fate and transport of nitrate (NO3−) were examined. During a growing season, soils derived from glacial material underlying either corn or soybeans were sampled
for levels of NO3− within the pore water. Measured levels of NO3− ranged from below detection limit to 14.9 g NO3− per kilogram of soil (g/kg). In fields with the same crop cover, the silty-clayey soil exhibited a greater decrease in NO3− levels with depth than the sandier soil. Crop uptake of NO3− occurs within the root zone; however, the type of crop cover did not have a direct impact on the fate or transport during
the growing season. The soils underlying soybeans had an increase in NO3− levels following harvest, suggesting that the decomposition of the soybean roots contributed to the net gain of NO3− in the shallow soil. For all of the soil types, conditions below 100 cm are conducive for microbial denitrification, with
both a high water saturation level (>60%) and moderate organic carbon content (1–2%). At depths below 100 cm, temporal differences
in NO3− levels of over a magnitude, up to a 95% reduction, were recorded in the soil units as the growing season progressed. Physical
properties that control the transport of NO3− or denitrification have a larger influence on NO3− levels than crop type.
Natural Hazards - As a concept, social vulnerability describes combinations of social, cultural, economic, political, and institutional processes that shape socioeconomic differentials in the... 相似文献
We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between energy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in large magnetic Reynolds number rotators, there must then be an escape of relative magnetic helicity associated with the mean field, accompanied by an equal and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity including the dissipation of magnetic energy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo simulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert 1 AGN coronae. 相似文献
The structure and function of alluvial Highly Dynamic River Systems (HDRS) are driven by highly variable hydrological disturbance
regimes, and alternate between resistant, metastable states and resilient, transitional states. These are in turn subject
to influences of feedback loops within hydrogeomorphic and biological processes. Here we consider how resistance and resilience
largely determine HDRS ecosystem trajectories and how these characteristics can be modified by natural and anthropogenic processes.
We review the mechanisms by which biodiversity can affect both resistance and resilience and introduce a conceptual framework
that incorporates some unique HDRS characteristics. We suggest that resilient and resistant patterns frequently coexist in
the active tract of these river systems, and that this coexistance promotes the return of metastable states after major disturbances.
In contrast, highly resistant and poorly resilient patterns dominate at their external boundaries. The loss of these natural
dynamics resulting from direct and indirect human impacts causes deviations to resistance and resilience patterns and therefore
to HDRS trajectory. We propose that understanding the role of interactions between biological and physical processes that
control resistance and resilience is crucial for system restoration and management. 相似文献
This paper presents model simulation results of vapor intrusion into structures built atop sites contaminated with volatile or semivolatile chemicals of concern. A three-dimensional finite element model was used to investigate the importance of factors that could influence vapor intrusion when the site is characterized by nonhomogeneous soils. Model simulations were performed to examine how soil layers of differing properties alter soil-gas concentration profiles and vapor intrusion rates into structures. The results illustrate difference in soil-gas concentration profiles and vapor intrusion rates between homogeneous and layered soils. The findings support the need for site conceptual models to adequately represent a site's geology when conducting site characterizations, interpreting field data, and assessing the risk of vapor intrusion at a given site. For instance, in layered geologies, a lower permeability and diffusivity soil layer between the source and building often limits vapor intrusion rates, even if a higher permeability layer near the foundation permits increased soil-gas flow rates into the building. In addition, the presence of water-saturated clay layers can considerably influence soil-gas concentration profiles. Therefore, interpreting field data without accounting for clay layers in the site conceptual model could result in inaccurate risk calculations. Important considerations for developing more accurate conceptual site models are discussed in light of the findings. 相似文献