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Groundwater flow model construction is often time-consuming and costly, with development ideally focused on a specific purpose, such as quantifying well capture from water bodies or providing flow fields for simulating advective transport. As environmental challenges evolve, the incentive to re-purpose existing groundwater flow models may increase. However, few studies have evaluated which characteristics of groundwater flow models deserve greatest consideration when re-purposing models for groundwater age and advective transport simulations. In this paper, we compare simulated age metrics produced by three MODFLOW-MODPATH models of the same area but with differing levels of complexity (layering and heterogeneity). Comparisons are made at three watershed scales (HUC 8 to HUC 12). Groundwater age metrics, specifically the young fraction and median age of the young and old fractions, are used for evaluation because they relate to intrinsic susceptibility of aquifers and are simpler to interpret than full age distributions used for advective transport. Results indicate that: (1) the young fraction is less sensitive to model layering than the median age of young and old fractions, suggesting that simple models may suffice for basic intrinsic susceptibility assessments; (2) water table mounding and associated discharge into partially penetrating boundaries, such as head-water streams, is important for simulating both the young fraction and the median age of the young fraction; and (3) the influence of partially penetrating head-water streams is maintained regardless of the porosity distribution. Results of this work should aid modelers with evaluating the appropriateness of re-purposing existing groundwater flow models for age simulations. 相似文献
75.
Bryan Shuman Anna K. Henderson Steven M. Colman Jeffery R. Stone Sherilyn C. Fritz Lora R. Stevens Mitchell J. Power Cathy Whitlock 《Quaternary Science Reviews》2009,28(19-20):1861-1879
The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability. 相似文献
76.
Jason?D.?ToftEmail author Charles?A.?Simenstad Jeffery?R.?Cordell Lenny?F.?Grimaldo 《Estuaries and Coasts》2003,26(3):746-758
The South American floating aquatic plant water hyacinth (Eichhornia crassipes) has a history of worldwide invasions, including a 1904 introduction into the Sacramento-San Joaquin Delta, California. The
native pennywort (Hydrocotyle umbellata) occupies similar habitats in the Delta and is extensively used by resident invertebrates and fish. We sought to discover
if an invader would be functionally equivalent to the native plant, by asking whether the encroadhing hyacinth modified the
invertebrate assemblage structure and fish-invertebrate food web relative to pennywort. We sampled epiphytic, epibenthic,
and benthic invertebrates, and plant canopy insects in patches of hyacinth and pennywort, and analyzed fish diets at three
sites in the Delta during 1998. We also measured habitat structure (leaf density, root biomass, and surface area). In 1999,
following control and absence of hyacinth, we again measured epiphytic invertebrates in pennywort. We found differences between
hyacinth and pennywort in structure, associated invertebrates, and fish diets. Most measurements inferred functional non-equivalency
between hyacinth and pennywort, although some functional equivalency and natural variation existed. Leaf and insect densities
were significantly higher in pennywort and there were also significant differences in insect assemblage compositions. Hyacinth
roots in the water column had significantly more surface area. Densities of epibenthic and benthic aquatic invertebrates were
typically greater in pennywort and taxonomic compositions of aquatic invertebrate assemblages showed significant differences.
Amphipods and isopods living epiphytically in the root masses were particularly abundant, including several newly discovered
introduced species: the amphipodCrangonyx floridanus and the isopodsCaecidotea racovitai andAsellus hilgendorfii. The native amphipodHyalella azteca was more abundant in pennywort and heavily preyed upon by fish, while the non-indigenousC. floridanus was more abundant in hyacinth and not prevalent in fish diets. The introduction of hyacinth to the Delta has caused significant
ecological alterations in the surrounding community, due to hyacinth being functionally different from native patches of pennywort. 相似文献
77.
Zhan Zhang Paul Fenter Jeffery G. Catalano James D. Kubicki David J. Wesolowski Neil C. Sturchio 《Geochimica et cosmochimica acta》2006,70(16):4039-4056
Adsorption of Zn2+ at the rutile TiO2 (110)-aqueous interface was studied with Bragg-reflection X-ray standing waves (XSW), polarization-dependent surface extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations to understand the interrelated issues of adsorption site, its occupancy, ion-oxygen coordination and hydrolysis. At pH 8, Zn2+ was found to adsorb as an inner-sphere complex at two different sites, i.e., monodentate above the bridging O site and bidentate between two neighboring terminal O sites. EXAFS results directly revealed a four or fivefold first shell coordination environment for adsorbed Zn2+ instead of the sixfold coordination found for aqueous species at this pH. DFT calculations confirmed the energetic stability of a lower coordination environment for the adsorbed species and revealed that the change to this coordination environment is correlated with the hydrolysis of adsorbed Zn2+. In addition, the derived adsorption locations and the occupancy factors of both sites from three methods agree well, with some quantitative discrepancies in the minor site location among the XSW, EXAFS, and DFT methods. Additional XSW measurements showed that the adsorption sites of Zn2+ were unchanged at pH 6. However, the Zn2+ partitioning between the two sites changed substantially, with an almost equal distribution between the two types of sites at pH 6 compared to predominantly monodentate occupation at pH 8. 相似文献
78.
Regional landslide-hazard assessment for Seattle, Washington, USA 总被引:13,自引:6,他引:13
Rex L. Baum Jeffery A. Coe Jonathan W. Godt Edwin L. Harp Mark E. Reid William Z. Savage William H. Schulz Dianne L. Brien Alan F. Chleborad Jonathan P. McKenna John A. Michael 《Landslides》2005,2(4):266-279
Landslides are a widespread, frequent, and costly hazard in Seattle and the Puget Sound area of Washington State, USA. Shallow
earth slides triggered by heavy rainfall are the most common type of landslide in the area; many transform into debris flows
and cause significant property damage or disrupt transportation. Large rotational and translational slides, though less common,
also cause serious property damage. The hundreds of landslides that occurred during the winters of 1995–96 and 1996–97 stimulated
renewed interest by Puget Sound communities in identifying landslide-prone areas and taking actions to reduce future landslide
losses. Informal partnerships between the U.S. Geological Survey (USGS), the City of Seattle, and private consultants are
focusing on the problem of identifying and mapping areas of landslide hazard as well as characterizing temporal aspects of
the hazard. We have developed GIS-based methods to map the probability of landslide occurrence as well as empirical rainfall
thresholds and physically based methods to forecast times of landslide occurrence. Our methods for mapping landslide hazard
zones began with field studies and physically based models to assess relative slope stability, including the effects of material
properties, seasonal groundwater levels, and rainfall infiltration. We have analyzed the correlation between historic landslide
occurrence and relative slope stability to map the degree of landslide hazard. The City of Seattle is using results of the
USGS studies in storm preparedness planning for emergency access and response, planning for development or redevelopment of
hillsides, and municipal facility planning and prioritization. Methods we have developed could be applied elsewhere to suit
local needs and available data. 相似文献
79.
Prediction of diffusion coefficients in porous media using tortuosity factors based on interfacial areas 总被引:1,自引:0,他引:1
Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media (tau5) as the ratio S/S(o) (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (tau(a)) is defined as a(aw)/a(aw),o (ratio of the specific air-water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on tau(s) and the same based on the Kozeny-Carman equation confirm the usefulness of the tau(s) parameter as a measure of tortuosity. 相似文献
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