ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Most state-of-the-art manipulations and visualizations of soil data use geographic information systems to portray soil landscapes in two dimensions (2-D). Nevertheless, soil attributes are distributed continuously in three dimensions (3-D) across landscapes. The objective of this study was to investigate the use of Virtual Reality Modeling Language (VRML), a 3-D graphics language suitable for stand-alone or browser-based interactive viewing, to create 3-D soil landscape models at different scales. Four different locations in southern Wisconsin were selected to represent pedon, catena, catchment, and soil region scales. Soil data, including texture, cone index, and depth of soil layers, were used in conjunction with topographic attributes to create 3-D soil landscape models. Spatial modeling techniques comprised 2-D and 3-D ordinary kriging. We used Environmental Visualization Software (EVS) to export the geometry of 3-D objects, which were enhanced to include: (i) viewpoints, (ii) Munsell colors, (iii) texture maps, (iv) 3-D cross-section animation, (v) animations such as zooming and rotation, and (vi) primitive shapes to highlight areas of interest. Virtual reality modeling language is capable of describing and visualizing extremely complex shapes, such as complex soil layers or terrain. Visualization of Munsell soil colors was difficult to implement because there is no hardware or software independent color-management system available in VRML. Animation techniques were valuable to high-light specific characteristics of each model. The accessibility of interactive VRML models via the World Wide Web and the portability of these models across platforms facilitate the entry of soil science into the virtual world of cyberspace.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

In many solute transport studies, either the flux or the resident concentration is used. In some cases, however, the transport parameters obtained from different concentration modes may not be identical, especially for soils having preferential pathways. In this study we investigated differences in the transport parameters between flux and resident concentrations by performing laboratory solute displacement experiments on a number of structured field soils. Breakthrough curves (BTCs) of flux and resident concentrations for a pulse injection of 10 g/L CaCl2solution were monitored simultaneously at the bottom and middle of soil columns using an EC-meter and time domain reflectometry (TDR) probes, respectively. Transport parameters were then obtained by fitting the convective lognormal transfer function (CLT) model to the observed BTC data and compared for different concentration modes. Flux concentrations predicted from the parameters of resident concentrations based on the CLT model were also compared with the observed BTC data. Comparison of transport parameters between the flux and resident concentrations showed substantial differences caused by preferential movement of solute through soil macropores. The predicted flux concentration BTCs also differed greatly from the observed BTCs in peak and travel time. This suggests that for structured soils having preferential flow, the TDR-measured resident concentrations are not representative of solute transport in the soil macropores but are primarily in the soil matrix region, and use of TDR for monitoring resident concentrations in such soils becomes limited.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Numerical models are being used increasingly to simulate water and solute movement in the subsurface for a variety of applications in research and soil/water management. Although a large number of models of varying degrees of complexity have been developed over the years, relatively few have been tested under field conditions. We tested the performance of the HYDRUS-1D computer model to simulate variably saturated water flow and chloride transport in a fine-textured Italian soil subject to a fluctuating saline groundwater table. The model was also used for estimating solute transport parameters using an inverse optimization scheme. Our results indicate that including the effects of immobile water produced better predictions of chloride transport compared with the traditional convection-dispersion transport approach. Including anion exclusion as well did not improve the model predictions appreciably. Occasional deviations between model prediction and field observation were attributed to unrepresented lateral groundwater flow processes and to preferential flow through macropores or other structural voids. The HYDRUS-1D model was found to be very useful for analyzing the relatively complex flow and solute transport processes at our field site and for estimating model parameters using inverse procedures.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Adsorption and desorption of herbicides are important processes that influence the amount of herbicide retained by the soil and that which is susceptible to runoff or movement in the soil profile. In this study, we examined the adsorption and desorption characteristics of metolachlor in two Louisiana soils: Sharkey clay and Commerce loam. Kinetic batch experiments were used for a wide range of initial concentrations and reaction times (up to 504 h). After adsorption of the metolachlor, desorption took place using successive dilution. Six consecutive desorption steps (6 day) were carried out. Metolachlor adsorption and desorption results were strongly kinetic for both soils. After the six desorption steps, the average amount of metolachlor desorbed was 30.5% for the Sharkey clay and 43.9% for the Commerce loam. The amount of metolachlor desorbed was also dependent on the initial concentration. Families of desorption isotherms were represented in two ways. The first family is based on the traditional approach, in which successive desorption isotherms are presented. The second family is time-dependent desorption isotherms, which are presented for each given reaction (adsorption plus desorption) time in a manner similar to those for adsorption isotherms. Two sets of Freundlich parameters were thus derived for the desorption isotherms. Moreover, a hysteresis coefficient, based on the difference in the area between the adsorption and desorption isotherms, was proposed. This coefficient was capable of quantifying hysteresis for both types of desorption isotherms.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

To understand the long-term fate of heavy metals applied to agricultural soils via sewage sludges, it is necessary to measure metal speciation and solubility in the soil for many years after application. With this as our objective, we measured total and dissolved trace element concentrations, including potentially toxic heavy metals, in the contaminated 0 to 5-cm surface soil layer of a long-term, no-till continuous bromegrass experiment about 18 years after the last application of three chemically different sewage sludges. For each particular sludge, long-term heavy metal solubility was generally linearly correlated to the remaining heavy metal concentration in the soil, with the nature of the sludges applied affecting the partition coefficient (KD) for some elements and heavy metals. Most of the dissolved Cu and Pb (generally >75%) was not labile by anodic stripping voltammetry (ASV), indicating a high degree of complexation of these two metals by soluble organic matter. A smaller degree of complexation (<50%) of dissolved Cd and Zn in nonlabile organic complexes was measured. Some reduction of both organic matter content and heavy metal concentrations in the 0 to 5-cm layer was measured in the time interval between 1979 and 1997. The KDvalues for Cu, Zn and Cd were all close to 104for the sludge-treated soils with the highest organic matter content, indicating a strong metal retention that may be attributable to binding to organic matter, but free calcium carbonate in the soil was probably important in limiting metal lability and solubility. A strong linear correlation between total Cd and sulfur in these soils suggests that Cd may be associated with organosulfur ligands. Heavy metal ion activities in the high-organic matter surface layer at this site were low compared with activities measured at other sites with comparable total metal loadings, suggesting the importance of the role of continuous grass and notill management in maintaining high organic matter and consequent low metal activity in high-lime soils.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Phosphorus (P) concentrations in soil solution are generally low, whereas those near fertilizer particles may reach much greater values. The process governing P sorption at these high concentrations is obscure. Thus, the sorption mechanisms at high P concentrations on a synthetic goethite were investigated to assess the influence of ionic strength on P sorption and release. In addition to classic isotherms, the adsorption was investigated by Fourier-transform infrared spectroscopy and electrophoretic mobility. At constant ionic strength, the maximum amount adsorbed was about 2.3 μmol P m−2, which remained constant even at the highest concentrations. With variable ionic strength, the sorption isotherm showed plateaus at 2.3 and 5.3 μmol P m−2. Zeta potential and particle size data indicated that the increased ionic strength generated by P addition caused a compression of the phosphate ion swarm closer to the oxide surface, resulting in a reduction of the electrostatic repulsive forces and leading to an increase in sorption. The results of P release and Fourier-transform infrared spectra of the goethite-P complexes confirm this hypothesis. Thus, at high concentrations, sorption is possible beyond the capacity of the goethite singly coordinated surface groups, suggesting that both chemisorption and physical mechanisms are involved.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Surface gamma-neutron gauges have been used to evaluate soil physical properties, mainly soil bulk density. However, their use for compacted soil layer detection is still not well understood. In this work, a surface gamma-neutron gauge was used to identify the presence of compacted layers in soil profiles. Variations of the gauge response were also investigated for layers located at different depths. The results showed that although this type of gauge yields an average value of the densities found along the gamma-ray path, the use of an algorithm permitted the transformation of gauge measurements into layer density values, which were in good agreement with gravimetric measurements. The application of the algorithm was found to yield the best results when there was a large difference in density between the compacted layers and the medium around them.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID