摘要:

The use of parallel two-rod time domain reflectometry (TDR) probes is widespread because the two-rod probe is the least destructive of the conventional TDR probes and has a larger sample volume than a three-rod probe of equal dimensions. However, in order to transform the electromagnetic signal from unbalanced to balanced at the point of connection with the cable, two-rod probes have been thought to require a balun, making the probe construction both more expensive and more complicated. In this study, a two-rod probe without a balun (unbalanced) was exposed to a rising air-water interface, creating a sharp dielectric permittivity boundary within the sample volume of the probe. The probes were horizontal, but they were located within a vertical plane, i.e., one rod was placed above the other. A shorting diode technique was used to improve the location of the end reflection on the TDR traces. Two experiments were carried out differing only in the connection of the coaxial cable to the probe rods. In one experiment, the conductor was connected to the lower rod and the shield was connected to the upper rod. In the second experiment these connections were reversed. Using a numerical model, the relative dielectric permittivity (K) responses of two-rod balanced and unbalanced TDR probes were predicted as a function of the fluid interface height. The measured and modeled responses of the unbalanced two-rod probe matched perfectly, and there was no observed increase in the spatial sensitivity of the probe adjacent to either rod. Furthermore, the modeled probe responses as well as the sample areas for the balanced and unbalanced probe configurations were identical. Based on these results, we suggest that baluns be omitted from two-rod TDR probe designs.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Time Domain Reflectometry (TDR) is a recognized and widely used technique for measuring electrical conductivity (EC) and volumetric water content (&thgr;) in porous media. The ability of TDR to measure both EC and &thgr; is especially appealing in solute transport in variably saturated media. It is commonly assumed that TDR measurements are representative of the EC and &thgr; in the horizontal plane in which the probe is located. The problem is that it is difficult to recognize heterogeneous solute transport with TDR, especially if it occurs outside the sample volume or in regions where the solute is giving little weight to the TDR-measured EC. To determine the presence of heterogeneous solute transport effectively, there is a need for a device to monitor the EC boundary conditions. In this study, a simple and easy-to-make coaxial flow cell is designed and tested for this purpose. The flow cell is made primarily of cheap, prefabricated, and readily available components, and the construction requires only a hacksaw and some welding skills. The idea is to make the effluent from a solute transport experiment pass through the coaxial flow cell, thereby obtaining a measure of the effluent EC. In addition to providing detailed information on the solute transport through the entire sample of porous medium, it will also detect, for example, bypass flow. A solute transport experiment was carried out in a PVC pipe packed with coarse silica sand under saturated conditions to calibrate the flow cell and to demonstrate its potential use. Step input breakthrough and breakdown functions were created using tap water and a KCl solution. Highly detailed measurements of EC in the effluent were obtained, from which solute transport parameters can easily be inferred.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

It is commonly believed that Time Domain Reflectometry (TDR) measures bulk soil electrical conductivity (EC) and volumetric water content within the same, well-defined sample volume. However, recent studies have shown that the sample volume is a function of the distribution of EC and dielectric permittivity near the probe. One result of this spatially distributed sensitivity is measurement-induced dispersion. That is, when TDR is used to measure a sharp advancing solute front, the measured EC is some average across the sharp front, leading to incorrect smoothing of the breakthrough curve. A reduction of the probe dimensions is the only solution to this artificial smoothing problem. In this study, a small scale TDR probe is presented and tested. The small probe dimensions produce a near point measurement of EC but make water content measurements unreliable. The small scale EC TDR (SEC-TDR) probe is simple, inexpensive, and made with readily available components. A solute transport experiment was carried out under saturated conditions in a plastic pipe packed with coarse silica sand. Five SEC-TDR probes were inserted, monitoring the EC at various positions along the column, and a coaxial flow cell was used to monitor the effluent EC. Step solute breakthrough and displacement breakthrough responses were created using tap water and a KCl solution. Highly detailed measurements of EC were obtained from which the dispersivity (λ) was inferred. The λ measured by the SEC-TDR probes was significantly lower than λ measured in the effluent by the coaxial flow cell, suggesting that the SEC-TDR probe can reduce the problem of TDR-induced dispersion under even the most challenging conditions.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

The equation of one-dimensional thermal conduction and convection in soil is solved analytically by applying the traditional harmonic method (HM) and the Laplace transform method (LTM). A simple method to determine accurate values of soil heat diffusivity and liquid water flux density is given. Using this method, we determined the soil thermal diffusivity,k= 0.85 × 10−6m2s−1, and liquid water flux density,W= 4.3 × 10−6m3s−1m−2, for the Naqu site in the summer of 1998. An experimental evaluation of the proposed methods is also given.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds that plays an important role in the movement of DOM-associated pollutants. In this study, transport and fractionation of DOM in soils was investigated in flow-through soil columns. Dissolved organic matter derived from spent mushroom substrate weathering was pumped through packed columns (2.5 cm × 10 cm) comprising a coarse-loamy subsoil (mixed, semiactive, mesic Typic Hapludult), and effluents were monitored for changes in the composition of DOM. Effluent DOM was characterized for UV absorbance, molecular weight, acidity, and hydrophilicity. Transport through the columns resulted in preferential retention of specific DOM constituents as indicated by comparison with a Br−tracer. During the transport process, effluent DOM exhibited decreasing values of E2/E3(from 10.3 to 6.2), acidity (from 20.8 to 13.1 mmolcg−1C), and hydrophilicity (39.0 to 28.4%), and increasing values of molar absorptivity (from 164 to 310 L mol−1C cm−1) and number and weight-averaged molecular weight (from 1770 to 3150 and 2450 to 4180 Da, respectively). These results indicate that DOM fractions with higher molecular weight, higher molar absorptivity, lower E2/E3ratio, lower acidity, and lower hydrophilicity were adsorbed preferentially by soil minerals, whereas the inverse fractions were transported preferentially. The adsorbed DOM could not be completely desorbed by DOM-free background solution, indicating a strongly bound fraction. Sorptive fractionation of DOM during transport likely affects the transport behavior of DOM-complexed constituents.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Water movement in soil was observed directly and recorded on videotape using soft X-ray technology with a liquid contrast agent and on continuous photos. The movement of soil water occurred in an Andosol sample, and quantitative analyses were carried out by image processing. The results shown here are:The fluids flowing in tubular macro pores connected with each other. The flow showed spatial motion in which the tubular pores were sometimes swollen. Soil water moved into small pores like a volcanic explosion and also moved, worm-like, according to the shape of the tubular pores. Soil water movement occurred only in some tubular pores. In most cases, soil water was not detected in the pores.The spatial structures of macropores shown in three-dimensional graphics were drawn from stereo-radiographs, based on the observation of the spatial contribution of soil pore structures and the calculation of their actual length and tortuousity.Applying Darcy’s law, various permeabilities (K2,K3,K4) of the soil were calculated using the diameters of the dominant macropores and actual velocities. The permeability was compared with the permeability (K1) tested independently by the constant head method. Differences amongK1,K2,K3andK4were not significant.The validity of Darcy’s law was tested using the constant head tank at different heads and samples. Results show that the validity of Darcy’s law was not confirmed.The Reynolds number was calculated using the actual diameter of the tubular pores and velocities of the agent measured by the stereo-graph obtained by the video. The flow seemed to be transitional between laminar and turbulent.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Nuclear magnetic resonance spectroscopy is used frequently for structural characterization of humic substances (HS). However, the study of relaxation phenomena of HS, including spin-lattice and spin-spin relaxation, has received little attention. The present study investigated proton spin-lattice relaxation times (T1) of humic acids (HA) in deuterated 0.5 M sodium hydroxide (NaOD) and dimethylsulfoxide (d6-DMSO) at various concentrations, temperatures, and pHs. The two HAs used were extracted from an Amherst peat soil and a New Hampshire peat soil. Overall, the proton T1s of humic acids used in this study ranged from 0.3 to 0.8 s in d6-DMSO, and from 0.26 to 2.3 s in 0.5 M NaOD/D2O. Aromatic protons had much longer T1than aliphatic protons in 0.5 M NaOD; however, both proton types exhibited similar T1values in d6-DMSO. These results indicate that humic acids might have different conformations in the two solvents. Proton T1generally decreased with increasing HA concentration. Only protons from methyl and methylene/methine groups exhibited a slight T1increase, with a temperature increase from 20 °C to 60 °C, with protons from other functional groups displaying a reduced T1, except for aromatic protons of New Hampshire HA in 0.5 M NaOD. Moreover, both HAs demonstrated distinct T1changes in alkaline solution as pH decreased. The proton T1values and their changes with environmental conditions can further the interpretation of segmental mobility and molecular conformation of dissolved HAs in different solvents.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Thermally synthesized polyaspartate (TPA, AmiSorb) is marketed as a crop growth enhancer. However, the stability and adsorption of this product in soils have not been fully evaluated. In this study, microbial degradation of TPA was determined in 10 different soils using14Clabeled TPA. Biodegradation, measured as mineralization (CO2production), varied with soil type ranging from 2.15% for a Hort Mix to 32.05% for the Pullman soil after 35 days incubation under the defined conditions. Percentages of degradation also varied with concentrations of the applied TPA. In most of the soils evaluated, TPA degradation rates were higher at 100 μg g−1than at 2 or 10 μg g−1after 35 days of incubation. In general, C and N supplements, as primary nutrients for microbial growth, reduced mineralization, suggesting that the level of readily available macronutrients in the soil influenced TPA utilization. Biodegradation of TPA was highly correlated with soil pH but was not correlated with any other soil parameters measured in this study, including Kd. The Kd values of the tested soils for TPA ranged from 10.6 to 98.8 mL g−1for the Norfolk and Bolfar soils, respectively.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID