摘要:

Drying of a soil surface can lead to cracking, thereby creating large channels for waste to leach out of a landfill site. Temperature affects the rate of drying. Water movement induced by thermal gradients was studied with physical experiments using soil columns and with numerical experiments using a computer program. Clay soil material was moistened to 0.407 or 0.392 m3/m3water content. The moistened soil was packed and compacted to a density of 1.45 Mg/m3into 0.3-m long columns. The soil columns were closed at the top end using black plastic discs and closed at the bottom end using plexiglas discs. The soil columns moistened at 0.407 m3/m3were subjected to natural radiation for 55 days and those moistened at 0.392 m3/m3were subjected to elevated radiation levels provided by heat lamps for 54 days. Both levels of radiation create periodic temperature boundaries at the ends of soil columns. The numerical model describes coupled heat and water transfer in the soils. The soil columns receiving elevated radiation had more net water movement from the hot ends toward the cold ends of the soil columns than did the soil columns exposed to natural radiation. The model predicted the soil water content distributions well along the soil column compared with the measured water content. The study shows that there may be some drying of compacted soil under a plastic landfill liner as a result of temperatures and thermal gradients created when the liner is exposed to the high periodic temperature regime in comparison with the low periodic temperature regime. The drying was limited to the surface 20 mm. The computer program was also used to simulate the condition of constant boundary temperatures. Temperatures of 50 and 20°C were used at the hot and cold ends, respectively. The numerical study with constant thermal gradients showed that, compared with periodic temperature regimes, large amounts of water migrated. The drying was extended to a depth of 70 mm. Decomposing buried waste that generates heat can lead to a constant thermal gradient in the clay liner. This can cause more severe drying and cracking than a liner surface exposed to the natural periodic heating and cooling of the environment. Generated heat may accelerate deterioration of the plastic liner as well.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Knowledge of wetting front location and cumulative infiltration during irrigation and rainfall events are important to many areas of soil science. We tested a new procedure using time domain reflectometry (TDR) to estimate wetting front location and cumulative infiltration. The method is based on an assumption of a uniform soil water content behind the wetting front. Measurements of wetting front location and cumulative infiltration were made with 0.144-m-long TDR probes vertically inserted into homogeneous loamy sand columns. Unlike previous procedures using TDR, the new procedure could be applied to short TDR probes (≤ 0.3 m) because the procedure did not require determination of a reflection of electromagnetic waves from the wetting front in TDR traces. The method gave satisfactory estimates of both wetting front location and cumulative infiltration.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

The bulk density of a soil changes with natural and artificial processes in a field. The larger the bulk density ρbof the soil, the larger the air entry suction heand the smaller the hydraulic conductivity Ks. Although both the Kozeny-Carman equation, based on Poiseuille's law, and Campbell's method, based on a similar media concept (SMC) by Miller and Miller, are able to predict bulk density dependencies of hydraulic conductivities theoretically, the applicabilities and limitations of them have been vague. This paper proposes a non-similar media concept (NSMC) model, composed of a characteristic length for the solid phase S and for the pore phase d, and a shape factor τ of the solid phase, to predict heand Ksvalues of a given soil as functions of its ρb. By comparing the NSMC model with the Campbell method and the Kozeny-Carman equation, it was clarified experimentally that the NSMC model is applicable to aggregated soils in the predictions of he(ρb) and Ks(ρb) as well as to dispersed soils. The shape factor τ was close to 1 when the soil was dispersed and sandy, whereas it was close to ρb/ρs, ρsbeing the density of soil particles, when the soil was aggregated.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Two weighing lysimeters were installed at the University of Arizona's Karsten Center for Turfgrass Research. The goals of the facility are to measure evapotranspiration (ET) and to perform water use and deep percolation experiments in a deep soil profile under both natural precipitation and irrigation conditions. Each lysimeter is 4.0 m deep, 2.5 m in diameter, and equipped with 96 sampling ports for soil solution samplers, tensiometers, time domain reflectometry (TDR) probes, and thermocouples. The weighing scales have a capacity of 45 Mg and can detect a 200-g mass change, equivalent to ± 0.04 mm of water on the surface. The lysimeters were filled with Vinton fine sand (sandy, mixed thermic Typic Torrifluvent). Monitoring the unvegetated lysimeters for 18 weeks indicated that about 66% of all precipitation percolated into the soil profile. In June 1994, the lysimeter surfaces and surrounding area were planted with `Tifway' bermudagrass (Cynodon dactylon x transvaalensis var. Tifway), and irrigation began. Three phases of turf water use were identified during the first 155 days after planting—establishment, full cover and overseeding. The percentage of water that percolated into the soil profile deeper than the root zone was 35.6%, 10.1%, and 48.3%, respectively, showing the effectiveness of active root water uptake in reducing deep percolation. The TDR, neutron probe and tensiometer methods provided very similar indications of the depth of the wetting front. Differences in depth estimates could be caused by differences in sampling volume among the three methods. The TDR and neutron probe provided estimates of soil water storage that usually were within 10% of one another, and consistent with changes in lysimeter mass.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

With the practice of continuous no-tillage, the question arises of whether, after a period of time, some sort of mechanical tillage will be required to alleviate compaction. Studies of maximum compaction using the Proctor test on a total of 36 samples from four Kentucky soils revealed that compactability was negatively related to the amount of organic carbon in the sample. Because organic carbon tends to be higher to a depth of 10-cm where no-tillage has been practiced, the tendency is towardslesscompaction with increasing years of no-tillage to that depth. Continuous sod also showed higher organic matter and less compactability. Therefore, nondisturbed systems such as no-tillage and permanent sod form a defense against shallow compaction over time because of the increase in organic matter that occurs.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Single and multiple metal ion reaction processes were studied on humic acids (HA) from various sources. A sequential sampling method (samples periodically removed from the bulk reaction volume) was used. Experimental parameters studied included pH, temperature, HA and peat sources, metal type and concentration. The Langmuir-Hinshelwood (LH) kinetics approach was used to describe the sorption kinetics of metal ions onto HA. Great differences were found among the metals in the extent of reaction and sorption kinetics. The extent of reaction was Cr(III) > Pb(II) > Cu(II) > Ag(I) > Cd(II) = Co(II) = Li(I) for single metal adsorption and Cr(III) >> Pb(II) > Cu(II) = Ag(I) = Cd(II) = Co(II) = Li(I) for multiple metal adsorption on Humic Acid No. 1. The multiple ion sorption kinetics were similar between HA from different sources. For peat reactions, the extent of multiple metal ion sorption was Cr(III) > Pb(II) > Cu(II) > Ag(I) > Co(II) = Cd(II) = Li(I).The sorption rates of all metal ions on HA were very rapid. Most adsorption occurred within 2 minutes, but sorption did not reach equilibrium in 1 day under competitive conditions. The presence of Cr(III) greatly decreased the adsorption of all other metals to the extent that some were not adsorbed at all. For all metal ions studied, less total sorption of each metal ion occurred in the presence of multiple competing ions than with single metal ions.Pearson's Hard Soft Acid Base principles and Klopman's generalized perturbation theory of chemical reactivity generally predicted the sorption preference between metal ions and humic substances. A nonlinearity concept was used to rationalize the use of the LH kinetic model.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

During sample preparation, the coarse fraction (> 2 mm) of soils is commonly excluded from further analytical determinations. Our results demonstrated, however, that the coarse fragments of soils derived from sandstone in Tuscany, Italy, are not chemically inert.From an intensive study of the clasts from three profiles in the Vallombrosa Forest 50 km east of Florence, we have established that the clasts of this fraction, especially those altered and partially altered, manifest properties that in some cases equal or surpass those of the more reactive fine earth. The profiles are underlain by Arenaria del Falterona, a sandstone of the Oligocene time intercalated with siltstone. Our research shows that the coarse fraction participates in a sequence of weathering events that involves dissolution of carbonates and formation and transformation of secondary minerals.Although fresh sandstone and siltstone display distinct mineralogical composition, these differences are not maintained once they undergo accelerated weathering in the soil. Dissolution of carbonate brings indirect enrichment of the noncarbonatic components. Formation of HIV (hydroxy interlayered vermiculite) and HIS (hydroxy interlayered smectite) is the dominant process in the fine earth and clasts. This process becomes particularly important in stabilizing the micropores of the clasts and maintaining a porosity that allows the soil solution to be adsorbed and circulate. Weathering processes thus create voids, release nutrient cations, and render the most weathered clasts similar to fine earth. The progression of weathering into the clasts is demonstrated by the levels of extractable Fe and Al obtained by selective dissolution. Furthermore, our research shows that the rock fragments contain C and N in considerable quantities and have a higher pH and a mineral assemblage less weathered than the fine earth. In addition, clasts constitute a reservoir for nutrients and possess a capacity for proton consumption. Moreover, although the fine earth, including organic matter, dominates the effective cation exchange capacity (ECEC) in the upper horizons, in the B and BC horizons the rock fragments represent from 20 to 55% of the total ECEC.In conclusion, our findings show that the clasts of these “reactive lithologies” are not inert but play an important role in soils as: (i) reservoirs of nutrients, including N, (ii) sources of cation exchange capacity, (iii) water supplies, (iv) adsorbers of organic pollutants, and in the consumption of protons.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID