摘要:

The transport of dissolved contaminants through the vadose zone is a major source of soil and groundwater contamination. Soil hydraulic properties must be determined to accurately describe water and contaminant transport and potential environmental impacts. Comparisons were made of three field and three laboratory methods for estimating soil-water retention, &thetas;(Ψ), and unsaturated hydraulic conductivity functions, K(&thetas;). Instrumentation was installed in 36 field plots, and two redistribution cycles were conducted. Field data obtained from each cycle were utilized in three outflow-based field methods; (i) instantaneous profile method, (ii) Libardi's method, and (iii) a nonlinear least squares approach. Undisturbed soil cores were extracted from 24 field plots at six depths and used in laboratory tests. Techniques consisted of (i) a multi-step outflow approach coupled with (a) “inverse methodology” for transient conditions and (b) a least-squares approach for equilibrium conditions and (ii) a particle size distribution model. Parametric models were coupled with the modeling efforts. The results obtained by the in situ instantaneous profile method for both soil hydraulic functions were considered to hold the greatest validity. However, the multi-step outflow methods produced feasible &thetas;(Ψ) curves, and the inverse methodology was time efficient. Libardi's method for determining K(&thetas;) relationships was accurate at deep profile depths but failed at shallow ones.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Analytical solutions to the convection-dispersion model (CDM) of solute transport require linear reaction terms, strict initial and boundary concentration conditions, and are often complex to evaluate because of the inherent mathematical functions. We present a flexible and mathematically very simple solution to the CDM at steady water flow, labeled a semi-analytical (SA) solution. The SA solution allows for nonlinear reaction terms, variable initial and boundary conditions, and is based on the recently presented moving concentration slope (MCS) model for solute transport. To derive the SA solution, a solute flux approximation at the upper boundary and a small, constant depth increment of 0.5 cm are used, and two features of the MCS model are exploited, i.e., an explicit, depth-integrated flux equation is already inherent in the model and all numerical error and stability equations are unique functions of the solute unit mean travel distance (SUMTD). The SA solution contains seven constants; one is the solute dis-persivity, and the remaining six are functions only of the SUMTD. Excellent agreement between the SA solution and ordinary analytical solutions to the CDM was obtained. For variable boundary conditions, the SA solution was also tested against data for chloride transport in sandy soil columns. Measured and calculated outlet concentrations compared well.The SA model allows for linear or nonlinear reaction terms without increasing the complexity of the solution. In the case of nonlinear reactions, the SA model offers a simple solution in situations where conventional analytical solutions are not available. This was illustrated by successfully comparing the SA solution, including a Michaelis-Menten reaction term, with measured data for simultaneous transport and reduction of nitrate in porous media columns.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Soil evaporation is an important hydrologic and energy transfer process, yet it has historically been difficult to measure. Weighing microlysimetry is a commonly employed and accepted technique that is accurate and inexpensive, but, it is time consuming and labor intensive, with lim-ited temporal resolution. We modified the method by fitting a time domain reflectometry (TDR) probe in each microlysimeter extending from the bottom to the top. When they are installed, the probes pierce a plastic film lining the bottom of the microlysimeter. The holes thus created allow free drainage when rainfall is sufficient to induce it, but they prevent unsaturated flow either direction. The use of an automated TDR system allowed continuous, unattended measurement of both evaporative losses and rainfall additions in multiple lysimeters. The TDR data compared favorably with water content changes measured at discrete intervals by weighing, and also with independent measurements of daily evaporation losses' and rain-fall additions measured by the Bowen ratio method and a rain gauge. Short-term (e.g., hourly) rate estimation exhibited considerable scatter, but further improvement should be possible. The TDR-microlysimeters were replaced less frequently than typically the case with standard microlysimeters, without serious consequences despite numerous rains. The method should be useful in many studies where information on soil/atmosphere water exchange is required.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

A study was conducted to determine the predictive capability of SPOT (Systeme Probatoire d' Observation de la Terre) satellite data for detecting areas that are subjected to salinity encroachment and nutrient deficiencies. The SPOT observations were related to the chemical characteristics of soil samples collected from a corn field located south of Torrington, in east central Wyoming. On a false color infrared composite (FCC) image, the saline areas were bright white patches; healthy vegetation was bright red to magenta in color. A nearby reservoir had a dark blue tone except where light blue tones indicate silty and/or shallow water. From the visual interpretation of the FCC image, it was not possible to predict soil salinity quantitatively. Statistical analysis of SPOT digital counts by bands indicated that the near infrared band (X-3) was superior to the visible bands for salinity detection. Of the three SPOT bands (XS-1, XS-2, and XS-3), XS-3 was significantly correlated with saturated paste electrical conductivity (EC) and water soluble Na. Brightness index (BI), the summation of digital count of the three bands, was positively correlated with soil EC, and water soluble Na, Ca, Mg and was negatively correlated with Mn, while the normalized difference vegetation index (NDVI = (NIR-Red)/(NIR + Red)) and ratio index (RI = NIR/Red) were negatively correlated with EC and water soluble Na, Ca, and Mg. All the spectral bands were significantly and positively correlated among themselves and with BI. Analysis of variance indicated that the sampling points possessing high BI values had higher EC, water soluble Na, Ca, and Mg and lower levels of Mn and Zn. Samples with low NDVI and RI values had high EC, water soluble Na, Ca, and low Mn, indicating that high salinity and nutrient deficiency can be detected with reasonable accuracy with BI, NDVI, and RI. Among the spectral indices, BI proved to be the best indicator of salinity and nutrient deficiency.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Oxygen has long been recognized as a control on the acid-generating reactions which produce acid mine drainage, and great effort has been made to reduce oxygen recharge to mine spoil. To investigate O2availability and the role of thermal convection in O2transport in mine spoil, spoil temperature, O2, and CO2concentrations of spoil gas were measured monthly throughout the entire thickness of mine spoil for 1 year at an abandoned strip mine in Clearfield County, Pennsylvania. O2concentrations deep inside mine spoil were found to be 18% or higher, despite active pyrite oxidation. The O2inside this mine spoil had not been lowered to a level that limited the generation of acid mine drainage. Temperature surveys showed thermal anomalies, which appear to result from heat generated by pyrite oxidation reactions. The field data and a simple model show that thermally induced air convection can be a dominant process maintaining high O2concentration in deep mine spoil.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Repeated applications of copper (Cu)-containing fertilizers and fungicides have contributed to Cu accumulation in several soils used for Florida citrus production. New plantings on such soils may result in Cu toxicity to young citrus trees. The objective of this study was to examine the reactions of Cu and SO4when applied as CuSO4to three Spodosols (Immokalee fine sand, Oldsmar sand, and Myakka sand) and one Alfisol (Boca fine sand) sampled (0–15 cm) either from citrus groves or from uncultivated locations under natural vegetation. The soil pH varied from 6.6 to 7.2 and from 4.8 to 7.6 among the grove and uncultivated soils, respectively. Sorption of Cu by the Boca-grove and uncultivated soils was much greater (99% of applied Cu) than that by the other soils. The soil pH, organic carbon content, and cation exchange capacity (CEC) were greater in the Boca fine sands (in grove and uncultivated conditions) compared with those of the rest of the soils examined in this study. The concentrations of Cu in the equilibrium soil solutions were under-saturated with respect to CuCO3, Cu3(OH)2(CO3)2and Cu2(OH)2CO3solid phases in all of the grove and uncultivated soils evaluated in this study, but were supersaturated with respect to CuFe2O4solid phase in all three grove soils and in the Boca-uncultivated soil. The quantities of Cu sorbed by the soils were 2− to 3-fold greater than that of SO4sorbed. The sorption of SO4was greater in high pH soils than in the lower pH soils. There was no evidence of release of hydroxyl ions in response to retention of SO4by the soils employed in this study. Simultaneous sorption of SO4along with that of Cu, although at much reduced quantity, may indicate that every one of three Cu ions sorbed may be associated with SO4. An increase in sorption of Cu increased the concentrations of Fe, Mn, and Zn in equilibrium solution.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

In this study,15N-labeled rice straw and green manure were used to feed goats and pigs separately, and the feces and urine from the animals were applied to rice plants as base fertilizer. It was shown that a total of 33.4–33.7% of the nitrogen in the feed straw was recovered by the goats' body and rice grain, which was significantly higher than the N recovery (9.9–14.5%) by rice grain when rice straw was returned directly to the field as a base fertilizer. The total N recovery from green manure as feed by the pigs' body and grain totalled 30.2–30.8%, which was significantly higher than that of grain (22.7–26.7%) recovered from green manure when it was directly applied to the soil. It was concluded that rice straw and green manure are used most economically by feeding goats and pigs and returning the feces and urine to the field.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID