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1.
Diffusion coefficients (D) of CO2 at 0–10 cm layers in undisturbed and tilled soil conditions were estimated using the Penman (Penman HL. 1940. Gas and vapor movement in soil, 1. The diffusion of vapours through porous solids. J Agric Sci. 30:437–463), Millington–Quirk (Millington RJ, Quirk JP. 1960. Transport in porous media. In: Van Baren FA, editor. Transactions of the 7th International Congress of Soil Science. Vol. 1. Amsterdam: Elsevier. p. 97–106), Ridgwell et al. (Ridgwell AJ, Marshall SJ, Gregson K. 1999. Consumption of atmospheric methane by soils: A process-based model. Global Biogeochem Cy. 13:59–70), Troeh et al. (Troeh FR, Jabro JD, Kirkham D. 1982. Gaseous diffusion equations for porous materials. Geoderma. 27:239–258) and Moldrup et al. (Moldrup P, Kruse CW, Rolston DE, Yamaguchi T. 1996. Modeling diffusion and reaction in soils: III. Predicting gas diffusivity from the Campbell soil–water retention model. Soil Sci. 161:366–375) models. Soil bulk density and volumetric soil water content (θv) at 0–10 cm were measured on 14 April, 2 June and 12 July 2005 at 0–10 cm depth in no-till (NT) and conventional till (CT) malt barley and undisturbed soil grass–alfalfa (UGA) systems. Air-filled porosity (ε) was calculated from total soil porosity and θv measurements. Both soil air porosity and estimated CO2 diffusivity at the 0–10 cm depth were significantly affected by tillage. Results of CO2 diffusion coefficients in the soil followed trends similar to those for soil ε data. The CT tended to have significantly greater estimated soil CO2 diffusion coefficients than the NT and UGA treatments. The relationship between D/D 0, and air-filled porosity was well described by a power (R 2 = 0.985) function. The model is useful for predicting CO2 gas-diffusion coefficients in undisturbed and tilled soils at various ranges of ε where actual gas D measurements are time-consuming, costly and infeasible.  相似文献   

2.
In column studies in the laboratory the apparent diffusion coefficients(D)of chloride and TOH for undisturbed and unsaturated soils were determined as a function of the pore water velocity. Parallel the effective diffusion coefficients (D+) of Cl? were measured as a function of water content. It was found that the values of D varied significantly between soil layers and soil types. This findings also held for the low flow velocities studied (0.3 and 1.0 cm/d). The impedance factors to calculate the effective diffusion coefficients (D+) from D values in the bulk water, decreased with decreasing bulk densities. In model calculations it was demonstrated that in spite of the differences of D values measured, for many purposes in the field good estimates of the solute distribution in the soils can be obtained by using simple relations between D and vo.  相似文献   

3.
Zhang  Jing  Wang  Ruimin  Cao  Xiaoyan  Li  Ying  Lan  Yeqing 《Water, air, and soil pollution》2014,225(7):1-10

Reversed-flow gas chromatography, a version of inverse gas chromatography, was applied for the study of the transfer of sulfur dioxide from pure and artificial sea water to the atmospheric environment. Using suitable mathematical equation rate coefficients, the transfer of SO2 from water to air (k c) as well as diffusion coefficients of SO2 vapors into nitrogen (D g) was determined. The rate coefficients increase with increasing temperature and decrease with increasing pH of the liquid phase. Moreover, they depend on the water environment, being bigger for the transfer of SO2 from pure water than those from artificial sea water. Finally, the values of D g and k c are compared with those given in the literature or calculated theoretically.

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4.
The permeable reactive barrier (PRB) filled with natural zeolite plays the role of a reactive treatment zone for remediation of contaminated groundwater. Based on column lab experiments, the volume of remediated solution, the distribution (Kd) and retardation (Rd) coefficients were evaluated, confirming successful removal and retention of zinc from contaminated groundwater. The effect of hydrodynamic dispersion on zinc capturing by zeolite in PRB was evaluated by the hydrodynamic dispersion coefficient (DL) and retarded hydrodynamic dispersion coefficient (DLR) using the Brigham method. For different assumed distances of the barrier, the simulation of one-dimensional zinc concentration profile from the point source through the barrier has been modeled by a simple analytical pulse model. The results show that the flow rate has the most significant effect on the concentration profile, peaks, and broadening of curves. The residence contact time (τ) corresponding to higher Kd and Rd as well as lower DL and DLR values outcomes the optimal range of 6.2–9.4 min. This interval corresponds to the experimental performance at the bed length of 8 and 12 cm and flow rate in the range of 6.38–9.57 PV/h. The calculated minimum thickness and longevity confirm the successful application of zeolite as a material in PRB for remediation of zinc contaminated groundwater.  相似文献   

5.
Phosphate diffusion coefficients (De) were determined by the quantity of P that diffused from a soil block with P addition into a soil block without P addition. To compare the results with theoretical concepts and to quantify the influencing factors, De was also calculated using the equation of Nye (1968). This equation takes into account the P diffusion coefficient in water, D1, the volumetric water content, θ, the impedance factor, f, and the buffer power, b, of the soil. The results show that De strongly depends on volumetric water content whereas the effect of bulk density on De values was relatively small. If the weighted average buffer power was used, calculated De values were in good agreement with measured values at higher soil moisture contents. At θ < 0.22 g cm?3 the measured values for De were smaller than the calculated. This effect is attributed to incomplete contact between the two soil blocks. The only small influence of bulk density on De is caused by the fact that bulk density affects both θ and b in a way which compensates each other.  相似文献   

6.
This study was to determine if diffusion of soil ammonium may explain why many sandy soils have greater nitrogen (N)–supplying capacity to rice than clay soils. A laboratory procedure using transient-state methods measured the linear movement of soil ammonium (NH4) in tubes packed with five field soils under aerobic conditions. Ammonium diffusion was measured by sectioning tubes after 48 h of equilibration and then measuring NH4 by steam distillation. Effective diffusion coefficients, De, and NH4 diffusion distance, d, per day ranged from De = 4.6 × 10?5 cm2 d?1 and 1.5 cm d?1 for Katy sandy loam to De = 2.9 × 10?7 cm2 d?1 and 0.11 cm d?1 for League clay. Ammonium diffusion distance d was strongly related to soil clay content and hence was predicted by d = Y × {[100/(% clay)] ? 1}, where Y is set to 0.1. Predicted d and measured d were highly related (R2 = 0.99).  相似文献   

7.
Investigation of the soil gas regime in a tillage experiment: 2. Apparent diffusion coefficients as a measure of soil structure In a soil tillage experiment with nursery stock on three different soils the apparent diffusion coefficients for CO2 were measured using soil cores of different depths at different times. Not-tilled, herbicide treated plots were compared with rototilled plots. The relationship between the relative apparent diffusion coefficient and the air content may be described by an exponential regression function Ds/Da = 0.0085 · e6.8EL, if all measurements are taken into the calculation. By dividing into different soil textures different regressions are obtained for the three studied soils: a sand, a silt and a clay loam. In many cases it is possible to show by the changing Ds(EL) regression changes of the soil structure with depth or as a result of tillage. In all these cases the soil of the not tilled plots turns out to be better structurized than that of the tilled ones, demonstrated by higher Ds-values at equal EL. The interpretation of the differences is being tried with the aid of soil pore tortuousity and continuity. Finally the measured Ds(EL) relationships are applied to characterize the soil gas regime for two seasons, using CO2 concentration profiles of the soil air on a day in summer and fall, respectively. It is shown, that CO2 production reaches farther down in summer than in fall.  相似文献   

8.
Gas diffusion through soil crumbs: the effects of compaction and wetting   总被引：3，自引：0，他引：3  
Samples of 1–2 mm crumbs from a clay loam under permanent pasture were equilibrated at -5 kPa water potential then compacted to varying degrees. Gas diffusion coefficients D, (hydrogen through air), were measured immediately on compaction, again after re-equilibration at -5 kPa, then at other water contents between saturation and dryness. The relationship between diffusion coefficient and air content, was, as elsewhere, in two parts (dD/d small for drainage of pores within crumbs; large for pores between crumbs), but the transition from one part to the other occurred at smaller air contents with increased compaction. The air content at which D approached zero as the samples wetted was greatest in the loosest soil. Compaction from a bulk density of 0.86–1.29 g cm?3 decreased the relative diffusion coefficient, D/D0 (D0 is the diffusion coefficient without impedance), from 0.35 to 0.22 (by 38%) at complete dryness, but from 0.19 to 0.035 (by 82%) in the soil initially at -5 kPa. On re-wetting and re-equilibrating at ?5 kPa, D/D0 decreased further to 0.008 (total 97%) because of extra water held in the now smaller pores of the compacted soil. No single relationship between D/D0 and fitted the results for even this one soil.  相似文献   

9.
Agrichemicals usually contaminate groundwater via preferential flow, therefore determination of the preferential flow characteristics of soil is needed. One model that predicts solute transport due to preferential flow is the mobile–immobile (MIM) solute-transport model, which partitions total water content (θ; m3 m?3) into mobile (θm) and immobile fractions (θim). In undisturbed soils, a method is proposed for determining the MIM model parameters, i.e. immobile water fraction (θim), mass transfer coefficient (α) and hydrodynamic dispersion coefficient (D h). Breakthrough curves were obtained for five different soil textures in three replicates, by miscible displacement of Cl? in undisturbed soil columns. Cl? breakthrough curves were evaluated in terms of the MIM model. Analysis suggests that the values of D h and α increased with lighter soil textures and θim increased with heavier soil textures. The values of θim ranged from 5.31 to 14.28% in different soil textures. Furthermore, values of θim were found to be related to soil clay content. Values of α ranged from 0.0257 to 0.32 h?1 and values of D h ranged from 0.36 to 11.2 cm2 h?1 in different soil textures. A significant linear correlation was obtained between α, θim, D h and soil saturated hydraulic conductivity (K s) and pore water velocity (v). A multivariate pedotransfer function was developed to estimate α, θim and D h based on the geometric mean (d g) and the standard deviation (σg) of the diameter of soil particles and soil organic matter content. The pedotransfer functions for D h, θim and α were validated by independent data sets from other investigators.  相似文献   

10.
The self diffusion coefficients of zinc (DaZn), determined by half cell technique, were found to be greatly influenced by variations in soil characteristics such as volumetric moisture content, bulk density, temperature, carrier zinc concentration and soil pH. The DaZn values showed exponential decrease with increase in soil pH but with increasing volumetric moisture content, DaZn values showed logarithmic increase. The highest DaZn value was obtained at a compaction of 1.50 g/cm3. Increase in temperature from 5°C to 30°C showed 4 to 20 times increase in DaZn values. An attempt was made to study the relationship between soil characteristics and DaZn values for 87 illitic soils differing in physical and chemical properties. The simple and multiple correlation coefficients did not account for more than 21 per cent of variations indicating that DaZn values cannot be predicted accurately from soil properties.  相似文献   

11.
Solute (NaCl) miscible displacement experiments are performed on long disturbed soil columns to determine the hydrodynamic longitudinal dispersion coefficient and correlate it with the variability of the local permeability. The solute concentration, averaged over several cross-sections along the soil column, is monitored by measuring the electrical resistance between rod electrodes. The measured solute concentration breakthrough curves are fitted simultaneously with the one-region and two-region analytical models of the 1-D advection–dispersion equation to estimate the longitudinal dispersion coefficient, D L, as a function of Peclet number, Pe, for common groundwater flow velocities (2?<?Pe?<?50). Macroscopic simulations of miscible displacement in 2-D porous media described by a periodic permeability field with low, moderate and high variability are employed to evaluate the predictability of the one-region and two-region models, and the sensitivity of the dispersion coefficients and flow velocities estimated from soil column displacement tests to the variance of local permeability. When the variability of the local permeability becomes high, the one-region model fails, while the two-region model is capable of reproducing satisfactorily the breakthrough curves, and providing reliable values of dispersion coefficients. The two mean pore velocities estimated by the two-region model represent, on average, a fast and a slow mean velocity of the dispersion front, whereas their difference is a measure of the transient evolution of the width of the equi-concentration dispersion front.  相似文献   

12.
If an exchangeable ion in soil diffuses along a liquid and solid pathway, its diffusion coefficient may be expressed as where D, v, f, C are diffusion coefficient, volume fraction, impedance factor, and concentration terms and the suffixes l,S refer to liquid and solid. The self-diffusion coefficient of the ion is then where D′, D′t, and D′s, are self-diffusion coefficients. D and D′ will vary with concentration. In diffusion out of the soil to a zero sink, the appropriate average diffusion coefficient is, approximately, the self-diffusion coefficient in the undisturbed soil. Diffusion of one ion species is influenced by other ions diffusing in the system through the diffusion potential set up. When ions are diffusing to plant roots, the diffusion potential is likely to be small. A more likely, though more complicated, expression for D than the first equation above is derived by assuming the ion to follow solid and liquid pathways in series as well as in parallel.  相似文献   

13.
This study determines the seasonal variability of metal partition coefficients [aluminium (Al), iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn)] and analyses the importance of suspended sediments (SS), dissolved organic carbon (DOC) concentrations, pH, and discharge (Q) on the seasonal variability of metal partition coefficients (KDs) in the headwaters of the Mero River catchment, which drains an agroforestry area in northwestern Spain. Metal partition coefficients were used as an approach to relate dissolved and particulate fractions. Water samples were collected over 3 years (2005–2008) at the catchment outlet. The mean metal dissolved concentrations were: Fe (43.5 μg L?1) > Al (23.3 μg L?1) > Zn (1.8 μg L?1) > Mn (1.2 μg L?1) > Cu (0.3 μg L?1). Partition coefficients followed the order Mn > Al > Fe > Zn > Cu, and their values exhibited low variability. Al, Cu, and Zn partition coefficients presented the greatest values in summer, except during 2007–2008, when the greatest KDs value was observed in autumn, whereas the KDs of Fe showed the greatest values in winter. The KD of Mn has no seasonality. For Al, Cu, and Zn, the seasonal SS concentrations were closely related to Kd. For Fe, Kd was more closely related to DOC concentration than to SS concentration.  相似文献   

14.
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15.
Evidence of anisotropy is reported for advective air and water permeabilities in soils. Thus, anisotropy is likely to exist also for diffusive gas fluxes. Information about direction‐dependent soil gas diffusivity is scarce and most modeling approaches assume isotropy. At hundreds of closely lying positions in a compacted and adjacent undisturbed forest soil, gas diffusivity (Ds/D0) was measured either in vertical or horizontal direction. The volume‐independent diffusion efficiency (i.e., diffusivity divided by air‐filled porosity) was fitted by a generalized additive model (GAM). Significant regressors were air‐filled porosity (?), soil depth, and the discrete diffusion direction. The model yields in all cases higher vertical diffusion efficiencies. The compaction factor did not yield a significant regressor of its own, i.e., the reduction of diffusivity in the compacted soil was the same as in low‐porosity samples of the undisturbed profile. To elucidate the role of sharing vertically and horizontally orientated pore space and a potential competition between diffusivity in different spatial directions, simple geometric models consisting of 3‐dimensionally crossed pores have been parameterized. These models provided a good explanation of the typical nonlinear Ds/D0(?) relationship. By simple one‐parameter correction (linear or power function), this mechanistic model could be fitted to the data. The one‐parameter correction of the geometric model could be a straightforward approach to consider direction dependence of measured diffusivities. However, by applying this approach to the observations the anisotropy effect was not clearly evident, which could be attributed to a changing Ds/D0(?) relationship with depth. As a reason for the preference of the vertical gas diffusion the dominance of vertical stresses and the activity of anecic earthworms are discussed. Direction dependency of gas diffusivity seems to be a basic feature of natural pore systems and has to be considered for modeling gas fluxes in soils. Generally, a preferential vertical diffusion direction reduces horizontal balancing and increases the heterogeneity of gas concentrations in the soil air.  相似文献   

16.
Abstract

We previously analyzed the effect of nitrate dispersion on the apparent nitrogen isotope fractionation factor associated with denitrification in soil (Kawanishi et al. 1993), and found that the dispersion effect was significant when the water flow was slow. In the previous report, we assumed that the dispersion coefficients of 14NO3 ? and 15NO3 ? were similar. However, when the water flow is slow, molecular diffusion will dominate mechanical dispersion and the above assumption may not be valid.  相似文献   

17.
The composition of the soil atmosphere is an indicator of biological processes, and soil CO2 gradients have been used to estimate CO2 efflux from the surface. Soil atmosphere samplers, constructed with gas-permeable materials, have been used to quantify soil CO2 concentrations. The type of material used can influence the perceived real-time concentrations of CO2 in the soil. Previous works have not directly compared different types of materials under the same conditions. The objective of this study was to determine the diffusion coefficient (D) and time of 95% equilibrium (teq) of CO2 through several materials, and to evaluate the effect of long-term soil burial (183 days) on diffusion characteristics. Materials tested included silicone, expanded Teflon (ePTFE), and ultra high molecular weight polyethylene (PE) tubing. The D of each material was determined using a closed-loop system consisting of a CO2-enriched (7800 ppm) chamber, a CO2 analyzer and an inner tube (experimental tubing) placed inside the chamber. Air was re-circulated through the inner tube, and as CO2 diffused from the chamber into the tubing, the analyzer recorded the increase in concentration. The silicone tubes had values of D ranging from 8.64 to 5.80×10−6 cm2 s−1 with corresponding teq between 3.9 and 9.7 h. Diffusion coefficients of the ePTFE (1.25×10−4 cm2 s−1) and PE (7.70×10−4 cm2 s−1) materials were 2 orders of magnitude greater, with teq<6 min. Exposure to the soil environment for 183 days did not visibly deteriorate the materials or significantly affect the D or teq values. Use of the ePTFE or PE materials, over the silicone materials, may allow for better characterization of dynamic CO2 concentrations in the soil based on the greater D and lesser teq values of these materials.  相似文献   

18.
The additional nutrient absorbed by a surface releasing a solubilizing agent is predicted. The ‘solubilizing effect’ is defined. The nutrient solubilized may diffuse away from the surface as well as towards it. If the change in solution concentration of nutrient and solubilizing agent at the surface are abrupt, the fraction of nutrient solubilized that is absorbed at a planar surface equals 1/(1 +√DY/DX), where DY, DX are the diffusion coefficients of the solubilizing agent and the nutrient. At a spherical surface this fraction has to be multiplied by 1/(1 +√πDY/t2a), and at a cylindrical surface, appropriate for a root, by approximately 1/(1 +√πDY/t4a), where a is the radius.  相似文献   

19.
 以管道理论为基础,运用主轴切割的方法,论述不同年龄小钻杨树体的结构特点。结果表明:树干生物量增加与高度的降低呈线性关系;同一层次内的叶量和枝量间存在显著的线性关系,其关系的确定性不受分层方式的影响;树冠不同层次内的累积叶量与支持的枝干总质量存在显著的线性关系;不同层次的总累积生物量与各层内的树干生物量密度之间的线性关系显著,对不同层次累积树干生物量与各层树干生物量密度进行指数回归,系数介于1.1～1.3,指数介于1～2,表明二者的关系是处于线性和指数关系之间的一种形式;在胸高以上,树干边材断面积与侧枝基部断面积总和呈线性关系,胸高之下线性关系不存在。建立不同年龄小钻杨边材与所支持叶量的树体管道模型,同时测得胸径（D1.3）与心材直径（Dx）间的关系为Dx=0.522 exp（0.169 8D1.3）,r=0.984 8。  相似文献   

20.
Reliable transport parameters of agrochemicals and soluble pollutants are crucial for modeling and management of soil and groundwater quality. This study investigated impacts of municipal wastewater on the transport parameters of five heavy metal/metalloid compounds (NaAsO2, Cd(NO3)2, Pb(NO3)2, Ni(NO3)2 & ZnCl2), two pesticides (cartap & carbendazim) and an inert salt (CaCl2) in four agricultural soils of Bangladesh. Solute-breakthrough concentrations were measured in repacked soil columns with time-domain reflectometry (TDR) both before and after wastewater treatment. Transport velocity (V), dispersion coefficient (D), dispersivity (λ) and retardation factor (R) of the solutes, and pertinent soil properties were determined. Wastewater reduced bulk density (γ) of the soils (from 1.32–1.37 g/cm3 to 1.26–1.35 g/cm3) by increasing organic carbon (OC) (from 0.37%–0.84% to 0.40–0.93%), increased pore-size distribution index (n) (by 0.02 unit) and reduced soil pH (from 6.32–7.45 to 5.92–6.46). D and λ decreased while V and R increased after wastewater treatment; D decreased and R increased linearly with decreasing bulk density. The correlations of V, D, and R with n improved significantly (p < 0.05) after wastewater treatment. The correlation between λ and OC improved markedly for Ca, Pb, Ni, and cartap. The observed indicative results have practical implications in developing pedo-transfer functions for solute-transport parameters using basic soil properties, which are subject to progressive modification due to agrochemicals application and wastewater irrigation.  相似文献   

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