Soil Science Society of America Journal

Veldkamp, Edzo; O'Brien, Joseph J.

doi: 10.2136/sssaj2000.6451549xpmid: N/A

Recently, a frequency domain reflectometry (FDR) was developed for measuring soil water content. It has a multivibrator that sends electromagnetic waves along its probes, and it measures the frequency of the reflected wave, which varies with water content. This FDR sensor has several advantages over time domain reflectometry (TDR); it is less expensive, has a lower power consumption, and continuous monitoring of soil moisture at several remote locations is easily automated using dataloggers. Our goal was to derive a calibration function for the FDR sensor with the following criteria: it should be applicable to soils with high clay and organic matter contents and with bulk densities between 0.7 and 1.1 g cm−3 We used undisturbed soil samples to account for the natural heterogeneity in soils. Our results show that the calibration functions derived from a three‐phase mixing model performed better than the manufacturer's empirically derived function for the soil volumetric content (θ) range of 0.45 to 0.70 m3 m−3 Separate values of the geometry parameter (α) and of the specific output period for soil matrix (Pers) were established both for the topsoil (0–0.5 m depth) and for the subsoil (>0.5 m depth). The manufacturer's calibration function underestimated the soil water content by up to 0.15 m3 m−3 The three‐phase mixing model uses a physical basis for the derivation of the calibration function in that the soil porosity is used for volumetric partitioning among soil components. This physical basis renders the calibration function widely adaptable.

Zhang, Z.Fred; Kachanoski, R.Gary; Parkin, Gary W.; Si, Bingcheng

doi: 10.2136/sssaj2000.6451554xpmid: N/A

In situ measurement of soil hydraulic properties remains a challenge. This study presents new analytical expressions for estimation of soil hydraulic properties below a surface line source by means of multi‐purpose time domain reflectometry (TDR) probes and existing quasi‐analytical, steady‐state solutions for infiltration from a surface line source. Inverse procedures are used to estimate the inverse macroscopic capillary length scale, α, and the hydraulic conductivity at saturation, Ks, from pressure head (ψ), water storage (W), and conservative ionic tracer travel time (T) measured via multi‐purpose TDR probes placed at several depths below a line source with constant flux of water. Soil water content at saturation, θs, can also be estimated if prior information is available. The parameter and spatial sensitivities of each solution were calculated by means of sensitivity coefficients. The uniqueness of possible combinations of measurements to estimate α, Ks, and θs was tested by means of two‐dimensional response surfaces. Significant correlation exists between Ks and θs, and thus it is not possible to estimate both Ks and θs by globally minimizing the objective function. Combination approaches with W (i.e., ψ and W, T and W, or ψ and W and T) give unique estimates of α and Ks if either θs is known or prior information on θs is available.

Zhang, Z.Fred; Kachanoski, R.Gary; Parkin, Gary W.; Si, Bingcheng

doi: 10.2136/sssaj2000.6451563xpmid: N/A

We designed and tested a field method to measure unsaturated soil hydraulic properties using multi‐purpose time domain reflectometry (TDR) probes below a surface line source with constant flux of water. The surface line source was produced with a moving irrigation system at a field site sheltered from precipitation. Two hundred multi‐purpose TDR probes were vertically installed in the soil beneath the line source to measure soil water pressure head (ψ), water storage (W), and tracer travel time (T). The soil hydraulic properties, the inverse macroscopic capillary length (α), hydraulic conductivity at saturation (Ks), and soil water content at saturation (θs) were estimated by inverse procedures with new analytical expressions. Five combinations of measurement sets, namely W‐only, ψ and W, ψ and T, and W and T, and ψ and W and T were used. Approximate confidence contours in the α–Ks plane were calculated to show the precision of the parameter estimates. For comparison, hydraulic properties were also measured by means of the Guelph Permeameter (GP) and the modified Guelph Pressure Infiltrometer (GPI) systems. Hydraulic parameters estimated from only W measurements were similar to those estimated from the combinations of W and ψ, or W and T, or W and ψ and T The estimated hydraulic parameters were similar to those obtained with three‐dimensional (3‐D) infiltration measurements by means of the GP and GPI systems.

Wilson, G.V.; Ely, D.M.; Hokett, S.L.; Gillespie, D.R.

doi: 10.2136/sssaj2000.6451570xpmid: N/A

Current recharge through the alluvial fans of the Nevada Test Site (NTS) is considered to be negligible, but the impact of more than 400 nuclear subsidence craters on recharge is uncertain. Many of the craters contain a playa region, but the impact of these playas has not been addressed. It was hypothesized that a crater playa would focus infiltration through the surrounding coarser‐grained material, thereby increasing recharge. Crater U5a was selected because it represented a worst case for runoff into craters. A borehole was instrumented for neutron logging beneath the playa center and immediately outside the crater. Physical and hydraulic properties were measured along a transect in the crater and outside the crater. Particle‐size analysis of the 14.6 m of sediment in the crater and morphological features of the crater suggest that a large ponding event of ≈63000 m3 had occurred since crater formation. Water flow simulations with HYDRUS‐2D, which were corroborated by the measured water contents, suggest that the wetting front advanced initially by as much as 30 m yr−1 with a recharge rate 32 yr after the event of 2.5 m yr−1 Simulations based on the measured properties of the sediments suggest that infiltration will occur preferentially around the playa perimeter. However, these sediments were shown to effectively restrict future recharge by storing water until removal by evapotranspiration (ET). This work demonstrated that subsidence craters may be self‐healing.

Hilhorst, M.A.; Dirksen, C.; Kampers, F.W.H.; Feddes, R.A.

doi: 10.2136/sssaj2000.6451581xpmid: N/A

A change in the relative proportions of the constituents of a porous material like soil will cause a change in its electrical permittivity. The measured permittivity reflects the impact of the permittivities of the individual material constituents. Numerous dielectric mixture equations are published, but none of these equations are generally applicable. A new theoretical mixture equation is derived, using the principle of superposition of electric (E) fields. This mixture equation relates the measured permittivity to a weighted sum of the permittivities of the individual material constituents and includes depolarization factors to account for electric field refractions at the interfaces of the constituents. The depolarization factors are related to physical properties of the material. Most other mixture equations contain one or more empirical factors. The concept of the depolarization factor is comparable with that of the “shape factor” of particles as described by other authors. A special case of the new mixture equation, for which the depolarization factors equals one (no depolarizations), appeared equal to a mixture equation for fluids derived from using thermodynamics. The new mixture equation is compared with other mixture equations. Comparison of the new mixture equation with measured data for glass beads and fine sand was promising. Concluding, depolarization factors in the new mixture equation relate the microstructural and compositional material properties to the measured bulk permittivity of a material. Although not shown, depolarization factors can be calculated from physical material properties.

Moldrup, P.; Olesen, T.; Gamst, J.; Schjønning, P.; Yamaguchi, T.; Rolston, D.E.

doi: 10.2136/sssaj2000.6451588xpmid: N/A

Investigations of gas transport and fate processes in packed soil systems require knowledge of the gas diffusion coefficient, DP, as a function of air‐filled porosity, ϵ. On the basis of the literature, data from six studies over the porosity range of 0.1 to nearly 1.0, it is reconfirmed that the Marshall (1959) model better predicts DP(ϵ) in completely dry, repacked porous media than do the Penman (1940) and Millington (1959) models. The smaller DP value in wet soil, as compared with dry soil at the same air‐filled porosity, is accounted for by introducing a water‐induced linear reduction (WLR) term, equal to the ratio of air‐filled porosity to total porosity, in the DP(ϵ) model. By adding the WLR term in each of the three DP(ϵ) models for dry porous media, the so‐called WLR(Marshall), WLR(Penman), and WLR(Millington) DP(ϵ) models for wet soil are developed. To test the three WLR models, DP was measured at different soil‐water contents in six differently textured (6–38% clay) repacked soils. The WLR (Marshall) model accurately and best described DP(ϵ) for all six soils and additional soils from the literature. All three WLR models performed better than previous DP(ϵ) models. This study implies that the smaller DP in a wet soil, which is due to water‐induced changes in air‐filled pore shape and pore connectivity, can be described by a simple, linear function of relative air‐filled porosity. The WLR(Marshall) model represents a conceptual and accurate model to predict DP(ϵ) in sieved, repacked soil.

Zhang, Renduo

doi: 10.2136/sssaj2000.6451595xpmid: N/A

The convection‐dispersion (CDE) equation and stochastic–convective models are the most commonly used process representations for predicting solute transport in the field. The convection–dispersion equation assumes that the solute is perfectly mixing in the lateral direction, whereas the stochastic–convective model assumes that the solute moves at different velocities in isolated stream tubes without lateral mixing. However, solute transport in heterogeneous porous media cannot always be conceptualized as being either a convective–dispersive or a stochastic–convective process. In this study, a generalized transfer function model (GTF) was proposed to describe various solute transport processes in heterogenous soils. The model is similar to the convective lognormal transfer function model, but two parameters, λμ and λσ, are introduced to characterize the depth‐dependency of the mean (μ) and standard deviation (σ) of the logarithm of travel time, respectively. The GTF can describe well the two extremes of solute dispersion, the convective–dispersive and stochastic–convective processes, and transport processes between the two extremes. In addition, the GTF can be used to characterize other solute transport processes in heterogeneous soils, such as those in which the mean of travel time increases with depth nonlinearly, and those in which the dispersivity is a scale‐dependent function of the travel distance with any power values.

Brouwer, Joost; Anderson, Heather

doi: 10.2136/sssaj2000.6451603xpmid: N/A

Water retention by coarse fragments in the soil is often ignored in agronomic and water balance studies. Following the calculation of inexplicably high water retention by the fine earth fraction, water contents at −20 and −1500 kPa and apparent bulk density were determined for remnant pisolithic ironstone gravel samples isolated from soils on the Dundas Tableland in southeast Australia. The volumetric water content of the ironstone gravel at −1500 kPa was found to vary between 0.12 and 0.24 m3 m−3, while at −20 kPa it varied between 0.16 and 0.36 m3 m−3 Available water holding capacity (AWHC) of the ironstone gravel varied between 0.03 and 0.15 m3 m−3 Both the AWHC and the water content at −1500 kPa of the ironstone gravel showed significant increases with depth. Magnetic ironstone gravel, found almost exclusively in the A and E horizons, was much denser than nonmagnetic ironstone gravel (average 3.38 vs. 2.64 Mg m−3), but had similar water retention characteristics. Ignoring the water retention characteristics of the ironstone gravel would have led to overestimation of the AWHC of the bulk soil by a factor 1.08 to 1.67 for various horizons. For the combined top 1.0 m of the soil, ignoring the water held by the ironstone gravel would have led to an estimated AWHC of 162 mm, while in fact it was only 129 mm. Water balance studies of soils with ironstone gravel clearly need to take into account the water holding characteristics of that gravel.

La Force, Matthew J.; Fendorf, Scott

doi: 10.2136/sssaj2000.6451608xpmid: N/A

Selective chemical extractions provide semiquantitative information on elemental partitioning within operationally defined soil fractions. In this study, the efficiency of common extraction steps was determined for a mining‐impacted soil by analyzing Fe transformations in the solid phase using x‐ray diffraction, scanning electron microscopy, and x‐ray absorption near edge structure (XANES) spectroscopy. Extractions involve the isolation of operationally defined double‐deionized water (soluble), magnesium chloride (exchangeable), sodium hypochlorite (organic matter), sodium acetate–acetic acid (carbonate), hydroxylamine‐hydrochloride–nitric acid (Mn‐oxides), ammonium oxalate in the dark (AOD) (noncrystalline material), hydroxylamine‐hydrochloride–acetic acid (Fe oxides), potassium perchlorate–hydrochloric–nitric acid (sulfidic), and hydrochloric–nitric–hydrofluoric acid (residual) fractions of the solid phase. Ferric Fe remained in the solid phase throughout the extraction sequence until its removal by hydrochloric–nitric–hydrofluoric acid (residual fraction). The hydroxylamine‐hydrochloride (1.0 M in 25% [v/v] HOAc) extraction may underestimate Fe associated with crystalline materials. Thus, selective sequential extractions need to be optimized for a given soil prior to implementation and should be used in conjunction with spectroscopic techniques, when possible, to fully ascertain elemental partitioning within the solid phase.

Jackson, Brian P.; Miller, W. P.

doi: 10.2136/sssaj2000.6451616xpmid: N/A

Phosphate and OH− are often used for the extraction of As and Se from soils, either as single extractants or as part of a sequential extraction scheme. However, the recovery of As and Se species and the integrity of the resulting solution speciation merit investigation. In this study the relative effectiveness of PO4 at 0.1 and 0.5 M and pH values of 3 and 6.7 and 0.1 M OH− to extract As(III), As(V), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), p‐arsanilic acid (p‐ASA), roxarsone (ROX), Se(IV) and Se(VI) sorbed to goethite and an amorphous Fe oxide were compared, and the speciation in the resulting extract was determined. The extent to which 0.1 M PO4 added to 0.25 M NH2OH·HCl or 0.175 M Na oxalate/0.1 M oxalic acid prevents readsorption of As(V) or Se(IV) to goethite during the dissolution of an amorphous Fe oxide was also assessed. Hydroxide was the most effective extractant for desorption of all species except As(III) from both oxide surfaces. Arsenite was extracted most efficiently by 0.5 M PO4 at low pH; however, amorphous Fe oxide exhibited a strong affinity for As(III) with a maximum of 18% of As(III) extracted by 0.5 M PO4 at pH 2.8. Partial oxidation of As(III) to As(V) occurred in all extractions where an Fe oxide solid phase was present, but only in the hydroxide extract in the absence of a Fe solid phase. Addition of 0.1 M PO4 to extractants used for the dissolution of the amorphous Fe oxide prevented the readsorption of As(V) and Se(IV) to goethite.