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We have examined the influence of the ionic composition of the soil solution on the movement of sulfate and calcium in 2 New Zealand soils with differing allophane content. For this study, we have carried out a series of miscible displacement experiments using repacked and intact soil columns, in which sulfate was applied in the presence of either calcium or potassium as the accompanying cation. Our results showed that sulfate leaching was significantly retarded in the soil with higher allophane content when applied with calcium. On the other hand, no effects were observed for all studied soils when potassium was used as the accompanying cation. In addition, in soils with high allophane content, calcium also had its retention increased when sulfate was present. The increase in sulfate adsorption was accompanied by corresponding increase in calcium adsorption. These findings evidenced the presence of co-adsorption, or ion-pair adsorption (IPA), in allophane-containing soils. The extent of this adsorption is dependent on the soil pH, the accompanying cation, and the allophane content of the soil. Additional keywords: breakthrough curve, calcium, ion-pair adsorption, miscible-displacement experiment, potassium., Introduction Sulfur deficiencies in soils have been increasingly reported in many countries worldwide including New Zealand (Nguyen and Goh 1994; Schnug et al. 1995; Scherer 2001; Edmeades et al. 2005), [...]

In Part I, we proposed a model for EDTA-facilitated transport of copper, and we tested the robustness of this scheme by reference to experiments with various initial and boundary conditions on a repacked soil. Now we examine a quite different repacked soil, and consider intact cores of the same soil as in Part I. Continuous leaching of an intact core of a contaminated orchard soil of volcanic origin under unsaturated conditions with excess 0.01 M EDTA (510 mm) reduced the copper concentration from 240 down to 80 µg/g in the top 25 mm. When a 480 µmol pulse of 0.001 M EDTA was applied to similar intact cores, 126 µmol of copper came out with immediate leaching, but only 18% of that was leached out from a core that was left for 1 month before leaching. The month's delay caused an increase in the amount of iron leached from 116 to 286 µmol. An experiment in which daily pulses CuEDT[A.sup.2-] were applied to a non-contaminated soil for 3 months indicated that these changes were due to CuEDT[A.sup.2-] slowly changing to Fe(III)EDT[A.sup.-], with the re-adsorption of the [Cu.sup.2+] released. Application of our model from Part I indicated that different rate constants are required for the chemical reactions for different soils, and also for the same volcanic soil when the pH is lower. But by changing just the dispersivity from 3 mm to 23 mm, the model with the same chemical parameters was able to simulate both the repacked columns and the intact core experiments using the volcanic soil. Additional keywords: iron, convection--dispersion equation, heavy metals, repacked soil, intact soil., Introduction In Thayalakumaran et al. (2003), which we will subsequently refer to as Part I, we described the transport, under near-saturated conditions, of copper in repacked columns of a contaminated [...]

The EDTA-enhanced remediation of copper contaminated sandy-loam soil of volcanic origin was investigated. The soil, from an orchard, was contaminated with about 250 mg/kg of copper due to the extensive use of copper sprays. Copper-contaminated soil was packed into 100-mm-long columns, and solutions of [Na.sub.2][H.sub.2]EDTA with Ca[Cl.sub.2], raised to a pH of 6.2, were applied at a flow rate of 24 mm/h. Application of an excess of 0.01 M EDTA leached about half the acid-extractable copper from the soil; most of it coming out in the first 3 liquid-filled pore volumes (PV). Also a 0.5 PV pulse of 0.001 M EDTA was applied to similar soil columns and then either leached immediately with 0.005 M Ca[Cl.sub.2], or left for periods of up to I month before leaching. With immediate leaching, 70% of the EDTA applied was complexed with copper in the leachate, but after a month's delay only 24% was complexed with copper in the leachate, the rest being complexed with iron. There was no evidence of EDTA retardation or adsorption. The experimental results were simulated using the convection--dispersion equation, incorporating a source/sink term. This described the competing time-dependent reactions of copper and iron with EDTA, and the reversion of CuEDT[A.sup.2-] to adsorbed [Cu.sup.2+] and Fe(III)EDT[A.sup.-] in solution. Reasonable simulations were achieved, mostly within errors of observation. Additional keywords: iron, convection-dispersion equation, heavy metals., Introduction There are a number of reasons why the interactions between copper and EDTA in soil are of interest. Copper compounds and EDTA are widely used and are persistent chemicals [...]

When time domain reflectometry (TDR) is used to measure soil water content ([theta]) and salinity, the probes can be installed either horizontally or vertically. We tested the common convention that [theta] values averaged from horizontal probes will be equal to a vertical direct measurement. In a laboratory experiment, a sandy loam soil, packed uniformly (0.02-m layers to 0.18-m depth) into a box, was gradually wetted to saturation by Ca[Cl.sub.2] solutions of 0, 0.5, 1.1, 2.1, 3.1, 4.5, 5.9, and 8.4 dS [m.sup.-1]. Horizontal and vertical TDR probes for measuring [theta] and electrical conductivity of the bulk soil ([[sigma].sub.a]) were installed during soil packing. A comparison between [theta] values averaged from three horizontal probes and from two vertical ones showed deviations of 0.02 (L [L.sup.-1]). A simple water redistribution model was used to attribute this deviation to the process of averaging the horizontal results. The best practical attainable reproducibility, under our experimental conditions, were close to the theoretical limit 0.005 (L [L.sup.-1]) but the average experimental reproducibility of [theta] was 0.01 to 0.02 (L [L.sup.-1]). Width of the soil layer affecting the moisture measurement was reconfirmed to be close to 30 mm. The resistors-in-series model was found to be a good approximation to describe the soil profile [[sigma].sub.a] from separately measured horizontal [[sigma].sub.a]. Final values of the electrical conductivity of the soil solution ([[sigma].sub.w]) after sufficient leaching were in good agreement with [[sigma].sub.w] values calculated by an empirical protocol that uses [[sigma].sub.a], [theta], and a soil texture property.

To measure soil water content (theta) by time domain reflectometry (TDR), one must know the relationship between the soil's dielectric constant (K(sub a)) and theta, Our objective was to determine K(sub a)-theta calibrations for sandy volcanic soils on the North Island of New Zealand. We collected samples from 24 horizons and 6 soil profiles. The soils were sandy loam and loamy sand textured, with bulk densities between 0.55 and 1.34 Mg m(super -3). Samples were air-dried and packed to their field bulk density in plastic boxes. Time domain reflectometry probes (100 mm long) were inserted, and TDR waveforms were recorded and analyzed. Water contents were increased in approximate steps of 0.05 m(super 3) m(super -3) volume to the liquid limit, which varied between 0.33 and 0.57 m(super 3) m(super -3). Samples were then submerged to obtain water contents as great as 0.646 m(super 3) m(super -3). Measured water contents were greater than those predicted by the Topp equation, but the differences varied according to soil texture. Pooled data from 17 of the samples provided a third-order polynomial calibration with an R(super 2) of 0.977 and root mean square error (RMSE) of 0.026 m(super 3) m(super -3). A third-order mixing model and a linear K(sub a)-theta expression gave greater mean errors. Although the calibration applied to a range of sandy volcanic soils, there were also two small groups of samples that showed distinct calibrations. These were coarser tephras, with at least 75% of particles >0.125 mm diam. By mass, and with bulk densities >l.0 Mg m(super -3).

A numerical model for axisymmetric flow of water and solute underneath a disk permeameter was developed based on experiments conducted in sand packed box containing a 1/4-sector disk permeameter. This is used to determine the chemical transport properties of soil by a removal and replacement of the disk. This activity created no disturbance in the model. The model, based on disk permeameter experiments, is also useful for parameter identification.

A technique for field measurement of the effectively mobile water fraction during near-saturated flow is presented. Soil is moistened with a disk permeameter until steady state is attained, then a tracer solution is applied. The ratio of the measured to applied concentration of tracer in the samples will yield the proportion of mobile water. Field testing of this method produced results similar to those obtained from long term studies at the same area.