Soil-structure Effects Research Articles

Effects of past copper contamination and soil structure on copper leaching from soil.

Copper contamination affects biological, chemical, and physical soil properties and associated ecological functions. Changes in soil pore organization as a result of Cu contamination can dramatically affect flow and contaminant transport in polluted soils. This study assessed the influence of soil structure on the movement of water and Cu in a long-term polluted soil. Undisturbed soil cores collected along a Cu gradient (from about 20 to about 3800 mg Cu kg soil) were scanned using X-ray computed tomography (CT). Leaching experiments were performed to analyze tracer transport, colloid leaching, and dissolved organic carbon (DOC) and Cu losses. The 5% arrival time () and apparent dispersivity (λ) for tracer breakthrough were calculated by fitting the experimental data to a nonparametric, double-lognormal probability density function. Soil bulk density, which did not follow the Cu gradient, was the main driver of preferential flow, while macroporosity determined by X-ray CT (for pores >180 μm) proved the best predictor of solute transport. Higher preferential flow due to the presence of well-aligned pores and small cracks controlled water movement in compacted soil. Transport of Cu was rapid during the first flush (≈1 pore volume) in association with the movement of colloid particles, followed by slower transport in association with the movement of DOC in the soil solution. The relative amount of Cu released was strongly correlated with macroporosity as determined by X-ray CT, indicating the promising potential of this visualization technique for predicting contaminant transport through soil.

Shrinkage Properties of Repacked Soil at Different States of Uniaxial Compression

Compaction can severely impair soil structure, which may in turn affect shrinkage properties. The few studies that have so far investigated compaction effects on soil shrinkage behavior have produced seemingly inconsistent results, reporting an increase, decrease, or no change in shrinkage. The objective of this study was therefore to examine the changes in soil shrinkage under controlled laboratory conditions across a wide range of compression. We analyzed shrinkage curves of repacked aggregated soil recorded at increasing levels of uniaxial compression after conditioning to different initial water potentials. With increasing compression, the shrinkage curves changed from a sigmoidal shape typical for structured soil to the simple shape typical for unstructured soil such as a clay paste, indicating the loss of structural pores. The slopes of the shrinkage curves were less steep in the structural and basic shrinkage domains at moderate compression than in the uncompressed initial state, indicating a strengthening of interaggregate bonds and thus an increase in hydrostructural stability. The highly compressed soil, in contrast, showed distinctly steeper slopes, indicating decreased hydrostructural stability due to the breaking of interaggregate bonds. The compression effects on the shrinkage curves became more pronounced with increasing initial water potential during compression. This strong dependence on the degree of compression and initial water potential may be one reason for the variation of compaction effects on soil shrinkage properties reported in the literature. The results show that soil shrinkage is well suited for the assessment of soil structural stability and compaction effects on structured soil.

Research on Soil-Underground Structure Interaction of Topography Variation in Local Site

Base on the studies of soil-underground structure interaction of topography variation, large shaking table model tests are conducted, and test results are analyzed contrastively. By considering the influences of earthquake type, vibration amplitude ,unidirectional or bidirectional inputs etc., natural dynamic characteristics, acceleration response, spectrum characteristics and vibration amplification effect of site soil and underground structure are studied, and soil-underground structure interaction is explored, according to methods of time and frequency domain combined, and comparison of tests no topography variation. Significant conclusions have been got, which can be served as a certain basis and reference for underground structure seismic design with local site of topography variation.

Effects of different soil structures on the decomposition of native and added organic carbon

Soil microbial communities live in the soil pore network and therefore the access they have to organic substrate, oxygen and water depends on how this network is structured. In this experimental study, the relationship between soil structure and soil organic matter dynamics was investigated by measuring the kinetics of organic carbon decomposition in samples that varied in the physical structure of the soil pore network. Soil cores with different structures (undisturbed, sieved and disaggregated by agitation in water), but with the same bulk density, were incubated and the mineralisation of native and added (13C-labelled substrates, fructose and vanillin) organic carbon was measured for a period of 127 days. The incubation was commenced after the immediate effects of the perturbations caused by the soil structural treatments had dissipated. At the end of the incubation, the microbial biomass and microbial community structure were determined. It was found that the respiration kinetics were not related to soil structural parameters, which were determined using X-ray micro-CT. The structure treatments had no significant effect on the mineralisation of soil or added organic carbon, despite the significantly different physical environments. However, the microbial community structure of the undisturbed and dispersed samples were significantly different. These results indicate that neither decomposer access to organic substrate nor the environmental conditions in which decomposition occurred were impaired by the soil structure treatments and suggest that the controls on carbon dynamics may take place at scales below those that were manipulated in this experiment.

Long-term Tillage and Nitrogen Fertilization Effects on Soil Properties and Crop Yields

Long-term Tillage and Nitrogen Fertilization Effects on Soil Properties and Crop Yields

Effects of land use, slope gradient, and soil and water conservation structures on runoff and soil loss in semi-arid Northern Ethiopia

Land degradation and recurrent drought are the major threats to rain-fed agriculture in the semi-arid Ethiopian highlands. Water harvesting has become a priority in the Tigray region since 1990. However, the success of water harvesting in reservoirs is limited due to reduced inflow. The aim of this study was to investigate the effects of typical land-use types, slope gradients, and different soil and water conservation (SWC) structures on runoff and soil loss at the runoff-plot scale. Six runoff measuring sites, corresponding to three slope gradients, were established for cropland (cultivated land for annual crop production) and rangeland (heavily grazed land on hillslopes with high rock-fragment cover) at Mayleba catchment in Tigray, Ethiopia. SWC structures tested were stone bunds, trenches, and stone bunds with trenches, in addition to control plots. In total, 21 large runoff plots (with lengths of 60 to 100 m) were monitored daily for runoff production and soil loss during the main rainy season (July–September) in 2010. The results show that the seasonal runoff coefficient (RCs) representing the fraction of rainfall measured as runoff was much higher for rangeland (0.38 < RCs < 0.50) compared to that for cropland (0.11 < RCS < 0.15). Seasonal soil loss (SLs) values were five to six times larger on rangeland (28.6 < SLs < 50.0 ton ha−1) compared to that for cropland (4.6 < SLs < 11.4 ton ha−1). Stone bunds with trenches were the most effective SWC structures in reducing runoff and soil loss. With the same SWC structures installed, RCs and SLs for both rangeland and cropland tend to decrease with increasing slope gradient mainly due to a corresponding increase in rock-fragment cover. The effects of SWC structures on runoff production and soil loss are considerable; hence, it is crucial to consider these effects for optimal design of water-harvesting schemes such as micro-dams that collect and store surface runoff for irrigation development in the Ethiopian highlands.

Intrinsic Compression Behavior of Remolded and Reconstituted Clays-Reappraisal

Evaluating the impacts of soil structure on mechanical behavior for natural sedimentary clays is an important issue in geotechnical engineering. Burland introduced void index for normalizing the compression curves of various remolded and reconstituted clays to obtain the intrinsic compression line, which provides a reference framework to assess the in-situ compression behavior. However, it does not quantitatively account for the effects of initial water content on compressive behavior of remolded and reconstituted clays and the initial water contents of clays are not always limited to 1.0 - 1.5 times the liquid limits defined by Burland. A modification based on collected tests data was presented on the expressions of and defined by Burland. Extensive oedometer test data were also collected on various remolded and reconstituted soils with distinct liquid limits and initial water contents to verify the validity of modified expressions. A normalized compression line deduced by intrinsic compression line is proposed in the e-log p plot, which can be used to evaluate the effects of soil structure quantitatively on the intact compressive behavior for natural sedimentary clays.

Virtual Soils: Assessment of the Effects of Soil Structure on the Hydraulic Behavior of Cultivated Soils

The hydraulic behavior of soil is determined by the spatial heterogeneity of its hydraulic properties. The interplay among parent material, pedogenesis, and tillage leads to characteristic structures in cultivated soils. Tillage‐induced features like a loosely aggregated seed bed, a compacted plow pan, and soil compaction beneath tractor ruts overlay natural features such as facies and horizons. Assessing the impact of such structural components on vadose zone hydrology requires an observation scale of several meters and a resolution in the range of centimeters, which is not feasible with experimental setups. An alternative solution is the generation of synthetic but realistic structures and their hydraulic properties as a basis for modeling the hydraulic behavior in response to different boundary conditions. With such “virtual soils” at hand, comparative studies are possible that help explore the relation between soil architecture and soil function. We developed a structure generator that provides great flexibility in the design of virtual soils with nested heterogeneity. Virtual soils with increasing complexity were generated to explore scenarios of precipitation and evaporation for a period of several months. The simulations demonstrated that the structure and the hydraulic properties close to the soil surface originating from tillage clearly govern atmospheric boundary fluxes, while the impact of heterogeneity on groundwater recharge is more complex due to threshold effects, hydraulic nonequilibrium, and the interaction with atmospheric forcing. A comparison with one‐dimensional, effective representations of these virtual soils demonstrated that upscaling of soil water dynamics becomes inaccurate when lateral fluxes become relevant at the scale of observation.

Performance of a reinforced embankment on a sensitive Champlain clay deposit

An existing elastoviscoplastic constitutive model is modified using concepts of the state-dependent fluidity parameters and the damage law, to incorporate the effect of soil structure and its destructuration. The model is employed to simulate the performance of a well-documented case study of the reinforced test embankment constructed over a sensitive Champlain clay deposit at Saint Alban, Quebec. The finite element calculations, using both the original (nonstructured) and modified (structured) elastoviscoplastic soil models, are compared with the observed field data from a test embankment brought to failure. The results from the structured elastoviscoplastic soil model show better agreements with the field data when compared with those analyzed using the nonstructured elastoviscoplastic soil model. The modified model captures many features of the reinforced embankment behaviour, such as vertical settlement, excess pore-water pressure responses, and reinforcement force. However, they also reveal the fact that another important characteristic of the natural clay deposit — anisotropy — needs to be considered to provide an adequate prediction of horizontal deformations. The role of geosynthetic reinforcement and its viscosity on short-term responses of the reinforced embankment examined in this study is also discussed

Investigation of Interactive Effects on Water Flow and Solute Transport in Sandy Loam Soil Using Time Domain Reflectometry

Surface-applied chemicals move through the unsaturated zone with complex flow and transport processes due to soil heterogeneity and reach the saturated zone, resulting in groundwater contamination. Such complex processes need to be studied by advanced measurement and modeling techniques to protect soil and water resources from contamination. In this study, the interactive effects of factors like soil structure, initial soil water content (SWC), and application rate on preferential flow and transport were studied in a sandy loam field soil using measurement (by time domain reflectometry (TDR)) and modeling (by MACRO and VS2DTI) techniques. In addition, statistical analyses were performed to compare the means of the measured and modeled SWC and EC, and solute transport parameters (pore water velocity and dispersion coefficient) in 12 treatments. Research results showed that even though the effects of soil structural conditions on water and solute transport were not so clear, the applied solution moved lower depths in the profiles of wet versus dry initial SWC and high application rate versus low application rates. The effects of soil structure and initial SWC on water and solute movement could be differentiated under the interactive conditions, but the effects of the application rates were difficult to differentiate under different soil structural and initial SWC conditions. Modeling results showed that MACRO had somewhat better performance than VS2DTI in the estimation of SWC and EC with space and time, but overall both models had relatively low performances. The means of SWC, EC, and solute transport parameters of the 12 treatments were divided into some groups based on the statistical analyses, indicating different flow and transport characteristics or a certain degree nonuniform or preferential flow and transport in the soil. Conducting field experiments with more interactive factors and applying the models with different approaches may allow better understanding of flow and transport processes in addition to the simulations of them in the unsaturated zone.

Assessment of soil structure in the transition layer between topsoil and subsoil using the profil cultural method

In France, agronomists have studied the effects of cropping systems on soil structure using a field method based on a morphological description of soil structure. In this method, called “profil cultural” or soil profile in English, the soil structure of the tilled layer is observed on a vertical face of a pit. Subsoil and more especially the transition layer between topsoil and subsoil have not always been given specific consideration. However, these layers undergo the effect of cropping systems through soil compaction or smoothing/smearing. A more accurate quantification of earthworm macropores and cracks in the transition layer would enable a more precise evaluation of the agronomical potential of the soil, manifest in root development or water retention. Thus, the aim of this paper is to present the profil cultural method in detail, along with the improvements we have made to quantify the ability of roots to penetrate compacted zones in the transition layer. We propose two indicators: (i) number of earthworm burrows per m2 counted on a horizontal surface at the bottom of the transition layer in the soil pit (ii) cracking quantified by taking a 50-mm×50-mm×100-mm sample of soil from the transition layer and examining the number of cracks. Results from experiments on different tillage treatments are used to demonstrate why it is worthwhile to take into account cracks and earthworm activity. Soil profiles were examined in mouldboard ploughed and no tillage fields, and described using the profil cultural method and the new indicators. Root maps were also traced to observe the effect of soil structure on root growth. Preliminary results show that the classification of cracking and the quantification of the number of earthworm burrows per m2 can explain observed root development in subsoil. This first approach towards a better observation of subsoil structure and the effect it has on roots needs to be confirmed through further research, and especially via quantification and scoring of soil structure impact on roots.

Moisture characteristics and their point pedotransfer functions for coarse‐textured tropical soils differing in structural degradation status

AbstractThe effect of soil structure on hydraulic pedotransfer functions (PTFs) in tropical soils with similar mineralogy and texture has not been well documented. Structurally contrasting soils from representative locations in southeastern Nigeria were analyzed for moisture retention at 0, 6, 10, 33, 100, 300 and 1500 kPa among other properties. They were grouped by depth (topsoils or subsoils) and also by their structural degradation status into low‐ and high‐stability soils, corresponding to organic matter (OM):[silt + clay] of &lt;7.5 and &gt;7.5%, respectively. Soil depth and structural stability influenced the soil moisture characteristic curves. The data were fitted to three tropical point PTFs, but none of them proved appropriate for predicting moisture retention in the soils. We therefore derived new ones using multiple linear stepwise regressions before and after the dataset grouping and compared their performances by means of cross‐validation. Moisture retention in the soils (sand content, 73.2–93.8%) could not be calibrated from texture and OM concentration alone, until when bulk density, total porosity and microporosity were included among the regressors. Microporosity's role was particularly outstanding at all matric potentials but the 1500 kPa. The ensuing PTFs represent a good fit for the soil moisture retention data. The two grouping strategies resulted in lower SE of the estimates in some cases, but this did not enhance the performances of the concerned PTFs. At the 1500 kPa, however, the PTF incorporating all datasets performed better than separate PTFs for topsoils/subsoils but worse than the one for high‐stability soils. Information on soil structure can therefore benefit PTF derivation for kaolinite‐dominated, coarse‐textured tropical soils at all the matric potentials considered. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Obituary: Professor H. R. Wallace—distinguished nematologist and plant pathologist 1924–2011

H.R. (Harry) Wallace was born on 12 September 1924, inLancashire, England. During World War II, he served in theRoyal Navy. Hesubsequentlytrained as azoologist andthenstudied wood-boring beetles for his Ph.D., which he re-ceived from the University of Liverpool. In 1952, he joinedthe School of Agriculture at the University of Cambridgeand began working on nematodes, studying seasonal emer-gence and the effects of soil structure, particularly aeration,on hatching in Heterodera schachtii. Whilst at Cambridge,Harry Wallace had extensive discussions with Sir JamesGray, Professor of Zoology, which led to his work onlocomotion in nematodes, commencing soon after he movedto Rothamsted Experimental Station (now RothamstedResearch) in 1955. However, Harry Wallace also contin-ued to investigate the effects of environmental factors onhatching and infectivity of juveniles, including attractionto roots, particularly in Heterodera spp. and Ditylenchusdipsaci. For some of these studies he worked collaborativelywith Audrey Shepherd and Jack Hesling. At Rothamsted healso supervised Cliff Blake’s Ph.D. project on Ditylenchus.In 1960, he was awarded a D.Sc. from the University ofLiverpool.In 1962, Professor W.R. (Buddy) Rogers visited Roth-amsted and encouraged Harry Wallace to move to Australia.He arrived at the then new CSIRO Division of Horticulturein Adelaide, where Alan Bird was already employed, in1963. There as Chief Scientist he concentrated his effortson Meloidogyne javanica and continued his work on envi-ronmental factors affecting movement of infective juveniles,studying the development, hatching and survival of eggs.He also became interested in factors affecting reproductionof M. javanica, and the effects of the nematode on its hosts.Seymour Van Gundy (University of California, Riverside,CA, USA) spent a sabbatical with Harry Wallace andAlan Bird in 1966, and collaborated with them on astudyofageingandstarvationinjuvenilesofM.javanicaandTylenchulus semipenetrans .In 1971, Harry Wallace was appointed to the Chair ofPlant Pathology at the Waite Institute, The University ofAdelaide. Whilst this meant he had to make contributions toundergraduate teaching and an increased administrative load(and hence less personal time for research), it also meantthat he had Ph.D. students and that he could broaden hisresearch interests. He continued his work on root-knot nem-atode and its effects on photosynthesis and nutrient demandin host plants. One major study, with Brian Stynes, involvedthe use of a synoptic approach to assess the relative impor-tance of various environmental factors on the growth andyield of plants. With Frances Reay he investigated thesusceptibility to and effects of M. javanica on various nativeplants and investigated the biodiversity of nematodes asso-ciated with the Australian bush.

Coupled effects of canal lining and multi-layered soil structure on canal seepage and soil water dynamics

Ponding tests were conducted in the Shiyang River Basin in Northwest China to assess canal leakage characteristics. Four anti-seepage constructions (concrete lining, pebble lining, clay lining plus compacted canal bed, compacted canal bed only) were performed on four canal sections, which were situated in multi-layered soils. The canal sections were tested using a two-stage approach: First, a stable water level was maintained; second, a stage where the water level in the canal section was permitted to drop. The canal seepage rate and the soil water content near the canal bed were monitored during each stage and in each canal section. Soil texture, bulk density and hydraulic conductivity were determined in each canal section and soil layer. Double ring infiltration tests were performed to investigate infiltration behaviour from the canal sections. The saturated–unsaturated flow model HYDRUS-2D was applied to simulate canal seepage and the local soil water response. The simulation results compared well with the monitored data, indicating that the model can reliably simulate canal seepage under these complex soil structures and different canal liners. Both experimental results and numerical modelling show that the clay lining plus compacted canal bed provides the best anti-seepage performance, followed by compacted canal bed only, then pebble and concrete lining. Simulation results also predicted that the soil water content was discontinuous at the interface of distinct soil layers, and that the range and form of wetting front varied greatly in the four canal sections, with a larger wetted area for the more permeable canal. Simulations were performed to study the sensitivity of canal seepage to the permeability of each soil layer and canal liner. The results, confirmed by the double-ring infiltration tests, indicated that the canal lining is not the only factor affecting canal seepage: The soil permeability can also influence the seepage, especially where there is a low permeability layer (e.g., compacted soil layer) close to the canal.

Vadose Zone Biodegradation of Benzene Vapors in Repacked and Undisturbed Soil Cores

Bench testing of petroleum biodegradation rates in vadose zone soils is typically associated with errors because transport conditions in laboratory systems are different from those found in the vadose zone. This work addressed the effect of soil structure and gas transport properties on hydrocarbon biodegradation in the unsaturated zone and we present a novel method for measuring biodegradation rates in intact and undisturbed soil columns of 100 cm3. To determine whether soil structure and gas diffusivity, defined as the ratio of the gas diffusion coefficient in soil to that in free air (Dp/D0), influence the outcome of aerobic benzene biodegradation experiments, measurements using identical sandy soils were performed on (i) undisturbed 100‐cm3 core samples; (ii) sieved (2‐mm) and repacked 100‐cm3 core samples; and (iii) soil samples (10 g) prepared as slurry microcosms. While slurry reactor experiments changed the first‐order rate constant (kw,1) significantly compared with undisturbed core samples, this was not the case for soil cores that had been sieved and repacked. This suggests that soil structure on a millimeter scale does not affect aerobic biodegradation in relatively unstructured sandy soils. Within differently textured soil cores, the biodegradation rate was found to increase with gas diffusivity when Dp/D0 &lt; 0.02. This establishes gaseous O2 and petroleum vapor diffusion and distribution in soil profiles as a controlling factor for natural biodegradation of petroleum vapors.

千葉市美浜区の液状化被害に与えた表層地盤構造の影響

千葉市美浜区の液状化被害に与えた表層地盤構造の影響

Design and Field Tests of an Access-Tube Soil Water Sensor

Accurate soil profile water content monitoring at multiple depths has heretofore been possible only using the neutron probe (NP) but with great effort and at infrequent time intervals. Despite the existence of several frequency domain electromagnetic (EM) sensor systems for profile water content measurements, accuracy and spatial representativeness has been precluded by fundamental problems related to soil conductivity and structure effects on the volume explored by the EM field of these sensors, which causes nonrealistic spatial variation in reported profile water contents. Time domain reflectometry (TDR) methods have the distinct advantage of employing a moving EM field that must pass through and be affected by both the drier and wetter soil structures in which the TDR electrodes are embedded. This article describes a profiling water content system based on TDR. The design, laboratory calibration, and field testing is detailed. The sensor system provides unattended, real-time, data acquisition. And, it can be installed without disturbing the soil around the access tube on the outside of which the TDR electrodes are embedded. The correlation coefficient between neutron probe and TDR measured soil water content was 0.94 with a slope of 1.40 and an intercept of -0.08 m3m-3. Bias between TDR and NP readings (TDR-NP) was positive at all depths below 10 cm, ranging from 0.021 and 0.096 m3 m-3. Uncertainty in data of ~0.012 m3 m-3 for soil water, and uncertainty in bulk electrical conductivity of 0.030 S m-1 (both partly due to unreliable mechanical electrical connections) shows that improvements must be made before such a system is acceptable for widespread use.

Nonlinear dynamic response of tall buildings considering structure–soil–structure effects

SUMMARYNumerous studies have shown that the interaction between adjacent buildings can result in changes in nonlinear dynamic response of structures, damage, and performance level, depending on the dynamic specifications of structures involved and the frequency content of the input motion.To study these effects, finite element method is used for the analytical investigations, and total soil–foundation–structure system is modeled all together. For modeling purposes and in order to realize the effects of the adjacent buildings on the dynamic response, two buildings, namely, 15‐story and 30‐story tall buildings, which were separated by distances of 1/4 and 1/8 of the width of the foundation and were located on hard and soft soil profiles, were considered.It was concluded that in the case where the soil and structure's periods were near to each other, the interaction of adjacent structures on increasing nonlinear responses (displacement and interstory drift) and structural damage indexes was noticeable and therefore was not negligible. Whereas in the case where periods are distant from each other, the interaction of adjacent buildings has a decreasing effect on damage indexes and nonlinear responses and therefore was negligible. Copyright © 2011 John Wiley &amp; Sons, Ltd.

A model to predict the effects of soil structure on denitrification and N 2O emission

► Dual porous model of soil that closely reproduces experimental water retention. ► Simplified to critical percolation path with hotspots positioned 10 μm or 100 μm away. ► Denitrification and nitrification assumed to occur in micro-porous hot-spot zones. ► A priori diffusion and kinetics reproduce experimental CO 2 , N 2 O and N 2 emissions. ► Model shows sensitivity of O 2 thresholds, rate coefficients and hotspot positions. A model of the void space of soil is presented, and used for the a priori biophysical simulation of denitrification. The model comprises a single critical percolation channel through a 5 cm stack of four unit cells of a dual-porous void structure. Together, the micro- and macro-porous structures closely replicate the full water retention characteristic of a sandy clay loam soil from the Woburn Experimental Farm operated by Rothamsted Research, UK. Between 1 and 10 micro-porous hot-spot zones of biological activity were positioned at equally spaced distances within 5 cm from the surface, and at either 10 μm or 100 μm from the critical percolation channel. Nitrification and denitrification reactions within the hotspots were assumed to follow Michaelis–Menten kinetics, with estimated values of rate coefficients. Estimates were also made of the threshold values of oxygen concentration below which the anaerobic processes would commence. The pore network was fully saturated following addition of an aqueous ‘amendment’ of nitrate and glucose which started the reactions, and which mirrored an established laboratory protocol. Diffusion coefficients for Fickian and Crank-Nicolson calculations were taken from the literature, and were corrected for the tortuosity of the micro-porosity. The model was used to show the amount of carbon dioxide, nitrous oxide and molecular nitrogen emerging from the simulated soil with time. Adjustment of the rate coefficient and oxygen threshold concentrations, within the context of a sensitivity analysis, gave emission curves in good agreement with previous experimental measurements. Positioning of the hot-spot zones away from the critical percolation path slowed the increase and decline in emission of the gases. The model and its parameters can now be used for modelling the effect of soil compaction and saturation on the emission of nitrous oxide.

Soil structure and flow rate-controlled molybdate, arsenate and chromium(III) transport through field columns

Preferential flow in soil causes rapid movement of chemicals and contamination at the tile-line level. Soil structure and flow rate effect on transport velocity of MoO 4 2−, H 2AsO 4 −1, and Cr + 3 was assessed for three soils differing in vadose structure. Triplicate, 50 cm long, 35 cm diameter field soil columns of Lamellic Hapludalf, Typic Udorthent, Typic Hapludalf from southwestern Germany were obtained. The metals solute prepared in 3.5 mM LiCl was applied as pulse, and flushed with synthetic rainwater of pH 5.5 at constant flow rates generated by + 10, −10, and −40 mm water heads at the column surface. The tracers' breakthrough data were fitted to a convective–dispersive flow equation with mobile and immobile soil water partition. Additionally, blue dye was rained on site, and X-ray tomography of one representative column from each soil was carried out to identify preferential flow paths in the field soils. Lamellic Hapludalf (sandy loam) had a dominantly massive structure, and blue dye flowed as vertical fingers. Typic Udorthent (silt loam) had well-connected macropores, and dye moved as macropore flow with radial diffusion. Typic Hapludalf (silty clay) had well-connected pores throughout the profile, and dye moved the deepest with smallest dyed area. Chloride moved sampling 0.30 to 0.33 fraction of porosity in the massive soil irrespective of flow rate; and depending on flow rate, it moved through only 0.05 to 0.15 fraction of porosity in the structured silt loam/silty clay soil columns. MoO 4 2−, H 2AsO 4 −1 and Cr + 3 leached as fast as Cl −1 under ponded flow and with large retardation under unsaturated flow suggesting macropore flow of the adsorbing solute in the well-structured soils. The massive soil effectively retained H 2AsO 4 −1 and Cr + 3 but allowed MoO 4 −2 to leach at all the flow rates. Retardations of the absorbing tracers were in the order of Cr + 3 > H 2AsO 4 −1 > MoO 4 −2 > Li + 1 . Depending on soil structure, the effective porosity and retardation increased, and dispersion decreased with reduced flow rate. The study concludes that strongly graded, coarse to medium angular/prismatic soil peds allow adsorbing solute to leach at reduced flow rate, and the field description of soil structure helps discern areas where preferential flow can potentially cause subsoil contamination.