O. H. Jacobsen

Relations Between Specific Surface Area and Soil Physical and Chemical Properties

The total specific surface area (SA) is a factor that can relate grain-scale properties to macro-scale physical and chemical properties of a porous medium. It is, therefore, advantageous to establish the relationships between SA and general soil physical properties. In this study we investigated

Particle transport in macropores of undisturbed soil columns

Abstract Particle-facilitated transport may be an important process in the leaching of contaminants such as pesticides, phosphorus and heavy metals. In this work particle transport in macropores through intact soil columns was quantified. Irrigation intensities corresponding to naturally occurring storm events were used. Intact soil columns (18.3 cm inner diameter, 20 cm length) were sampled at two different depths (2–22 cm and 42–62 cm) from a structured sandy loam. Infiltration experiments, consisting of leaching of naturally occurring particles and infiltration with two types of colloidal suspensions were performed on each column. The active macroporosity was estimated in a dye experiment. A significant transport of particles (especially clay and silt) through macropores was observed at both depths. The total amount of mobilized particles at a certain amount of water outflow was found to be higher at depth 42–62 cm than at depth 2–22 cm, but unaffected by irrigation intensity. The particle size in the effluent was found to decrease over time during both the leaching of naturally occurring particles and during the subsequent leaching of colloids from the infiltration with colloidal suspensions, but seemed to stabilize at a particle size 10 μm) into account, was found to describe the leaching of natural particles well using initial particle concentrations on the macropore walls and detachment coefficients for small and large particles as calibration parameters. ©1997 Elsevier Science B.V.

Glyphosate sorption in soils of different pH and phosphorus content

The sorption mechanism of glyphosate, one of the most frequently used herbicides in the world, resembles that of phosphate. This study quantifies the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH. Using batch experiments, glyphosate adsorption and desorption isotherms were determined on soil samples taken from long-term field experiments that received different additions of phosphorus and lime during 60-year (coarse sand) and 100-year (sandy loam) periods. Sorption isotherms were non-linear and manifested adsorption desorption non-singularity. The isotherms were best fitted with an extended Freundlich model, which had earlier been shown to describe phosphate sorption data well. The phosphate content in the soils had a significant influence on the sorption of glyphosate. With 0.5 M bicarbonate extractable P (pH 8.5) increasing from 6.2 to 58.7 in the loamy sand and 9.1 to 87.4 in the coarse sand, the extended Freundlich adsorption coefficient (Kf,MF,ads) decreased from 214.7 to 106 and from 154.0 to 83.5, respectively. Liming of the coarse sandy soil resulted in stronger glyphosate sorption because of an increase of reactive amorphous aluminum and iron hydrous oxides with increasing pH values. Glyphosate competes with phosphate for sorption sites, a quality that might result in glyphosate being sorbed more weakly in soils with high phosphorus levels.

Diffusion-limited mobilization and transport of natural colloids in macroporous soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

[14C]Glyphosate transport in undisturbed topsoil columns

Although glyphosate (N-(phosphonomethyl)glycine) is one of the most frequently used herbicides, few controlled transport experiments in undisturbed soils have been carried out to date. The aim of this work was to study the influence of the sorption coefficient, soil-glyphosate contact time, pH, phosphorus concentration and colloid-facilitated transport on the transport of [14C]glyphosate in undisturbed top-soil columns (20 cm height × 20 cm diameter) of a sandy loam soil and a sandy soil. Batch sorption experiments showed strong Freundlich-type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore flow. Consequently, amounts of glyphosate leached from the macroporous sandy loam soil were 50–150 times larger than from the sandy soil. Leaching rates from the sandy soil were not affected by soil-glyphosate contact time, whereas a contact time of 96 h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relatively unimportant with respect to total glyphosate leaching. The contribution of colloid-facilitated transport was <1 to 27% for the sandy loam and <1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosate leaching from the top-soils seems to be limited to conditions where pronounced macropore flow occurs shortly after application. © 2000 Society of Chemical Industry

Modelling mean nitrate leaching from spatially variable fields using effective hydraulic parameters

When using simulation models for estimating the mean nitrate leaching on different soil types, the common approach is to interpret the field as a single equivalent soil column using effective hydraulic parameters, which are estimated from point measurements. The use of effective hydraulic parameters was evaluated on a coarse sandy soil and a sandy loam using the one-dimensional mechanistic model, DAISY. On each location, texture, soil water retention and hydraulic conductivity from 57 points were measured within an area of ca. 0.25 ha. The following approaches for estimation of effective hydraulic conductivity were examined: (1) geometric mean; (2) arithmetic mean; (3) estimated arithmetic mean from a lognormal distribution; and (4) mean estimated from a stochastic large-scale model for water flow, similar to the Richards equation in one dimension, but with large-scale effective parameters accounting for the local three-dimensional flow. The approach of interpreting the field as a number of non-interacting columns was examined by calculating the mean of the field as the mean of the 57 soil columns. The nitrate concentrations simulated by DAISY were compared with nitrate concentrations measured by ceramic suction cups at the 57 points at 25 cm and 80 cm depths during the winter period 1989/1990. At both locations, the nitrate concentrations simulated by the geometric mean, the stochastic approach and the mean of the 57 simulations matched the observed nitrate concentrations while the other approaches gave unreliable results on the coarse sand. Hence, to simplify the calculations the geometric mean can be used.

Transport modes and pathways of the strongly sorbing pesticides glyphosate and pendimethalin through structured drained soils

Leaching of the strongly sorbing pesticides glyphosate and pendimethalin was evaluated in an 8-month field study focussing on preferential flow and particle-facilitated transport, both of which may enhance the leaching of such pesticides in structured soils. Glyphosate mainly sorbs to mineral sorption sites, while pendimethalin mainly sorbs to organic sorption sites. The two pesticides were applied in equal dosage to a structured, tile-drained soil, and the concentration of the pesticides was then measured in drainage water sampled flow-proportionally. The leaching pattern of glyphosate resembled that of pendimethalin, suggesting that the leaching potential of pesticides sorbed to either the inorganic or organic soil fractions is high in structured soils. Both glyphosate and pendimethalin leached from the root zone, with the average concentration in the drainage water being 3.5 and 2.7 μg L(-1), respectively. Particle-facilitated transport (particles >0.24 μm) accounted for only a small proportion of the observed leaching (13-16% for glyphosate and 16-31% for pendimethalin). Drain-connected macropores located above or in the vicinity of the drains facilitated very rapid transport of pesticide to the drains. That the concentration of glyphosate and pendimethalin in the drainage water remained high (>0.1 μg L(-1)) for up to 7d after a precipitation event indicates that macropores between the drains connected to underlying fractures were able to transport strongly sorbing pesticides in the dissolved phase. Lateral transport of dissolved pesticide via such discontinuities implies that strongly sorbing pesticides such as glyphosate and pendimethalin could potentially be present in high concentrations (>0.1 μg L(-1)) in both water originating from the drainage system and the shallow groundwater located at the depth of the drainage system.

High-resolution time domain reflectometry : sensitivity dependency on probe-design

When using the time domain reflectometry (TDR) technique in laboratory experiments, e.g., with packed soils columns, it is often of great importance to obtain high depth resolution with minimal disturbance of the soil and to be able to,measure close to the soil surface. This requires the use of fairly small TDR probes that can be placed near each other. In laboratory experiments on packed soil, we have examined the importance of relations between the probe-rod diameter, probe-rod length, distance between probe rods, and distance from the probe to the soil surface for accurate determination of volumetric water content. The experiments were conducted on a coarse sand with three different rod diameters (1, 2, and 3 mm), three different rod spacings (10, 20, and 50 mm), two rod lengths (50 and 150 mm), and at distances to the soil surface varying from 5 to 50 mm. Theoretical work by Knight et al. (1994, Symposium and workshop on time domain reflectometry in environmental, infrastructure and mining applications, North-western Univ., Evanston, Illinois, Sept 7-9, 1994, pp. 93-104) has been used to evaluate the results. In general, theory and measurements agreed very well. Both measurements and theory showed that the volume of soil contributing to the measurement is highly dependent on the spacing of the rods and, to a lesser degree, on the rod diameter. For rod spacings of 10, 20, pod 50 mm, measurements were accurately made as close to the soil surface as 10, 15, and 20 mm, respectively

Transport and Distribution of Salmonella enterica Serovar Typhimurium in Loamy and Sandy Soil Monoliths with Applied Liquid Manure

ABSTRACT A leaching experiment, where liquid manure spiked with Salmonella enterica serovar Typhimurium (Tet+) DSM554 was applied to soil surfaces, was conducted on intact soil monoliths (60 cm in diameter and 100 cm long). A total of 6.5 × 1010 CFU was applied to each column. We found that Salmonella serovar Typhimurium could be transported to a 1-m depth in loamy soil at concentrations reaching 1.3 × 105 CFU/ml of leachate. The test strain was found in concentrations ranging from 300 to 1.35 cells/ml in loamy soil throughout the 27 days of the experiment, while concentrations below 20 cells/ml were sporadically detected in the leachates from sandy monoliths. Real-time PCR targeting invA DNA showed a clear correspondence between the total and culturable numbers of cells in the leachate, indicating that most cells leached were viable. On day 28, distribution of Salmonella serovar Typhimurium at five depths in the four monoliths was determined. The highest recovery rate, ranging from 1.5% to 3.8% of the total applied inoculum, was found in the top 0.2 m.

Variation of MCPA, metribuzine, methyltriazine-amine and glyphosate degradation, sorption, mineralization and leaching in different soil horizons

Pesticide mineralization and sorption were determined in 75 soil samples from 15 individually drilled holes through the vadose zone along a 28km long transect of the Danish outwash plain. Mineralization of the phenoxyacetic acid herbicide MCPA was high both in topsoils and in most subsoils, while metribuzine and methyltriazine-amine was always low. Organic matter and soil pH was shown to be responsible for sorption of MCPA and metribuzine in the topsoils. The sorption of methyltriazine-amine in topsoil was positively correlated with clay and negatively correlated with the pH of the soil. Sorption of glyphosate was tested also high in the subsoils. One-dimensional MACRO modeling of the concentration of MCPA, metribuzine and methyltriazine-amine at 2m depth calculated that the average concentration of MCPA and methyltriazine-amine in the groundwater was below the administrative limit of 0.1mug/l in all tested profiles while metribuzine always exceeded the 0.1mug/l threshold value.