Parameterization of the MACRO model to represent leaching of colloidally attached inorganic phosphorus following slurry spreading (original) (raw)
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Applied Geochemistry, 2018
Surface complexation modelling (SCM) is a powerful tool to estimate speciation and fate of solutes in soil, provided sufficient model validation. This study aims to describe phosphate (PO 4) leaching with SCM. The leachate phosphorus concentrations ([P]) of 120 unsaturated columns of contrasting agricultural soils were measured and modelled. Leachate [P] ranged 0.7-240 μM. Leachate [P] increased as the ratio of P to iron and aluminium (+ P Al Fe) in acid oxalate soil extracts increased and as leachate Fe and Al concentrations ([Al + Fe]) increased. SCM was used to describe PO 4 sorption to ferrihydrite (CD-MUSIC model). This yielded adequate description of leachate [P] (RMSE log10 = 0.39), but only when reactive PO 4 was described from isotopically exchangeable PO 4 , when organic matter was included as the main competing adsorbate and when mobile colloidal ferrihydrite was included. The model reveals that colloidal PO 4 transport enhanced leachate PO 4 concentrations up to a factor 50 at small soil P content and small calcium (Ca 2+) concentration in solution, as a large Ca 2+ concentration enhances colloidal stability. This modelling approach explained that long-term application of organic fertilisers with higher Ca content reduced P leaching, likely due to the effect of Ca 2+ on colloidal stability. A two-parameter empirical Langmuir model, based on soil Fe and Al oxyhydroxides, fitted data better than any SCM, suggesting that the empirical model might be advocated for application at large scale. This study revealed the power of SCM to better understand colloidal transport of P in soil.
Impact of Macropores and Gravel Outcrops on Phosphorus Leaching at the Plot Scale in Silt Loam Soils
Transactions of the ASABE, 2017
In response to increased nutrient loads in surface waters, scientists and engineers need to identify critical nutrient source areas and transport mechanisms within a catchment to protect beneficial uses of aquatic systems in a costeffective manner. It was hypothesized that hydrologic heterogeneities (e.g., macropores and gravel outcrops) in the vadose zone play an integral role in affecting flow and solute transport between the soil surface and shallow alluvial aquifers. The objective of this research was to characterize phosphorus (P) leaching through silt loam soils to alluvial gravel aquifers in the floodplains of the Ozark ecoregion at the plot scale. Solute injection experiments used plots (1 m 1 m, 3 m 3 m, and 10 m 10 m) that maintained a constant head for up to 52 h. Solutes in the injection water included P (highly sorptive), Rhodamine WT (slightly sorptive), and chloride (conservative). Electrical resistivity imaging identified zones of preferential flow. Fluid samples from observation wells indicated nonuniform subsurface flow and transport. The surface soil type, ranging from silt loam to clean gravel outcrops, had a significant impact on P leaching capacity, with gravel outcrops resulting in high infiltration rates and rapid solute detection in wells (e.g., 4 min). Even in silt loam soils without gravel outcrops, macropore flow resulted in rapid transport of P. Maximum transport velocity for soluble reactive P in one silt loam plot was 810 cm h-1 , compared with a mean pore water velocity in the range of 25 to 130 cm h-1. Soluble reactive P concentrations in observation wells reached up to 0.54 mg L-1 in silt loam plots and 1.3 mg L-1 in gravel outcrop plots, demonstrating that a highly sorbing solute can be mobile.
Science of The Total Environment, 2005
A model of phosphorus (P) losses in a small dairy farm catchment has been set up based on a linkage of weather-driven field-scale simulations using an adaptation of the MACRO model. Phosphorus deposition, both in faeces from grazing livestock in summer and in slurry spread in winter, has been represented. MACRO simulations with both forms of P deposition had been calibrated and tested at the individual field scale in previous studies. The main contaminant transport mechanism considered at both field and catchment scales is P sorbed onto mobile colloidal faeces particles, which move through the soil by macropore flow. Phosphorus moves readily through soil to field drains under wet conditions when macropores are water-filled, but in dry soil the P carrying colloids become trapped so losses remain at a low level. In the catchment study, a dairy farm is assumed to be composed of fields linked by a linear system of ditches which discharge into a single river channel. Results from linked simulations showed reasonable fits to values of catchment outflow P concentrations measured at infrequent intervals. High simulated outflow P concentrations occurred at similar times of year to high measured values, with some high loss periods during the summer grazing season and some during the winter when slurry would have been spread. However, there was a lack of information about a number parameters that would be required to carry out a more exact calibration and provide a rigorous test of the modelling procedure. It was nevertheless concluded that through soil flow of colloid sorbed P by macropore flow represents a highly plausible mechanism by which P is transported to river systems in livestock farming catchments. This represents an alternative to surface runoff transport, a mechanism to which high P losses from livestock farming areas have often been attributed. The occurrence of high simulated levels of loss under wet conditions indicates environmental benefits from avoiding slurry spreading on wet soil or during rain, and from some forms of grazing management. D
Adsorption controls mobilization of colloids and leaching of dissolved phosphorus
European Journal of Soil Science, 2004
Loss of phosphorus (P) from agriculture contributes to the eutrophication of surface waters. We have assessed the magnitude and controls of P leaching and the risk of colloid-facilitated transport of P from sandy soils in Mu¨nster. Concentrations of soluble reactive P in drainage water and groundwater were monitored from 0.9 to 35 m depth. Total P concentrations, P saturation, and P sorption isotherms of soil samples were determined. Concentrations of dispersible soil P and colloidal P in drainage water and groundwater were investigated. The concentrations of soluble reactive P in drainage water and groundwater were close to background concentrations (< 20 g P l À1 ). Median concentrations in excess of 100 g P l À1 were found down to 5.6 m depth at one of four research sites and in the lower part of the aquifer. Experimentally determined equilibrium concentrations and the degree of P saturation were good predictors of P concentrations of drainage water. Large concentrations of dispersible P were released from soil with large concentrations of oxalate-extractable P and addition of P induced further dispersion. Colloidal P was transported in a P-rich subsoil when there was a large flow of water and after nitrate had been flushed from the soil profile and total solute concentrations were small. We conclude that the concentration of soluble reactive P in drainage water is controlled by rapid adsorption in the sandy soils. Subsurface transport of dissolved P contributes substantially to the loss of P from the soils we investigated. Accumulation of P in soils increases the risk of colloid-facilitated leaching of P.
Phosphorus Transport during Transient, Unsaturated Water Flow in an Acid Sandy Soil
Soil Science Society of America Journal, 1996
Sorbent-sorbate interactions heavily retard the movement of P during water flow in most soils. The effect of soil/solution ratio on sorption kinetics and movement of P during unsteady unsaturated water flow were investigated. A series of batch experiments with soil/ solution ratios ranging from 0.1 to 6.4 Mg m~3 were conducted to obtain sorption rate coefficients. Aqueous P solutions (100-800 g m~3) were applied at two constant fluxes (1.4 x 10~* and 6.9 X 10"' m s" 1) to columns of an air-dry spodic soil. The experimental data were simulated with a parallel two-site nonlinear, nonequilibrium transport model during unsteady, unsaturated water flow. Phosphorous sorption followed Freundlich-type reversible kinetics with a very fast reaction occurring in Type I sites and a slow reaction occurring in Type II sites. The sorption reaction of P in batch experiments was satisfactorily described by the model but the rate coefficients varied with the soil/ solution ratio. Experimentally determined rate coefficients described P movement in column experiments at an influx rate of 6.9 x 10"' m s" 1 , but not for the slower influx rate of 1.4 x 10" 6 m s" 1 , hence calibration of rate coefficients was necessary for describing P movement during the slower water influx. We developed a technique for determining rate coefficients under water saturated-unsaturated conditions that provides a way to validate P transport models during transient, unsaturated water flow. E XCESSIVE APPLICATIONS of P fertilizers, animal wastes, or treated municipal wastewater to acid, sandy soils with low sorption capacities and high permeabilities create a conducive environment for P leaching during periods of heavy rainfall (Mansell et al., 1991). Downward seepage of P-laden water through unsaturated soil to groundwater, subsequent lateral saturated subsurface flow to streams, and runoff during periods of shallow water tables are the major routes of P entry in the lakes (Mansell et al., 1995), which might deteriorate water quality through eutrophication (Collins and Young, 1987). Sorption of P by soils is a dynamic process and might retard the transport of P during the flow of P-laden water through soils. Phosphorus-soil interactions exhibit kinetic, reversible, nonlinear sorption often with some degree of irreversibility (Selim et al., 1975; Mansell et al., 1993). This process is characterized by a very fast initial transfer of P molecules from solution to sorbed phase, followed by a slow transfer (Barrow, 1983). Quantities of P sorbed during miscible displacement of P-laden solutions in soil columns tend to decrease with increasing water flow velocity (i.e., decreasing contact time) (
Finite element modeling of long-term phosphorus leaching through macropores in the Ozark ecoregion
2014
Phosphorus (P) is a critical nutrient for plant growth in agriculture, but is also responsible for surface water enrichment that leads to toxic algal growth. While P loading to surface waters has traditionally been thought to occur from surface runoff, contributions from subsurface transport can also be significant. While P transport through many soil types is well-documented, the presence of highly conductive gravel outcrops and macropore networks can have a significant, yet poorly-documented effect on P movement to the aquifer. Floodplain soils in the Ozark ecoregion generally contain coarse chert gravel layers that exhibit macropore behavior. Previous research has evaluated short-term P transport in plot trials ranging from 1 m 2 to 100 m 2 across many Ozark ecoregion floodplain sites. Traditional methods of estimating P loading and soil saturation do not account for macropore flow and likely underestimate P transport to the water table. To address this concern, long-term P modeling was performed in HYDRUS-2D/3D using data collected from short-term plot experiments. Calibration was performed using single-and dual-porosity models with both homogeneous and heterogeneous gravel profiles. The dual-porosity model with heterogeneous hydraulic conductivity best matched experimental data, although the dual-porosity model with homogenous soil layers also performed well. Long-term P transport to a 3 m-deep water table was simulated using 9 years of both daily and 5 minute rainfall data with a P flux consistent with yearly poultry litter applications. Long-term simulations with 5 minute rainfall data found that 113 kg ha-1 reached the water table over 9 years, or 21% of P applied.
Water Environment Research, 2007
In this study, several columns of different lengths were filled with composite soils sampled from the field at corresponding depths and then loaded intermittently with influent of a high phosphorus concentration to evaluate phosphorus fate and transport in soil. The results indicate that the height of the mass transfer zone, solvent pore velocity, and soil's life expectancy for phosphorus removal increased with depth, while the retained phosphorus per kilogram of soil and the linear adsorption equilibrium coefficient, R, decreased with depth. An equation was developed to link liquidphase phosphorus with solvent traveling time and soil depth. The results of X-ray diffraction and washout tests indicate that calcium-phosphorus precipitation and/or crystal growth occurred in the columns. The new protocol is useful for evaluation of phosphorus fate and transport in other subsurface systems, because it allows flexible adjustments in hydraulic loadings, feed solution, and sampling schemes.
Mode of Transport of Surface‐Applied Phosphorus‐33 through a Clay and Sandy Soil
Journal of Environmental Quality, 1999
Phosphorus (P) is the limiting nutrient for primary production in most freshwater ecosystems. The magnitude of P leaching from agricultural soils is therefore critical. Preferential flow has been proposed as a major cause for high P losses in structured clay soils. Undisturbed soil of two texturally different soils, that is, a day soil in which preferential flow was expected to be the main mode of water transport and a sandy soil where piston flow is the dominant process, were used in this study. Use of labeled P made it possible to determine the origin of leached P. An equivalent of 100 kg P ha−1, labeled with 33P, was added to the soil surface of each lysimeter. Water equivalents to 100 mm were added on five occasions with 7 d between each watering event. Ponded flow conditions were established during periods when water was added, to trigger preferential Bow behavior. Phosphorus leaching loads from day columns were much higher than P loads from sand columns. The average P leaching...
Phosphorus in soil treatment systems: Accumulation and mobility
2014
Septic tanks with subsequent soil treatment systems (STS) are a common treatment technique 18 for domestic wastewater in rural areas. Phosphorus (P) leakage from such systems may pose a 19 risk to water quality (especially if they are located relatively close to surface waters). In this 20 study, six STS in Sweden (11 to 28 years old) were examined. Samples taken from the 21 unsaturated subsoil beneath the distribution pipes were investigated by means of batch and 22
Vadose Zone Journal
Animal slurry application to agricultural land can be a threat to the quality of groundwater and nearby surface water bodies by percolation of solutes from slurry sources. We hypothesized that local-scale processes, such as mass exchange between preferential flow paths and matrix pore regions, can play a substantial role in relation to slurry application and nutrient leaching. To improve understanding of these mass exchange mechanisms, soil column leaching data of nonreactive slurry components after injection of dairy slurry were analyzed under different initial and boundary conditions with single-and double-porosity model approaches. The data set was from nine intact soil columns (20-cm i.d., 20-cm height) of the plow layer of arable loamy topsoil that were percolated under unsaturated steady-flow conditions with a suction of 5 cm applied at the bottom. Both single-and double-porosity water flow and mobile-immobile solute transport models could describe these experimental breakthrough curves. Rainfall interruptions mimicking more natural conditions and variably saturated intermittent flow led to higher leaching of injected slurry compounds than steady-flow conditions. These observations could be explained by an increased mass exchange of dissolved injected slurry components from the immobile to the mobile pore water regions during interruptions. The results suggest that column tests under steady-flow conditions could lead to false predictions of solute leaching after slurry injection in structured soils. Furthermore, local-scale processes, such as mass exchange between pore regions, should be included in larger scale model predictions of nutrient losses from agricultural fields.