MODELING SALINE SOIL REMEDIATION THROUGH FLUSHING USING HYDRUS-1D AND AN ANALYTICAL SOLUTION (CDE)
By
ABSTRACT
Soil contamination is not only a great threat to the environment but to sustainable agriculture
as well; it can be devastating for the economical production of crops crucial to humankind’s
survival all around the world. Out of the different soil remedial techniques, soil flushing may be
the most environmentally friendly and cost effective, as well it poses very little risk for the on-
site workers. It uses water to accelerate geochemical dissolution reactions such as
adsorption/desorption and solution/precipitation along with accelerating a number of
subsurface contaminant transport mechanisms, normally found in unsaturated flow systems,
including advection, dispersion, molecular diffusion and volatilization or solubilization. In this
study irrigation water use efficiency (WUE) for salt flushing was estimated by calculating liters
of water/kg of salt flushed by modeling solute transport in HYDRUS 1-D. Soil flushing was
simulated for a period of five years ranging from 1997 to 2001 under two different climatic
regions Ottawa and Saskatoon, to flush out 80% of three different initial masses of salt, based
on three different soil salinity levels of EC equaling 4, 8 and 12, under natural conditions using
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the actual daily average precipitation, maximum and minimum temperature data for sand, silt
loam and clay loam soils and four different irrigated conditions. For irrigation condition 1,
during the month of June 10 mm/day of water was applied continuously along with the
precipitation data, which amounted to 300 mm/year for sand, silt loam and clay loam. For
irrigation condition 2, during the month of June water was applied continuously along with the
precipitation data, at a rate equal to 1/4th value of Ks, which amounted to 810 mm/year for silt
loam and 468 mm/year for clay loam. For irrigation conditions 3.a, during the months of May
and June water was applied continuously along with precipitation data, at a rate of 10 mm/day,
which amounted to 610 mm/year for sand only. And for irrigated condition 3.b, during the
month of June water was applied continuously along with precipitation data, at a rate of 20
mm/day, which amounted to 600 mm/year for sand only. To validate HDRUS-1D, first an initial
solute concentration was developed in MATHCAD through an analytical solution of CDE for
resident concentration under evaporation conditions. Then the same initial concentration was
used in HYDRUS-1D (by developing steady-state conditions) and in MATHCAD through a
convolution-based analytical solution of CDE using identical flow and transport conditions.
Comparisons of results showed that HYDRUS-1D duplicated the analytical solution nearly
identically.
Under Ottawa weather the objective of flushing 80% of the initial solute mass was achieved for
sand, silt loam and clay loam under the natural condition. In terms of WUE, sand did not need
any irrigation added to the precipitation data to achieve the target of flushing 80% of the solute
mass for all three salt concentrations, even under irrigated conditions; 1, 3.a and 3.b. In terms
of total time to flush out 80% of initial solute mass silt loam and clay loam showed a
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improvement of 34% under irrigated condition 1 versus natural condition. WUE improved by
26% under irrigated condition 1 versus condition 2 for Silt loam, whereas clay loam showed
16.5% increase in WUE under irrigated condition 2 versus irrigated condition 1. Under
Saskatoon conditions for sand again the objective of flushing 80% of the solute mass was
achieved under the natural conditions. In terms of total time to flush out 80% of the initial
solute mass, the objective was achieved a lot quicker (65%) under the irrigated condition 1
versus natural conditions; however, poor WUE (7441, 3720 and 2480 lit/kg of salt for mass 1, 2
and 3 respectively) was estimated by HYDRUS-1D. If time is not a big constraint then even
under Saskatoon conditions, flushing of salts from sand can be done quite efficiently without
applying any irrigation water. The target to flush out 80% of the initial solute mass was not
achieved for silt loam and clay loam under natural condition. Silt loam and clay loam used 5.5%
and 1% more water/kg of salt flushed under irrigated condition 2, versus irrigated condition 1. The major finding of the study was that under both climatic regions for all three different soil
types, WUE in terms of liters of water/kg of salt flushed, was found to be directly proportional
to the increase in the initial salt mass in the soil profile.
ABBREVIATIONS / KEY WORDS:
Ks, saturated hydraulic conductivity. 1/4th of Ks, one fourth of saturated hydraulic conductivity.
Vadose zone, between soil surface and groundwater. In situ soil flushing, in place extraction of
contaminants from the soil with water or other suitable aqueous solutions. WUE, water use
efficiency. CR, resident concentration. Ci, initial concentration.
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INTRODUCTION
Soil and groundwater contamination is an ever increasing widespread problem. Each year tens
of millions of dollars are spent worldwide to either remediate or to limit and prevent the future
contamination of the subsurface soil layers and groundwater. Being one of the major
contributors, the fate and transport of agricultural chemicals (salts, pesticides, pathogenic
micro-organisms, fertilizers, heavy metals) in the soil and groundwater systems, has become a
major concern. Fertilizers and pesticides applied to agricultural lands may move below the soil
root zone and not only accumulate there, but may contaminate the underlying groundwater
reservoirs. Chemicals migrating from municipal and industrial disposal sites may also leach and
thus pose an environmental hazard. Radionuclides emanating from energy waste disposal
facilities also pose a significant risk.
Most of the subsurface contamination problems arise from the activities involving the
unsaturated (vadose) zone, between soil surface and groundwater. The vadose zone plays a
fundamental role in many aspects of hydrology, including solute and water movement,
infiltration, soil moisture storage, evaporation, plant water uptake, groundwater recharge,
erosion and runoff, etc. Considering all these factors the unsaturated zone provides the best
opportunity to limit or prevent the future contamination of groundwater because once these
contaminants enter the groundwater, pollution is virtually irreversible or can be remediated
only at extremely high costs. Hence not only does the unsaturated zone play a crucial role in
regards to the soil remedial process, but its importance has long been recognized as a buffer for
preventing groundwater contamination.
Once released into the subsurface environment, industrial as well agricultural chemicals are
generally subjected to a large number of simultaneous physical, chemical and biological
processes, including sorption-desorption, volatilization, photolysis, and biodegradation. The
extent of degradation, sorption and volatilization largely determines the persistence of a
pollutant in the subsurface (Chiou, 1989).
The processes of evaporation and plant transpiration also exert a major influence on the water
and solute distributions in near-surface environments. These processes concentrate salts by
decreasing the amount of water in the soil and when combined with irrigation in arid regions
can lead to highly saline conditions.
Under natural field conditions water and solute movement may be occurring under
unsaturated, partially saturated, or fully saturated porous media and the flow region may be
composed of non-uniform, heterogeneous soils. Flow and transport can occur in the vertical,
horizontal, or in a generally inclined direction. This flow may be occurring in a dual-porosity
fashion in which one fraction of the water content is mobile and another fraction is immobile or
dual-permeability type flow involving two mobile regions, one representing the matrix and
other macropores.
In order to facilitate the removal of organic contaminants and salts from soil as well
groundwater, a number of physical, chemical and biological remedial treatments are available.
Physical remediation technologies include incineration, air stripping, soil flushing/washing, ion
exchange and membrane separation. Chemical treatments include neutralization, reduction-
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bioaugmentation, or a combination of both (Bicki and Felsot, 1994).
Soil flushing is the enhanced in situ (in place) mobilization of contaminants in a soil for the
purpose of their recovery and/or treatment (Anderson, 1995). This method uses water to
accelerate the same geochemical dissolution reactions that alter contaminant concentrations in
groundwater systems, namely: adsorption/desorption and solution/precipitation. In addition it
accelerates a number of subsurface contaminant transport mechanisms that are normally
found in unsaturated flow systems, including advection, dispersion, molecular diffusion and
volatilization or solubilization.
The flushing fluids can be imported from an off-site source and/or drawn from the groundwater
in the immediate area. They can be introduced to the soil either through spraying, surface
flooding, subsurface leach fields, or subsurface injection. After the flushing of contaminants,
contaminated fluids can be removed from the groundwater by pump-and-treat systems in the
saturated zone.
In situ soil flushing has some advantages over other remediation techniques, like excavation
which requires access to the contaminated soil, soil replacement and/or disposal costs. It
results in minimal disruption of the local ecosystem, has cost advantages with greater depths of
contamination and minimized worker exposure to contaminants (Anderson, 1995). However
some site-specific conditions might limit the overall efficiency of soil flushing such as, soils with
pockets of low hydraulic conductivity, pipes and other underground utilities.
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Past studies in Ontario (Szweminska, 1998), based on soil column experiments have shown that
under unsaturated flushing conditions, metolachlor was successfully removed from a soil near
Cambridge, Ontario under constant water irrigation at a rate equal to one fourth of the
saturated hydraulic conductivity of the soil. Simulations conducted during the same study, for
interrupted flushing and constant infiltration rate at a rate equal to one fourth of the saturated
hydraulic conductivity of the Cambridge soil, suggest that the efficiency of the contaminant/salt
removal, in terms of total volume of the infiltrated water, was increased compared with
continuous flushing.
Even though several analytical solutions have been published for simplified transport systems
involving consecutive decay reactions (Cho, 1971; Wagenet et al., 1976; Harada et al., 1980;
Higashi and Pigford, 1980; van Genuchten, 1985), for more complex situations like transient
water flow or the non-equilibrium solute transport with nonlinear reactions, analytical solutions
are either not available or cannot be derived. In this scenario numerical models are the only
solution and must be employed. However such numerical models must allow for different
reaction rates to take place in the solid, liquid and gaseous phases, as well for a correct
distribution of the solutes among the different phases.
The past several decades have seen considerable progress in the conceptual understanding and
mathematical description of water flow and solute transport processes in the unsaturated zone
and computer models are now increasingly used to predict the future behavior of these
chemicals. A variety of numerical and analytical models are now available to predict the water
and/or solute transfer processes between the soil surface and groundwater. The most popular
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models remain the Richards equation for variably saturated flow and the Fickian-based
convective-dispersion equation (CDE) for solute transport. Deterministic solutions of these
classical equations have been used to predict water and solute movement in the vadose zone
and for analyzing specific laboratory or field experiments involving unsaturated water flow and
solute transport. One of the numerical models which uses Richards equation and the CDE is
HYDRUS-1D, which was used in this study, along with an analytical solution of the CDE used in a
convolution integral to validate the simulated estimates of HYDRUS-1D.
OBJECTIVES
The goal of the study was to not only flush 80% of the three different initial concentrations of
salt in a five year span, but to determine the water use efficiency (WUE), by calculating the
amount of water in liters, needed to flush per kg of salt. Apart from salt flushing, WUE has
several other important implications when considering irrigation, soil and water conservation,
crop productivity and sustainability of irrigated agriculture (Ayars et al., 1999). In terms of crop
productivity it is defined as the ratio of dry matter yield (Y) per unit of water evapotranspired,
(E + T) for a non-water stressed crop, whereas for salt flushing WUE as used in this study can be
defined as the amount of water in liters/kg of salt flushed. To achieve this goal, two approaches
were adopted. In terms of any past studies specifically regarding the modeling of WUE in terms
of liters of water/kg of salt flushed, nothing was found in the literature.
Analytical Modeling
Before the age of digital computers, the standard method for the analysis of soil water
problems or solute transport involved the analytical solution of a partial differential equation,
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with the corresponding initial and boundary conditions, meaning that the appropriate method
of solution had to be determined for each particular problem. For example, for problems of
steady flow in two dimensions, the most powerful method appears to be the method of
complex variables, which involves such concepts as the velocity hodograph and conformal
mapping. Non-steady problems, in general, include a time derivative of the dependent variable,
usually the pressure head. A powerful method to deal with this complication is the Laplace
transform technique. The complete solution of a problem often requires the application of
integral transformation techniques (Fourier transformation, Hankel transformation, etc). A
typical analytical solution may then be in the form of an infinite series of algebraic terms, or
even a double infinite series, or an infinite series of definite integrals. Only in some special
cases, such as flow in a region of very simple form (a circular region or an infinite strip), can the
solution be expressed in the form of a single analytical formula. A great advantage of an
analytical solution, apart from its immediate availability, is that it can give good insight into the
dependence of the solution on various physical parameters, such as dispersivity, hydraulic
conductivity, porosity, or geometrical dimensions. This advantage may be partially or
completely lost, when the form of the solution is very complicated and the evaluation of data
requires a computer program, paving the way for a numerical solution (Jacob, 1987).
Considering all that, the first objective of this study was to test HYDRUS-1D by determining if its
estimates of solute transport under flushing conditions are in agreement with the analytical
solution for the same simple conditions (Billstein et al., 1998). The results from an analytical
solution though, depend upon the particular approach and solution used. During a study of the
analysis of the transport of heavy metals in unsaturated soil at both steady and unsteady states,
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Dawood et al. (2009), validated the analytical solution of the fractional advection dispersion
equation (FADE) with experimental data for steady state conditions and then used a numerical
solution of FADE with MATLAB (comparable with HYDRUS-1D) for the unsteady state. Validation
tests showed that FADE improved modeling of the transfer of the solutes in the unsaturated
zone versus the classical CDE.
Numerical Modeling
Even though numerical modeling has become a common means to simulate water and solute
transport in porous media especially while making long-term future predictions, the reliability
of its estimates remains a substantial question mark against the actual field results due to the
heterogeneous climatic and soil conditions and other site/region specific characteristics.
Rungvetvuthivitaya et al. (2007) reported a large discrepancy between the observed and model
estimated concentrations of dibromochloropropne (DBCP) during a study revisiting
groundwater contamination predictions made by researchers at the University of Hawaii in
1988 for selected wells of the Honolulu Board of Water Supply. During the post-audit study,
initiated in the same area, different modeling approaches were undertaken for simulating flow
and contaminant transport in the vadose and saturated zones in the wake of new data. It was
concluded that the previous model predictions (1988) for contaminant travel times to reach the
water table and the approximate recovery times for wells were less than the new model
predictions made in 2007. Factors such as parameter uncertainties and model limitations were
considered important for the range in model predictions. Also, even though new predictions for
the concentrations matched the sample data better than the older predictions, there were still
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uncertainties related to modeling which remained unsolved. Having said that, proper
calibration along with use of numerical models is also very critical as the future reliance on
models will grow (Shackelford et al., 2006). So the second objective of this study was to
determine the WUE for flushing of salts from soil through HYDRUS-1D under natural and three
different irrigation conditions for three different soil textures in two different climatic regions
of Canada. Modeling flushing for different soil textural classes is particularly important as the
estimates/predictions may vary significantly according to the soil type. Park and Shin (2009)
reported that solute transport/extraction along with regular water flow are restricted due to
the low permeability and adsorption capacities of the salt-effected soils, which pose a
significant challenge to the flushing process specifically in the case of heavily textured soils.
Climatic conditions also influence flushing greatly due to differences in minimum and maximum
temperatures, average precipitation etc from region to region. A changing climate may also
alter chemical processes in the soil, including chemical weathering (White and Blum, 1995).
Avila et al. (1996) simulated a significant increase in the weathering rate of base cations in
Spain when temperature and precipitation increased (although if precipitation was reduced,
the effects of the higher temperature were offset). This resulted in an increase in concentration
of base cations such as calcium, sodium and potassium and an increase in streamwater
alkalinity. On the other hand, warmer or drier conditions promote mineralization of organic
nitrogen (Murdoch et al., 2000), thus increasing the potential loss of nitrogen to the surface or
groundwater. Load is also influenced by the processes by which water reaches the river
channel. For example nitrates are frequently flushed into rivers during intense storms following
prolonged dry periods (Compagnucci et al., 2001). In summary, climate may affect efficiency of
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salt flushing; such as in drier climates due to the different nature of the salts present
(NaCO3/NaHCO3), their solubility in the soil water, chemical amendments needed, whereas in
areas with high rainfall rates per year due to the saturated conditions, and should be
considered in designing remedial systems for salt-contaminated soils.
For the second objective, two different climatic regions of Canada, Ottawa and Saskatoon,
Saskatchewan were simulated in this soil flushing investigation with HYDRUS-1D. Ottawa
represents temporal climatic conditions with an annual average temperature of 6.0 C and an
annual average precipitation of 943 mm, whereas Saskatoon represents semi-arid climate with
an annual average temperature of 2.2 C and annual average precipitation of 350 mm
(Environment Canada, 2006, Canadian Climate Normals or Averages 1971 to 2000). Sand, silt
loam and clay loam soils were selected to estimate the volume of water required to flush out
80% of three different initial concentrations of soluble salts under natural and irrigated
conditions using HYDRUS-1D.
METHODOLOGY
To start with, using simple boundary conditions, a fabricated saline soil profile was developed
under evaporation to then simulate the efficiency of soil flushing through HYDRUS-1D and test
the simulation results with an analytical solution of the CDE for identical conditions. Following
the validation of HYDRUS-1D, the efficiency of flushing of salts from three different soil types
was investigated using HYDRUS-1D with climate data from two different climatic regions,
Ottawa and Saskatoon, Saskatchewan. These two regions were chosen because of their
different climatic conditions, with Saskatchewan representing a semi-arid climate, whereas
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Ottawa being a temporal climate zone. HYDRUS 1-D was used as a tool for estimating the
amount of water required to flush 80% of three different initial concentrations of salts from the
three soil profiles under natural and irrigated conditions; so essentially there will be two
modeling approaches: analytical and numerical.
1. Analytical Approach
The CDE and stochastic-convective are commonly used models representing solute transport in
the field. The CDE and stochastic-convective models represent two extreme processes for
solute dispersion; the CDE 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. These models are limited to describing the
solute transport characterized by either a linear or a quadratic increase in the travel time
variance with depth (Liu and Dane, 1996). However, solute transport in heterogeneous porous
media cannot always be conceptualized as being either a convective-dispersive or a stochastic-
convective process.
The CDE is the most common model used to describe solute movement through soils (van
Genuchten and Wierenga, 1976). As solute movement in the soil is influenced by the variation
of pore-water velocity, a number of solute transport models have been developed for the
classical, one-dimensional CDE to represent velocity variations (Bresler and Dagan, 1981; Parker
and van Genuchten, 1984). The CDE assumes a high degree (Fickian) of mixing among the flow
paths; therefore, it is generally used for solute transport when the water velocity and solute
concentrations are relatively uniform within the modeled region among flow paths. Often it is
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applied to describe the transport process with well developed lateral mixing with constant V
and D.
Under conditions of salt flushing for this study, HYDRUS-1D solute transport output was
validated through an analytical solution of CDE, by using convolution process. By definition
convolution is a mathematical operation on two functions f and g, producing a third function
that is typically viewed as a modified version of one of the original functions. The convolution
of, for example, f and g is written as f*g and is defined as the integral of the product of the two
functions after one is reversed and shifted. It has several applications that include statistics,
computer vision, image and signal processing, electrical engineering and differential equations.
In this study convolution has been used as a process of adding individual solutions of linear
differential equations having constant coefficients, as CDE is also an example of a linear partial
differential equation to which convolution theory can be applied if V and D are fixed, meaning
solutions are summed by integration (Jury and Roth, 1980). In the validation process
convolution (superposition) was used to convert a delta spike solution to an initial value
problem in which salt has already accumulated in a soil profile and is subsequently flushed out
by flowing water.
2. Numerical Approach
Numerical modeling is becoming a very important tool for analyzing complex problems, which
involve water flow and contaminant transport in the unsaturated zone. Many models of varying
degree of complexity and dimensionality have been developed during the past several decades
to quantify the basic physical and chemical processes affecting water flow and contaminant
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transport in the unsaturated zone. These models are now being used increasingly for a wide
range of applications in research and management of natural subsurface systems. Modeling
approaches range from relatively simple analytical and semi-analytical models to more complex
numerical codes that permit consideration of a large number of simultaneous nonlinear
processes (Simunek, 2008). While analytical and semi-analytical solutions are still used for
relatively simple applications, the ever-increasing power of personal computers, as well as
supercomputers, and the development of more accurate, numerically stable, and often
parallelized (Hardelauf et al., 2007) solution techniques have given rise to the much wider use
of numerical models in recent decades (Simunek, 2005).
HYDRUS-1D
In this study HYDRUS-1D (version 4.14) was used to simulate one-dimensional variably
saturated water flow, heat movement and the transport of solutes. HYDRUS-1D consists of the
HYDRUS computer program and HYDRUS-1D interactive graphics-based user interface. The
HYDRUS program numerically solves the Richards equation for saturated-unsaturated water
flow:
H (1)
In this equation there are two dependent variables: soil volumetric water content (θ) and the
pressure head (ψ) and two independent variables time (t) and depth (z); K is the soil hydraulic
conductivity and H is hydraulic head.
Convection-dispersion (CDE) type equation for solute transport is:
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(2)
Where V is velocity, D is hydrodynamic dispersion coefficient, and C is the concentration in the
liquid phase.
The water flow equation may incorporate a sink term to account for water uptake by plant
roots. It may also consider dual-porosity type flow in which one fraction of water content is
mobile and another fraction is immobile, or dual-permeability type flow involving two mobile
regions, one representing the matrix, the other macropores. The CDE considers convective-
dispersive transport in the liquid phase and diffusion in gaseous phase. It is written in a very
general form by including provisions for nonlinear non-equilibrium reactions between the solid
and the liquid phases and the linear equilibrium reaction between liquid and gaseous phases.
Hence both adsorbed as well the volatile solutes such as pesticides can be considered. The CDE
may also incorporate the effects of zero-order production, first-order degradation independent
of other solutes and first-order production/decay reactions that provide the required coupling
between the solutes involved in the sequential first-order decay chain. The transport models
also account for convection and dispersion in the liquid phase and diffusion in the gas phase,
thus permitting to simulate solute transport simultaneously in liquid as well gaseous phases.
The governing flow and transport equations are solved numerically using standard Galerkin-
type linear finite element schemes, or any modifications.
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DESIGN
As a starting point an arbitrary initial solute concentration was developed using a constant
concentration of salt initially throughout the entire soil profile. Evaporation conditions were
applied to accumulate salts at the soil surface after five days, through an analytical solution of
the resident concentration of the CDE in MATHCAD:
CR Ci V t
2 D t⋅⋅
+

2 D t⋅⋅
(3)
Where CR is resident concentration and Ci is the initial constant resident concentration. The salt
concentrations versus depth developed with Eq. 3 were used as the initial conditions for
comparing estimates of salt flushing using HYDRUS-1D and convolution-based analytical
solution of the CDE:
4 D⋅ T⋅
V z⋅ D
4 D⋅ T⋅ −
d
(4)
Where Ci (h) represents the initial concentration of salt developed with Eq. (3) and T represents
a specific time. Both HYDRUS-1D and the analytical solution modeling were performed under
steady-state conditions. Steady-state was established in HYDRUS-1D by first running it for a
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long time to determine the steady-state water content (θss) for a homogeneous soil profile. The
constant flux of water applied at the soil profile in HYDRUS-1D was then adjusted until the ratio
of the surface flux to θss was equal to V used in the analytical solution of the CDE. Once the
appropriate θss was established it was entered as the initial water content throughout the soil
profile to establish steady-state flow conditions immediately throughout the soil profile. The
values of input parameters for HYDRUS-1D and Eq. (4) are given in Table 1.
Table 1. Values for the soil hydraulic parameters used in HYDRUS-1D and MATHCAD to validate HYDRUS-1D estimates with CDE in MATHCAD
Parameter HYDRUS-1D MATHCAD Residual Water Content (θr) 0.078 ___ Saturated Water Content (θs) 0.43 ___ α 0.036 cm-1 ___ n 1.56 ___ Saturated Hydraulic Conductivity (Ks) 24.96 cm/d ___ Water Flux (q) -0.485 cm/d ___ Dispersivity (d) 1.33 cm ___ Steady-State Water Content (θss) 0.3233 ___ Average Soil Water Velocity (V) 1.5 cm/d 1.5 cm/d Hydrodynamic Dispersion Coefficient (D) 2 cm2/d 2 cm2/d
Here α and n are the parameters in the van Genuchten (1980) soil water retention function.
After successfully completing the validation, then HYDRUS-1D was used to estimate vertical salt
flushing; sixty different situations were simulated, by using Ottawa and Saskatoon climatic data
for sand, silt loam and clay loam soils, for three different salt concentrations, under natural and
four different irrigated conditions. The simulated soil depth was 2 m with a time period of five
years. For the natural conditions meteorological data was used for the two regions including
daily minimum and maximum temperatures (C) and daily precipitation (mm/day) from 1997 to
2001. The first irrigated condition, was during the month of June added to the precipitation
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data, in which irrigation water was continuously applied at a rate of 10 mm/day, which
amounted to 300 mm/year, for sand, silt loam and clay loam, for all five years. Under the
second irrigated condition, during the month of June and added to the precipitation data,
under which irrigation water was continuously applied at a rate equal to one fourth of the
saturated hydraulic conductivity in HYDRUS-1D (27 mm/day for silt loam and 15.6 mm/day for
clay loam), which amounted to 810 mm/year for silt loam and 468 mm/year for clay loam, for
all five years. This approach was not adopted for sand, considering a very high value of the Ks
for sand in HYDRUS-1D (712.8 cm/d), which would give an impractical irrigation rate (1782
mm/day for 178.2 cm/day), which will amount to 53460 mm/year even at 1/4th Ks. Under the
third irrigated condition, first (3.a) during the months of May and June irrigation water was
continuously applied at a rate of 10 mm/day and added to the precipitation data, which
amounted to 610 mm/year, for sand only, for all five years, whereas for the second sub-
condition (3.b) under third irrigated condition during the month of June, irrigation water was
applied continuously at a rate of 20 mm/day and added to the precipitation data, which
amounted to 600 mm/year, again for sand only, for all five years (Table 2).
In the time information option of HYDRUS-1D, Sinusoidal Variation of the Precipitation (to
approximate the variations in the precipitation rate over the course of each day using a cosine
function) was selected. Also the Hargreaves model was selected in HYDRUS-1D to estimate
potential evaporation. To produce daily rate in the output files, Print at Regular Time Interval
option was selected under the Print Information. No crop was simulated assuming that the
saline soil would not support it. For the soil hydraulic model, van Genuchten – Mualem was
selected along with no hysteresis. In terms of values for soil hydraulic parameters, default
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values of HYDRUS-1D were used for all three soil types with a uniform soil profile. For water
flow upper boundary, Atmospheric with Surface Run Off was selected with lower boundary
being Free Drainage. For solute transport, upper boundary was Concentration Flux BC, whereas
lower boundary was Zero Concentration Gradient. Initial solute concentration was established
as Mass of Solute/Volume of Water in Liquid Phase Concentrations. For the different salt
concentrations, first the solubility of salts was determined by assuming NaCl as the base salt; its
solubility is 0.36 g/cm3 of water. Then soil EC was taken as the basic guideline with values of 4,
8 and 12 microsiemens/cm, for the three different initial salt concentrations in the upper 0 to
50 cm of soil. This depth range was selected to represent the root zone of most crops. These
values of EC were then converted into ppm by multiplying by a conversion factor (for NaCl) of
500, which gave 2000, 4000 and 6000 ppm or mg/liter (HANNA Instruments, 2000. Instruction
Manual HI 9810 Portable pH/EC/TDS Meter, accessed at
http://www.hannainst.com/manuals/manHI_9810.pdf).
Table 2. Details of the different precipitation and water application rates and timings for different soil types under irrigated and natural conditions for the five year simulation period from 1997 to 2001.
Soil Condition Precipitation/Water Water Application Timing Sand
Silt Loam Clay Loam
Sand Silt Loam Clay Loam
Irrigated# 1 (10 mm/day) Precipitation + Irrigation June 1st to 30th every year
Silt Loam Clay Loam Irrigated# 2 (1/4th of Ks) Precipitation + Irrigation June 1st to 30th every year
Sand Sand
Irrigated# 3.a (10 mm/day) Precipitation + Irrigation May 1st to June 30th every year June 1st to 30th every year Irrigated# 3.b (20 mm/day)
Note: 1/4th Ks for silt loam and clay loam are 27 and 15.6 mm of water/day.
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For HYDRUS, these values were converted to g/cm3 and finally values of 0.002, 0.004 and 0.006
g/cm3 were used as the three different initial salt concentrations in the HYDRUS simulations.
RESULTS AND DISCUSSIONS
HYDRUS-1D Validation
Salt concentration versus depth was calculated with Eq. 3 and used as the initial condition for
comparing estimates of salt flushing using HYDRUS-1D and a convolution-based analytical
solution of the CDE (Eq. 4). Almost identical results were obtained using the two methods as CR
(z, T) matched each other nearly perfectly (Figure 1). Therefore, the objective to check the
validity of HYDRUS-1D for flushing of a salt-contaminated soil was achieved.
Figure 1. Comparison of resident concentration calculated with Eq. 4 in Mathcad and HYDRUS-
1D as a function of depth at four different times.
0 0.5
1 1.5
2 2.5
3 3.5
4 4.5
de nt
22
It is very important to note here, that by validating the estimation of HYDRUS-1D with the
analytical solution of CDE, it is not being implied by any means, that results produced through
HYDRUS-1D simulation will accurately represent natural phenomena. This is due to the fact
that, natural systems are never closed, models can only be evaluated in relative terms and their
predictive value is always open to question (Naomi et al., 1994). The main purpose of this
validation was to confirm HYDRUS-1D under a simplified salt flushing scenario using an
analytical solution of the CDE.
Under Natural Conditions
The period selected for this study ranged from 1997 to 2001. For Ottawa year 2000 had the
highest annual precipitation of 962 mm and 2001 being the lowest with an annual precipitation
of 780 mm. Throughout the five year period, months of January, March, June and September
remained the peak precipitation months with monthly average of 89 mm. In terms of number
of days when it rained again year 2000 was the highest with 179 days and year 1999 being the
lowest with 147 days. In terms of average daily rainfall intensity (when it rained) 1999 was the
highest with 6 mm with year 1997 being the lowest with an average daily rainfall of 4.92 mm.
For Saskatoon year 1999 was the highest with an average annual precipitation of 401 mm with
2001 being the lowest with 160 mm. In each year, the May to September period remained the
highest throughout the simulation period with an average monthly precipitation of 38 mm. In
terms of number of days when it rained again year 1999 was the highest with 120 days with
year 2001 being the lowest with 87 days. In terms of average daily rainfall intensity (when it
23
rained) 1998 was the highest with 3 mm with year 2001 being the lowest with an average daily
rainfall of 1.8 mm.
Even though under natural conditions, no irrigation was added to the precipitation data, 80% of
the salt was flushed through the simulated depth of 200 cm for Ottawa region for all three soil
types (98 days for sand, 834 days for silt loam and 822 days for clay loam) and all three salt
masses, which was mainly due to high average annual rainfall of 860 mm for the simulation
period. Silt loam and clay loam, took approximately the same time to flush 80% of the salt
initially residing in the 0-50 cm soil depth range. In the case of salt mass 3, for silt loam and clay
loam insignificant quantities of solute still persisted in the lower 25 cm of the soil profile. The
maximum solute concentrations estimated by HYDRUS-1D after the simulated period of 1826
days are shown in Figure 2. Note that solute did not reach 200 cm depth as well it failed to
reach the maximum steady-state cumulative mass flux levels at the end of the simulation
period for the two fine-textured soils at Saskatoon due to low drainage . Peak concentration is
directly proportional to the mass of solute applied. Salt flushing fared much better in the case
of sand versus silt loam and clay loam due to the rapid downward movement of solute. Soil
drainage was also understandably higher versus evaporation for sand, due to the higher value
of Ks, whereas time duration for leaching to occur also considerably increased for silt Loam and
clay loam versus sand due to the lower values for Ks. Evaporation rates almost doubled versus
drainage for silt loam and clay loam (Table 3) due to the slower soil drainage because of the
finer texture of soils.
24
For the Saskatoon region not only the time period to flush 80% of the initial salt concentration
increased considerably versus Ottawa for sand, but for silt loam and clay loam the objective to
flush 80% of the initial salt concentration was not achieved even after the completion of the
five year simulation period. All three salt masses, for silt loam and clay loam, failed to reach soil
profile depth beyond 160 and 150 cm and thus are not plotted for the maximum cumulative
bottom solute flux at 200 cm after 1826 days (Figure 2).
Figure 2. Maximum cumulative bottom solute flux at 200 cm depth as estimated by HYDRUS-1D after 1826 days for the three different initial solute masses for Sand, Silt Loam and Clay Loam for Ottawa and Saskatoon regions under natural conditions.
This was due to the slower soil drainage versus sand, very low total 5-year rainfall of 1499 mm
versus 4330 mm for Ottawa, plus increased or equal evaporation versus drainage (Table 3). This
is in line with results of Samarasinghe and Lennon (1986), who found that high evaporation, low
rainfall, gulfs and embayments in the arid coastal regions of Australia resulted in highly saline
0
0.2
0.4
0.6
0.8
1
1.2
25
water masses. Drainage rates were almost double versus evaporation for sand, whereas in case
of silt loam and clay loam either evaporation rates were equal or higher than drainage. Higher
drainage rates due mainly to a higher value of Ks produced better results in terms of flushing
80% of the initial solute concentrations for sand, taking 555 days in Saskatoon.
The reason for these contrasting results between Ottawa and Saskatoon for the same soil types
and same initial salt masses was the difference in the climatic conditions. A changing climate
may alter the soil chemical processes (White and Blum, 1995) and can greatly affect, the way
solutes are moved in the soil. A changing climate can result in an increased concentration in
base cations like calcium, sodium and potassium, due to the increase in the base cation
weathering rates (Avila et al., 1996).
Table 3. Water budget data/year as estimated by HYDRUS-1D for Ottawa and Saskatoon regions for sand, silt loam and clay loam under natural conditions as estimated by HYDRUS-1D.
Year Region Soil *P (mm) *D (mm) *R (mm) *E (mm) ∗S (mm)
1997
Ottawa
823 823 823
433 313 274
0 0 4
306 519 505
83 -9 40
160 160 160
49 14 13
0 0 0
126 185 184
-15 -39 -38
*P is precipitation, D drainage, E evaporation and S is the change in storage. Overall it took
more time to flush out 80% of the initial solute masses for sand in Saskatoon region versus
Ottawa, again due to the low average annual precipitation rates, resulting in not enough water
to drain through the entire simulated depth of 200 cm. Also one thing which was found
common for all three soils at Ottawa and sand at Saskatoon, that even though the total time in
days to flush out 80% of the initial salt concentration varied for the three different soil types
under Ottawa (98, 834 and 822 days for sand, silt loam and clay loam) and Saskatoon (555 days
for sand) climatic conditions, however it did not change at all or the change was very
insignificant such as, difference of one day, with the increase in the initial salt mass.
Under Irrigated Condition 1 (10 mm/d from June 1st to 30th)
Under this irrigated condition, during the month of June, irrigation water was applied
continuously at a rate of 10 mm/day and added with the precipitation data, which amounted to
300 mm/year for the five year simulation period to determine WUE to flush each kg of salt. For
Ottawa region sand did not need any addition of water to flush out 80% of the solute mass for
all three initial solute masses as it only took 98 days to achieve that target. This may be because
27
of the fact that majority of the contaminants are concentrated in the finer-grained materials or
on the surfaces of the larger soil particles (AbdEl-Sabour, 2007), thus making it easy for the
water to flush the salt mass in a sandy soil versus a finer textured soil. Under this particular
irrigated condition, water was added from day 152nd to 181st. Under Saskatoon weather the
number of liters of water/kg of salt used to flush 80% of the initial salt concentration was more
than 100% higher versus the liters of water/kg of salt used to flush 80% of salt mass under
Ottawa weather irrespective of the soil type and mass level. It was also observed for all three
soil types, as well both climatic regions, that as the initial salt concentration was increased, so
did the WUE in terms of using fewer liters of water to flush out each kg of salt (Figure 3). For
example in the case of Ottawa for silt loam as well as clay loam, WUE to flush out 80% of the
salt mass doubled when the solute initial concentration from 0 – 50 cm depth was doubled
from 0.002 g/cm3 to 0.004 g/cm3. WUE further increased 1.5 times as the initial concentration
was again increased 1.5 times from 0.004 g/cm3 to 0.006 g/cm3. This means that WUE is directly
proportional to the increase in the initial solute concentration. In the case of Saskatoon, WUE
again doubled when the concentration was doubled from 0.002 g/cm3 to 0.004 g/cm3, whereas
it showed an improvement of 1.5 times when salt concentration was increased 1.5 times from
0.004 g/cm3 to 0.006 g/cm3, proving the point that irrespective of the different climatic regions
or the soil types, WUE is directly proportional to the increase in the initial solute concentration.
The time to flush 80% of the initial salt concentration for sand under Saskatoon weather was
96% higher (98 versus 189 days) than Ottawa, whereas for silt loam and clay loam it was 271%
and 254% higher (548 versus 1487 and 542 versus 1377) than Ottawa.
28
Figure 3. Volume of water in liters used to flush each kg of salt as estimated by HYDRUS-1D for
the three different initial solute masses fluxes for sand, silt loam and clay loam for Ottawa and
Saskatoon regions under irrigated condition 1 equal to 10 mm/day from June 1st to 30th every
year during the simulation period.
With the addition of 300 mm of water per year for the simulation period, 80% salt removal was
achieved for all three salt masses and soil types under both climatic regions, which was a
significant improvement in case of silt loam and clay loam soils for Saskatoon region under this
irrigated condition versus the natural condition. However, even though salt masses were
completely flushed at Ottawa for all three soils, in Saskatoon salt still persisted in the soil profile
even after the completion of the five year simulation period for silt loam and clay loam. Mason
et al. (1997) reported 75-90% efficiencies in terms of flushing uranium from contaminated soil
(Ohio, USA) by using 0.5 M sodium bicarbonate as a dominant reagent along with irrigation
0
1000
2000
3000
4000
5000
6000
7000
8000
29
water. It is also important to note here that the efficiency of contaminant removal or WUE for
the salt flushing varies with the nature as well the form of the salt/contaminant. Duff et al.
(1998) investigated several leaching solutions for the removal of uranium (U) from
contaminated media such as soil and military catch-box sand used for the ballistics testing of
uranium-containing projectiles. Batch leaches with six uranium-contaminated media and seven
leach solutions (of concentrated sulphuric acid and of carbonate, with and without oxidants
such as peroxides used in the mining of uranium from high-grade ore deposits) were
conducted. After the leach solutions were reacted with the media (five soils and a catch-box
sand), the solutions were analysed for dissolved U, Ca, Si, Fe, Mn, Pb and Cr. When the ability of
the acidic, basic, acidic-oxidizing and basic-oxidizing solutions to leach uranium from the
contaminated media was analyzed, the results suggested that the removal efficiency of soil
uranium for each leach solution varied with the nature of the uranium-contamination in the
media and the media composition. It was likely that the forms of uranium in the contaminated
media such as reduced, absorbed, complexed and solid species influenced the solubilisation of
uranium by the leaching agents.
Under Ottawa region all three salt masses reached the soil profile depth of 200 cm and the
steady-state levels for the cumulative maximum bottom solute flux were achieved,; however,
under Saskatoon region excluding sand, in case of silt loam and clay loam even though all three
salt masses reached the soil depth of 200 cm they failed to reach the steady-state maximum
cumulative bottom solute flux levels (Figure 4).
30
Figure 4. Maximum cumulative bottom solute flux as estimated by HYDRUS-1D at 200 cm depth after 1826 days for the three different initial solute masses for Sand, Silt Loam and Clay Loam for Ottawa and Saskatoon regions under irrigated conditions 1 equal to 10 mm/day from June 1st to 30th every year.
Under irrigation condition 1, drainage again remained dominant versus evaporation in case of
sand for both regions whereas evaporation surpassed drainage in case of silt loam and clay
loam for Ottawa as well Saskatoon. Runoff was almost negligible and occurred only in the case
of clay loam for both regions (Table 4).
Table 4. Water budget data/year for Ottawa and Saskatoon regions as estimated by HYDRUS- 1D for Sand, Silt Loam and Clay Loam under Irrigated Condition 1 equal to 10 mm/day from June 1st to 30th every year.
Year Region Soil *P+I (mm) *D (mm) *R (mm) *E (mm) ∗S (mm)
1997
Ottawa
1123 1123 1123
610 454 435
0 0 5
430 658 636
84 11 47
613 613 613
299 240 219
0 0 1
277 484 458
37 -111 -66
460 460 460
227 100 117
0 0 0
248 378 358
-14 -18 -16
*P+I are precipitation plus irrigation water, D drainage, E evaporation and S is the change in
storage. With the addition of water under this particular irrigated system, it was observed again
that though there was a change in total time to flush out 80% of the initial salt concentration
for three different soil types under Ottawa (98, 548, 542 days for sand, silt loam and clay loam)
and Saskatoon (189, 1487 and 1377 days for sand, silt loam and clay loam) climatic conditions,
but the total time did not changed with the increase in initial salt mass.
Under Irrigated Conditions 2 (equal to 1/4th of Ks)
The amount of water applied to flush each kg of salt under this irrigation condition was slightly
higher versus irrigated condition 1, where a total of 300 mm/year was added to the
32
precipitation data every day from June 1st to 30th. Under this condition even though the timing
for the water application remained the same as condition 1, the amount of water applied
increased considerably with 27 mm/day and 15.6 mm/day for silt loam and clay loam
respectively, which amounted to 810 mm/year for silt loam and 468 mm for clay loam due to
the water quantity being equal to ¼ of Ks (2.7 and 1.56 cm/day for silt loam and clay loam
respectively).
In terms of WUE in litres to flush each kg of salt, under Saskatoon weather silt loam used 36%
more liters of water/kg of salt (4710, 2354 and 1570 liters) than Ottawa (3007, 1504 and 1002
liters) to flush 80% of the initial salt concentration whereas clay loam used 61.5% more liters of
water/kg of salt (4491, 2245 and 1497 liters) than Ottawa (1725, 863 and 575 liters) to achieve
the same objective. In terms of the comparison between irrigated conditions 1 and 2, in Ottawa
region, silt loam used 26% more water/kg of salt under irrigated condition 2 (3007, 1504 and
1002 litres) than irrigated condition 1 (2228, 1114 and 743 litres, Figure 4) to flush out 80% of
the three different salt mass, whereas under Saskatoon climate and under this particular
irrigated condition, WUE improved a bit and it only used 5.4% more water/kg of salt (4710,
2354 and 1569 litres) than irrigated condition 1 (4454, 2227 and 1485 litres) to flush out 80% of
the three different salt masses (Figure 5). This effect might be induced because of the fact that
under Ottawa weather, silt loam received more precipitation for the entire simulation period
than Saskatoon. Under Ottawa climatic conditions, WUE to flush salts from clay loam soil on the
other hand improved under this irrigated condition by using 16.5% less water/kg of salt (1725,
863 and 575 litres, Figure 3) than irrigated condition 1 (2065, 1032 and 688 litres, Figure 3) to
flush out 80% of the initial salt concentration, whereas it used 1.5% more water/kg of salt
33
under this irrigated condition (4491, 2245 and 1497 litres) than irrigated condition 1 (4429,
2214 and 1476 litres) under Saskatoon region.
Szweminska in 1998, conducted numerical simulations (LEACHMP) as part of one-dimensional
soil column experiments, under unsaturated flushing conditions, to remove metolachlor from a
sandy loam soil (86% fine to very fine sand and 14% silt and clay) near Cambridge, Ontario
under constant water irrigation rate of 132 mm/day, equal to the actual ¼ Ks of the
experimental soil (0.55 cm/hour), measured by falling head permeameter method (Carter,
1993). In that study, it was reported that about 82% of the initial metolachlor concentration
was removed with the lower soil boundary being fixed depth water table of 182 cm below the
soil surface.
Figure 5. Volume of water in liters used to flush each kg of salt as estimated by HYDRUS-1D for the three different initial solute masses for sand, silt loam and clay loam for Ottawa and Saskatoon regions under irrigated condition 2 equal to 1/4th of Ks/day from June 1st to 30th every year during the simulation period.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
34
Improvement in WUE was again directly proportional to the increase in the initial salt
concentration irrespective of the climatic regions and soil types. WUE doubled when the salt
concentration mass was doubled from 0.002 g/cm3 to 0.004 g/cm3, whereas it increased 1.5
times when the salt concentration was further increased 1.5 times from 0.004 g/cm3 to 0.006
g/cm3.
All three maximum cumulative bottom solute fluxes reached steady-state levels along with
reaching the simulated depth of 200 cm at the end of the simulation period of five years for silt
loam under Ottawa and Saskatoon and clay loam under Ottawa (Figure 6). The addition of the
irrigation water with the precipitation data seems to be the obvious reason for that. In the case
of clay loam under Saskatoon, even though the maximum cumulative bottom solute flux
reached the 200 cm depth for initial mass 1, it did not fully reach its maximum steady-state
level for all three salt masses even at the end of the simulation period. Finer texture of the soil
and low Ks seems to be the reason for that. There was also a 285% improvement in terms of
number of days to flush 80% of the initial salt concentration under Ottawa climate and this
irrigated condition for silt loam as it took 192 versus 548 days under irrigated condition 1,
whereas in case of clay loam there was a improvement of 17% as it took 462 days in case of this
irrigated condition versus 542 days under irrigated condition 1. Under Saskatoon as well, in case
of silt loam there was an improvement of 279% under this irrigated condition, as it took 533
days versus 1487 days under irrigated condition 1, whereas for clay loam time to flush 80% of
the initial salt mass improved by 153% under this irrigated condition taking 899 days versus
1377 days under irrigated condition 1.
35
Figure 6. Maximum cumulative bottom solute flux as estimated by HYDRUS-1D at 200 cm depth after 1826 days for the three different initial solute masses for Sand, Silt Loam and Clay Loam for Ottawa and Saskatoon regions under irrigated condition 2 equal to 1/4th of Ks/day from June 1st to 30th every year.
In terms of soil water budget data, drainage remained as high as 40% versus evaporation for silt
loam under Ottawa weather and mostly soil gained water storage, whereas for clay loam the
difference remained between 19 to 9%. This was due to the higher average annual
precipitation, temporal climatic conditions, finer texture of the soil and the lower Ks.
Drainage again remained about 30% higher versus evaporation for silt loam under Saskatoon
weather, whereas for clay loam evaporation remained on an average 16% higher versus
drainage. This seems to be because of the drier Saskatoon weather conditions versus Ottawa
(Table 5).
36
Table 5. Water budget data/year as estimated by HYDRUS-1D for Ottawa and Saskatoon regions for Silt Loam and Clay Loam under Irrigated Condition 2 equal to 1/4th Ks (Silt Loam = 27 mm/day and Clay Loam = 15.6 mm/day) from June 1st to 30th every year.
Year Region Soil *P+I (1/4th of Ks) in mm
*D (mm) *R (mm) *E (mm) ∗S (mm)
1997
Ottawa
1671 1329
1044 728
1 10
618 589
8 2
1714 1372
1011 718
0 9
657 632
47 13
1772 1430
1078 725
9 33
688 661
-3 11
1590 1248
975 664
8 22
597 575
10 -13
1090 748
662 331
3 17
396 374
29 27
1211 869
734 406
1 17
505 477
-28 -31
1155 813
668 352
0 2
468 446
18 12
970 628
599 284
0 1
386 360
-15 -17
*P+I are precipitation plus irrigation water, D drainage, E evaporation and S is the change in
storage. In terms of the total time to flush out 80% of the initial salt concentration under
Ottawa climatic conditions silt loam and clay took 192 and 462 days, thus showing an
improvement of 65% and 15% respectively under this irrigated condition versus condition 1
(548 and 542 days respectively), whereas at Saskatoon same soil types took 533 and 899 days,
showing an improvement of 64% and 13.7% versus irrigated condition 1 (1487 and 1377 days).
Again total time to flush out 80% of the initial salt concentration did not change with the
increase in the initial salt mass.
37
Under Irrigated Conditions 3.a and 3.b (10 mm/d from May 1st to June 30th and 20 mm/d from June 1st to 30th)
Under this condition applied to sand only two approaches were adopted; first 10 mm/day
water was added to the precipitation data from May 1st to June 30th, totalling 610 mm of
water/year (3.a), whereas in the second approach irrigation water quantity was doubled to 20
mm/day, applied from June 1st to 30th, meaning 600 mm/year (3.b). The total quantity of water
though remained more or less the same for both conditions; however, the timing of application
was different. In the case of Ottawa, sand again did not need any application of water under
both of the irrigated conditions, as it only took 98 days to flush out 80% of the salt under
natural conditions. In the case of Saskatoon it actually took less time for the irrigated condition
3.a versus 3.b (157 days versus 169) which may be due to the fact that irrigation application
started 31 days earlier in case of 3.a versus 3.b. However irrigated condition 3.b still showed a
2.7 % increase in WUE versus the condition 3.a under the Saskatoon climatic conditions (Figure
7). These results are a bit different than those obtained by Persson and Berndtsson (1999) who
evaluated the effects of water application frequency on the parameters derived using steady-
state models by studying bromide transport in an undisturbed soil column of loamy sand
exhibiting fingered flow. Four solute displacement experiments were carried out under quasi
steady-state flow conditions with a mean water flux in all experiments being 1.42 cm/d-1.
Though water flux was constant, application time was different for each interval; one, two,
three, or six times daily. Water content and solute concentration were measured by TDR and
CLT and CDE models were fitted to the solute transport data, with CDE being the best-fit.
Results for the mass recovery were 40% for all depths and did not change with the differing
38
intervals between water applications. WUE was again found directly proportional to the
increase in the initial salt concentration, irrespective of the climatic differences or soil types.
Figure 7. Volume of water in liters used to flush each kg of salt as estimated by HYDRUS-1D for the three different initial solute fluxes for sand, silt loam and clay loam for Ottawa and Saskatoon regions under irrigated condition 3.a and 3.b equal to 10 mm/day From May 1st to June 30th and 20 mm/day from June 1st to 30th every year during the simulation period.
The condition of reaching the maximum steady-state cumulative bottom solute flux at 200 cm
depth was met for all three solute concentrations under Ottawa as well Saskatoon regions.
Coarser nature of the soil giving better soil drainage conditions played an active role (Figure 8).
Drainage remained predominant versus evaporation at Ottawa as well Saskatoon regions,
under 3.a and 3.b irrigated conditions. Under Ottawa climatic conditions drainage was 45%
higher than evaporation under irrigated conditions 3.a, whereas under irrigated condition 3.b it
was 60% higher versus evaporation. Under Saskatoon climatic conditions the gap decreased a
0
2000
4000
6000
8000
10000
39
bit and on average drainage remained 25% higher versus evaporation under irrigated condition
3.a, whereas under condition 3.b it was 55% higher versus evaporation (Table 6).
Figure 8. Maximum cumulative bottom solute flux as estimated by HYDRUS-1D at 200 cm depth after 1826 days for the three different initial solute masses for Sand, Silt Loam and Clay Loam for Ottawa and Saskatoon regions under irrigated condition 3.a and 3.b equal to 10 mm/day from May 1st to June 30th and 20 mm/day from June 1st to 30th every year.
Table 6. Water budget data/year as estimated by HYDRUS-1D for Ottawa and Saskatoon regions for Sandy soils under 10 mm/day (May 1st to June 30th) and 20 mm/day (June 1st to 30th) type irrigated condition 3.a and 3.b every year.
Year Region Irrigation Type *P+I in mm *D (mm) *R (mm) *E (mm) ∗S (mm)
1997
Ottawa
1471 1461
939 1053
0 0
516 398
16 10
1514 1504
967 1083
0 0
542 422
6 -1
1572 1562
1084 1145
0 0
468 402
19 14
1390 1380
903 985
0 0
496 409
-8 -14
890 880
498 629
0 0
376 244
16 7
1011 1001
635 703
0 0
375 303
2 -4
955 945
526 631
0 0
412 305
16 9
770 760
416 532
0 0
368 248
-14 -20
*P+I are precipitation plus irrigation water, D drainage, E evaporation and S is the change in
storage. Weather again played a decisive role, as it took 157 days for the 80% of the initial salt
concentration to flush out of the profile under condition 3.a under Saskatoon climate versus
condition 1 (189 days), showing a improvement of 17% whereas under condition 3.b it took 169
days versus 189 days it took under condition 1, thus showing a improvement of 10.5%.
SUMMARY AND CONCLUSIONS
Ottawa Region
In Ottawa region sand did not need any application of irrigation water as it took only 98 days to
achieve the objective of flushing 80% of the initial salt mass. In terms of WUE for liters of
water/kg of salt flushed, silt loam showed better results under irrigated condition 1, using 300
mm of water/year versus the irrigated condition 2 equal to 1/4th of Ks using 810 mm of
water/year and improved the WUE by using 26% less liters of water/kg of salt flushed. This
means that under the Ottawa climate where the average annual precipitation was 866
mm/year for the five year simulation period, silt loam does not need more than 300 mm of
water/year to have 80% of salt flushed.
41
Clay loam on the other hand showed improved WUE under irrigated condition 2 equaling 1/4th
of the Ks value, using 468 mm of water/year versus irrigated condition 1 using 300 mm of
water/year by using 16.5% less liters of water/kg of salt flushed. This is due to the fact that
there is not a big difference in the water application rate in the case of irrigated condition 1
(300 mm) versus condition 2 (468 mm) for clay loam (468 mm versus 300 mm) as well
considering the finer texture of the soil and the lower value of Ks. So, Under Ottawa weather,
irrigated conditions equal to 1/4th of the Ks are the best option for clay loam.
Saskatoon Region
In Saskatoon region sand gave the best results, under irrigated condition 1 using 300 mm of
water/year, by using 19% less liters of water/kg of salt flushed versus condition 3.a and 16.5%
less versus irrigated condition 3.b, using 610 mm and 600 mm of water/year. It is also
important to note here, that even without any use of irrigation the objective of flushing 80% of
the salt was achieved for sandy soil with average annual precipitation of 300 mm/year under
natural conditions after 554 days of simulation. So if time is not a crucial factor, then the
objective of flushing of 80% of the salt concentration can be achieved without applying any
water in case of sandy soil even under Saskatoon weather conditions.
Silt loam used 5.5% more liters of water/kg of salt flushed in terms of WUE under irrigated
condition 2 equal to 1/4th of Ks versus irrigated condition 1 using 300 mm of water/year;
however, considering the fact that under irrigated condition 1, even though maximum
cumulative bottom solute flux did reach the simulated depth of 200 cm, it failed to reach a
steady-state level and salts persisted in the profile even after the completion of the five year
42
simulation period. Whereas under irrigated condition 2, salts were completely flushed from the
profile before the completion of the simulated period; irrigated condition 2 seems to be a
better option for silt loam under Saskatoon climatic conditions.
In the case of clay loam again under irrigated condition 2 equaling 1/4th of soil Ks, 1% more
liters of water/kg of salt flushed were used versus the irrigated condition 1 using 300 mm of
water/year; however, under condition 1, again the maximum bottom cumulative solute flux
failed to reach a steady-state level and salts persisted in the profile even after the completion
of the simulated period. Under condition 2, salts were completely flushed before the
completion of the simulated period. Considering that as well the small difference in the WUE,
for clay loam under Saskatoon, irrigated condition 2 seems a better option versus condition 1.
For both of the regions for all three soil types under all of the irrigated conditions, the WUE was
found to be directly proportional to the increase in the initial salt concentration. WUE doubled
when initial salt concentration was doubled from 0.002 g/cm3 to 0.004 g/cm3. It again showed a
1.5 times increase, when the salt concentration was further increased 1.5 times from 0.004
g/cm3 to 0.006 g/cm3.
FUTURE QUESTIONS
Even though under the simulated conditions, not only the estimated results of CDE matched
the results of HYDRUS-1D, but the estimated output of HYDRUS-1D for all sixty situations gave
quite logical results; however, the important points to note here are that the simulated
conditions were very simple and only involved a vertically directed one dimensional solute flow,
so this brings the following questions:
43
1. What would be the WUE in terms of liters of water/kg of salt flushed, if the simulated
conditions are more complex such as, a layered heterogeneous soil, two or three
dimensional solute flow or a highly reactive solute with its byproducts?
2. As the estimated results show, WUE was directly proportional to the initial salt
concentration, so the next question is that up to what level in terms of soil salinity, this
increase in WUE continues. Will it level off at some concentration and if so, then at what
concentration. Also will that concentration be similar under different climatic conditions
found at Ottawa and Saskatoon?
3. It is again a question of interest that whether WUE will increase or decrease in the case
of chemically enhanced in situ soil flushing using surfactants, alkalis and polymers,
chemicals commonly used in the petroleum industry (Clark et al., 1988; Nelson, 1989) to
reduce the oil saturation below the residual oil saturation by modifying the pore-level
physical forces that inhibit oil removal, as in this study only irrigation water was used to
flush the salts?
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H. Hardelauf, M. Javaux, M. Herbst, S. Gottschalk, R. Kasteel, J. Vanderborght and H. Vereecken, 2007. PARSWMS: A Parallelized Model for Simulating Three-Dimensional Water Flow and Solute Transport in Variably Saturated Soils, Published in Vadose Zone...
Ihssan Dawood, Laurent Lancelot and Isam Shahrour, 2009. Application of the fractional Advection Dispersion Equation (FADE) at both Steady Unsteady States.
J.E. Ayars, C.J. Phene, R.B. Hutmacher, K.R. Davis, R.A. Schoneman, S.S. Vail and R.M. Mead, 1999. Subsurface Drip Irrigation of Row Crops. A review of 15 years of research at the Water Management Research Laboratory, Agriculture Water Management 42 (...