1 INTRODUCTION
Agricultural
irrigation on the Heifangtai terrace has induced frequent loess
landslides, and more than 70 large-scale landslides have occurred since
the 1960s (Xu et al., 2014). Numerous studies have been conducted to
investigate the loess landslides in Hefangtai. Their results indicate
that the infiltration of irrigation water causes a rise in the
groundwater table (GWT), which triggers a large number of landslides on
the terrace (Cui, Pei, Wu, & Huang, 2018; Qi, Xu, & Liu, 2018; Xu et
al., 2011; Xu, Qiao, Wu, Iqbal, & Dai, 2012). Since the groundwater
depth in the Loess Plateau is mostly 30-80 m (Zhu, Li, Peng, & Zhang,
1983), the GWT is higher than the gully surface runoff and the confined
water head of the bedrock, so there is no lateral groundwater recharge
(Li, 2001). Therefore, the rise of the GWT is only induced by vertical
infiltration, i.e. atmospheric precipitation and agricultural
irrigation. These studies do not agree on how surface water infiltrates
into the GWT and recharges the groundwater. Several scholars have
suggested that preferential water flow paths such as loess fissures,
sink holes, and macropores are the main methods of recharge (Xue, 1995).
However, these features are observed only on the edge of the loess
tableland in the unloading area (Xu et al., 2011). In addition,
continuous water flow cannot occur in these preferential paths, because
the water flowing into these paths is quickly absorbed by the
surrounding soil due to differences in the hydraulic gradient, which
indicates that these preferential paths have difficulty connecting with
the groundwater. Li et al. (2013)
found that the depth of rainfall infiltration was very limited based on
manual drip experiments. In addition, they found that there was an
obvious increase in the moisture content of the deep paleosol while the
moisture content of the upper soil changed only slightly, which
indicates that the unsaturated seepage in the loess recharges the
groundwater. However, few scholars have been able to draw on any
systematic research in unsaturated seepage.
Zimmermann
et al. (1966) used the tracer method for a groundwater recharge study
and proposed that soil water is transported by piston flow in the
homogeneous vadose zone. Subsequently, the tracer method was widely
applied in the study of water flow and solute transport in the vadose
zone. Davis et al. (1980) gave a comprehensive review of injected
tracers, in which
anionic
tracers (Cl‾, Br‾, and I‾) were used for a broad range of ground-water
tracing applications. Cameron and Wild (1982) used36Cl‾, 15NO3‾, and
HTO (titrated water) to label water molecules, studied solute transport
under field conditions following the application of irrigation water and
under winter rainfall conditions. Bowman (1984a and 1984b) found that
low molecular weight anions (Cl‾, Br‾, and NO3‾) do not
interact with most natural porous media and seem to be an ideal
artificial tracer for soil water studies. Porro
and Wierenga (1993) analyzed water
flow and solute transport in soil using tritiated water and bromide in a
large scale soil column. Cook et al. (1994) estimated the recharge rate
of groundwater in southern Australia by tracing the soil water movement
with 36Cl, 3H, and chloride. Gates
et al. (2011) estimated the groundwater annual recharge amount in Yanan,
Zhifanggou agricultural land using a chloride mass balance. Huang et al.
(2017) used soil chloride profiles and multiple tracers to investigate
groundwater recharge in the arid western Ordos Basin, NW China. Cl‾ and
Br‾ are widely used as tracers to analyze water flow and solute
transport, and good results have been obtained from soil column tests
and field experiments.
Therefore, in order to understand unsaturated seepage in the loess
vadose zone, a 6 m high remolded loess column test using chloride (Cl‾)
and bromide (Br‾) as the artificial tracers was carried out under the
effect of irrigation, and a numerical simulation was crested using
COMSOL Multiphysics. The findings are important to further studies in
the field of unsaturated seepage and in the investigation of landslide
mechanisms in Heifangtai.
2 STUDY AREA
Located in the central part of Gansu Province, China (Fig.1), Heifangtai
is situated at the confluence of the Yellow River and the Huangshui
River,
on
the class IV terraces of the Yellow River. It extends east to west from
the Huangshui River to Fangtai and north to south from the Moshi Gully
to the Yellow River. The lithologic profile of this area is as follows
from top to bottom (Fig. 2). (1) The top layer is mantled by Malan loess
(26–48 m) with a loose structure and well-developed vertical fissures.
(2) An alluvial clay layer (3–19 m) with a dense structure and low
permeability is present below the loess layer. (3) Overlying the bottom
is a fluvial gravel layer (1–6 m) mixed with sands, which has good
permeability. (4) The bottom bedrock layer consists of mudstone and
sandstone partings. The primary loess used in the column test was
collected from the Heitai area. In order to avoid the influence of crop
roots on the loess, the sampling depth was below 4 m. In addition, the
saturated gravimetric moisture content of the primary loess was 35%,
which was measured in-situ. A series of indoor physical property tests
was carried out on the loess to obtain its basic physical properties
such as the natural density, moisture content, specific gravity, and dry
density (Table 1). Then, the ground up soil was air-dried to obtain the
residual moisture content and the particle grading curve (Fig. 3).
3 METHOD
3.1 Soil column test
3.1.1 Test preparations
A large number of scholars have observed loess vertical profiles after
rainfall or artificially simulated rainfall infiltration experiments in
loess areas (Li, Li, & Vanapalli, 2016; Tu, Kwong, Dai, Tham, & Min,
2009). It has been shown that the depth of direct rainfall infiltration
is limited and rarely exceeds 4 m. Therefore, considering the
persistence of infiltration under the effect of irrigation and based on
a better observation of unsaturated seepage in the loess vadose zone, a
6 m high soil column was applied.
In order to study the infiltration and the ion tracing under the effects
of irrigation in the soil column, the device shown in Fig. 4a was
designed to carry out the test in the laboratory. The device mainly
consists of a soil column, moisture probes, water potential probes, and
an automatic data acquisition system. The resolution of the EC-5 soil
moisture probe is 0.1%, the resolution of Teros-21 soil water potential
probe is 0.1 kPa, and the collection frequency of both probes is 1 min.
The Em-50 data acquisition system automatically stored the moisture
content data and the matric suction collected by the probes. The soil
column was constructed from a transparent plexiglass cylinder, with an
outer diameter of 320 mm, an inner diameter of 300 mm, a height of 6 m,
and a sealed bottom. Fifteen small holes with a diameter of 1 cm were
made in the cylinder’s wall for the probes, and 12 sampling holes with a
diameter of 2.5 cm were made for real-time sampling.
3.1.2 Test procedures
After air drying, the loess was sifted using a
sieve (d=2 mm). Distilled water was
added to the loess, and it was fully stirred until the target
gravimetric moisture content of the loess (13.7%) was achieved with a
dry density of 1.40 g/cm3, which is approximately the
same as that of the in-situ loess. The column was filled with the
prepared loess, which was tamped down and shaved every 5 cm. In
addition, the probes were placed into the center of the column through
the 1 cm holes. The moisture probes were installed along the column at
heights of 0.15 m, 0.55 m, 1.15 m, 1.55 m, 2.15 m, 3.15 m, 4.15 m, 5.15
m, and 5.4 m; and the water potential probes were installed along the
column at heights of 0.15 m, 1.15 m, 2.15 m, 3.15 m, 4.15 m, and 5.15 m.
The sampling holes were installed along the column at heights of 0.35 m,
0.95 m, 1.35 m, 1.95 m, 2.35 m, 2.95 m, 3.35 m, 3.95 m, 4.35 m, 4.95 m,
and 5.35 m. The remaining gaps in the 1 cm holes and the sampling holes
were sealed with glass glue. After every 20 cm of testing loess was
added, the soil column was allowed to stand for 12 hours to reach a
uniform moisture content. This was continued until the height of the
remolded soil column reached 5.6 m. The column was prepared as shown in
Fig. 4b.
After the preparation of the column, two layers of filter paper and a 5
cm gravel layer were placed on the top of the soil. This was done to
prevent erosion of the surface loess when water was added. Before the
infiltration, an ion chromatograph was used to measure the initial Cl‾
and Br‾ concentrations of the remolded soil sample and the infiltration
water sample. The soil samples were air-dried, triturated, and filtered
using a sieve (d=1 mm). Thereafter, 50 g of filtered soil sample was
added to a 250 ml volumetric flask, and then, the flask was filled with
deionized water, stirred, and shaken for 1 hour. The mixture was allowed
to settle for 24 hours. After filtering the mixture through a 0.22-μm
membrane, the supernatant was stored in a 10 ml reagent tube until
analysis using an ion chromatograph. The results indicate that the Cl‾
concentration of the remolded loess was 141 mg/L and its Br‾
concentration was 0 mg/L. No Cl‾ or Br‾ were present in the infiltration
water.
To analyze the water flow and solute transport of Cl‾ and Br‾ under the
effect of irrigation, a falling head permeability test was designed to
control the infiltration in the column, that is, an infiltration head of
16 cm was added at a fixed time every day. The infiltration water was
prepared with NaCl (AR) and KBr (AR) ensure a Cl‾ concentration of 50
mg/L and a Br‾ concentration of 100 mg/L. According to the in-situ
irrigation in Heifangtai, the infiltration test was carried out for 45
days. The test was divided into three stages according to the
advancement of the wetting front and the real-time data measured by the
probes.
Stage
I (irrigation ⅰ):
saturated
wetting front advance (1–13 days);
Stage II (non-irrigation): without adding infiltration water (14–24
days); and
Stage III (irrigation ⅱ): the second irrigation of moist soil (25–45
days).
As the infiltration was conducted, samples were collected in real-time
using the sampling holes along the column. After each 100 g soil sample
was collected, the sampling hole was quickly sealed with glass glue to
prevent the influence of external air pressure on the column. Four
stages of sampling were conducted. (1) Samples were collected at
different depths as the saturated wetting front advanced (1–10 days);
(2) samples were collected at
different depths after the saturated wetting front reached the bottom of
the column on the 11th day; (3) samples were collected
at different depths in Stage II on the 20th day; and
(4) samples were collected at different depths after the second
irrigation stable in Stage III on the 45th day. After
processing the samples using the methods described above, the samples
were placed in an ion chromatograph to measure their ion concentrations,
and the columns’ Cl‾ and Br‾
profiles were obtained over time.
3.2 Numerical simulation of the column
test
Due to the limited number of sampling times, the process of solute
transport could not be demonstrated in detail, and thus Comsol
Multiphysics was used to simulate the process in detail. Based on Comsol
Multiphysics, which is a multi-physical field coupling software, a 3D
soil column was constructed using a 2D axisymmetric component and a time
dependent study was used to investigate the variation in the moisture
content and ion concentration with respect of time. The Richards’
Equation interface was used to analyze the water flow in the porous
media, and to model the infiltration in the column in combination with
the Van Genuchten (1980) Equation. Many
efforts to simplify and improve the modeling of water flow through
variably saturated media have produced a number of variations on the
Richards’ equation since its appearance. The form of the equation that
COMSOL Multiphysics solves is very general and allows for time-dependent
changes in both the saturated and unsaturated conditions
(Bear,
2012; Bear, 2013):
. (1)
Where the pressure, p (Pa), is
the dependent variable. In Equation (1), Cm (1/m)
is the specific moisture capacity, Se is the
effective saturation, S (1/Pa) is the storage coefficient,k s (m/s) is the hydraulic permeability, μ(m2/s) is the fluid dynamic viscosity,k r is the relative permeability, ρ(kg/m3) is the fluid density, g(m/s2) is the acceleration due to gravity, D(m) is the elevation, and Qm is the fluid source
(positive) or sink (negative).
The transport of the dilute species interface was used to calculate the
ion concentration in the porous media, and the solute transport was
modeled by taking into consideration the solute sources.
The following equations for the
concentrations, ci , describe the transport of
solutes in a variably saturated porous medium for the most general case,
when the pore space is primarily filled with liquid, but also contains
pockets or immobile gas (Bear, 2012; Bear, 2013):
. (2)
On the left-hand side of
Equation
(2), the first three terms correspond to the accumulation of the species
within the liquid, solid, and gas phases, respectively, while the last
term describes the convection due to the velocity field u (m/s).ci (mol/m3) is the
concentration of species i in the liquid,cP, i is the amount adsorbed onto (or desorbed
from) the solid particles, and cG, i is the
concentration of species i in the gas phase.ρb (kg/m3) is the bulk density,ρb=(1-εp )ρ . εp is the
porosity, and ρ(kg/m3) is the
solid phase density. For saturated
porous media, the liquid volume fraction θ is equal to the
porosity εp , but for partially saturated porous
media, they are related by the saturation s as θ =sεp . The resulting gas volume fraction isav = εp − θ =
(1-s )εp .
On the right-hand side of Equation (2),
the first term introduces the spreading of the species due to mechanical
mixing, diffusion, and volatilization into the gas phase. The tensor is
denoted as DD (m2/s) and the
effective diffusion as De(m2/s). The last two
terms on the right-hand side describe the production and consumption of
the species, respectively. Where Ri is a reaction
rate expression, which can account for the reactions in the liquid,
solid, or gas phase, and Si is an arbitrary
source term, e.g., due to a fluid flow source or sink.
The research method and physical field were chosen by taking into
consideration the solute transport in saturated or partially saturated
porous media under the effects of diffusion, convection, dispersion,
adsorption, etc. Fig. 4c shows the schematic diagram of the soil column
model. The model contains two layers. The upper layer is a 16 cm water
layer, which has a set Cl‾ source
concentration of 50 mg/L, and the lower layer is a 5.6 m loess layer
with a set Cl‾ initial concentration of 141 mg/L. The initial moisture
content of the model was defined as the pressure head. In addition, the
left side (r = 0) was set as the axisymmetric boundary, the right side
as the no-flow boundary, the upper surface as the pressure head
boundary, and the interlayer as the pervious water layer boundary. It
should be noted that the bottom boundary was set as the small flux
boundary becuase the column’s bottom contained sampling holes and sensor
holes. The simulation parameters
were obtained from the results of the column test, in which the
saturation permeability coefficient Ks was
determined from the average velocity of the saturated wetting front
advance, and the parameters a and n in the VG model were
obtained using the pressure plate method. Its soil water characteristic
curve (SWCC) fitting is shown in Fig. 5. The remaining parameters were
chosen according to the optimal solution of a large number of simulation
results. All of the parameters used in the simulation are listed in
Table 2.
4 RESULTS
4.1 Moisture content and matric
suction
The infiltration began with an obvious saturated wetting front (Fig. 6).
In Stage I (Figs. 7a and b), and the maximum moisture content of the
soil at different depths would be reached at a certain time and would be
greater than the saturated volumetric moisture content (49%). This is
attributed to the change of the soil mass from dry to wet in a short
time, which caused a large amount of water to accumulate. Therefore, the
data measured by the moisture probes were the maximum values within the
measuring range, which was greater than the saturated volumetric
moisture content of the test loess and caused a significant fluctuation
in the moisture content of the soil layer within 2 m depth. Meanwhile,
the matric suction decreased rapidly, and then gradually stabilized.
Once the water began to flow downward into the dry soil again, the
moisture content in the upper part decreased as the accumulated water
gradually dissipated. Finally, the moisture content remained at a stable
unsaturated value. The infiltration water reached the bottom of the
column about 10 days later. Due to the sealed bottom of the container,
it continued to accumulate and formed the GWT. A significant saturated
wetting front advanced and moisture content variations in Stage I. In
order to simulate the in-situ irrigation in Heifangtai, infiltration
water was no longer added in Stage II (14–24 days). As shown in Fig. 7c
and Fig. 7d, the moisture content of the soil layer above a depth of
3.15 m continued to decrease during Stage II. In addition, the variation
in the soil moisture state also caused the moisture content of the upper
soil layer to fluctuate significantly. The moisture content in the
shallow vadose zone changed significantly with the addition of water.
However, the moisture content of the deep soil layer remained the same,
and the content at a depth of 4.15 m increased to near saturation at a
slow rate, and the matric suction gradually became stable. This
indicates that cutting off the water supply was conducive to forming
perched water at a depth of 4.15 m. After the 25th day
(Stage III), the irrigation head at 16 cm was added again at a fixed
time every day. Figure. 7e illustrates that only the probes above 3.15 m
had a short response to the second irrigation, and the moisture content
no longer fell back after this, but continued to increase. Meanwhile,
the moisture content of the perched water layer at 4.15 m, which acted
as an impervious layer to divide the soil column into two parts,
remained saturated and stable. The moisture content of the upper soil
gradually increased but still remained constantly low because of the
reapplication of the irrigation head, while there was no change in the
high moisture content of the lower soil. Although the apparent moisture
content remained unchanged in the deep vadose zone, water still flowed
there, i.e., unsaturated seepage occurred, because the irrigation head
added to the top of the soil column continued to fall each day.
Through the analysis of the above different infiltration stages, we
concluded that unsaturated seepage in the deep vadose zone is
characterized by a stable apparent moisture content and a low velocity.
In addition, because of these characteristics, the direct infiltration
depth of the surface water into the column under the effect of
irrigation is less than 4 m, which is the same as the observed results
obtained under the condition of rainfall (Li et al., 2016; Tu et al.,
2009). The reason for this is that below a depth of 3.15 m, the moisture
probes did not respond to the second irrigation, so that the advance
rate of the wetting front could not be calculated according to the
change in the moisture content in Stage III (Fig. 8). Therefore,
traditional methods of monitoring the moisture content and matric
suction cannot accurately describe the process of unsaturated seepage.
4.2 Chloride and bromide ion
profiles
In addition to the monitoring of the moisture content and the matric
suction, the Cl‾ and Br‾ profiles at different infiltration stages were
also analyzed using the column test. The Cl‾ initial concentration in
the loess was 141 mg/L and the Br‾
initial concentration was 0 mg/L, indicating that as shown in Fig. 9a
and Fig. 9b, a large amount of Cl‾ is leached while Br‾ mainly
accumulates in the soil layer. On the 11th day of
Stage I, the wetting front reached the bottom of the column. At this
time, the solute front did not reach the bottom along with the wetting
front, and a large amount of Cl‾ was detected near a depth of 4 m. It
was observed that the ions in the column were not transported in real
time with the wetting front, but had a certain lag time, which is
consistent with the results of Porro and Wierenga (1993) and Ghuman
(1980), i.e., the solute fronts significantly lag behind the moisture
fronts. This phenomenon was also clearly reflected in the advance of the
saturated wetting front (1–10 days). Only a small amount of Cl‾ was
detected below a depth of 3 m, indicating that the Cl‾ in the soil was
almost completely leached out. In the upper layer (above a depth of 3
m), a certain concentration of Cl‾ was detected, which is consistent
with the results obtained from the lower layer because the advance of
the solute front is highly dependent on the soil moisture content
(Ghuman, Verma, & Prihar, 1975; Kirda, Nielsen, & Biggar, 1973).
Namely, the moisture content of the upper layer fluctuated significantly
as the rapid solute front advanced rapidly, so the Cl‾ was not fully
leached from the soil. The moisture content of the lower layer increased
gradually to saturation while the solute front advanced slowly, so the
Cl‾ could be completely leached out. However, the Br‾ profile did not
follow the same pattern, probably because the soil had been accumulating
Br‾, and less than half of the prepared Br‾ in the infiltration water
was absorbed by the soil in Stage I. In Stage III (Figs. 9c and d),
since the Br‾ concentration at each depth was close to 0 mg/L, it was
difficult to obtain the trend in Br‾ and use it to describe the
unsaturated seepage. In the Cl‾ profile, the Cl‾ in the upper layer was
almost completely leached, and its concentration was close to 0 mg/L;
while in the lower layer, the accumulation intensified, and then the Cl‾
started to become enriched toward the deep vadose zone. The above ion
profile analysis reveals that although the apparent moisture content of
the vadose zone was unchanged (Fig.10), the Cl‾ continued to be
transported downward and its concentration at different depths varied
with the water flow, which demonstrates the existence of unsaturated
seepage.
4.3 Simulation results
Excessive sampling would affect the internal pressure of the column,
resulting in an imbalance between the water and air in the vadose zone,
and consequently, the ion profile obtained from the column test would
not fully show the unsaturated seepage. Therefore, the transport of Cl‾
during infiltration was simulated.
Fig. 11 shows the temporal variation in the Cl‾ trajectory in the
longitudinal section and the concentration variation in the 3D soil
column obtained from the simulation. At the critical time, the
simulation results were basically consistent with the column test
results, e.g., Cl‾ reached a depth of 4 m on the 10thday, and remained at a depth of 5 m on the 45th day.
The simulated results are idealized, while the column test includes many
human and machine errors, e.g., the influence of external air pressure
and temperature on the column during the sampling process, the
mechanical error when the ion concentration is detected by an ion
chromatograph, etc. However, by
comparing
the test and simulation results for the Cl‾ at different infiltration
stages (Fig. 12), we concluded that the trends of the ion profiles are
basically consistent, so the model parameters obtained in the column
test are applicable to this simulation.
Fig. 13 summarizes the variation in the Cl‾ profile with respect of time
from the numerical simulation. In this figure, it can be seen that Cl‾
is enriched at a certain depth every day. In order to show its
enrichment law more simply and accurately, the peak of the Cl‾
concentration was extracted to obtain an enrichment curve (Fig. 14). The
daily data is shown in Table 3. In Stage I, the solute front rapidly
advanced downward, and the daily peak in the Cl‾ concentration varied
significantly due to the large fluctuation in the moisture content in
the saturated wetting front mentioned in Section 4.2. Interestingly, the
peak concentration began to decrease when the enrichment depth reached
about 4 m, which is consistent with the depth at which the perched water
formed in the column test. Both the solute front and the wetting front
changed significantly at 4 m depth because the water began to flow into
the deep vadose zone in the form of unsaturated seepage. The advancing
rate of the solute front gradually decreased and the peak concentration
decreased at an almost constant rate each day in Stage II. The
irrigation head was reapplied at the beginning of Stage III, but the
solute front and the peak concentration remained basically the same as
that in Stage II, which indicates that the second irrigation did not
affect the deep unsaturated seepage. In the simulation, on the
30th day, the enrichment depth of the Cl‾ began to
increase at a slow rate, and the decreasing rate of the peak
concentration only fluctuated slightly. The reason for this may be that
the GWT began to rise during this process because of the accumulation of
water at the bottom of the column.
5 Discussion
5.1 Tracing the effects of different
ions
Because the initial concentration of Cl‾ and Br‾ in the test loess are
different, the infiltration water was prepared with different
concentrations of Cl‾ and Br‾. This was done to fully balance the
tracing effects. Although a large amount of Cl‾ was leached while the
Br‾ mainly accumulated in the soil layer in Stage I, as is shown in the
Fig. 15, both of the ion profiles exhibit similar tracing effects on the
11th day, both ions have a strong transport ability
with water in the loess, and most of the ions are transported rapidly to
the bottom of the column, which indicates that both Cl‾ and Br‾ can be
used as soil water tracers in the column test. Davis et al. (1980) and
Bowman (1984a) also recommend the use of Cl‾ and Br‾ as common soil
water tracers, especially Br‾, because Br‾ is usually present in natural
water at levels of less than one percent of the Cl‾ concentration and is
not absorbed by most soils. During the second irrigation (Fig. 15b),
since the Br‾ concentration at each depth was close to 0 mg/L, it was
difficult to obtain the law of Br‾ to describe the unsaturated seepage.
The reason for this is that it is difficult for the low concentration of
Br‾ to precipitate out and be adsorbed onto the surface of the soil
during recrystallization, so a large number of Br‾ was not detected in
the soil samples. However, Cl‾ exhibits a regular downward transport and
its tracing effects are better (Fig.15a), which may be due to its
initial concentration in the loess. Of course, the differences mentioned
above in the ion tracing effects discussed in this paper are only for
the test loess, but different test results can also be obtained because
of the ion adsorption capacity of the soil, the effect of convection,
hydrodynamic dispersion, etc. Therefore, no in-depth discussion is
presented in this paper. However, the above analysis still suggests the
following. (1) The adsorption capacity of Cl‾ in the test loess is
stronger than that of Br‾. (2) During the in-situ monitoring of the
water flow in the vadose zone in Heifangtai, recommend that Cl‾ be used
as the soil tracer and Br‾ be used as the water tracer.
5.2 Characteristics of unsaturated
seepage
According to the results of the column test and simulation, under the
effect of irrigation, the direct infiltration depth of the surface water
into the column is less than 4 m and a large amount of water accumulates
at a depth 4 m, forming perched water, which acts as an impervious layer
and divides the column into two parts. Then, the infiltration into the
deep vadose zone is dominated by unsaturated seepage and its
characteristics are as follows: (1)
the apparent moisture content remains unchanged; (2) it has a low
velocity;
(3) it is not affected by the second irrigation; (4) the effect of the
rising GWT is small and limited.
According to the moisture content profile (Fig. 10), the apparent
moisture content remains unchanged during unsaturated seepage, and the
remaining characteristics will be discussed in this section. Solute
transport is substantially dominated by hydrodynamic dispersion (Bear,
2013), so for a large velocity, mechanical dispersion plays a
significant role and distributes the solute along the direction of the
average flow velocity. Its effect is related to the pore velocity. In
Stage I (Fig. 14), when the soil moisture content (above a depth of 3 m)
fluctuates significantly (Fig. 7a), the average velocity in the soil
pores is larger, and the solute front can advance downward quickly; when
the soil moisture content (below 3 m depth) slowly increases to
saturation (Fig. 7a), the average velocity gradually decreases in the
pores, and the advancement rate of the solute front decreases. However,
in the Stage III, the infiltration is dominated by unsaturated seepage
and the apparent moisture content remains unchanged, so the solute front
advances at an almost constant rate, which can be seen from the change
in the Cl‾ enrichment depth in Table 3. Therefore, unsaturated seepage
occurs at low velocities.
For a low velocity, the proportion of molecular diffusion in the total
diffusion increases. The peak concentration decreases at a constant rate
in Stage II (Table 3), and when the irrigation head is reapplied at the
beginning of Stage III, although the molecular diffusion has been
aggravated, the peak concentration remains almost the same as that in
Stage II, which indicates that the second irrigation does not affect the
deep unsaturated seepage.
As the GWT rises, the Cl‾ enrichment depth starts to increase, while the
advancement direction of the solute front remains downward. This occurs
because the rate at which the GWT rises is very slow and the dilution
only affects the transition zone between the saturated zone and the
unsaturated zone. Molecular diffusion always makes the solute
concentration uniform in the pore channel, so the decreasing rate of the
Cl‾ peak concentration only fluctuates slightly. It can be concluded
that the influence of the rising GWT on unsaturated seepage is small and
is limited to the transition zone.
6 Conclusions
The main conclusions of this study are as follows.
(1) The column test results show that the unsaturated seepage in the
deep vadose zone is characterized by a stable apparent moisture content
and a low velocity, so it is difficult to investigate it using real-time
monitoring of the moisture content and matric suction. However, based on
our analysis of the different infiltration stages, the direct
infiltration depth of the surface water into the column is less than 4
m, and as the daily irrigation head falls, the existence of unsaturated
seepage can be indirectly proven.
(2) The solute transport in the deep vadose zone was investigated using
artificial tracers. When the apparent moisture content remains almost
unchanged, the regular downward enrichment of the ions demonstrates the
existence of unsaturated seepage. In addition, according to the
different tracer effects of the chloride and bromide ions, the
adsorption capacity of the chloride ions in the test loess is stronger
than that of the bromide ions. We recommend that chloride ions be used
as the soil tracer and bromide ions be used as the water tracer during
the in-situ monitoring of unsaturated seepage in Heifangtai.
(3) To analyze the chloride and bromide ion profiles using the results
of the column test and simulation, since the solute transport is
essentially affected by hydrodynamic dispersion, the results suggest
that the unsaturated seepage is not affected by the second irrigation,
and the effect of the rising GWT is small and is limited to the
transition zone. The rise in the groundwater table, which is caused by
the stable seepage in the loess vadose zone, induces landslides in
Heifangtai terrace.
References
Bowman, R. S. (1984a). Evaluation of some new tracers for soil water
studies1. Soil Science Society of America
Journal, 48 (5), 987-993.
http://doi.org/10.2136/sssaj1984.03615995004800050007x
Bowman, R. S. (1984b). Analysis of soil extracts for inorganic and
organic tracer anions via high-performance liquid
chromatography. Journal of Chromatography A, 285 (3),
467-477.