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IV. Field Studies of Solute Transport

IV. Field Studies of Solute Transport

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176



WILLIAM A. JURY AND HANNES FLUHLER



have been performed over a large surface area, only the most basic of

the process assumptions can be evaluated. Several of these are discussed

below.



A. MEAN SOLUTE

VELOCITY

When a mobile anionic tracer such as bromide or chloride is added as a

pulse to the surface of a field and is subsequently leached by irrigation or

rainfall, the mean velocity V , of the area-averaged pulse can be estimated

by monitoring the downward movement of the chemical with solution

sampling or soil coring. If the field is receiving water at a net flow rate q ,

then the observed field-mean velocity should be comparable to the socalled piston flow velocity V, = q / 8 , where 8 is the mean volumetric water

content, provided that all of the water is participating in the transport.

However, field studies that have made this comparison have come to

somewhat contradictory conclusions, as shown in Table 11.

The study by Biggar and Nielsen (1976) took place on 20 continuously

ponded plots located within a 150-ha field area. The chloride tracer was

added to each plot as a pulse and was monitored by solution samplers to

1.8 m. The measured solute velocity was calculated by fitting the breakthrough curve from a given sampler to the solution of the convection

dispersion model. The 20 calculated solute velocities averaged over all of

the samplers at a given site compared favorably with the piston flow

velocity (slope = 1.09; r 2 = 0.84) estimated from the steady infiltration rate

and the saturated water content of each site.

The studies by Butters et ai. (1989) and Ellsworth et aZ(l991) took place

on the same field at different times. Butters’ study involved applying a

bromide pulse to the entire surface and leaching it downward by spatially

uniform bidaily sprinkler irrigation, while monitoring the pulse with solution samplers in a 4 x 4 grid at different depths between 0.3 and 4.5 m. He

calculated the solute velocity as the ratio of the depth of observation to the

mean solute pulse arrival time, and found it to be substantially less than the

net applied water flux divided by the volumetric water content in the top

1.8 m, below which the estimates agreed. When the arrival time of the

peak concentration rather than the mean arrival time was used to estimate

the solute velocity, the agreement with piston flow was much closer, but

still was about 30% slower near the surface.

The study by Ellsworth et aZ(1991) involved leaching massive plumes of

solute under bidaily irrigation, after the plumes had been injected in the

soil slowly in an approximate cubic configuration 1.5 or 2.0 m on a side.

The plumes were sampled by soil coring at various times after application



Table II

Published Comparisons between the Measured Average Solute Velocity V, and That Predicted from Piston Flow V,

Water application

Reference



Soil



Method



Rate

(cm day-')



Sampling

method



Study area

(m2)



Biggar and Nielsen (1976)



Clay loam



Butters er al. (1989)



Loamy sand



Ellsworth et al. (1991)



Loamy sand



Jaynes et al. (1988)



Sandy loam



Steady

ponding

Bidaily

sprinkler

Trickler ,

sprinkler

Ponding



Rice er al. (1988)



Sandy loam



Ponding



Starr ef al. (1978)



Layered

sandy loam

Loam



Ponding



47



Solution



Sprinkler



0.25-2.5



Soil cores



1.5 X 1.5

(14 plots)

1.8 X 1.8

(4 plots)

24.4 x 18.3

(14 plots)

4.6 X 6.1

(4 plots)

3x3



Layered

sandy loam



Steady

tricklers



Solution



8x8



Wild and Babiker (1976)

Van der Pol et al. (1977)



14.6



Solution



1.1



Solution



1.1



Soil cores



35

1.4



2.0



Solution

Soil cores



6.1 x 6.1

(20 plots)

80 X 80



Depth

V,lV,



(4



1.0



0.3-1.8



1.6- 1.9

1.0

1.o



0.3-1.8

3.0-4.5



0.6-0.7

1.o

0.2



0-0.6

1.0-3.0

0-2.0



0-6



2.1



0-0.6



1.o



0-0.8



1.o



0-1.5



178



WILLIAM A. JURY AND HANNES FL-LER



in an irregular grid encompassing the plot boundaries. The solute velocity

was estimated by the location of the center of mass of the plume, and

agreed almost perfectly with the local piston flow estimate calculated from

the local water application rate and the local volumetric water content

profile.

The studies of Rice et al. (1988) and Jaynes et al. (1988) were performed

on the same field, but different relationships were observed between V, and

Vp. In a bromide leaching study under intermittent ponding, Rice et al.

(1988) measured a solute velocity that was almost five times greater than

the one predicted by piston flow. In contrast, Jaynes et al. (1988) observed

a solute velocity that was only moderately higher than the piston flow

prediction in the surface 0.6 m, and found them to be similar below that

depth. They postulated that soil cracking was responsible for the difference

that they observed between the two studies.

The study of Starr et al. (1978) was conducted under ponding in a layered

soil. They calculated the solute velocity from the arrival time of the solute

peak at a solution sampler location, finding that this measurement was only

about half as large as the piston flow prediction. The authors noted that

very early arrival was detected at deep samplers, indicating that solute was

moving also by preferential flow. In contrast, Van der Pol et al. (1977)

found good agreement between piston flow and solute velocity calculated

from fitting solution sampler observations to the convection dispersion

model in a layered clay soil irrigated by tricklers. Wild and Babiker (1976)

also found good agreement between the two estimates when they calculated solute velocity from the mean location of the solute pulse in a soil

core. When they used the location of the peak to estimate V,, however, it

was less than half of the piston flow prediction.

The research studies summarized above illustrate how the complexity of

the field regime can obscure the identification of even the most basic of

solute transport processes, the mean convection rate. In those cases in

which piston flow underestimates the solute velocity, the most likely reason

appears to be that part of the wetted pore space is not active in transport.

This would not affect the mean arrival of a solute pulse viewed by soil

samplers, because the early arrival would be offfset by the late tailing of an

outflow curve, provided that all of the solute was observed as it passed the

depth of observation (Jury and Sposito, 1985). However, it would affect

the mean position of a resident concentration pulse viewed by soil coring

(Jury and Roth, 1990). Moreover, the solute peak velocity of the flux

concentration viewed by solution samplers is much faster than the mean

velocity of the pulse (as found by Butters et al., 1989), whereas the peak

velocity of the resident concentration pulse can be much slower than the

center of mass velocity (as found by Wild and Babiker, 1976). Therefore,

at least part of the confusion between the solute velocity and the piston



TRANSPORT OF CHEMICALS THROUGH SOIL



179



flow velocitv relates to the frame of viewing the pulse (flux or resident),

and part derives from the way in which solute velocity is defined.

When the area-averaged solute pulse arrives later than predicted by

piston flow, one explanation offered for the disagreement between the two

velocities is that part of the water flux is moving through preferential flow

channels that are not observed by shallow solution samplers, as suggested

by Starr et al. (1978) and Roth et al. (1991). In this case the piston flow

velocity calculated by assuming that all of the water enters the matrix flow

region would lead to an overestimate. This effect could account for the

difference in behavior observed on the same field by Butters et al. (1989)

and Ellsworth et al. (1991), because this field is known to experience

substantial preferential flow in the surface zone (Ghodrati and Jury, 1990).

It is possible that the shallow preferential flow was not detected by the

solution samplers used by Butters et al. (1989), but was detected by the soil

coring method used by Ellsworth et al. (1991).



B. SOLUTE

DISPERSION

With the exception of the study of Ellsworth and Jury (1991a), the

transverse dispersion of a solute plume has not been monitored at the field

scale in the unsaturated zone. Therefore, the discussion below will focus

on the vertical or longitudinal spreading of an area-averaged solute pulse

after application to the surface of a field. As noted above, the dispersion

mechanism can only be revealed by monitoring solute spreading at progressive times by soil coring, or at progressive distances by solution

sampling.

A large-scale study of solute dispersion was conducted by Butters and

Jury (1989) on a 0.64-ha alluvial loamy sand soil. A pulse of solution

containing KBr was added by sprinkler irrigation to the field surface and

was leached downward thereafter by irrigating every other day. The field

had been placed in an initial state of quasi-steady water flux by several

weeks of bidaily irrigation at the same net flux rate ( = l cm day-') as the

flux during the experiment. Solution samplers were located in a 4 x 4 grid

at six depths from 0.3 to 3.0 m, and six additional samplers were placed

at 4.5 m. The samplers (16 or 6) at a given depth were averaged to produce a one-dimensional concentration representing the entire field area as

a function of depth and time. The dispersion hypothesis was tested by fitting the model parameters of the CDE [Eq. (25)]and CLT [Eq. (28)] solutions to the area-averaged solute concentration at the shallowest (0.3 m)

depth, and then comparing the predictions of the models (without further parameter adjustment) with the observed concentrations at greater

depths. Figure 5 shows the comparison at 1.2, 1.8, and 3.0 m, illustrating



0



20



40



60



80



100



Net Applied Water (cm)



120



140



0



20



40

60

80

100

Net Applied Water (cm)



120



140



Figure 5. Comparison between area-averaged solute concentrations from solution samplers with CDE [Eq. (25)] and CLT [Eq. (28)]

model predictions after calibration at 30 cm. Adapted from Butters (1987).



TRANSPORT OF CHEMICALS THROUGH SOIL



181



the close degree of agreement between the CLT model and the data, and

the extremely poor agreement between the CDE model and the data.

Recalling the foundational basis of the two dispersion models discussed in

Section II,C,4, we conclude that on this field the lateral mixing time for

solute is much longer than the travel time of the solute to the 3-m depth.

As a result, the CLT model, which neglects mixing and treats the soil as

consisting of a group of parallel independent columns with different local

water velocities, provides a good description of the pulse spreading with

time. Therefore, in this region the apparent dispersion coefficient is growing proportionally with distance below the point of entry. Indirect confirmation for this interpretation was offered by the transverse dispersion

measurements of solute plumes by Ellsworth and Jury (1991ab), who

found that the lateral spreading of solute was insignificant in the top 3 m

during downward movement of the plume.

Butters and Jury (1989) found that the initial stage of increasing dispersion persisted for considerable time and distance below the surface. The

apparent dispersion coefficient continued to increase until the final soil

sampling, when the pulse was centered at about 13 m, except for a decrease of 40% between 3 and 4.5 m. They attributed this temporary decrease to the influence of a thin layer of soil high in silt content located

between the two regions. Figure 6 shows the apparent dispersivity A = D / V

measured at each depth by refitting the CDE to the area-averaged data.

A contrasting result to the one observed by Butters and Jury (1989) was

found in the rainfall-driven field experiment reported by Roth etal. (1991).

In this study, C1- movement was monitored with 110 solution samplers

located in a grid arrangement extending 10 m horizontally and at depths

between 0.4 and 2.3 m along the side wall of a tunnel under the field. The

soil was extremely heterogeneous in the vertical direction, having eight

distinct horizons in the top 3 m that varied substantially in texture and

structure. The applied chloride pulse split into a main part that moved

slowly downward through the soil matrix, and a series of fast pulses that

moved by preferential flow, bypassing the solution samplers until they

terminated at a subsurface layer of fine texture at about 2.2m. These

pulses were sporadic and only present during the first part of the experiment. However, they transported approximately 58% of the total applied

solute mass. The remaining 42% that moved slowly through the matrix

was detected by the solution sampler network, and could be evaluated by

dispersion models after the mass balance was corrected. In this case, the

longitudinal dispersion of the area-averaged pulse moving within the soil

matrix was described more accurately by the CDE than the CLT. This

finding implies that the transverse mixing time in the soil matrix is much

shorter on this field than on the one in which the study of Butters and Jury



WILLIAM A. JURY AND HANNES F L m L E R



182



0



0 Solution Samplers



0 Soil Cores



d



0



.



.



2



4



6



8



10



12



14



16



Depth (m)

Figure 6. Apparent dispersivityobserved as a function of distance below the soil surface.

After Butters and Jury (1989).



(1989) was conducted. This difference is undoubtedly a consequence of the

soil layering in the field of Roth et al. (1991), which can greatly enhance

lateral mixing of solute.



c. SOLUTE hSORFTION

There have been very few field-scale studies evaluating solute adsorption. Those that have been conducted usually involved pesticides, and

often were confined to shallow monitoring for persistence of residues. One

of the earliest studies that monitored transport was the plot scale experiment of Rao et al. (1974). In this study, picloram was added in solution to

the surface of two plots (2.74 X 2.74 m2) on a silty clay soil and was leached

by separate ponded irrigations thereafter while the pulse was monitored

by solution samplers between 0.03 and 1.43 m. They noted two unusual

features of the leaching process. First, they observed traces of picloram

at all depths after the first 24cm of irrigation. The maximum depth of

appearance (1.43 m) was substantially deeper than that at which they

expected to find the compound. Second, they found that the herbicide



TRANSPORT OF CHEMICALS THROUGH SOIL



183



peak migrated more slowly than they expected it to based on its modest

retardation ( R = 1.6) in this soil. They attributed both phenomena to the

presence of macropores, which caused part of the herbicide mass to move

rapidly, and to significant depths, but which also caused subsequent irrigations to -be less efficient in leaching herbicide that had diffused into the

microstructure.

The first major field study of pesticide movement was reported by Jury

ef al. (1986b), in which a pulse containing C1- and a relatively strongly

adsorbed herbicide (napropamide) was added to the surface of a 0.64-ha

loamy sand field and was subsequently leached with daily sprinkler irrigation for 2 weeks until 23 cm of water had been added. Then, soil cores

were taken at 36 locations on the field to a depth of 3 m and were analyzed at 0.1-m intervals for C1- concentration. Nineteen of these cores

were also analyzed for napropamide concentration.

The laboratory measurements of equilibrium sorption Kd averaged

1.97 cm3 g-' on 36 samples taken near the coring locations, with a coefficient of variation of about 31% (El Abd et al., 1986). This produced a

mean retardation factor R of about 9.6. Based on the variability of the

water flow as revealed by the chloride profile, the authors predicted that

the maximum depth the pesticide could reach if adsorbing to equilibrium

was about 30cm. The pesticide profile contained a large peak at the

surface depth of 10 cm, and approximately 77% of all mass averaged over

the field area was found in the top 30 cm. However, the remaining 23%

was located between 30 and 180cm, with a distinct secondary peak at

about 90 cm. The deep movement was found in all but two of the 19 cores

analyzed for pesticide.

A second study was performed on field plots at the same site by Clendening (1985), who added four chemicals ranging in relative retardation from

R = 1 (bromide) to R = 18 (prometryn), simultaneously in a spray to individual plots, and sampled to 3 m after 10 or 18 cm of water had been

added. The study again showed both shallow and deep leaching of the

strongly adsorbed pesticides, regardless of whether the plots were leached

immediately after pesticide application, or whether a 72-hr equilibration

period was allowed after application.

A final study on the same field was performed by Ghodrati (1989), this

time using three pesticides and chloride as the tracers, while varying the

water application method (continuous or intermittent ponding or sprinkling), the pesticide formulation method (dissolved in water, emulsified

concentrate, or wettable powder), and the condition of the soil surface

(undisturbed or rototilled to 30 cm and repacked). Sixty four 1-m2 plots

received 12 cm of water after chemical application and were subsequently

sampled to 1.5 m with three cores that were consolidated in 10-cm depth



184



WILLZAM A. JURYAND HANNES FLmLER



intervals prior to analysis. There were no significant differences in the

leaching patterns caused by the various water application methods, and all

chemicals showed both shallow and deep leaching patterns. Breaking up

and repacking the surface 30 cm of soil did not extinguish deep leaching of

the pesticides, except when wettable powder was used. Ghodrati (1989)

attributed this to physical filtering of the powder out of the solution by the

smaller pores within the packed soil matrix.

Other studies of pesticide transport under field conditions have reported

higher than expected mobility for compounds that adsorb to the soil. Gish

et al. (1986) reported that the laboratory-measured adsorption coefficient

for atrazine greatly underestimated its mobility relative to bromide in a

field study on a cropped silt loam soil. Hornsby et al. (1990) monitored

bromide together with aldicarb and its metabolites for 200 days under a

citrus grove in a sandy soil in Florida. They observed that bromide and

aldicarb had comparable mobility, even though the latter was predicted to

have a retardation factor of between 1.1 and 2.0 based on laboratory

measurements. Because of the large amount of rainfall in this area, both

the bromide and aldicarb reached groundwater at 7.2 m during the 200-day

monitoring period.

Bowman and Rice (1986) conducted a large study on a sandy loam field

in which a water tracer (PBFA) and a mildly adsorbed herbicide (bromacil)

were applied and leached under periodic ponding. Nearly all of the individual soil cores showed the bromacil to be retarded with respect to the

water tracer, but both were moving faster than predicted by piston flow.

The apparent retardation of bromacil relative to PBFA varied between

0.83 and 4.9 among 85 cores based on model fitting, the former number

indicating that the bromacil moved faster than the companion nonsorbing

tracer.

Tile drains have been used as a means of monitoring nutrient and pesticide leaching losses below cropland for many years (Johnson et al., 1965;

Pillsbury and Johnson, 1965; Devitt et at., 1976). Recently, they have been

used to detect extremely high mobility of dissolved chemicals. Richard

and Steenhuis (1988) reported chloride outflow in the 80-cm-deep drain

effluent of a sandy loam, with the first outflow following application of

the chemical to the surface. Subsequent rainfalls were also accompanied

by additional outflow pulses of chloride. A similar phenomenon was reported by Kladivko etal. (1991) on a tile-drained silt loam soil. Following a

single application to the surface, traces of the pesticides appeared in the

drain tile after only 2 cm of net drainage. These pulses tapered off before

the water quit flowing out of the tile, but returned with subsequent rainfall

events through the season. Everts et al. (1989) conducted an analysis of

four ions (including the very strongly sorbed organic dye rhodamine WT)

moving to the tile drain of a loam soil, and found that traces of all four



TRANSPORT OF CHEMICALS THROUGH SOIL



185



appeared at the effluent during the first irrigation. These concentrations

were as high as 30% of the input solution concentration for the bromide

tracer and 6% for the rhodamine WT, indicating that substantial preferential flow with little mixing was occurring.

The consensus among all of these studies is that transport of sorbing

compounds through field soils cannot be completely described by a linear,

equilibrium adsorption model, even when the spatial variability of the

water flow regime is taken into account. Instead, a portion of the adsorbing

solute displays much higher mobility than could be accounted for with a

laboratory-measured adsorption coefficient. For the more strongly adsorbing compounds, extremely high mobility is often regarded as an indirect

observation of preferential flow.



D. P~FERENTIALFLOW

Field studies of preferential flow have consisted of two different types:

direct monitoring through dye trace observations, and indirect monitoring

by sorbing or nonsorbing chemical tracers. In the latter case, the concept of

preferential flow is invoked to explain unusual mobility relative to the

extent of transport expected to occur if the compound was undergoing

normal convection and adsorption.

Although preferential flow of water and chemicals has been recognized

as a contributor to transport under field conditions for well over 100 years

(Schumacher, 1864; Lawes et al., 1882), its significance was largely overlooked during the years of experimentation and modeling under laboratory

conditions. When researchers returned to the study of transport through

soil with natural structure, awareness of this phenomenon returned. With

the development of the monolith lysimeter, Kissel et al. (1974) were able

to monitor chemical leaching in a saturated swelling clay soil, observing

5% recovery of a chloride pulse in the effluent at 132 cm after only 5 cm

of drainage. Starr et al. (1978) observed the movement of dyes through a

layered (sandy loam over coarse sand) field site leached by ponding, and

concluded that the majority of the water moved through narrow channels

5-20 cm in diameter that were produced through flow instabilities at the

layer interface. A similar finding was observed by Quisenberry and Phillips

(1976), who observed a marked increase in the coefficient of variation of

solute concentration below a plow pan. They concluded that the solutes

were moving in isolated channels below the interface.

Aside from unstable flow arising from transport across layered soil

interfaces, preferential flow has been shown to arise because of geometric

voids in the soil. Ritchie et al. (1972) used a visible dye to demonstrate that



186



WILLIAM A. JURYAND HANNES FLWLER



much of the water moving through a swelling clay soil was migrating

through vertical cracks. Omoti and Wild (1979) used florescent dyes in a

weakly structured loamy sand to determine that earthworm channels,

fissures with apertures between 0.05 and 0.10 mm, and loosely packed soil

were all acting as conduits for the rapid transport of the adsorbing dyes

they used in their field study. Johnston et al. (1983) used rhodamine WT to

mark the pathways of water recharge to ground water in a lateritic weathering profile in Australia. They found that the water moved relatively

uniformly through the 1-2 m of permeable sandy gravel comprising the

surface zone. However, in the underlying clay-rich saprolites, the transport

occurred mainly within cylindrical vertical channels up to 30 mm in diameter filled with coarse-textured material. The origin of the channels was

believed to be decayed tree roots. Scotter and Kanchanasut (1981) reported movement of chloride to a mole drain at 0.4 m under continuous

ponding within 5 min after introduction of the chemical into the infiltrating

water. Dye tracing revealed that root and worm channels and occasional

fracture planes were carrying the flow.

The above studies for the most part describe preferential flow through

structural voids. However, even structureless sandy soils have been found

to exhibit preferential flow, arising from a variety of causes. Saffigna et al.

(1976) used rhodamine WT dye to reveal a highly nonuniform water

infiltration pattern under a potato crop on Plainfield sand in Wisconsin.

They attributed the nonuniformity to stemflow along the plant and runoff from the sidewall of the hills on which the potatoes were planted. A

later comprehensive study was conducted on Plainfield sand by Kung

(1990a,b), who excavated a field plot after dye application. By detailed

observation of the dye flow patterns, he was able to show that water

funneled into increasingly narrow zones at greater depths, eventually carrying the bulk of the flow in a small fraction of the cross-sectional area.

Kung (1990b) postulated that the funneling mechanism was discrete coarse

sand lenses that acted as barriers to downward movement, causing the

water to flow around them and focus at the edges. The high permeability of the sand allowed very high local flows in the focused regions. A

similar phenomenon was reported by Ghodrati and Jury (1990) on the

Etiwanda loamy sand field where deep leaching of pesticides had been

reported by Jury et al. (1986b,c). Using a soluble anionic dye (amine red),

Ghodrati and Jury (1990) observed that preferential flow regions comprising a small fraction of the cross-section were moving more than twice as

deep as the main front under both ponded and sprinkler irrigation in

undisturbed field plots. When the surface was tilled and repacked, distinct

fingers formed at the plow layer.

Certain porous median contain two or more distinct flow domains that



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