multiple tracing of fast solute transport in a drained grassland soil
TRANSCRIPT
Multiple tracing of fast solute transport in a
drained grassland soil
Christian Stamm a,*, Raphael Sermet a,1, Jorg Leuenberger a,Hans Wunderli a, Hannes Wydler a, Hannes Fluhler a, Mathias Gehre b
aInstitute of Terrestrial Ecology, Soil Physics, Grabenstrasse 3, CH-8952 Schlieren, ETHZ, SwitzerlandbUFZ-Umweltforschungszentrum Leipzig-Halle, Sektion Analytik, Permoserstr. 15, 04318 Leipzig, Germany
Received 4 July 2001; received in revised form 20 February 2002; accepted 17 May 2002
Abstract
Fast transport of fertilizers and other agrochemicals into subsurface drainage systems has been
recognized as a serious threat to surface waters. We report on a tracer experiment carried out on a
7.3� 20 m2 plot on a loamy grassland soil to determine the flow paths to a tile drain at 1 m depth.
The experiment consisted of a series of consecutive tracer applications including seven solutes and
liquid manure that were applied either on the entire plot or on limited bands. Based on the
discharge behavior under natural conditions, we estimated the effective hydraulic conductivity of the
subsoil to be in the order of 8–29 cm day � 1. Under experimental conditions, the soil
transmitted 120 mm day� 1 into the subsurface drain and two vertical profiles without producing
surface runoff. Only part of the soil water, corresponding to 6–27 mm of the soil depth,
contributed to the fast hydrological response. The transport of the tracers was very fast. Within 7–
16 h after application of the conservative Br � , Cl� and HDO and the slightly sorbing substances
brilliant blue (BB) and amino-G-acid (AG), these tracers reached relative concentrations in the
outflow between 19% and 35% of the input concentrations. From the mass balance for water and
solutes, it follows that the tracers were quickly transported over lateral distances of several meters.
The manure constituents dissolved reactive P (DRP), NH4+ and Cl� , applied as liquid manure on the
surface on a 1 m wide band above the tile drain, reached the drain within 5 min after application.
After the early peak of DRP and NH4+ , their concentration in the drain decreased quickly to
background levels, whereas Cl � exhibited a second peak. Despite the fast transport and the small
soil volume conducting water and solutes, the interaction between irrigation water and soil matrix
0016-7061/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0016 -7061 (02 )00178 -7
* Corresponding author. Fax: +41-1-633-11-23.
E-mail addresses: [email protected] (C. Stamm), [email protected] (R. Sermet),
[email protected] (J. Leuenberger), [email protected] (H. Wunderli),
[email protected] (H. Wydler), [email protected] (H. Fluhler), [email protected] (M. Gehre).1 Present address: ProCert Safety AG, Thunstr. 17, 3000 Bern 6, Switzerland.
www.elsevier.com/locate/geoderma
Geoderma 109 (2002) 245–268
was intimate enough to retain the two sorbing tracers. From the stained flow paths, the hydrologic
behavior of the field under natural conditions and the hydrometric data during the experiment, it
follows that the fast lateral tracer transport occurred mainly close to soil surface and not through the
subsoil. Only in the immediate vicinity of the tile drain and of two lateral pits at the edge of the
experimental plot water was redirected downwards and discharged from the tile drain and the bottom
parts of the profiles, respectively. Hence, effluent from tile drains may not be representative for water
reaching the subsoil or shallow ground water in undisturbed soils.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Tile drains; Agriculture; Preferential flow; Surface runoff; Sprinkling experiment; Dye tracers
1. Introduction
The flow patterns occurring in natural and managed soils are in many, if not most cases,
highly irregular. This irregularity is often called preferential flow, which means that part of
the water and solutes move rapidly through a small portion of the soil without much
exchange with the surrounding soil matrix over a substantial length of the flow domain
(Fluhler et al., 1996). The flow velocities are large and even strongly sorbing solutes like
pesticides (Flury, 1996; Flury et al., 1995; Kladivko et al., 2001), phosphorus (Addiscott et
al., 2000; Heckrath et al., 1995; Hergert et al., 1981a,b; Sims et al., 1998; Stamm et al.,
1998) or radionuclides (Bundt et al., 2000) may reach groundwater or subsurface drainage
systems before being sorbed or biologically degraded.
Due to the large sampling volume, subsurface drainage systems are sometimes taken
as ideal experimental systems for transport phenomena in soils (Czapar and Kanwar,
1991; Flury, 1996; Gish et al., 2000; Lennartz et al., 1999). Such studies rely on the
assumption that sampling large volumes averages out the small-scale spatial hetero-
geneities. Classical drainage theory assumes a vertical transport through the unsaturated
and a predominantly lateral flow through the saturated zone towards the drain. If this
concept was true and preferential transport into the drains observed an interconnected
system of vertical and lateral preferred flow paths should exist. This has been shown,
e.g., for some heavy clay soils (Inoue, 1993; Ruland et al., 1991) or for forest soils
(Luxmoore et al., 1990).
We have found preferential P transport into subsurface drains in weakly structured
loamy soils (Stamm et al., 1998). Based on infiltration experiments, we concluded that
vertical worm burrows are the main macropore structures in our study region. In one case,
we observed lateral transport of a dye tracer and we could attribute this to a dense network
of well-preserved ancient root channels in the saturated zone. Such structures are rather
exceptional features. Therefore, it is not evident what the lateral preferred flow paths
towards a tile drain are in weakly structured soils. The purpose of this paper is to
investigate the fast transport into a subsurface drain in such a soil and to study how the
lateral distance to the drains affects the fast solute transport. We carried out a sprinkling
experiment on a drained grassland plot. The flow paths were assessed based on the
breakthrough behavior of spatially separated tracers, hydrometric data and infiltration
patterns in the soil.
C. Stamm et al. / Geoderma 109 (2002) 245–268246
2. Study site and methods
2.1. Site description
The study site is situated in a subcatchment (‘‘Kleine Aa’’) of Lake Sempach in the
central Swiss Plateau at an altitude of 585 m a.s.l. (Schweizerische Landestopographie,
coordinates 659,100/221,000). The mean air temperature is about 7.5 jC and the mean
annual precipitation amounts to 1200 mm. The soil has developed from glacial till (Wurm
glaciation) and is classified as a loamy, frigid to mesic Oxyaquic Eutrochrept (Soil Survey
Staff, 1992), characterized by periodic waterlogging as well as strong dryout of the profile.
Some of the soil properties are summarized in Table 1.
The tile drains of the field were installed in the 1940s to a depth of 100–155 cm.
Natural soil material was used as backfill. Even after the 50 years of settlement, the bulk
densities at 5–15 cm depth were different for samples from the backfill material
(1.24F 0.08 mg m� 3) compared to undisturbed soil (1.36F 0.03 mg m � 3). Below that
depth, no differences in bulk densities could be detected. The water characteristic
exhibited slight differences between undisturbed soil and backfill material down to a
depth of 35 cm and disappeared below (Fig. 1).
The experimental plot was part of a 1.02 ha field used as permanent grassland for
many years (Fig. 2). The usual practice in this region is to cut the grass five to seven
times during the growing season (April–October). After each mowing, liquid manure is
applied. For some weeks each year, the parcel is used as a pasture for cows. The average
slope of the experimental plot was 2.5% parallel to the tile drain and 5% perpendicular
to it.
2.2. Methods
The experiment was carried out on a plot of 146 m2 (Fig. 3). From January to March
1996, we monitored the temporal and spatial variability of the groundwater table within
the entire 1 ha field. In June, we compared the hydrological response of the experimental
plot under natural conditions with that of the entire parcel. From August 12 to 19, we
conducted the multitracer experiment on the experimental plot. During the final period
Table 1
Soil properties of the field
Horizon Depth
(cm)
Bulk density
(mg m� 3)
Clay
(%)
Sand
(%)
Organic
mattera (%)
pHb
A (A) 0–10 1.25 – – – 5.6
B (B) 10–30 1.50 28 29 2.7 5.5
Bw (B-Sw) 30–80 1.55 27 27 0.6 6.4
C (Sd-C) > 80 1.75 17 40 0.6 7.9
The nomenclature refers to the US soil taxonomy (Soil Survey Staff, 1992). The symbols corresponding to the
German nomenclature are added in parentheses (Arbeitsgrupped Bodenkunde, 1982).a Determined from weight loss after digestion with H2O2.b Measured in 0.01 M CaCl2 solution.
C. Stamm et al. / Geoderma 109 (2002) 245–268 247
Fig. 1. Soil water characteristics of the undisturbed soil (six samples per depth from one profile) and the backfill
material (10–15 samples from two profiles) for three depths.
Fig. 2. Experimental area with tile drains, contours, location of piezometers and location of the experimental plot.
C. Stamm et al. / Geoderma 109 (2002) 245–268248
(August 20–September 3), we excavated 17 soil profiles to study the stained infiltration
patterns of the dye tracers.
2.2.1. Discharge and water table measurement in the entire field
We installed 29 piezometers arranged in three downslope rows over the entire parcel
(Fig. 2). The locations of the piezometers were chosen such that half of them were in
immediate vicinity to the drains whereas the others were in-between two neighboring
drains. From January 10 to March 11, we determined the level of the water table by weekly
manual measurements. Daily precipitation values for this winter period were obtained
from a weather station 2 km away.
In June, discharge from the main drain leaving the field was measured with an inductive
flow meter (DISCOMAG TDMI 6731, Endress & Hauser, Reinach, Switzerland). Rainfall
intensity was recorded on-site by a rain gauge at 15 min intervals. Samples of the drainage
water were taken by an automatic sampling device (ISCO 2900, Lincoln, NEB) whenever
a predefined discharge level was exceeded. Samples were filtered within 24 h using 0.45
Am filters (Schleicher & Schuell, FP 030/20, Keene, NH). Sampling and discharge
measuring from the experimental plot are described below.
2.2.2. Sprinkling experiment
The experimental plot had an area of 20� 7.3 m2 covered by a tent during the
experiment in August. Parallel to the long, lower side of the plot, there was a tile drain
installed 50 years ago (see above) at 100 cm depth (Fig. 3) at a lateral distance of 0.5 m
from the lower edge of the irrigated area (inside the plot). Two pits, at the upper (PU) and
the lower (PL) ends, each 1.15 m deep, were excavated over a distance of 2.5 m at the
Fig. 3. Set-up of the experimental plot.
C. Stamm et al. / Geoderma 109 (2002) 245–268 249
short sides of the plot to install instruments (see below). The profile walls were protected
and stabilized by planks. The rest of the boundary was undisturbed soil.
The plot was sprinkled by a moving spray bar that was oriented perpendicular to the tile
drain. Sprinkling was stopped at 0.25 m from the profile walls. We irrigated the plot at a
rate of approximately 5 mm h� 1 (Fig. 5). The sprinkler was running on two aluminum
tracks and consisted of 24 nozzles (TeeJet 80010LP, Spraying Systems, Wheaton, IL)
installed 30 cm apart and 30 cm above ground. The sprinkling solution was applied at a
pressure of 2.2 bar inside the nozzles.
Before applying the tracers, we irrigated the plot with tap water for 2 days. Afterwards,
we sequentially applied different tracer combinations for periods of 7–16 h (Table 2). In
order to detect lateral transport towards the tile drain, we simultaneously applied different
tracers on two bands parallel to the drain (Fig. 2): the lower band (LB) covered the surface
directly above the tile drain, while the upper band (UB) was about 2.5 m from the drain. The
first tracer combination consisted of the three conservative tracers Br� , Cl � and HDO
(Table 2). With the second combination, slightly sorbing fluorescent dyes were applied
[amino-G-acid (AG; 7-amino-1,3-naphthalene disulfonic acid, Aldrich, Buchs, Switzer-
land), brilliant sulfaflavine (SF; Sigma, St. Louis, MO) and sulforhodamine B (SB; C.I.
45100, Siegfried, Zofingen, Switzerland)]. Next followed 20 l of liquidmanure spread with a
watering can on the area of LB just above the tile drain. Finally, we applied two dye tracers
[acid red 1 (AR; C.I. 18050, Sigma) and brilliant blue FCF (BB; C.I. 42090, Hoechst,
Frankfurt, Germany)] to stain the infiltration pathways. Immediately afterwards, the
irrigation was terminated. Between the different tracer combinations, tap water was applied.
The tracer concentrations in the sprinkling solution and in the manure are given in Table 3.
Discharge from the tile drain was measured by a tipping bucket. Surface runoff was
collected right above the tile drain being the lowest location of the soil surface inside the
plot and measured by a tipping bucket. To prevent tracer transport at the soil surface from
area UB towards the drainage area, a PVC panel was inserted 5 cm into the topsoil at the
lower side of the area UB. The seepage rate from the upper profile PU into the upper pit
Table 2
Experimental schedule of the tracer applications
Time (days) Tracer
Lower tracer band (LB) Upper tracer band (UB) Main area (MA)
0.00–2.00 Water Water Water
2.00–2.67 HDO, NH4Br NaCl, NH4Br NH4Br
2.67–2.92 Water Water Water
2.92–3.63 No application No application No application
3.63–4.13 Water Water Water
4.13–4.80 Amino-G-acid and
sulforhodamine B
Brilliant sulfaflavine Water
4.80–5.13 Water Water Water
5.13 Liquid manure Water Water
5.13–6.00 Water Water Water
6.00–6.29 Brilliant blue Acid red 1a Water
Start of the experiment (t= 0) was on August 12 at 4 PM. For the experimental design, see Fig. 3.a Applied only over a distance of 2 m from the lower end. The rest of UB was not irrigated during this period.
C. Stamm et al. / Geoderma 109 (2002) 245–268250
was determined by the rate of water table increase in the deepest part of the pit. This rate
was measured occasionally using a stopwatch and yardstick installed in the pit. Seepage
from the lower profile PL was measured with bucket and stopwatch at one location in the
pit, where the water was channeled and flowing into the deepest part.
Samples from the drainage effluent were taken by automatic samplers (ISCO 2700 and
2900). The sampling interval varied from 3 min (for the conservative tracers) or 5 min (all
other tracers) immediately following the changes in tracer composition to 60 min later on.
Samples were taken manually from the seepage water at the locations PU and PL. The
samples were filtered within few hours (0.45 Am; Schleicher & Schuell, FP 030/20) and
stored at 5 jC under dark conditions. Samples for HDO analysis were stored in sealed
vials to prevent gas exchange. Samples for the fluorescent dyes were stored in brown snap-
cap vials.
Two-rod TDR probes of 15 cm length were installed horizontally at four depths (15, 30,
50 and 80 cm) in the two profiles PU and PL. The center of each rod was placed at 50 cm
horizontal distance from the profile faces. In both profiles, two columns of TDR probes
were installed in duplicates, one row adjacent to the tile drain and the second at 2.5 m
uphill. Tensiometers were inserted into the profiles PU and PL corresponding to the layout
of the TDR probes. The center of the cup (High Flow Porous Ceramic Cub 653� 1B1M3
1bar, Soil Moisture Equipment, Goleta, CA) was placed at 50 cm from the profile. Within
the plot, additional tensiometers were installed vertically at seven locations. At each
location, one or two tensiometers were installed at the same depths as in the profiles. The
matric potential was measured every 5 min by pressure transducers (26 PCCFA3D,
Honeywell, Minneapolis, MN).
2.2.2.1. Infiltration patterns. We prepared and photographed 17 soil profiles of 1�1 m2
stained by the dye tracers to analyze the infiltration patterns according to Forrer et al.
Table 3
Tracer concentrations in the sprinkling solutions
Tracer Mean concentration (mg l� 1) Coefficient of variation (%)a
Conservative tracers
Br 60.8 1.3
Cl 1670 1.9
HDO 2244 5.5
Fluorescent dyes
Amino-G-acid 17.2 8.3
Sulforhodamine B 17.9 1.7
Sulfaflavine 12.3 4.5
Manure
Cl 1830 < 0.1
NH4 218.8 b
DRP 57.3 11.1
Nonfluorescent dyes
Acid red 1 12,000 b
Brilliant blue 4100 13.7
a Measured in the different cups sampling the sprinkling water on the plot.b Only one sample was measured.
C. Stamm et al. / Geoderma 109 (2002) 245–268 251
(2000). After preparing a profile face as smooth as possible, the pit was covered with a
light tent to produce diffuse illumination. A uniform gray frame was mounted around the
profile to correct the pictures afterwards for inhomogeneous illumination. We used
Ektachrom Elite (100 ASA) and Ektachrom Panther (200 ASA) film for color slides.
On five profiles, the dye patterns were categorized according to structure types (e.g., worm
burrows or plant roots) that had caused the preferential dye transport.
2.2.2.2. Laboratory analysis. Phosphorus was determined as soluble-reactive P accord-
ing to Vogler (1965) and NH4+ was measured according to DIN 38,406 (1993). We
measured brilliant blue concentrations with a spectrophotometer at 630 nm (PU 8620,
Philips Scientific, Cambridge, UK) and the fluorescent dyes with a spectrofluorimeter
(Jasco FP 821, Tokyo, Japan). Amino-G-acid (excited at 355 nm) was determined from the
emission difference between 400 and 420 nm, sulforhodamine B (excited at 565 nm) from
the difference between 575 and 580 nm and brilliant sulfaflavine (excited at 420 nm) from
the difference between 505 and 525 nm. We measured the anionic tracers (Cl� and Br� )
by an ion chromatograph (DX-100, Dionex, Sunnyvale, CA). The HDO concentrations
were measured by mass spectroscopy according to Gehre et al. (1996a,b).
3. Results
3.1. Natural conditions
3.1.1. Spatial and temporal variation of the water table in the field
As expected, the level of the water table was substantially higher between two drains
than in the immediate vicinity of the drains (Fig. 4). The drainage system was functional
despite its age of about 50 years. During February 1996, snow (62.5 mm) fell on frozen
ground and melted at the end of the month. The melting water raised the water table from
February 6 to 28 close to the surface followed by a pronounced decrease during the dry
period from February 28 to March 11. The dynamics of the water table position (Fig. 4)
allows estimating the effective hydraulic conductivity of the subsoil in that field. This
estimation is based on the equation for the water table dynamics in the half-space between
two drains developed by Youngs (1999):
Hm ¼ H0mð1þ tKeff
S ða � 1ÞðH0mÞ
a�1S�1D�aÞa�1 ð1Þ
where Hm is level of the water table between two drains relative to the depth of the drain
expressed as a function of time t, Hm0 is the value of Hm for a given time zero, a is the ratio
between the depth Zb of an assumed impermeable layer below the drain and D the half of
the distance between the two drains, Kseff is the effective saturated hydraulic conductivity
and S is the specific yield, which is the change of the water storage in the profile given a
change of the level the water table of a given unit.
Assuming a constant value for S over the profile and over time, we can estimate an
upper limit for S based on the amount of precipitation and the change in the water table
from the dry condition on February 6 to the wet condition after snow melt on February 28.
C. Stamm et al. / Geoderma 109 (2002) 245–268252
The values range from 0.091 to 0.112 mm mm� 1 for eight pairs of piezometers. Solving
Eq. (1) for K seff, using the obtained values for S and varying Zb over a range from 0.01 to 3
m allows estimating of the hydraulic conductivity. Based on the data for the eight pairs of
neighboring drains with the required time series, we obtained for the draining period
(February 28–March 5) the mean values of 1.3F 0.3 and 0.6F 0.1 cm day � 1 for the two
extreme values of Zb, respectively. For the prolonged draining period until March 11, the
corresponding results were of a similar order of magnitude, being 0.6F 0.1 and
0.36F 0.09 cm day � 1, respectively.
These values correspond to the behavior of the integrated subsoil drainage system. In
order to estimate the hydraulic properties of the soil itself, one has to consider the nonideal
behavior of real drains (Dierickx, 1999) and account for the influence of the limited
openings for the water to enter real drain tubes. Based on the analysis of Youngs (1974),
the saturated conductivity of the soil Kssoil can be expressed as a product of the
conductivity of the soil drainage system and a factor accounting for the flow geometry
and the entry resistance b:
KsoilS ¼ Keff
S
2pb þ logð DpR0
Þlogð D
pR0Þ ð2Þ
Fig. 4. Spatial and temporal variability of the water table along the second transect comprising piezometers 11–
20. The date of February 6 corresponds to a dry period, February 28 to the end of the melting period and March 5
to the first measuring date of the dry draining period.
C. Stamm et al. / Geoderma 109 (2002) 245–268 253
with R0 being the radius of the tile drain (5 cm). Assuming a large value for the entry
resistance of the tiles of five (Youngs, 1974), the estimated hydraulic conductivity of the
subsoil ranges between 8 and 29 cm day � 1. These values correspond well with data
reported from the literature (Leij et al., 1996) for loamy soils.
3.1.2. Natural rainfall event
A single natural rainfall event occurred during the observation period of June 1996
(June 10). It was a short but intensive thunderstorm. Prior to the storm, there was no
discharge from the experimental plot at 1 m depth. In contrast, the main drain at the field
outlet was discharging, probably caused by the larger depth (1.55 m) of the drain as
compared to that of the lateral plot drain and caused by the uphill position of the
experimental plot (Fig. 2).
The drainage outflow responded quickly to rainfall. Peak discharge was measured 15
min after peak precipitation intensity. Discharge from the plot drain decreased rapidly after
precipitation had ceased. Flow from the whole field decreased more slowly and continued
for days at rates higher than those prior to the event. The discharge/rainfall ratio was much
larger for the whole field (13.7% for the period of 3 h during which discharge occurred
from the plot) than for the plot (1.3%). The about 10-fold larger discharge/rainfall ratio for
the whole field suggests that substantial lateral flow towards the tile drain was induced in
those parts of the field where the water table was above the tile drain.
As found in a previous study (Stamm et al., 1998), the dissolved reactive P (DRP)
concentration increased with discharge. The values from the experimental plot (maximum
value 680 Ag l� 1) were about 2.5 times higher than those from the entire field. This
indicates that at the larger scale the outflow of subsurface water low in P was contributing
relatively more than at the plot scale.
3.2. Sprinkling experiment
The sprinkling period lasted for 6.25 days with one major interruption of 18.5 h after
2.9 days due to technical problems with the sprinkler (Fig. 5). Additional short
interruptions of 2–5 min were due to switching tracer tanks and a electrical power failure.
The sprinkling rate varied between 4.44 and 5.50 mm h� 1. The spatial variability of the
sprinkling rate perpendicular to the tile drain had an average coefficient of variation of
8.9%. During the entire experiment, 95,300F 3600 l of water and tracer solutions were
applied, which is equivalent to 652F 25 mm. This corresponded to about 1.5 pore
volumes of the soil down to the depth of the tile drain. The large amount of irrigation was
necessary to apply several tracers sequentially under conditions as close as possible to
steady state.
Between days 2.33 and 2.90, the length of the sprinkled area was reduced from 20 to
14.2 m. The nonirrigated area was adjacent to the upper profile PU. Irrigation was
completely stopped between days 2.90 and 3.69. Thereafter, the period prior to this
interruption is called the first stage of the experiment and the period after the interruption
until the definite end of irrigation is called the second stage.
Immediately before the start of irrigation (t= 0), the matric potential at 80 cm depth was
about � 4.5 kPa at both profiles and about � 3.7 kPa in the center of the plot. After the
C. Stamm et al. / Geoderma 109 (2002) 245–268254
start of sprinkling, a water table developed quickly. In the middle of the plot, the water
table was close to soil surface at a depth of about 15 cm. It declined towards the two
profiles PU and PL as expected. The influence of the tile drain was much less than that of
the two profiles. Even at 0.8 m distance from the drain, the level of the water table was not
affected by the drain such as to be measurable. After about 12 h, the hydraulic head
remained practically constant for the rest of the experiment, except during the mentioned
interruption period (Fig. 5).
3.2.1. Discharge behavior
Discharge occurred from the tile drain and from the two profiles PU and PL. Based on
the hydraulic conductivity estimated from the piezometer data (see above), one can
calculate the outflow from the two profiles and the tile drain according to Hooghoudt’s
(1937) equation for an elliptical water table. The expected outflow is in the order of 2–7
mm day� 1 or only few percent of the applied amount of irrigation. Despite this limited
discharge capacity, we did not observe any surface runoff at the lowest point of the plot
during the entire experiment. The outflow from the two profiles as well as from the tile
drain was much larger than expected based on the estimated hydraulic conductivity.
Discharge from the profile PL contributed the major portion of the total outflow volume.
For the last day of the experiment, we have data of all three outflow components to
Fig. 5. Sprinkling intensity, flow rate from the tile drain and level of the water table in the center of the plot during
the experiment of August 1996. The discharge data values indicate moving averages over five measurements.
C. Stamm et al. / Geoderma 109 (2002) 245–268 255
compare. At 16:00 h of that day, 5.7 l min� 1 (or 56 mm day � 1) were measured from
profile PL, discharge from the tile drain was 1.9 l min� 1 (or 18.7 mm day � 1) and from
PU only 1.5 l min� 1 (or 14.7 mm day � 1). With the sprinkling rate being 12.2 l min � 1
(or 120 mm day � 1), this results in a discharge ratio of 75% for the entire plot.
Total discharge from the plot during the whole experiment was 392 mm. The increase in
water content measured by TDR accounted for 44F 11 mm and evapotranspiration (ET) for
less than 10 mm. There remained 207F 28 mm or 32F 4% of the input that was not
accounted for. The loss includes deep seepage, lateral flux out of the plot and nondetected
seepage draining from profile PL into the sinkhole of the pit where we could not measure
discharge. Influx from outside the plot could be excluded because the weather was dry.
According to the three discharge outlets, the whole plot can be divided into three
subcatchments CatchPU, CatchPL and CatchD drained by the outlets upper profile (PU),
lower profile (PL) and the tile drain (D), respectively. The relative sizes of these
subcatchments can be estimated from the discharge volumes and were therefore 16%
for CatchPU, 63% for CatchPL and 21% for CatchD. These figures are obtained by
assuming that the 32% of the water not accounted in the water balance (see above) affected
the three subcatchments proportionally to their relative size. Hence, the contributing area
of the lower profile PL CatchPL was very large and almost four times the size of CatchPU.
Considering the moderate slope of only 2.5% between the two profiles, such a large
discrepancy was surprising. The average width of the contributing area for the tile drain
was 1.5 m.
Discharge was very sensitive to any changes in the irrigation rate. It decreased within
minutes after interruptions of sprinkling (Fig. 5). From the total outflow after stopping
irrigation, one can calculate the volume Vcontr of the water stored in the soil that was
contributing to discharge. Dividing this outflow volume through the area of the subcatch-
ment results in the average thickness of Vcontr. For the tile drain, this contributing volume
was extremely small: after sprinkling was stopped at the end of Stage 1, only 57 l were
discharged until outflow ceased completely. This corresponds to only 2.7 mm of water or
the amount irrigated within about 30 min (at reduced sprinkling distance of 14.2 m). At the
end of the experiment, a total discharge of 102 l was measured, which was equivalent to 3
mm of water over the entire plot length of 20 m. From the lower profile PL, 1236 l of
discharge was measured at the end of the experiment until the flow rate dropped below 0.1
l min � 1 or less than 5% of the maximum flow rate. This volume is equivalent to 13.4 mm
water or a soil layer of about 27 mm thickness.
Given the contributing water volume Vcontr, the expected mean travel time of an inert
tracer is given as the ratio between Vcontr and irrigation rate I. With I of 5 mm h� 1 and the
volumes as given above, the mean travel times into the tile drain and towards the lower
profile PL, respectively, are expected to be very short. For the drain, this expected value
was less than 1 h and about 3 h for PL. These estimates will be compared to the measured
data in Section 3.2.2.1.
3.2.2.1. Solute transport. The tracer applications started on day 2 with the conservative
tracers Br � , Cl � and HDO (Table 2). Bromide was applied onto the whole plot, HDO
onto the lower tracer band (LB) above the tile drain and Cl � onto the upper tracer band
(UB). The comparison of the three tracers should therefore indicate whether lateral
C. Stamm et al. / Geoderma 109 (2002) 245–268256
transport towards the tile drain occurred. In absence of a fast lateral transport, only HDO
and Br are expected to exhibit a fast breakthrough in the tile drain. It was further expected
that the two tracers should reach the same relative concentrations.
The shape of the breakthrough curves for Br and HDO were indeed very similar (Fig.
6). The concentrations started to increase after about 1 h and peaked at the end of the tracer
application with a slight increase of HDO after the end of application. After the end of the
tracer application, the concentrations decreased quickly. Chloride was only measured at
low concentrations never exceeding 1.5% of the input concentration. After 16 h or about
0.18 pore volumes being applied, the maximum Br� concentration was as high as 32% of
the input concentration, underlining the quantitative importance of the fast solute transport.
The lower value of 19% for HDO indicated that Br� was rapidly transported laterally
towards the tile drain from areas where no HDO was applied. Although the tracer
concentration in the effluent increased rapidly to large values, the measured travel times
were much larger than those expected based on the contributing water volume Vcontr (see
above). This is only possible if the volume relevant for the solute transport is larger than
Vcontr. Hence, despite the fast transport and the very wet conditions, there was significant
mixing between the irrigation water and the soil solution. However, only part of this
mixing volume contributed to the fast hydrological response. The plot behaved like a dual-
porosity system.
Fig. 6. Breakthrough of the conservative tracers Br� , Cl� and HDO in the drainage effluent.
C. Stamm et al. / Geoderma 109 (2002) 245–268 257
Two distinct concentration peaks occurred for both tracers during their application (Fig.
6). Such peaks are completely unexpected during a constant tracer input. These peaks
occurred immediately after the two short periods when the sprinkler was mechanically
blocked and stood still for a few minutes. During these periods, sprinkling continued but
the tracer application was concentrated at one location. This suggests that not only the
flow rate but also the tracer concentrations in the effluent responded very sensitively to
changes of the irrigation conditions. The two peaks may well have been caused by local
ponding due to the high local irrigation intensity during the stop of the spray bar.
In the effluent from profile PL, the conservative tracers were detected at high
concentrations as well. In Fig. 7, the breakthrough of Br � in the tile drain and in the
effluent from profile PL is shown. The concentrations in the two different effluents were in
phase, although the maximum concentration in the effluent from profile PL was about
25% higher than in the outflow from the tile drain. Chloride was also measured at high
concentrations (up to 8% of the input concentration) in the effluent from PL. These high
tracer concentrations show that large amounts of the applied substances were transported
quickly over lateral distances of several meters.
The behavior of the fluorescent dyes (AG, SB and SF) applied on day 4 (Table 2)
confirmed the results of the conservative tracers. The two substances AG and SB were
Fig. 7. Breakthrough of Br� in the drainage water and the effluent from the lower profile PL.
C. Stamm et al. / Geoderma 109 (2002) 245–268258
applied to the band LB above the tile drain, while SF was sprinkled on UB. Despite
Kd values determined in batch experiments ranging from 3.1 (AG) to 23.7 l kg� 1 (SB),
the breakthrough of the tracers was very fast, and the relative concentrations reached
values of 31% and 9% for AG and SB in the tile drain and 9% for SF in the effluent
of PL: The very low concentrations of AG and SB in the water from PL ( < 1x)
and of SF in the drain (2%) show that the two tracer bands belonged to separated sub-
catchments.
At the end of the experiment, we applied two dye tracers to stain the infiltration
pathways. BB solution was sprinkled onto the lower tracer band (LB). Fifteen minutes
after the beginning of the application, its concentrations in the drainage effluent started
to increase slightly. After 1 h, an almost linear increase set in for about 5 h. The relative
BB concentrations of 21% reached at the end of application after 7 h were higher than
that for HDO after 16 h (19%) and similar to that of AG applied earlier on the same
tracer band. Accordingly, the dye export was also quite high, amounting to 15% of the
applied mass during the time of application (Table 4). Acid red 1 was applied on a
length of 2 m on band UB. It was visually detected in the outflow from PL but not
analyzed in detail.
Corresponding to the fast increase of the concentrations of the tracer after their
application, these values decreased rapidly after the irrigation solution had been changed
to tap water. Basically, the decrease was the inverse of the breakthrough curve. The
increasing tracer concentrations C(t)pred can be expressed as a function of the decreasing
values C(t + Tend)meas after stopping application at time Tend:
CðtÞpred ¼ CðTendÞ � Cðt þ TendÞmeas; for 0VtVTend ð3Þ
In Fig. 8, we have shown the comparison between the measured and the predicted
values for Br, AG and SB in the drainage effluent. Generally, the agreement is good with
Table 4
Tracer export from the plot and the lower profile (PL)
Tracer Export from plot (percent of applied mass)
During
entire
experiment
Duration of
sampling period
after application (h)
During tracer
application
Duration of
application (h)
HDOa 43 74 12 16
Brb 29 74 14 16
Bra 36 74 24 16
Cl (manure)a 19 32 – pulse
Amino-G-acida 36 40 20 16
Sulforhodamine Ba 13 40 12 16
Sulfaflavineb 41 40 4 16
Brilliant bluea 19 4 15 7
DRPa 6 32 – pulse
NH4 (manure)a 12 32 – pulse
a Export through the tile drain.b Export through the tile drain and exfiltrating from the profile PL.
C. Stamm et al. / Geoderma 109 (2002) 245–268 259
certain deviations: AG exhibited strong tailing after the application, which was not
observed for the breakthrough. SB showed a shift between the observed and the predicted
concentrations due to a retarded concentration maximum after the end of the tracer
application. Nevertheless, the slopes of the increasing and decreasing branches were very
similar.
At the beginning of the irrigation, the DRP concentration increased with increasing
flow rate in the drainage effluent. The DRP peak concentration (280 Ag l � 1) was rather
small compared to the maximum value of about 680 Ag l � 1 reached during the natural
event of June 10, 1996. One possible explanation could be the effect of Ca2+ -rich tap
water used for the irrigation, which may have reduced the P mobility (Scharer et al., 2001).
Although discharge continued to increase, the DRP concentrations decreased after some
hours and reached a fairly constant value of about 100 Ag l� 1 until manure was applied on
day 5 of the experiment.
Immediately after the manure application, the DRP concentration increased dramati-
cally. In the first sample collected at the drain outlet 5 min after spreading liquid manure
on the soil surface of the lower tracer band LB, the DRP concentration had already
increased substantially. After 10 min, it reached a maximum value of 800 Ag l� 1.
Thereafter, the values rapidly decreased and reached background concentrations after
Fig. 8. Comparison of the observed concentrations of the tracers AG, Br� , and SB with the predicted values
according to Eq. (3).
C. Stamm et al. / Geoderma 109 (2002) 245–268260
some 5–6 h (Fig. 9). About 6% of the DRP spread with the manure were lost to the tile
drain during the experiment (Table 4). We observed a similar breakthrough for NH4+ and
initially also for Cl� , which is abundant in manure (1830 mg l � 1 in our case). In
contrast to DRP and NH4+ , Cl � exhibited a second concentration peak, lasting much
longer than the first one. The relative amount of Cl � lost to the tile drain was larger
(19%) than that for NH4+ (12%) and DRP (6%) as expected in case of a conservative
solute.
3.2.2.2. Infiltration patterns. We applied two dye tracers to stain the infiltration
patterns. BB was intended to stain the flow paths in the immediate vicinity of the tile
drain, AR should reveal possible lateral flow paths towards the tile drain. However, it
turned out that AR lost its visibility after some days and could not be analyzed properly.
Therefore, we only present results obtained from the infiltration patterns from the lower
tracer band irrigated with BB solution.
The flow patterns were highly irregular with stained patches down to below 100 cm
depth (Fig. 10). On five profiles, we visually classified the stained areas according to the
type of soil structure that was the likely cause of preferential flow. Earthworm burrows
Fig. 9. Breakthrough of the manure constituents Cl� , NH4+ and DRP in the drainage effluent.
C. Stamm et al. / Geoderma 109 (2002) 245–268 261
were the most abundant types of these structures. Second were aggregate surfaces (Table
5). The other categories were encountered much less frequently. Most of the stained
structures were concentrated in the upper parts of the profiles and their numbers declined
strongly with depth. Worm burrows were the only structures with sizable frequency at the
depth of the tile drains.
Table 5
Predominantly stained soil structures
Structure type Number Percentage of
stained structures
Earth worm burrows 182 46.7
Surfaces of aggregates 94 24.1
Root channels 31 7.9
Animal burrowsa 20 5.1
Stone surfaces 17 4.4
Undefined structures 46 11.8
Total 390 100
a Animals other than earthworms, taxa not identified.
Fig. 10. Bitmap of the infiltration pattern of brilliant blue on a vertical soil profile perpendicular to the tile drain.
The dark areas indicate the dye-stained areas.
C. Stamm et al. / Geoderma 109 (2002) 245–268262
4. Discussion
In our experiment, solutes were quickly transported vertically as well as laterally over
considerable distances of several meters. This fast transport affected not only a small
percentage of the solutes but also up to 20–40% of the applied tracer mass. Because the
hydrometric data showed that the contributing water volume was small—in the order of
few millimeters to centimeters only—one may ask where this transport took place within
the soil.
In an earlier experiment at a smaller scale (2 m2), we observed brilliant blue to enter an
open drainage ditch at a lateral distance of 4.5 m from the lower end of the plot after only
75 min or 12.5 mm of irrigation (Stamm et al., 1998). Inspecting the soil profiles
afterwards showed that the dye infiltrated vertically into the initially dry soil until it
reached the saturated zone. At that depth, a well-preserved network of ancient root
channels seemed to act as conductors for the fast lateral transport downhill. In the present
study, no such soil structures were detected. The only structures related to the dye patches
at the depth of the tile drain were vertical worm burrows.
The hydrometric data obtained in the experiment also clearly contradict the possibility
of lateral flow in the subsoil being the major mechanism of rapid transport. Despite the
moderate slope of the terrain, the discharge rates from the upper and lower pits differed
almost by a factor of four. Using the SWMS-2D code (Simunek et al., 1994), we simulated
the steady-state discharge behavior through a cross-section from the upper to lower pit
given a constant infiltration rate of 5 mm h� 1. In order to simulate lateral preferred flow
paths, we introduced a layer of large hydraulic conductivity at a depth of 95–107 cm. By
varying the conductivity of this high-conductivity layer, we modified the level of the water
table and the outflow ratio between the upper and the lower profiles. As long as we
assumed the fast flow to occur in the subsoil, it was impossible to reconcile the disparity in
the flow rates with the high water table in the center of the plot (Fig. 11). Due to the slope
of only 2.5%, the flow rates from the two pits would have been similar for high levels of
the water table. Only by moving the high conductivity zone close to the soil surface could
a high water table be reconciled with a strong discrepancy in the outflow rates from the
upper and lower profiles.
The same conclusion has to be drawn by comparing the discharge behavior in the tile
drain and the movement of the water table. During the main sprinkling interruption (2.9–
3.67 days), discharge from the tile drain ceased very quickly. However, the water table in
the middle of the plot was still 70–75 cm above the level of the drain at a lateral distance
of only 0.8 m from the drain. The same result was observed at the end of the experiment.
Obviously, the drain discharge was not governed by the hydrostatic pressure in the soil
matrix below 20–25 cm depth. Hence, the critical soil layer causing the fast solute
transport into the tile drain was the topsoil and not the subsoil. The infiltration patterns also
indicate that the water and solutes were flowing in lateral directions only close to the
surface.
This is plausible if one considers the actual local and instantaneous irrigation rates. For
the average sprinkling rate being 5 mm h� 1, it took the spray bar 24 runs an hour. The
nozzles irrigate a width of maximum 0.1 m at one given position. Therefore, the irrigation
C. Stamm et al. / Geoderma 109 (2002) 245–268 263
rate during each irrigation pulse of 0.75 s at a given location had an instantaneous intensity
of about 750 mm h� 1 followed by a period of an average duration of 120 s without
irrigation. This suggest that water was flowing intermittently at the soil surface during the
periods of the spray bar passing by.
The fact that we did not observe any surface runoff at the outlet of the plot indicates that
the lateral transport was redirected downwards in the drainage area and in the vicinity of
the open profiles. A few macropores connected to the tile drain or the open profiles were
sufficient to discharge the observed outflow volumes. Hence, we may summarize our
findings in the conceptual model depicted in Fig. 12.
Despite the fast transport and substantial tracer export, the mixing behavior between the
irrigation water and the soil solution was important. Most of the tracer mass was retained
in the soil (Table 4) and sorption effects were observed for SB, DRP and NH4+.
Similar results to ours, regarding the flow paths, have been presented for heavy clay
soils. Haria et al. (1994) concluded that in their arable field lateral transport occurred in the
A-horizon or occasionally by surface runoff towards the mole drains. Similar results are
reported by Spoor and Leeds-Harrison (1999). The work by Øygarden et al. (1997)
highlighted the importance of the disturbed soil above the drain itself for channeling water,
solutes and eroded soil particles into subsurface drains. For rice paddy fields, similar
effects are described by Iwata et al. (1995). That the drainage behavior reflects the
response of the coupled soil drainage system was also demonstrated by Turtola and
Fig. 11. Relationships between position of the water table and outflow ratio between discharge from the upper and
the lower profiles. The three curves indicate the relationship obtained by modeling a high-conductivity zone at
95–107 cm depth, at 35–50 cm depth or at the soil surface.
C. Stamm et al. / Geoderma 109 (2002) 245–268264
Paajanen (1995). They have reported on the change in the percentage of surface runoff
compared to drainage discharge from different fields after reinstalling the tile tubes.
The connectivity between soil surface and a tile drain may be of major importance for
the fate of solutes. Shipitalo and Butt (1999) have shown that the connectivity of worm
burrows to tile drains was limited to a narrow band of only about 1 m. The hydraulic
conductivity of the connected burrows was larger than those farther away from the drain. If
the connectivity is not given, P losses may be small even with fast flow occurring. In a
recent study, we have found little P losses from free-draining lysimeters despite the
occurrence of preferential flow (Sinaj et al., 2002). It seemed that P lost from the topsoil
was retained in the strongly fixing subsoil because the preferred flow paths ended in the
capillary fringe above the free draining boundary.
It might be argued that our findings are mainly due to the experimental boundary
conditions and that the transport we have observed occurred only with the large amount of
applied water. However, measurements on similar sites in the study area show that the very
wet conditions as produced during the experiment also occur naturally (von Albertini et
Fig. 12. Conceptual model of the flow paths for the fast transport into a tile drain in a weakly structured loamy
soil as at our study site.
C. Stamm et al. / Geoderma 109 (2002) 245–268 265
al., 1993) and that they are especially critical for surface runoff (von Albertini, 1990). In
the experiment, we reproduced these important conditions that in nature are mostly
transient states but extended its duration to make it accessible to experimental purposes
under steady-state conditions.
The two pits excavated for experimental purposes influenced of course the direction of
flow in the plot. However, they only affected the direction of the main lateral flow but not
the depths at which transport took place.
5. Conclusions
Often, surface runoff and subsurface flow are treated as separate processes in the sense
that water or solutes are thought to be transported to open waters by either one of the two.
Considering our findings, this clear distinction gets blurred. Instead of two parallel
processes, transport may be a sequence of two where water may (first) move laterally
as (near-) surface runoff that is intercepted by preferred flow paths in the vicinity of
subsurface drains.
This has also consequences for the interpretation of drainage water quality data. The
results of this experiment demonstrate that the assumption of subsurface drains being ideal
measuring devices for subsoil processes may be strongly violated not only for heavy clay
soils. The fast lateral transport in this loamy soil occurred in the topsoil and by-passed the
soil where the vertical macropores were connected to the tile drain or open profiles. Under
natural conditions, this could also be a river bank. Hence, the water quality in the drain as
well as in the profile effluent during storm flow was strongly influenced by the
composition of water coming from the topsoil. It is important to realize that the behavior
of solutes in the drainage effluent does not merely reflect the processes in the undisturbed
soil but is a combined response of the soil and the artificial drainage system. Hence, it may
well be that the large P losses reported in several drainage studies do not represent P losses
in the subsoil and shallow ground water but the losses in the immediate drainage area
where preferred flow paths directly connect to the tile drain below.
Therefore, we suggest that more attention should be given to the actual drain
configuration in order to complete the picture of the relevant factors determining solute
transport in drained soils. This is agreement with Richard and Steenhuis (1988) who
concluded that tile drains are useful for sampling over larger areas but suffer from the
possible problems due to soil disturbances or with Thomas and Barfield (1974) who
pointed out that nitrate concentrations in tile drains do not necessarily represent in a
reliable way the processes in the subsoil.
Acknowledgements
We would like to thank the many people who helped in the field or in the laboratory: F.
Denoth, H. Feyen, M. Fluhler, F. Funk, G. Gal, H. Hoffmann-Riehm, J. Hollinger, A.
Keller, B. Kulli, S. Lampert, H. Laser, P. Lehmann, A. Mares, R. Meuli, B. von Steiger, B.
Studer and V. Vouets. R. Hofling helped us by analyzing the deuterium samples. P.
Lazzarotto provided the precipitation data of the Sempach weather station. We want to
C. Stamm et al. / Geoderma 109 (2002) 245–268266
thank also the farmer and landowner, Mr. Rindlisbacher, who agreed on the experiment
carried out on his land and who often supported us when we encountered practical
problems in the field. The people from the Cantonal Administration of Lucerne, especially
M. Achermann, J. Blum and P. Stadelmann, provided many helpful information
concerning the local situation. M.C. Jensen and an anonymous referee helped to improve
the manuscript by their comments. The project was funded by the Swiss National Science
Foundation (grant nos. 21-39314.93 and 20-49552.96).
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