flow and transport processes in a macroporous subsurface-drained glacial till soil i: field...

ELSEVIER Journal of Hydrology 207 (1998) 98- 120

Journal

&rology

Flow and transport processes in a macroporous subsurface-drained glacial till soil

I: Field investigations

K.G. Villholth”,*, K.H. Jensena, J. Fredericiab

aDepartment of Hydrodynamics and Water Resources, Technical University of Denmark, 2800 Lyngby, Denmark bGeological Survev of Denmark, Thoravej 8. 2400 Copenhagen NV, Denmark

Received 5 March 1997; accepted 18 February 1998

Abstract

The qualitative and quantitative effects of macropore flow and transport in an agricultural subsurface-drained glacial till soil in eastern Denmark have been investigated. Three controlled tracer experiments on individual field plots (each approximately 1000 m*) were carried out by surface application of the conservative chloride ion under different application conditions. The subsequent continuous long-term monitoring of the rate and chloride concentration of the drainage discharge represented an integrated and large-scale approach to the problem. In addition, point-scale determination of macropore structure and hydraulic efficiency, using image analysis and tension infiltration, and of soil water content, level of groundwater table, and chloride content of soil water within the soil profile yielded insights into small-scale processes and their associated variability. Macropore flow was evidenced directly by the rapid (within 10 mm of water input) and abrupt chloride breakthrough in the drainage water at 1.2 m depth in two of the tracer experiments. In the third experiment, the effect of macropore transport was obvious from the rapid and relatively deep penetration of the tracer into the soil profile. Dye infiltration experiments in the field as well as in the laboratory supported the recognition of the dominant contribution of macropores to the infiltration and transport process. The soil matrix significantly influenced the tracer distribution by acting as a source or sink for continuous solute exchange with the macropores. An average field-determined active macroporosity constituted 0.2% of the total porosity, or approximately 10% of the total macroporosity. 0 1998 Elsevier Science B.V. All rights reserved.

Keywords: Structural soils; Macropores; Subsurface-drainage; Field-scale experiments; Conservative tracer; Flow and transport processes

1. Introduction

The predominance and effect of macropore flow processes in soil and groundwater systems have been studied and documented in numerous reports during the past couple of decades (for a review, see Thomas and Phillips, 1979; Beven and Germann,

* Corresponding author.

1982; White, 1985; Villholth, 1994). Macropore flow, or alternatively bypass flow or fracture flow, is attributed to structural pore systems that provide pathways for relatively rapid transport of water and dissolved or suspended constituents through the porous medium. These ‘express routes’ (e.g. worm and root channels, and planes and openings created by fracturing and dissolution) are numerous and ubiquitous in most natural soils and many aquifer

0022-1694/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOO22- 1694(98)00 129-2

K.G. Villholth et al./Joournal of Hydrology 207 (I 998) 98-120 99

materials. Thus, a better understanding of these preferential flow and transport processes is needed.

In shallow unsaturated soils, which is the main focus of this study, the effect of macropore flow can be manifested through very short arrival times of surface-applied or derived substances to the groundwater. The retarding or buffering capacity of the unsaturated zone is further diminished for many degradable and sorbing chemicals because the retention time and contact possibilities with the porous medium and the resident water are small. Drainage studies in which the leaching of surface-applied chemicals is traced in subsurface drains, have pre- viously been used and recommended as a tool for analyzing transport processes in the upper predominantly unsaturated soil (Bottcher et al., 1981; Everts et al., 1989; van Ommen et al., 1989; Kladivko et al., 1991; Vinten et al., 1991; Jayachandran et al., 1994; Southwick et al., 199.5), including macropore flow phenomena (Richard and Steenhuis, 1988; Utermann et al., 1990). The advantages of the drainage study approach are as follows.

The measured drainage response integrates the effects of spatial variability with respect to flow and transport characteristics within a large soil volume. A measure is obtained of the flux concentration (Parker and van Genuchten, 1984; Utermann et al., 1990) which is of concern when evaluating the contamination load to the shallow groundwater. Existing subsurface drains provide easy access for sampling and monitoring equipment, installed without disturbing the soil volume of interest.

Finally, the agricultural practice of subsurface drainage which is widespread on heavy soils charac- terized by macroporosity provides on its own merits a justification for research into the hydrological and water quality impacts of cultivation and soil management.

In previous reported drainage studies the documen- tation of the effect of macropore flow has primarily been based on observations of rapid breakthrough of surface applied chemicals in the drainage effluent and, only to a smaller extent, has this presumption been substantiated with further experimental documenta- tion. This study aims at extending the conventional

drainage tracing approach in order to improve the interpretation of the flow and transport processes responsible for the movement of water and a conservative tracer (chloride, Cl-) from the soil surface to the drain and shallow groundwater in an agricultural glacial till soil with visible macroporosity. As opposed to previous drainage tracer studies this study also includes:

1.

2.

3.

4.

5.

6.

hydraulic and geometric characterization of the macropores; point-scale observation of important variables, such as soil water content, soil water tension, piezometric head, and concentration of tracer in soil water: continuous monitoring on short as well as long time scales of the drainage response; varying conditions for the soil surface application of the tracer; a qualitative analysis of the main recipient of the tracer (groundwater or surface water): a model analysis of the field tracer experiments with special emphasis on the effect of macropore flow and transport.

The experimental procedure was implemented in order to specifically observe and evaluate the macropore flow and transport processes on a small and large scale, spatially as well as temporarily. A comprehen- sive monitoring scheme was designed in order to provide a consistent and relevant data set for the model analysis. The aim of the model application was to assess more quantitatively the significant flow and transport components in the investigated soil on the basis of a numerical, dynamic double- porosity flow and transport modeling concept. The results of this study are reported in an accompanying paper (Villholth and Jensen, 1998). In the present paper, the results of the field investigations are reported.

2. Materials and methods

2.1. Field site

The experimental site is located in an agricultural area in the Syv Creek Catchment (1170 ha) in the eastern part of Denmark, 40 km south-west of

100 K.G. Villholth et al.Noumal of Hydrology 207 (1998) 98-120

Copenhagen (Latitude: W, 40’). The Quaternary- deposited till sequence consists of interbedded layers of moraine clay and alluvial sand and gravel resting upon a primary Paleocene limestone aquifer at lo- 15 m depth. The soil is classified as an Inceptisol (Soil Taxonomy, Soil Survey Staff, 1975). The soil texture ranges from silty to sandy loam (FAO, 1977) with a maximum clay content of approx. 30% by weight. The macroscopic soil structure was observed and evidenced from vertical as well as horizontal cross- sections exposed in three large pit excavations. The visible macropores are dominated by predominantly vertical earthworm channels (0.5- 10 mm in diameter, approximately 15 per square decimeter), but also root patterns and a ped and block structure in the till give rise to macropores in the upper approximately 1 m of the soil. Below approximately 0.8 m depth a pseudo gley zoning (mottled grey/brown coloring of the soil) indicates temporary saturation and that the structural pores have generated heterogeneous saturation and oxidation conditions in the soil. Further insight into the macropore structure was gained from dye infiltration studies and image analysis of horizontal cross- sections of excavated soil columns.

The drained field site is relatively flat (<3% slope in the south to north direction) and has been under conventional tillage prior to the investigations. The drainage system was established in the 1940s at a depth of approximately 1.2 m. The drain lines consist of cylindrical sections of clay tile (5.5 cm in diameter) laid end to end with gaps between adjacent sections to permit water entry. The area above two adjacent parallel drain lines (DRAIN 1 and DRAIN 2) which were located 20 m apart was investigated (Fig. 1). Three separate rectangular sub-areas. PLOT 1, PLOT 2 and PLOT 3, each comprising a theoretical

sub-catchment approximately 1000 m’ in area to one of the drains, were delineated for individual tracer experiments. An access manhole was installed at the downstream end enabling monitoring and sampling of effluent from both drains. DRAIN 2 extends only partly through PLOT 3. In addition, from the downstream end of PLOT 3 to the manhole DRAIN 2 was replaced with an impervious plastic pipe due to the diversion of the original drain line to the manhole.

The access manhole was instrumented in order to measure continuously (smallest time resolution. 5 min) the drainage flow rate and the chloride concentration in the drainage water. Continuous monitoring is required due to the highly transient nature of macropore flow. The drainage flow rate was determined from registered and data logged water level changes in an upright cylindrical receiving tank (70 1). Upon filling of the tank, its content was rapidly discharged to the downstream end of the drain by a submerged automatic pump. The chloride concentration in the drainage water was measured using a sensitive (range 10-j to 1 M Cl-) chloride-specific (AgCl) electrode with a calomel (HgCl) electrode as a reference. The electrodes (Radiometer. Copenhagen) were installed in a small (24 cm3) reservoir supplied with drainage water directly from the drain to the bottom of the reservoir and overflowing into the large tank. The small sample volume combined with total mixing by continuous automatic stirring ensured sampling of the prevailing drain water concentration. The concentration was calculated from a recorded potential difference between the two electrodes. A calibration curve was established by taking drainage water samples at intervals governed by the intensity of the drainage discharge ( 1 - 14 days) and determining the chloride concentration by argentometric titration in the

Fig. I. Plot layout at the field site.

K.G. Villholth et al./Joumai of Hydrology 207

- Drain line 0 Pwometer pi LOT 1

1998) 98-120

Plot boundary x Soil sampling spot

- Cl application stnp ~3 Soil sampling spot @@ XsfS X MB

0 s23

Access manhole + piezometer X xx xx xx Cl2 C11 Cl0 “4R,,

10

N

/

PLOT2

X x X X

El28 X 825 & O 827

/I

t 5m

Fig. 2. Schematic plane view of PLOT 1 and PLOT 2.

PLOT 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~.““““““““‘“““““““““” rJ ‘c

“““‘,““..““’

i” @,g@ ‘r;, % ‘0 00 s s CXI

bo b

” Plot boundary

0 Soil sampling spot

5m

Fig. 3. Schematic plane view of PLOT 3.

P / I 33

102 K.G. Villholth et al./Journal of Hydrology 207 (1998) 9% 120

laboratory (Standard Methods for the Examination of Water and Wastewater. 198.5).

Besides the drainage monitoring. the held site was instrumented to allow monitoring of local conditions within the soil profile. The monitoring program included: (1) piezometric head at 1.0-2.6 m depth measured manually, or automatically with pressure transducers, in 16 mm diameter piezometers: (2) soil water tension at 0.3 and 0.6 m depth measured with tensiometers; (3) chloride concentration determined by argentometric titration in soil water collected in either porous suction cups (0.3 and 0.6 m depth). in piezometers. or from disturbed soil samples (50 g) collected at O-3 m depth prior to and after the tracer applications (2-12 and 4-8 replications each time prior to and after. respectively (Fig. 4)); and (4) soil water content (down to 2-3 m depth) measured with ,eutron logging equipment and from the collected soil

samples. Figs. 2 and 3 show the layout of the plots including the measurement locations. On-site measurement of rainfall was provided by a recording tipping bucket rain gauge (resolution, 0.1 mm) and an integrating rain gauge at ground level used to correct for aerodynamic effects.

2.2. Field tracer experiments

The experimental work commenced in the fall of 1989 and lasted through the spring of 1992. Drain flow was a prerequisite for the drainage experiments, which dictated that the tracer experiments were initiated in the fall-to-spring period. The tracer experiments on PLOT 1 and PLOT 2 were initiated late in the drainage season of 1989-1990 (SEASON 1 ), and on PLOT 3 the tracer application was performed in the 199 1 - 1992 drainage season (SEASON 3). During the intervening drainage season (1990- 199 1, SEASON 2) no tracer applications were performed due to unfavorable weather conditions. The hydraulic response of PLOT 3 was, however, monitored during SEASON 2 (Fig. 4).

The tracer experiments on PLOT 1 and PLOT 2 were considered preliminary tests of the immediate effect of macropore flow on discharge and concentration in the drain and hence the monitoring focused on the drainage outflow in SEASON 1. However, grab samples were taken occasionally during the following two drainage seasons from DRAIN 1. which combines

the response of the tracer applications on PLOT 1 and PLOT 2. In addition, analysis of soil samples collected in the plots between the seasons gave information on the long-term response. The tracer experiment on PLOT 3 was conducted to evaluate. with better temporal and spatial resolution, the short- and intermediate-term response of the drainage effluent as well as the water and solute transport inside the test area. Hence the installation and monitoring program on PLOT 3 during SEASON 3 was quite intensive. A reliable and consistent comparison of tracer movement in different drain plots hinges on an assumption of similar hydraulic characteristics. A linear correlation between the instantaneous drainage flow rate in DRAIN 1 (Qdm,) and the discharge rate in DRAIN 2 (Qdll12) during SEASON 2 and SEASON 3 was deducible (Qdml = 2.49Qd,,: + 0..13 1 min-‘: R = 0.89: II = 32). The linearity suggests that the hydraulic responses of the adjacent drain plots are similar. In addition, the slope of the line corresponds to the ratio of the drained areas.

The tracer was applied manually on the soil surface in 2 m wide rectangular strips along the drains. The area of the tracer application strips were 236. 142 and 56 m’ for PLOT 1, PLOT 2 and PLOT 3, respectively. The strips were displaced 1 .O- 1.5 m from the location of the drain line (Figs. 2 and 3). The strategy was to avoid tracer infiltration into the soil volume directly above the drain that could have been disturbed during the past drain installation (Hergert et al.. 1981: Richard and Steenhuis. 1988; Kladivko et al.. 1991: Chow et al., 1993). A fast breakthrough in the drainage water upon tracer application would then be indicative of macropore flow effects in a representative part of the held soil. At the same time, the close proximity of the tracer application area to the drain was chosen to allow investigation of effects dominated by flow in the unsaturated zone. High tracer input doses were used due to relatively high initial background chloride concentrations in the soil. ranging from 24 to 292 mg ll’. Amounts ot 620-790 g Cl (as CaClZ,2H20) per square meter were applied. When mixed with 3 mm of water. as in the experiment on PLOT 3, this corresponded to input concentrations of about 180 g Cl 1-l.

The circumstances of tracer application varied among the three experiments (Table 1). In PLOT 1 and PLOT 2 the calcium chloride was applied in solid

K.G. Villholth et al./Journal of Hydrology 207 (1998) 98-120

PROFILE la

PROFILE 1 b PROFILE 2a

PROFILE 2b

FEB.28 MAR.8

APR.PO APR.27

JUN.21-

gg PLOT 1

m PLOT 2

PLOT 3

PROFILE lc PROFILE 3a AUG.1Z

DEC.19

PROFILE ld PROFILE 3b APR.24

SEASON 1

SEASON 2

qcl SEASON 3

103

Fig. 4. Experimental sequence for the tracer experiments. Length of bars represents the duration of continuous monitoring in the drain. 4 and date indicate time of tracer application. PROFILE no. and date indicate plot and time of soil core sampling.

104

Table 1 Tracer application conditions

K.G. Villholth et al./Journal of Hydrology 207 (199X) 9% 120

PLOT I PLOT 2 PLOT 3

Tracer form solid Soil water content field capacity Water input w. tracer none

solid > field capacity

irrigation

dissolved > field capacity

rain

form as flakes, directly on the ground. In PLOT 2 this application method was immediately succeeded by irrigation of 11 mm of water to the tracer area. In PLOT 3 a solution of calcium chloride was sprayed from sprinkling cans directly on the tracer area during a 20 min period. Irrigation on PLOT 2, and also on PLOT 1 later in the experiment, with low Cl-content water (50 mg Cl 1-l) was achieved by hoses supplied from a large tank. The rates (6- 11 mm h-‘) were high enough to cause transient surface ponding but no significant surface runoff. The soil water content prior to the tracer applications was lowest in PLOT 1, and was artificially increased in PLOT 2 by irrigation of 44 mm of water during the preceding 2 day period. In PLOT 3, the soil water content was naturally high due to high rainfall during the preceding days, and chloride application was performed during rain (onset of a 4 mm rain event lasting 6 h).

It was possible to perform two tracer tests involving the same drain line (PLOT 1 and PLOT 2 on DRAIN 1) for three reasons. Firstly, no immediate chloride breakthrough in the drainage water was observed in the first experiment on PLOT 1. Secondly, the experiment on PLOT 2 lasted only a short time and was mainly influenced by irrigation. Thirdly, PLOT 1 was kept covered with plastic during the experiment on PLOT 2, thereby minimizing leaching of tracer from this plot.

2.3. Physical and hydraulic characterization of the field soil

The tracer experiments were supplemented with physical and hydraulic characterization of the field soil. The effect of macropore flow on water transmission in the upper soil layers was investigated by performing in-situ infiltration measurements under ponded and non-ponded conditions at random locations throughout the site. The non-ponded experiments were carried out using a tension infiltrometer by which the water is applied at a small negative

pressure through a porous plate. The basic principle described and tested by a number of researchers (for example, Watson and Luxmoore, 1986; Perroux and White, 1988; Ankeny et al., 1990; Messing and Jarvis, 1993) is that as the tension of the applied water is increased, pores of increasingly smaller size will be excluded from the infiltration process. A comparison with infiltration rates during ponded conditions thus gives a measure of the contribution of macropores to the overall infiltration rate.

The ponded infiltration tests were performed in a single stainless steel infiltrometer cylinder (diameter. 0.3 m) with the lower rim pushed 50 mm into the soil. Ponded water was applied to a constant depth (approximately 80 mm) via a Mariotte bottle type reservoir. The tension infiltration measurements were performed with a tension infiltrometer similar to the one described by Watson and Luxmoore (1986). The 3.2 mm thick H.D. polythene porous plate (diameter, 0.3 m) had a maximum average pore size of 80 mm and a conductivity of 170 mm he’. A proper hydraulic contact with the soil surface was ensured by placing the infiltrometer on an approximately 20 mm layer of medium sand, which in turn was placed on a fine-meshed nylon cloth (mesh size, 0.2 mm) to prevent the sand from occluding any macropores at the soil surface. The initial volumetric soil water content was relatively low, approximately 0.20 m3 m-3. Individual infiltration tests lasted between 80 and 160 min.

The temporal evolution of the infiltration rate for both types of infiltration tests was fitted to the exponential Philip (1957) infiltration equation. The long-term asymptotic infiltration rate was assumed to approximate the effective vertical hydraulic conductivity, corresponding, in the case of the tension infiltration, to the tension under which water was applied (3, 6 or 12 cm).

In addition, two dye infiltration tests (ponded and under 3 cm tension) were conducted in the field to

K.G. Villholth et al./Journal of Hydrology 207 (1998) 98-120 105

identify and visualize the flow paths. The infiltration water was mixed with 1 g 1-l Rhodamine B and 15 and 10 mm of the solution was infiltrated through the infiltrometer ring and tension infiltrometer, respectively. The soil was covered and after l-3 months gradually excavated manually from the vertical plane of a pre-dug soil pit.

Macropores and macroporosity were quantified in the laboratory from image analysis of horizontal cross-sections of two undisturbed soil columns (0.3 m in diameter, 0.5 m in height) collected at the field site (Wildenschild et al., 1994). Horizontal planes at 8 cm intervals were successively cleared and vacuumed before macropores evident on the surface were traced on a transparent sheet, video recorded, and analyzed for total number, total area1 fraction, and area frequency distribution of

macropores using automated image-processing software (Johansen, 1990). A measure of the hydraulically effective or active macroporosity was obtained from analyzing pores that were stained by a Rhodamine B solution applied at the upper surface of the column prior to the slicing. The dye was supplied to the columns as a slug of 14 mm of ponded solution or continuously as 10 mm h-’ for 4 h through a rain simulator. The latter produced intermittent ponding at the surface.

The effect of natural infiltration on changes in water content in the field soil profile was investigated by performing repeated neutron loggings at three locations in PLOT 3 during a shorter period in SEASON 3 in which relatively many rain events occurred (six events during 12 days; maximum hourly rain intensity, 3.5 mm h-l).

26 27 28 29 Apr-90

2

0 \ I 26

AL, 27 28

, 29

3 E . E 8 d

1400 -

1200 -

1000 -

800 -

600 -

400 -

200 -

0-I 26

Apr-90

28 29 Apr-90

Fig. 5. Observed time series data from PLOT 2, SEASON 1. A indicates time of tracer application. Hollow bars indicate artificial water application.

106 K.G. Villholth et al./Journal of Hydrology 207 (1998) 98-120

Water retention properties and the saturated hydraulic conductivity of the soil were obtained from conventional laboratory analysis (stepwise equilibration during sample drainage in a pressure cell apparatus and constant head measurements, respectively) of small (100 cm3) undisturbed soil core samples collected at three depths during SEASON 1 (0.1, 0.45, and 0.8 m, four replications

at each depth). In addition, in-situ saturated hydraulic conductivity in PLOT 3 was obtained from slug tests in which the recovery rate of water levels in the piezometers (nine replications at 1.2 m and 10 at 2.3 m depth) after a sudden injection of a volume of water was monitored. This data (described in more detail by Villholth (1994)) provided input to the model analysis of the field tracer experiments.

10

8

2

o--r Sep.91 act-91 ’ Now91 ’ Oec-81 ’ Jan-92 ’ Fcb-92 Mar-92 Apr-92

10, ,

a- % Oc = ._ 6- CE ‘6 : b 4-

2-

0-T I I_,,, A_.- Sep-91 Ott-91 Now91 Dee-91 Jan-92 Feb-92 Mar-92 ’ Apr-92

0 Sep-91 Ott-91 Now91 Dee-91 Jan-92 Feb-92 Mar-92 Apr-92

44-

E 43 - Ground

40 Sap-91 Ott-91 Now91 Dac-91 Jan-92 Fab-92 Mar-92 Apr-92

Fig. 6. Observed time series data from PLOT 3, SEASON 3. A indicates time of tracer application. Hydraulic head elevation is monitored at 2.6 m depth. Inset shows the period immediately following tracer application.


3. Results and discussion

3. I. Initial chloride breakthrough, PLOT 2 and PLOT 3

Very rapid breakthroughs were observed in the tracer experiments in PLOT 2 and PLOT 3. The chloride concentration in the drainage effluent from PL,OT 2 and PLOT 3 increased abruptly by factors of 20 and 400, respectively, within a few hours of the surface application (Figs. 5 and 6, and Table 2). A decrease in the chloride concentration from 180 000 mg 1-l in the application solution in PLOT 3 to a maximum peak value of 4170 mg I-’ in the breakthrough solution indicated dilution due to mixing with resident soil water inside as well as outside the tracer application area takes place (Fig. 6). The rapid drain responses, however, leave little doubt that macropores participated in the initial solute transport process. Considering a minimum travel distance from the soil surface to the drain of 1.9 m, 3.3 mm of net precipitation occurring from the onset of the tracer application to the time of drainage breakthrough in PLOT 3 and assuming no change in storage of water within the profile during the breakthrough, this corresponds to a hydraulically active average macroporosity of 0.17% of the total soil volume or approximately 0.5% of the total soil porosity. Furthermore, the rapid breakthroughs in PLOT 2 and PLOT 3 indicate a fair degree of continuity and connectivity of the conducting macropores in the upper l- 1.5 m of the soil profile (Figs. 5 and 6). The efficiency and deep reaching capacity of the macropores were also evident from the dye infiltration studies in the field. Here, single stained pores (earthworm channels) could be traced to 0.9 and 1 .l m depth. Other researchers have reported similar findings based on early drainage

Table 2

Chloride application and breakthrough data

breakthroughs of surface-applied chemicals. Kladivko et al. (1991) determined a 3% soil volume contributing to initial drainage breakthrough of four pesticides using the displacement approach. Rogowski (1988) used distributed sampling from a clay liner to show that the effective overall porosity was less than 1% over 44% of the site area.

3.2. Active macroporosity

In the following it is hypothesized and theoretically substantiated, based on the image analysis and the tracer experiments, that a few macropores connecting the drain lines could be responsible for the initial breakthroughs. In addition, the analysis provides support for a perception of macropore water transport governed by irregular, tortuous and unsaturated flow conditions that do not comply with a conventional Poiseuille interpretation.

The approach of determining a hydraulically efficient or active macroporosity introduced by Watson and Luxmoore (1986) was applied to the observed tension infiltration data. Assuming that the conductivity increase from a supply tension of 3 cm to ponding conditions is attributable to laminar gravitational full flow in a number of equally sized (diameter, 1 mm) cylindrical vertical tubes according to the Poiseuille equation, the maximum efficient macroporosity is estimated to be 0.038%. Including the pores contributing between 3 and 12 cm tension (diameter between 0.25 and 1 mm) only increases the macroporosity by 0.003%. According to this analysis, the efficient macroporosity comprises only about 0.04% of the total soil volume. Similar values were determined by Watson and Luxmoore (1986).

The active macroporosity determined on the basis of the tension infiltration tests is considerably smaller

Chloride input First solute peak

Dose Background Time to Water input to Max. water Max. Cl lost cont. breakthrough breakthrough input rate Cl cont.

(kg m-‘) (mg 1-l) (min) (mm) (mm h-‘) (mg I-‘) (kg)

PLOT 1 0.73 47 _ _ 10 _

PLOT 3 0.79 59 134 9.4 9 1360 9 PLOT 3 0.63 10 26 3.3 4 4170 4

108 K.G. Villholth et al./Journal of H.vdrology 207 (1998) 98-120

than the active macroporosity determined from image analysis of the stained soil columns (0.34%). Although dye was supplied to the soil columns under different conditions (by ponding or sprinkling the soil surface), the active macroporosity was comparable in the two columns indicating that for the relatively high input rates used the overall flow processes in the two columns were comparable. The conditions for infiltration in terms of the soil sample size, the initial water content and the application rates were comparable in the two types of experiments. Also, the variability of the active macroporosity estimates was small within the determination method (standard deviation equal to 0.025% and 0.22% macroporosity for the tension infiltration and the image analysis, respectively). These facts indicate that the large difference in active macroporosity estimates from the two experimental procedures is due to the method of analysis rather than to different soil or experimental conditions.

The estimate based on the tension infiltration tests is a maximum active macroporosity considering full flow in pores of minimum diameter 1 mm, according to Poiseuille’s equation and the capillary equation. If the macropores are generally not full-flowing (see below), the assumptions of the analysis are not satis- fied, and the estimate of an active macroporosity based on the tension infiltration tests should be considered uncertain.

The estimate based on the dye tests includes the area1 porosity of stained macropores. This figure is overestimated if (1) not all stained macropores are active at the same instant. and if (2) macropores are not full-flowing. Facts supporting these suppositions are the following: (1) the flow distribution across the outflow surface of the soil columns used in the dye tests was non-uniform in time, indicating that flow in macropores changed temporally, presumably as a consequence of air entrapment or local flow convergence phenomena (Wildenschild et al., 1994); (2) the average macropore diameter determined from the dye tests was 3.6 mm, and full-flow in just three of such pores per square meter would, according to the Poiseuille equation. represent a conductivity attributable to macropore flow as assumed in the analysis (24 mm h-l). In contrast, the average number of colored macropores was 338 per square meter.

Assuming, alternatively. that the colored

macropores (average diameter, 3.6 mm) contribute with flow along their walls only, a minimum, average flow wall thickness of 0.1 mm would be required to generate the measured macropore conductivity. depending on the number of pores participating in the flow process (according to the Poiseuille law). Hence, the dye test estimate should be considered a temporally accumulated active macroporosity including any internal macropore volume not necessarily participating in the flow. Considering a range of flow regimes from full flow in three instantly active macropores to constant wall flow in all colored macropores and correcting the active macroporosity accord- ingly indicates that only 0.02 to 0.003%, respectively. of the soil volume is participating in the flow process.

An estimate of 0.17% for the active macroporosity was obtained from the initial breakthrough in PLOT 3. Although not a definite quantity, the estimate is expected to be more representative as the evaluation is based on field conditions of minimally manipulated flow and assuming total displacement of the porosity actively contributing to water transmission.

The large discrepancy between the field estimate of active macroporosity on the one hand and the estimate based on the tension infiltration and the corrected porosity based on the dye experiments on the other hand is attributed to the simplifying assumptions inherent in the Poiseuille equation for tube flow. By neglecting effects such as macropore constrictions. tortuosity of flow. and air entrapment the flux rate in the macropores are overestimated which in consequence leads to an underprediction of the active macroporosity. Assuming a representative active macroporosity of 0.17%. the macropore flux in the dye experiments is overestimated by a factor of approximately 100.

In summary, the macropore flow process is highly irregular in time as well as in space and flow is generally restricted to a small fraction of the total visible macroporosity. An average total macroporosity within the top 0.5 m of the soil was estimated to 1.6% based on the image analysis. Applying the held estimate for the active macroporosity (0.17%). the macropores contributing to flow at drain level generally constituted approximately 10% of the total macroporosity. In addition, the analysis indicated that due to non-ideal flow conditions in the macropores the Poiseuille equation for tube flow had little


applicability in the estimation of active macroporosity. The use of tension infiltration tests for the evaluation of macroporosity is thus discouraged. Estimates based on considerations of pore volume displacement during tracer breakthrough under wet field conditions are recommended as it is inde- pendent of how rate quantification and because only hydraulically active macroporosity is included.

To test the hypothesis of a few macropores being fully connected to the drain and responsible for the rapid response and breakthrough in the drain on PLOT 3 the time required to conduct the tracer through the macropores is calculated. The assumption is that all solute in the initial breakthrough peak arises from plug-flow transport through a minimum of active average-sized

12

10 ~-

a -- 6 --

4 --

11. I I I I . . . .l”LL I Mar-90 II Apr-90

10 -.

a-

6-

4-

2-

0 I Mar a-90 Mar-90 Apr-90

1200 -

1000 -

800 -

600 -

400 -

200 -

0-r * ,goeee Q ee Co- Mar B-90 Mar-i0 Apr-90

;$I _*Oao “,:’ oDrai&

Mar 8-90 Mar-90 Apr-90

Fig. 7. Observed time series data from PLOT 1, SEASON 1. A indicates time of tracer application. (I) indicates irrigation. Hydraulic head elevation is monitored at 1.3 m depth.

110 K.G. Viliholth et d/Journal of H_ydrology 207 (1998) 98-120

continuous macropores, without exchange with the matrix.

The 100 g of chloride detected in the initial breakthrough peak corresponds to 556 ml of undiluted input solution with a concentration of 180 g I -‘. With a total of three active macropores (d = 3.6 mm) per square meter over a contributing area of 3.3 m’ (estimated from an observed volume of drainage water of 10 1 from the time of the tracer application to the time of the breakthrough which is assumed displaced in a box-shaped soil volume of 1.9 m depth and effective porosity of 0.17%), each macropore would have to transport 60 ml of the undiluted solution. Considering plug-flow in the macropores consistent with a total active macroporosity of 0.170/c, the time required for a minimum travel distance of 1.9 m is 1 min. The time required to transmit the 60 ml of solution through the pores is 3 min.

The calculation shows that a few continuous macropores could theoretically be responsible for the rapid breakthrough. The corresponding observed values were 26 min for the breakthrough time and 12 h for the peak duration, indicating that additional dispersion in the macropores is taking place, primarily due to diffusion with the soil matrix.

3.3. Initial breakthrough, PLOT I

The immediate drain response on PLOT 1 (Fig. 7) significantly deviated from the response on PLOT 2 and PLOT 3 because no instant breakthrough of the chloride tracer was observed. Despite intense monitoring when the drain was still discharging following the tracer application, no significant increase in the drainage chloride concentration was observed. Initially this response was interpreted in terms of

Table 3

Data from first profile sampling after tracer application

0.6 E

-n-PLOT1

~ -A-PLOT3

1.6

0 1 2 3 4 5 6 7

Cl-cont., g/l

Fig. 8. Average chloride concentration distribution with depth in the three plots. Observation from the first sampling after the tracer application (see Table 3 ).

lacking initial macropore transport of the tracer due to the relatively dry surface soil and due to the fact that water was not applied in immediate connection with the tracer application (Table 1). When rainfall (11 mm during 6 h; maximum intensity, 3.4 mm h-‘) occurred 24 h after the tracer application, the dissolved calcium chloride apparently was taken up by the soil matrix at the soil surface. leaving relatively ‘clean’ water to infiltrate into macropores and thereby bypassing the high concentration zones.

The first soil samplings performed after the tracer applications (PROFILE lb, PROFILE 2b and PROFILE 3b), however, revealed that the average chloride concentration in the deeper part of the profile

Plot Profile Time Water Ave. elapsed input Cl cunc. since since at I .4 m tracer tracer depth application application (mgl ‘1 (days) (mm)

PLOT 1 PROFILE lb 45 54 357 f 81 PLOT 2 PROFILE 2b 56 IO3 207 -t 92 PLOT 3 PROFILE 3b 126 224 37 i- 14

K.G. Villholih et d/Journal of Hydrology 207 (1998) 98-120 111

in PLOT 1 was higher than in PLOT 2 and PLOT 3 Evidently, the drainage approach in the case of (Fig. 8). With less time and infiltrating water available PLOT 1 failed to capture the macropore effects. for tracer transport in PLOT 1 (Table 3) the relatively Although the circumstances under which the tracer deep chloride penetration is interpreted as a result of was applied to this plot, e.g. initial soil water content, macropore facilitated transport in this plot as well. application method and amount of rain or irrigation in The individual chloride concentration profiles from connection with or subsequently to the application the first sampling in PLOT 1 show, in four out of time, may in part contribute to the missing response, six cases, a tendency for two separate concentration a likely reason for the shortcoming may be a macro- maxima, one at the soil surface and one at 0.8- 1.2 m pore structure that displays variability on a scale com- depth (Fig. 9). This double peak feature which is not parable to the size of the drainage catchment area. If a evident in PLOT 2 and PLOT 3 also indicates that few continuous and drain-connected macropores are some rapid bypassing has occurred in PLOT 1. sufficient to immediately translocate the surface Since no rain or irrigation was provided in immediate solute to the drain, as hypothesized based on the connection with the tracer input on PLOT 1, subse- results of PLOT 3, a lack of direct macropore connec- quent rain and irrigation must be responsible for the tion is probable and would mean a somewhat slower relatively deep percolation. In this case 24 h elapsed tracer breakthrough in the drainage. Drain flow ceased before the first arriving rain showing that soluble only 37 days (and 35 mm of water input) after the chemicals, surface-applied in solid form, can be tracer application in PLOT 1 which therefore was susceptible to rapid leaching for an extended period the maximum time available for a positive dete’ction following application. of a breakthrough in the drainage effluent in this plot.

0.0

0.5

E “’ L = 1.5

al p 2.0

2.5

3.0

823 818 0.0

0.5

1.0

1.5

2.0

2.5

3.0

822

0 2 4 6

Cl-cone., g/l

0 2 4 6 0 2 4 6

B21 0.0

2.5

3.0 L 0 2 4 6

Cl-cont., g/l

0.0

0.5

1.0

1.5

2.0

2.5

3.0

r 820

0 2 4 6

Cl-cone., g/l

Fig. 9. Chloride concentration distribution with depth in the PLOT 1. Observation from the first sampling after the tracer application (PROFILE lb).

112 K.G. Villholth et ai.Nounuzl of Hydrology 207 (1998) 9% 120

The interpretation indicates that the size of the drainage area in relation to the scale of variability of the macropore structure may be important in the drainage breakthrough approach. However, in order to render unnecessary the inclusion of extended drainage areas, internal profile sampling of resident soil water quality to deeper layers as performed in this study is recommended as a means of documenting the effect of macropore flow. The modeling efforts are consistent with the interpretations of the experimental findings (Villholth and Jensen, 1998) indicating that macropore flow and transport effects are of importance in all three plots.

3.4. Macropore efjCect at low input rates

The fact that the observed deep initial tracer movement was attributed to macropore effects in all three tracer experiments in spite of a variety of input and antecedent conditions may suggest that macropore flow is a generally occurring transport mechanism at this site. The large contribution of macropore flow close to soil saturated conditions was supported by the tension infiltration experiments. A IO-fold decrease in the hydraulic conductivity from conditions of ponded water application (25 mm h-l) to conditions of application at 3cm tension (2.6 mm h-l) was found. This indicates a strongly bimodal pore system (Messing and Jarvis, 1993) and that 89% of the saturated flux occurs through the largest pores. Watson and Luxmoore (1986) found a

Table 4

Hydraulic conductivity and summary statistics for infiltration tests

nearly 73% contribution of macropores of a similar tension interval in a forest soil.

The results indicate that macropore effects occur under various hydraulic input conditions. The field experiment with dye infiltration under 3 cm tension revealed upon excavation of the underlying soil that flow through preferential flow paths coinciding with structural pores had occurred. A hydraulically active wormhole was traced to 0.8 m depth. The staining pattern at depth. however, was not as intense compared to the test with ponded application which could be explained by a smaller number of active macropores. Less dye solution is available for flow convergence and macropore interception close to the soil surface due to the tension of the applied solution. The experiment showed that macropore flow probably occurs at tensions of approx. 3 cm and at hydraulic conductivities of at least one order of magnitude less than the saturated values (Table 4). Jarvis et al. ( 1987) similarly observed stained macropores upon dye infiltration at 3 cm tension through a tension infiltrometer. Scatter and Kanchanasut ( 198 1) detected a coincidence of preferential flow paths stained with one dye (Methylene blue) under ponded. saturated conditions with macropores stained with another dye (Rhodamine B) under unsaturated conditions.

The influence of macropore flow at comparably low input rates (3-3 mm h-l) was investigated on the basis of water contents measured by neutron probe. During a sequence of rain events, successive loggingx

Hydraulic conductivity (mm he’)

Supply tension -8 cm (ponded) 3cm 6 cm I’cm

Arithmetic mean Standard deviation (s.d.) Coefficient variation Geometric mean Geometric s.d.

25.1 25.0 48.7 4.3 27.8 18.0 14.8 14.5 0.58 20. I 2.3

4.1

2.1 I .h

2.6 1.9 0.8 1.3 0.8 0.3 0.50 0.40 0.30 2.4 I .8 0.8 1.6 I .6 I .4

2.6 2.1

1.1

I.1

0.x 0.6

K.G. Villholth et al./Journal of Hydrology 207 (1998) 98-120

5

a-

0-, 1 I I I I I A A A

-B E 833 -‘129’30’31’1 ‘2 ‘3 4 5 6 7 6 9 10 11 12 13 1 ‘15

Nov-91

Fig. IO. Precipitation and times of monitoring during the neutron log campaign

were performed to investigate moisture changes in the the soil, more or less evenly throughout the unsatu- soil profile. Results indicate that after an initial rated zone. Additional infiltration subsequently results imbibition accompanied by a general increase in the in groundwater rise only. After the initial matrix wet- soil water content and an increase in the water table ting which may arise from partial interception of height, additional water input did not increase the soil water flowing in the macropores, the soil primarily moisture content significantly even when the logging wets from below, and the groundwater rise reflects was performed immediately following the rain event filling of the macropores and adjacent portions of (Figs. 10 and 11). Rather a concurrent rise in the the soil matrix. Andreini and Steenhuis (1990) noticed groundwater table was observed. The interpretation preferential flow effects on undisturbed soil columns of these findings is that infiltration primarily occurs at a constant application rate of 0.8 mm h-l. (Ither through the macropores. The water content in the researchers have reported threshold values of rain matrix increases during the start of the wetting of intensity or water application rates to promote

0

0.6

E ; 0.6 X p” 1

1.6

Boring N45

3

2

1

Boring N46

113

0.6

0.6

0

0.2

0.4

0.6

0.6

1

1.2

1.4

1.6

Boring N47

0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70

Vol.% Vol.% Vol.%

Fig. 11, Volumetric water content and groundwater level in three profiles during the neutron log campaign.

+1

542

-A--3

3-e-4

2-X-6

+Porosity

1

114 K.G. Villholth et ul.Nournal of Hydrology 207 ( IYYLT} 98-120

macropore flow in the range of 2-3 mm h-’ (Trudgill et al., 1983; Kneale and White. 1984: Coles and Trudgill, 1985: Tsuboyama et al., 1994). Factors found to influence this threshold value. that hence is not a constant. were antecedent moisture content, duration of application. and soil structure. There seems to be increasing evidence and recognition of macropore flow in unsaturated. non-ponded soil (Radulovich et al., 1992).

3.5. Mucropore effect in dp soil

Macropore flow in very dry soil was evidenced from incipient drain flow during a 5 year rain event (41 mm in 15 h; maximum intensity, 9 mm h-‘) in the summer of 1991. The groundwater table in PLOT 3 which was 0.9 m below the drain depth prior to the onset of the storm showed only a minor and delayed increase in response to the rain input. The abrupt drain response before the saturated zone intercepts the drain indicates that relatively continuous and drain- connected macropores were responsible for the rapid downward movement of the infiltrating water. In addition, an internal catchment of infiltrating water from the macropores to the soil matrix is likely due to large sorption capacity of the dry soil and to macropores ending at a depth above the antecedent groundwater table. Steenhuis et al. (1988) attributed drain flow preceding a groundwater rise above the drain level to direct pathways from the surface layer to the tile line at 0.8 m depth. Urbanek and Dolezal (1984) demonstrated by gypsum casting and excavation the physical connection between a worm channel and a drain tile at 0.95 m depth in Czecho- slovakian soil. Similarly, Scatter and Kanchanasut (198 1) found by dye experiments cracks and other macropores in conjunction reaching a mole drain at 0.4 m depth.

Conclusions based on these findings are that macropore flow in the investigated soil is promoted at low and commonly occurring input rates. on the order of one tenth of the saturated hydraulic conductivity (approximately 2 mm h-l). in wet, but not necessarily ponded soil. In addition, macropore flow will be prominent in the dry soil when the input rates are larger than one-tenth of the saturated hydraulic conductivity and the input duration is relatively large (minimum 6 h).

3.6. Effect of soil surface conditiom

A factor significantly influencing the effect of macropore flow is the degree of continuity and connectivity of the structural pores. The non-systematic incident of immediate tracer breakthrough in the three plots was attributed to variable extent of direct macropore connection to the tile drains. Surface connectivity of the macropores also appears to affect the transport pathways. In PLOT 3 the drain response generally displayed a more spiky appearance with peaks of relatively sharper leading edges in SEASON 2 compared to SEASON 3 (Figs. 6, and 12 ), a fact that is not explainable by significantly different weather conditions. Rather, an interpretation is the surface sealing and the soil surface compaction that arose during the spring and summer of 1991, between SEASON 2 and SEASON 3 when the soil in PLOT 3 was partly bare and a fallow crop was establishing. Macropore openings at the soil surface possibly became sealed due to impact of rain and human activity. Vertical macropores emerging just below the soil surface were in fact observed during the infiltration tests performed in the summer of 1991. Also. the porosity at the soil surface was observed to decrease from SEASON 2 to SEASON 3 using the gamma/neutron logging, indicating a general compaction of the topsoil. No manual irrigation with hoses was conducted on PLOT 3 and hence a possible sealing from surface flooding of the soil is not likely. A tracer experiment was conducted on PLOT 3 in SEASON 3 only. and hence the observed hydraulic influence of the surface conditions could not be substantiated by evident changes in the tracer behavior. The effect of time-variable surface macropore structure on soil infiltrability has been documented in an experimental study by Messing and Jarvis ( 1993 ).

3.7. Macropore-matrix interactiorz

The soil macropores may primarily act as conduits for water and solute and the matrix porosity may have the primary role of retaining and storing the soil solution. However, the system apparently functions as a dynamic interacting bi-continuum. Support for such a concept is obtained from interpretation of the observed continuous time evolution of the chloride concentration in the drainage effluent. The singular

K.G. Villholth et al.Nournal of Hydrology 207 (1998) 98-120 115

PLOT 3, SEASON 2

0 h Now90 Dee-90 Jan-91 ’ Feb-91 ’

44

43.5 Ground c 43 .z __._~_---____~.

m 42.5

PE m 42 .o*e.,, e&C+ c P 41.5

P Drain

41

40.5

40 1 Nov-90 Dee-90 Jan-91 Feb-91

Fig. 12. Observed time series data from PLOT 3, SEASON 2. Hydraulic head elevation is monitored at 2.6 m depth.

and transient peaks following tracer application in response to rain events indicate that pulses of surface- derived solute are moving through the macropore system (Fig. 6). The peak response to rain events ceases after a relatively short time (4 days) indicating that the surface source has been depleted and less concentrated solution is moving through the macropores. An exchange of solute between the macropores and the matrix is evident from the relatively constant drainage concentration following the initial peaks. Only during heavy rainfall later in the season is a tendency for dilution of the drainage effluent apparent, indicating that chloride release from the matrix to the macropores is limited by diffusive

(Leeds-Harrison, 1995) or slow convective transport in the matrix.

3.8. Mass balance of chloride

The amount of chloride contained in the initial breakthrough peak represents 0.1% and 0.3% of the applied total masses for PLOT 2 and PLOT 3, respectively, implying a very small initial tracer loss. How- ever, the ensuing drainage concentration in PLOT 3, except for a few secondary steep and short-lived peaks within the first 4 days following the initial breakthrough, is elevated approximately lo-fold in comparison to pre-experimental conditions and remains

116 KG. Villholth et al./Joumal of Hydrology 207 (19%) 9% I20

fairly constant throughout the rest of the drainage period. Rather than a significant initial loss of tracer to the drain, the main impact of the initial fast macropore transport is the partial bypassing of the unsaturated zone and the relatively rapid and deep penetration of the solute into the profile. From the lower, variably saturated part of the profile an increased and fairly persistent source of contamination to the drain is provided. From an overall mass balance analysis of chloride the accumulated drain loss amounts to approximately 20% of the applied total mass (including chloride in rain) in DRAIN 1 (PLOT 1 + PLOT 2) as well as DRAIN 2 (PLOT 3) after the first effective drainage season (Fig. 13) indicating that the long-term drain response at the plots were fairly similar (SEASON 1 and SEASON 2 combined is considered as one effective

drainage season as applications in SEASON 1 took place shortly before the cessation of drainage discharge). The initial pathways of the tracers thus appears to be a primary factor influencing the long- term leaching.

Significant chloride leaching persisted in DRAlN I (PLOT 1 and PLOT 2) until the end of SEASON 3 when concentrations in soil water and drainage water were approaching pre-experiment levels. SEASON I and SEASON 2 contributed two-thirds of the total drain loss with the remaining drain leaching occurring in SEASON 3. The temporal displacement of the leaching load towards early times of the breakthrough event is a feature of transport dominated by initial macropore effects which is generally recognized based on small-scale. short-term column experiments. This overall transport picture has hence been

PLOT 1 AND PLOT 2, SEASON 1 - 3

t Tracer Ra1ll Tracer

136

i i i

SEASON 1 A = 103(75%) I & =66(75%) o-1

SEASON2 A = 27(20%) *:7 u - 47 30%

SEASON 3 A ~0 Az? II - 19

PLOT 3 , SEASON 3

A =21 (55%)

Fig. 13. Chloride mass balances for the tracer experiments. Figures in kg. 9% iigures represent fractions of total input mass.

K.G. Villholth et al.Nournal of Hydrology 207 (1998) 98-120 117

documented for a larger-scale (temporally as well as spatially) in a subsurface-drained field soil.

3.9. Loss to sugace or groundwater?

Aside from being strongly governed by the macropore flow and transport processes, an overall picture of the tracer behavior includes the effects of the drainage and the dynamic interaction between the unsaturated and the saturated zone. An essential question is whether the tracer which represents a conservative contaminant is primarily lost to the deeper groundwater or to a surface water recipient via drainage water or shallow groundwater flow. The lateral transport in the upper shallow groundwater appeared to be significant and to dominate the short as well as the long-term solute transport. From post-application chloride profiles taken on a perpendicular line from the drain including the gap zone between the drain and the tracer area in PLOT 3, a significant translocation of the solute in the horizontal direction into the gap zone is evident with only little concurrent vertical displacement (Fig. 14). In addition, the chloride profiles within the tracer area showed a tendency for skewness towards the upper soil layers, indicating that dilution from upstream. unaffected water had occurred in the lower saturated part of the profile. Analysis of the hydraulic gradients determined from the piezometers revealed that the horizontal gradient close to the drain during high flows was a factor of 5 larger than the vertical gradient.

DIO 0.0

0.5

1.0

1.5

2.0

2.5

D9 D9

3.0 t ” ” ” 1 3.0 a 0 1 2 3 4 0 1 2 3 4

Clconc., g/l Clconc., g/l

Evidence of lateral flow on a larger scale was based on the mass balance analyses in conjunction with the chloride profiles. At the termination of

I the momtormg, after SEASON 3, approximately 30% of the total applied chloride mass in PLOT 1 and PLOT 2 had been recovered in the drainage effluent from DRAIN 1. At this time the profiles in PLOT 1 revealed only minor increases in the chloride concentration within the tracer area, and at the bottom of the profiles, at 3 m depth., the background concentration was maintained indicating non-significant downward movement past 3 m depth (Fig. 15). This finding suggests that approximately 70% of the chloride input was lost laterally from the tracer area or taken up by plants, and only little mass was transported to deeper groundwater.

The lateral flow paths presumably were provided by shear planes between structural blocks and peds. In addition, sand pockets in the till matrix were observed during the piezometer installation and soil core sampling, indicating that increased textural porosity could give rise to lateral locally increased flow, par- ticularly if the pockets were connected to the (drain line. Contour maps of the hydraulic head elevation measured at 1.3 and 2.6 m depth in 22 locations in PLOT 3 imply a regional baseflow governed by the topography. Apart from a dominating lateral hydraulic gradient, a lack of macropore continuity to depths below the drain could control the overall flow pattern. However, this could not be established from the study. Other studies involving isolated cores from

2.5

D4

0 1 2 3 4 0 1 2 3 4

Clconc., g/l Clconc., g/l

Fig. 14. Chloride concentration distribution with depth in a transect perpendicular to the drain in PLOT 3. SEASON 3. Observation from the first sampling after the tracer application (PROFILE 3b).

118 K.G. Villholth et al./Journal of Hydrology 207 (1998) 9X-120

the same geological formation suggest that vertical transport through macropores in deeper saturated layers is possible (Jorgensen and Fredericia, 1992). Whether this is occurring in the field remains to be investigated, especially in contrasting areas dominated by regional recharge of groundwater. In a weathered and fractured clay till McKay et al. (1993) found the hydraulic movement to be predominantly horizontal in response to water injection to vertically dug trenches in the upper 4 m of the saturated zone. However, there was hydraulic and isotopic evidence at the site and other sites of vertical flow along widely spaced (meters to tens of meters) vertical fractures extending up to 6 m into the unweathered clays (Ruland et al., 1991; McKay and Cherry, 1992). In summary. the conditions at the site in question appear to cause larger potential loads of agricultural contaminants to the surface water than to the deeper groundwater, either as drainage loss or as shallow lateral

4. Conclusions

An extended drainage study in a macroporous glacial till soil has been conducted to document and describe effects of macropore flow and transport under field conditions. Macropore flow and transport effects were evidenced by the rapid responses in the quantity and quality of the subsurface effluent at 1.2 m depth upon soil surface application of water and a conservative tracer. Penetration of the chloride tracer to depths below 1 m in the unsaturated zone after a few events of high drain flow was also taken as a strong indication of macropore transport. The effect of the initial macropore transport of the solute was a relatively small immediate load to the drainage system (less than 1% of the applied tracer amount) and an ensuing steady drain leaching on a longer time-scale governed by the subsequent release of solute retained in the micropore domain of the soil.

For chemically and biologically reactive constituents a rapid initial transmission to the drain may groundwater discharge.

c12 Cl0 c9 Cl1

t 0.0

0.5

2.0

2.5

3.0 0 500 1000 1500 2000

0.0

0.6

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1 f

E 1.0

K

2.5

3.0 0 500 1000 1500 2000

IL_ 0 500 1000 1500 2000

Cl6

2.6

Cl3 0.0 1 I

2.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0 L 0 SW 1000 1500 2OW

Cl-cone., mgll

0.5

E 1.0

E:

f 2.5

t ” “’ 3.0 0 500 1000 1500 2000

Cl-cont., mgll 0 500 low 1500 2ooo

Cl-cone., mgll

0 500 1000 1540 2000

Cl-cone., mgll

Fig. 15. Chloride concentration distribution with depth in the PLOT 1. Observation from the last sampling after the tracer application (PROFILE Id).


be detrimental despite a relatively small initial loss of mass to the surface water. In case of substances of relatively high biological toxicity such as certain pesticides an instant flush of highly concentrated substance may impact the surface water quality and natural flora and fauna more than a steady leaching of contaminant at elevated but moderate concentrations.

The significance of macropore flow in the investigated soil was found to extend to relatively low and commonly occurring rainfall rates. In addition, soluble agents applied in solid form on the soil surface are liable to leaching for an extended period of time. Hence, surface application of agricultural chemicals needs special attention in order to minimize leaching through macropores. In as much as the soil surface conditions are to a large extent manageable in cultivated soil, the dynamics of macropore structure at the soil surface and the influence on chemical transport deserve additional research efforts.

The analysis of flow and solute distribution indicated a minor loss of contaminant to the deeper groundwater at the location investigated. In terms of the soil and drainage conditions, the experimental site was representative of much of the cultivated land in the eastern part of Denmark. However, a general con- clusion on the fate of surface-applied chemicals cannot be based on a single study. It is speculated that the flow governed by the subsurface drainage system is superimposed on a regional groundwater flow pattern that greatly influences the long-term pathways of the tracer.

Applying the Poiseuille equation for gravitational flow in vertical cylindrical tubes (representing the ubiquitous earthworm channels) appeared to significantly underestimate the field-observed active macroporosity due to non-ideal flow in the macropores. Even so, the active macroporosity based on the drainage experiments (0.2%) which was considered more representative, comprised no more than 10% of the total visible macroporosity under situations of high natural flow. The tension infiltration approach gave valuable information of the matrix and macropore hydraulic conductivity and contribution to flow in the soil surface layers which was used in the model interpretation of the tracer experiments (Villholth and Jensen. 1998).

Acknowledgements

W e

thank our technician, Klaus Fa?ster Hansen, for his assistance with the field experiments. We also thank Chris Ogden and an anonymous reviewer for their constructive comments and suggestions.

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